Physics mid term, Last half

Scales of size and time

- 10^-15m to 10^29m and 10^-18s to 10^17s - Most of these size and time scales are way beyond our every-day experience (1m and 1s). - Quarks > Atoms > Person > Area > City > Country > Continent > Planet > Star > Solar System > Solar Interstellar Neighbor hood > Milky way Galaxy > Local Galactic Group > Virgo Supercluster > Local Superclusters > Observable Universe

Beatrice Tinsley

- 1941 - 1981 ○ Raised in New Zealand ○ PhD at the UT Austin (1966) - Worked on galaxy evolution, cosmology ○ Key papers in the field - Galaxies evolving as a function of time

Expanding Universe

- 5 years between "discovering" galaxies ○ And learning that the universe expands - Our "picture" of the universe grows dramatically - Another Copernican revolution ○ We are no longer at the centre of an island universe - Today ○ We use redshift as a measure of distance

Jupiter

- 5.2 AU from sun - Consists of Hydrogen and helium - Low density (whole planet is just a massive ball of atmosphere) - 10 hour rotational period - Has rings (but not as extensive as Saturn) - Has a magnetic field - can see aurora at the poles (the most massive magnetic field in the solar system) - Has a relatively small core. - Pressure is so high that the hydrogen can become a metal (solid).

The active Earth

- 70% of Earth's surface is covered by water. - Mountains are relatively rapidly eroded away by the forces of water.

Red giants and binaries

- 75% of stars have at least 1 companion - First two observed supernova progenitors were in binaries - Can loss some mass onto companion The evolution of one star in binary system can affect the evolution of the companion star

Hubble's "constant"

- A little piece of a curve always looks straight - Does the expansion rate change with time? - If so, distant parts of the universe expand at a different (faster) speed... - Standard matter - Expansion slows down

Parallax equations (PARallax arcSECond )

- A parsec is the distance at which 1 AU subtends an angle of 1 arcsecond - p=d/D where d is the Earth-Sun distance, D is the Sun-star distance, and p is the parallax angle

Thermal equilibrium

- A positron and electron can annihilate with each other ○ Total charge is zero ○ And for two photons (gamma-rays) - Two gamma rays can convert into a particle-antiparticle pair ○ If they have enough energy: "equilibrium" ○ As the universe cools, energy of photons drops ○ Eventually process goes one way

Solar wind and impact on Earth

- A prominence can erupt can 'snap' and then reconnect, but this can send a pulse of solar wind particles towards earth - Solar wind from sun gets deflected by earth's magnetic field and gets deflected away from earth so that is doesn't impact surface of the earth o What we see at the poles of the earth, some of the particles strike atmosphere of earth= produce aurora

Andromeda, 1885

- Andromeda is a spiral galaxy, bigger than the Milky Way ○ A supernova occurred in Andromeda in 1885 ○ Easily visible in a small telescope - Argument went: ○ Andromeda spectrum very similar to individual stars (stellar in nature) ○ Supernova occurred but Andromeda was just thought to be a nearby object so the cause was through to be much less luminous (thought it was a nova) ○ This was all wrong It was a supernova within this galaxy that caused it, not just a regular nova.

Earth's tectonic history

- Another form of evidence for tectonic plates. - Earth started off being a single land mass which spread around the Earth due to plate tectonics. - Can see this in fossil/species record. - Creatures with the same ancestor can be explained if the areas of land was a single land mass in the beginning.

Kepler's Laws of Orbital Motion

- Applies to all orbits. 1. Planetary orbits are ellipses with the Sun at one focus. 2. A line between the planet and the Sun sweeps out equal areas in equal times. 3. (T1 / T2)^2 = (R1 / R2)^3 or comparing planets to Earth: (Orbital Period / year)^2 = (Distance from Sun / A.U.)^3

Recombination

- Around 380,000 years after the Bang Bang ○ Universe cooled enough for hydrogen atoms to be stable ○ Universe now transparent ○ Called "recombination", really just "combination" - Microwave background photons can now move freely ○ Provides us with a "baby photo" of the universe

The Cosmological Principle

- Assume the universe is homogeneous and isotropic ○ "Cosmological principle" ○ Isotropic: same in every direction ○ Homogeneous: same in every place - True on average; universe is not exactly the same everywhere ○ This coincides with the "Great Debate" / island universe

Power Source: Nuclear fusion (what is the energy of the sun?)

- At low temperature: with 2 charged particles are moving slowly, when they get close they will repel (like charged particles repel) - The sun is at high temperature: two charged particles at high speeds in high temperature and when they are close enough, their electric field in unable to repel them and when they fuse and energy is released

The Stable Sun:

- At the core of the sun, fusion reactions are occurring which produces lots of energy and radiates out of sun - This energy (from core of sun) heats balls of gas of the sun which becomes plasma (flows outwards), is constantly fighting against the inward pool of gravity (in equilibrium as a stable object

Georges Lemaître

- Belgian, 1894-1966 - Expanding universe: 1927 ○ "Primeval atom" ○ "Cosmic egg" ○ Universe began at a fixed point in time - Work known to Hubble ○ Widely discussed in 1930s ○ Independent of Friedmann

Escape Velocity

- Escape velocity is the speed that the particle must achieve in order to escape an object's gravitational pull. - Smaller mass objects = smaller escape velocity. - Eg. If you throw a ball up into the air, if you do not surpass the escape velocity, then the tennis ball will come back down. However, if you throw it so fast/hard that it exceeds the escape velocity, the Earth will not be able to pull the ball back own with its gravitational pull.

Four main stages of Evolution

- Gravitational collapse of proto stellar cloud, with disc. - Condensation of gas cloud to chondrules. - Accretion of gas and dust to form planetesimals. - Accretion of planetesimals

Distance Ladder

- If one rung is off, the rest will be out (errors propagate) ○ Leads to systematic errors... ○ Many different, overlapping options ○ But all measurements involve several rungs - Hubble flow: bound objects don't expand ○ Observationally + theoretically - Can also infer Hubble's constant from observations of the microwave background

23 hours, 56 minutes (Sagittarius A)

- Knew that this constellation was in the sky and not the ground because it appeared every 23h 56mins - Earth spins on its axis every 24 hours ("day"!) - Without spin, sun would rise and set once a year - Day + orbit: star rises every 23 hours and 56 mins - 4 mins x 365 ~ 24 hours - Source reappearing every 23hrs 56mins is in the sky (something different)

Saturn

- Known for its extensive ring system. - Ring appear edge-on to Earth every 15 years. - Rotational period 17.25 hours. - Has magnetic fields (we know this because they have Aurora). - Has many moons

Redshift

- Light "stretched" in an expanding universe ○ Cosmological redshift different from kinematic redshift ○ All cosmological observers think they are at rest ○ Redshift is induced by the growth of the universe

Elliptical Galaxies

- Little gas and dust ○ No ongoing star formation ("red and dead") - old stars ○ Stars orbit at all angles ○ No spiral arms - Most massive galaxies known are ellipticals ○ Range from 107 - 1013 solar masses - Giant elliptical: typically at the centre of clusters

Nebula: What are they?

- Look up into sky and see stars ○ But also see "extended objects" - nebulae, or "clouds" - All stars are (basically) the same ○ But "nebulae" are very diverse - Two questions: ○ What are they made from, and how do they shine? ○ If something giving off light, something must be paying the power bill (metaphorically speaking) - Supernova is way bigger than just a nova

Galaxies

- Most stars in center ○ But velocity ~ constant. (speed does not die away with distance) ○ This is different to our knowledge from the solar system ○ (Period) ~ (Radius) - Vera Rubin (1975-80) ○ Spiral galaxies: outer stars move faster than they "should" Does it apply to the rules of Newtons Law: - Most stars in the centre - Speed of outside of galaxies (outer stars) do not die away with distance (velocity constant) - (Period)~(Radius)

Tides

- Moon causes tidal bulges of water. - If we had no moon or stars, the oceans would be completely symmetrical all over the world. - However since we have the moon, the ocean gets drawn towards the edges and towards the moon and also in the opposite direction too. - The Earth is spinning so we get the two tides at the back and the front. We actually have two tides during the period of a day.

Neutron star merger (to discover more of in future)

- Neutron star mergers most commonly associated with gamma ray bursts ○ Gravitational waves move at similar speed to speed of light ○ Detected this through gamma ray burst and neutron star merge

What is Dark energy?

- Nobody knows: this is a deep mystery ○ Density does not change as universe expands ○ Consistent with energy conservation ○ Associated with "negative pressure" [like a stretched spring] - Seem to need a good idea... ○ May be explainable as a modification to gravity ○ Or the interplay between quantum mechanics and gravity

Origin of Spiral Arms

- Not just that outer parts orbit more slowly - Or spirals would "wind up" with time - And bars would be unstable - Also, bright stars in spiral arms are young § Many with lifetimes less than time to make one orbit - Density wave and/or shock wave triggers star formation - "Rotates" around galaxy

Planetary Nebula

- Nothing to do with planets S-process nucleosynthesis Don't need to know this process

The Early Universe

- Nucleosynthesis - The Cosmic Microwave Background - Inflation: universe undergoes expansion

What is a white dwarf star?

- One end result of a low-intermediate mass star's life - Composed mostly of electron-degenerate matter - Very dense. About mass of Sun, about size of Earth - No fusion reactions at core - Not very luminous (HR diagram), still hot, bluer

Galileo Galilei (1594 - 1642)

- One of the evidence is Planetary motion. - Reinvented the modern view of science: Transition from a faith-based 'science' to an observation-based scientific method. - Greatly improved on the newly improved telescope technology (Did not invent it). - First to report telescope observations of the sky to support the Copernican model of the universe.

How is Hubble's constant measured

- One way: Start with Cepheids ○ Nearby Cepheids distances be measured with parallax ○ Up to 1500 light years away; 15 stars - Cepheids found in galaxies which host supernovae ○ Establish distance to galaxies; calibrate supernovae ○ Supernovae themselves "standardizable", not standard. - Hubble's constant: 73.00 ± 1.75 km/s/Mpc ○ Second way: microwave background (to come)

Pluto

- Orbital period 248 years. - Rotational period 6.4 days (tidally locked with its moon Charon). - Still has an atmosphere despite being so small. - Discovered by Clive Tombaugh - Tidally locked with its moon Charon - Has an atmosphere

Solar system

- Outer planets go further because they have a bigger orbit ○ And go more slowly because gravity is weaker as you go further away from the centre of the solar system - The inner planets move faster - Speed ~ (distance)0.5 - Planets further from the sun - ve more slowly... - All as predicted by Newton - Almost all the mass of the solar system is in the sun

Stellar parallax

- Parallax is the observed apparent change in the position of an object resulting from a change in the position of the observer. - Specifically, in the case of astronomy it refers to the apparent displacement of a nearby star as seen from an observer on Earth.

What Can We Do About This? (Darkness at night)

- Live in a finite galaxy in an infinite, otherwise empty space ○ That works! (But does not look like our universe) - Light from distant stars is absorbed by gas and dust ○ Except the gas and dust heats up and so dust will heat up to the temperature where it will no longer absorb heat ○ In an infinitely old universe, that won't help - Assume the universe has a finite age ○ Speed of light limits how far we can see. ○ That will work

What Bad luck

- "Spiral nebulae" avoided the plane of the Milky Way ○ Zone of avoidance ○ Looks like they are connected with our galaxy ○ Now know that the Milky Way is opaque § Lots of galaxies in the milky way, we just cannot see them (light is absorbed) - Adriaan van Maanen (1916) claimed to see rotation in M101 ○ IF it is as big as the Milky Way, stars move faster than light ○ Now know: it is rotating, but not fast enough for him to see it ○ It was actually an optical distortion between two images ○ Was actually just an observational error

Novae and Supernovae

- "Temporary stars" or Novae (singular: nova) ○ Commonly observed in the Milky Way ○ No supernova in Milky Way observed since 1600s ○ Gas accreting in a thin layer on a white dwarf ○ "Burns" via "normal" nuclear fusion - Supernovae can also be caused by accretion on white dwarf - But white dwarf collapses to a neutron star (kaboom)

Where we are now

- 100 thousand light years in diameter (disk) - Milky Way contains between 1011 and 5x1011 stars (??) ○ Lots of very small, faint stars (which are hard to count) hence the variability - If you shrink Earth's orbit to the diameter of a human hair.... ○ α Centauri is 13.4m away ○ Milky Way is 250km in diameter

size of star and its evolution time + candle analogy

- Stars spent most of their time on main sequence (mass determines where they are on the main sequence), when they run out of fuel at the core, they evolve towards the right seuqnece into red giants, redder (lower temperature) - All stars formed roughly same time but they all have different masses, larger stars evolve off main seuqence faster, burn their fuel at the core sooner, by looking at a globular cluster we can give an age for that custer due to a similar birth date - Red= 1 Mega year (1 million year), blue= 10 million years etc... - The heavier the star= less time spent on main sequence

Galaxies evolve with time

- Stellar populations get older ○ With no star formation, only small old stars left (ellipticals) - Old stars return gas to interstellar medium ○ Either by supernova explosions ○ Or stellar winds (see planetary nebula) ○ Chemical composition changes with time ○ Process is nuclear physics, not chemistry

What the Universe Looks Like

- Structure of the Milky Way Galaxy and our place within it ○ 1700s - 1930s - One galaxy or many? ○ Settled in 1920s - Large scale structure: stars live in galaxies; how are galaxies arranged in space... ○ 1980s - now...

There is a cycle for sun spots: Period of 11 years

- Sun goes through busy and quiet times (sometimes it is more magnetically active than others, more sunspots= more solar magnetic activity - Sun is dynamic cycle, magnetic fields correlate to number of sunspots

Loops

- Sunspots arise from interactions between intense magnetic field and material on surface of sun - Magnetic field coming out of surface of the sun looping out of sun (N) and to surface of the sun (S) - Loops of plasma follow the magnetic field lines through the sunspots via the photosphere - Loops slow down the Convection loops in the convection zone (which transported in these loops), the magnetic loops poking through the photosphere , the convection transport gets suppressed so no more rising of materials= leaving sunspots - Material trapped in the magnetic field lines forming the prominences

What we see (sky)

- Temperate: 2.7 Kelvins - Temperature differences: parts in 100,000 - "Foreground subtraction" - hot gas, dust - Signal:noise > 100:1 at large scales ○ And it reflects the history of the universe - First (knowingly) detected in 1965 ○ Each point in the sky ~3 degrees above absolute zero ○ The universe is slightly "warmer" than expected

Inverse square law

- The energy we receive is inversely proportional to square of the distance. - Can predict the temperatures of planets. - The energy we receive is inversely proportional to square of the distance. - The larger the distance, the larger the energy received.

How the Universe Changes With Time

- The expansion of the universe ○ Discovered in 1929 - The Microwave Background and Confirmation of the Big Bang ○ Discovered in 1965 ○ Microwave background is that the atmosphere is slightly warm 2.7 kelvin above absolute zero - Understanding the physics of the very early universe ○ 1940 - through to today

The future: our local universe

- The fraction of dark energy increases with time ○ Hubble's constant evolves towards a fixed value - Galaxies getting further away from each other ○ Apparent motion speeds up (over billions of years) - Eventually, distant galaxies would "move" faster than light ○ And they become invisible ○ So after billions of years, only the local group will be visible

The fusion processes at centre of sta

- The fusion processes at centre of star, depends temperature and pressure of core of star, depennds strongly on mass of star (how much gravity is pulling matter into core of the star, changing pressure and temperature), drives what fusion reactions can occur at core of star and drives which elements can be mashed together to form additional energy What quantities at the centre of a star dictate what fusion reactions can occur? - Temperature - pressure

Motions of Sky (Moon)

- The illumination of the moon changes during its orbit of the Earth (due to the Sun).

Different Kinds of Atoms

- The kind of atom depends on the number of protons in the nucleus. - Most abundant: Hydrogen (one proton + one electron). Next abundant: Helium (two protons and two neutrons + 2 electrons) - Number of protons determines what type of element it is. - Number of neutrons determines what type of isotope it is. (Isotopes are when there are different number of neutrons but same number of protons.)

Roche Limit

- The radius around a celestial body (planet, star etc.), within which a second body held together only by gravitational forces will be torn apart by tidal gravitational forces. - Sets a boundary of which moons can form around planets. - Hence we find the ring system between the planet and the moons. - There are a few exceptions of which can exist inwards of the Roche limit. - Due to sticky and adhesive forces other than gravitational forces

Deceleration of hubbles constant

- The slope of the line gives Hubble's constant ○ The shape of the line tells us how the "constant" changes ○ We can find acceleration without the distance ladder - We can't see individual stars in the distant universe ○ And galaxies come in sizes

Slowing down of Earth's rotation

- The tidal bulges are actually slightly off centred. - This is because the Earth's ocean is constantly trying to slow down the Earth due to the moon's gravitational pull, whereas the Earth is trying to rotate in its normal 24 hour rotation. - There is friction between the Earth, the Earth's water and Moon's gravitational pull.

Maori Astronomy

- There is clear evidence that the Maori had extensive knowledge of the night sky, well before the arrival of the Pakeha. - Includes details of seasons and using the stars for navigation. - Knowledge and understanding has been mixed with colonial knowledge and lost over the past two centuries.

Earth has magnetic field

- These are caused by circulating currents from the interior of the Earth. - Magnetic fields deflect the solar winds around the Earth. - Prevents the particles from the sun (radiation) from striking the surface of the Earth. - For a planet to have life, it may require magnetic fields to stop high energy radiation from disrupting life patterns. - Magnetic fields can sometimes become trapped and strike the atmosphere at the poles of Earth. When this happens, the gas becomes excited and then de-excites itself by emitting photons of light. You see this as an Aurora.

Why do we only see one side of the Moon? What is with the 3 : 2 resonance on Mercury?

- This is because there is a distance dependence in the force due to gravity (differential gravity field). - One side of the plant feels a stronger gravitational field than the opposite side (the moon towards the Earth and Mercury towards the Sun). - The forces try to squeeze the force and material out, making a slightly oblate sphere. - It is non-spherical and hence, not symmetrical meaning a bit more mass in one particular direction. This means a higher force due to gravity will act on the bulge (extra mass). - So, when the moon and mercury try to spin in their own orbit, due to the additional forces from the Sun and Earth, they are unable to and hence keep facing the same way.

Radio Astronomy

- This method is used because it is hard to see through the dust in the milky way - Radio invented in the 19th century ○ A complicated story... ○ Tesla, "Hertzian waves", Marconi, even Rutherford - 1930s: Karl Jansky founded radio astronomy ○ Need directional detector (not so easy for radio) ○ Or timing as the source rises and sets ○ Or even when source passes behind the moon (quasars, 1963) Brightest source at "low" frequencies: Sagittarius A

The cosmic pie

- This picture changes as a function of time ○ Dark energy has a constant density ○ Matter density drops by a factor of eight each time the universe doubles in size - Dark energy will overtake and dominate the universe - The coincidence problem - 70% dark energy, 25% dark matter, 4% gas and dust, 1% stars - This picture changes as a function of time ○ Dark energy has a constant density ○ Regular Matter density drops by a factor eight each time the universe doubles in size - Ten billion years ago, just a few % dark energy ○ Ten billion years from now, just a few % normal matter ○ On our way for universe to be dark energy dominated

Orbit/Rotation of Venus

- Venus rotates clockwise, with period slightly longer than orbital period. - Orbital motion is anticlockwise, so Venus has opposite orbital and rotational motion. Possible reasons of this: - Off-centre collision with massive protoplanet (more likely). - Tidal forces of the Sun on molten core.

The Surface of Mercury

- Very similar to Earth's moon. - Heavily battered with caters, including some large basins. - No wind, water, plate tectonics > means there are no erosive forces > the surface must be OLD. - A record of impacts tell us about early formation history of the solar system, and these craters and old surfaces are the records. - The records allows us to reconstruct what happened when the planets were forming.

Motions of sky (Sun)

- Stars rise in the East and sets in the West. - Southern celestial pole: Stars rotates around here in southern hemisphere. - The sun's apparent path on the sky is called the ecliptic. - Star signs are the stars which were up in the background while you were born. - Earth's rotation axis is tilted by 23.5 degrees with respect to ecliptic (hence why Northern and Southern hemispheres have different seasons)

The atmosphere of Mars

- Very thin: Only 1% of pressure on Earth's surface. - 95% CO2. - Due to the tiny mass, the surface gravity is too low to hold onto a thick atmosphere. - Can see thin Martian atmosphere through a haze and clouds covering the planet. - There is a magnetic field but it is not strong like on Earth and Mercury. - Can sometimes see dust storms. Tiny mass = lower escape velocity = cannot hold onto an appreciable amount of atmosphere

What is at work on the Moons

- Volcanism (volcanoes). - Venting o water or frozen mixtures of carbon and hydrogen. - Impact craters. - Push-me pull-me gravitational stress (tidal forces). - Plate tectonics (in this case ice fracturing and folding).

Wavelength

- Wavelength of light is very short, hence it cannot pass through walls/corners. - Two important properties: Wavelength of light (how much energy the light carries), The intensity. - Shorter the wavelength, the more energy it carries Eg. X-ray, gamma rays, UV rays. - Longer wavelengths have small amounts of energy Eg. Infrared Visible spectrum: 400nm ~ 700nm (nano 10^-9)

Absolute magnitudes

- We measure the brightness of a star (m) - We also measure its parallax (p) - We put all the stars at a distance of 10pc and then see how bright they are - 10pc corresponds to a parallax of 1/10 or 0.1 arc seconds. - These magnitudes are called absolute magnitudes (Mv) - At 10pc the Sun is a very ordinary star of absolute magnitude 4.6. - The brightest stars maybe at magnitude -10 or 3 lots of 100 (100^3) or a million times brighter. - The faintest stars may be at magnitude 15 or 100 x 100 fainter or 10,000 times fainter than the Sun. For example, the Sun has apparent magnitude of -27. At 10 parsecs: M(sun)= -27 - 5 log(4.8×10-6) + 5 = 4.6

Sky is not completely black at night... (disproves infinite universe)

- We see some spots of light (stars) ○ Now imagine the "galaxy" was infinitely large ○ And infinitely old, and with no gas or dust ○ With the same density of stars everywhere, at all times. ○ Then there would be a star wherever you looked ○ Hence, should be bright at night aswell - Olber's Paradox ○ Also Kepler's paradox, and Halley's paradox

Sun Spots

- When the sun is slightly more active - Correspond to Slightly cooler spots on the sun that appear darker

Galaxies Evolve

- Will see that the universe is around 13.8 billion years old ○ Galaxies billions of light years away younger than Milky Way - Study evolution of galaxies by looking deep into space ○ Although distant galaxies are very faint - Hubble Extreme Deep Field [25 September 2012] ○ 50 days of telescope time ○ Most visible light shifted into the infrared

When the Sun and the moon are aligned (Full Moon/New Moon)

- there are strong tides (Spring tides). - This strong tide occurs because both the moon and Sun have the same direction of gravitational pull.

Hubble- galaxies theory that is false

- thought that galaxies grew in a sequence (not just stay elliptical for the whole life) - Thought galaxies were in a evolutionary track

High-mass star (life cycle)

1. Interstellar cloud (light-years across) 2. Collapses down to a proto-star. 3. Starts main-sequence. 4. Forms helium core, becomes red giant. 5. Blue loop during helium burning. (He>CO) 6. CO core, burning stages continue to iron-group elements core. 7. Star explodes in a core-collapse supernova. (run of reactions to sustain itself, release of gravitational energy) 8. Leaves neutron star or black hole.

Major Discoveries of Galileo

1. Moons of Jupiter (4 Galilean Moons) 2. Rings of Saturn 3. Moon has shadows, could estimate the height of mountains on the moon (it is lumpy): Had imperfections even though according to authority it was meant to be PERFECT AND DIVINE. 4. Sun spots (proves that sun is not perfect). 5. Phases of Venus, proving that Venus orbits the sun and not the Earth.

Two sources of heat in Earth's interior (Produces heat)

1. Potential energy from material falling into the Earth (Gravitational potential energy turning into kinetic energy). 2. Decay of radioactive material (decay of such material produces heat). Most traces of bombardment (impact craters) now destroyed by later geological activity.

Score card for dark matter existing

1. Rotation curve 2. Cluster dynamics 3. Cluster lensing 4. Galaxy/cluster interactions 5. Structure formation 6. Microwave background

But why was the Earth thought to be stationary (Greek)?

1. The Earth is not part of the heavens (today we know it is one of 8 planets and our sun is quite ordinary). 2. The celestial objects are bright points of light while Earth is an immense nonluminous sphere of mud and rock. 3. The Greeks saw little change in the heavens while Earth was the home of birth, change and decay. 4. Finally, our senses show that the Earth appears to be stationary; There isn't a strong wind that we should technically feel since the Earth is rotating so fast.

Human effects on Earth's atmosphere

1. The greenhouse effect - Earth's surface is heated by the sun's radiation. - Heat energy is re-radiated from Earth's surface as infrared radiation. - Greenhouse gases such as H2O and CO2, but also other gases in the atmosphere, absorb infrared light (heat is trapped in the atmosphere). - It occurs naturally an is essential to maintain a comfortable temperature on Earth. - Without greenhouse gases, the Earth would be about 33K colder on average. - However, human activity, in particular CO2 emissions from cars and industrial plants, is increasing the concentration of greenhouse gases. 2. Destruction of the Ozone layer - Ozone absorbs UV radiation (which has damaging effects on human and animal tissue). Chlorofluorocarbons (CFC's) which are used in industrial processes, refrigeration and air conditioning, destroy the ozone layer.

Two Laws of Black Body Radiation (1)

1. The hotter an object is, the more luminous it is: Where; A = surface area Sigma = Stefan-Boltzmann constant L = Luminosity T = Temperature

Planets are discovered by (mainly):

1. Transit technique - Light from host star becomes periodically blocked by the planet orbiting around it. 2. Radial Velocity · Light from a star gets blue shifted (towards shorter wavelengths (higher velocity), towards you) and red shifted (towards longer wavelengths, away from you). · If the source in relation to observer is moving towards you the sound/light increase frequency and if move away it will decrease frequency 3. Microlensing (wikipedia) - Light from a distant star is bent and enhanced by the gravitational field of a closer star. The presence of an exo-planet rotating around that closer star bends the light of the star farther away periodically. 4. Direct imaging take a direct image of the planet.

Type 1a Supernovae

1. Two normal stars in binary 2. More massive star becomes giant 3. Giant spills gas on to secondary star and engulfs in envelope 4. Both stars spiral towards within common envelope 5. Common envelope ejected while separation between stars decrease 6. Remaining core of giant collapses and becomes white dwarf 7. Aging companion star swells and spills gas on to white dwarf 8. White dwarf increases in mass until critical mass then explodes 9. Companion star ejected away

Olber's Paradox

A paradox pointing out that if the universe were infinite in both age and size (with stars found throughout the universe), then the sky would not be dark at night.

Chinese (SN1006), Arabic, Indian Astronomy

CHINESE - Star exploded. - Catalogue of supernovae. "Guest star" used for when a new star is seen ARABIC Al-Battani (CE 850-929, Baghdad/Cordoba) made 4% error in measuring orbit of Earth around the Sun. Also made very accurate measurement of circumference of Earth. INDIAN Considerable expertise in observational astronomy. Most records lost in religious wars.

Albert Einstein (1879 - 1955)

Came up two theories of relativity. 1. Special relativity (1): - Based on one assumption: that the speed of light is the same for ALL observers no matter how they're moving. - Everything is now moving at CONSTANT velocities. 2. Special relativity (2): - E = mc^2 - Energy has mass. When you move, you gain kinetic energy and hence increase in mass. However, by Newton's 2nd law, it would be more difficult to accelerate with higher mass. This is why it is not possible to travel faster than the speed of light. 3. General Relativity - Given tools to consider accelerations. - Gravity is not a force but more due to curved/bent space time. - Objects "think" they feel a force of gravity but they are actually moving in straight lines. However, the space they move through is curved/bent hence appearing to move in a curved trajectory. - Curvature changes. Anything that is orbiting causes ripples in space time. - Changing ripples would give waves oscillating.

Chromosphere and Corona both seen in eclipse

Chromosphere: Layer above the photosphere Corona: Stream of plasma from sun

Discovery of gravitational waves

Confirmation of general relativity

How do most massive stars die?

End result: neutron star orbiting each other (binary stars) > spiral inwards > releasing energy as they merge together > black hole

Habitable Planets, planetary radius

Habitable Planets - Habitability is strongly linked to whether a planet can support liquid water on its surface. Planetary radius - Too small: planet loses atmosphere. - Too large: planet becomes a gas giant.

Stefan-Boltzmann Law

Luminosity is proportional to fourth power of temperature.

Great Debate - 1920

National Academy of Sciences, Washington DC - Harlow Shapley - nebulae are "local" - Heber D. Curtis - nebulae are galaxies

Population 1 vs population 2 stars

Population 2 are from star-formation a long time ago, population 1 are from more recent star-formation

Habitable zones

Right temperature for water to exist. Mars (and Venus both) in the habitable zone. Although we need the right temperature range, habitability is more than just the idea of a planet around its host star. Need to consider other things such as; magnetic fields, change in atmosphere over the years, plate tectonics

Rotation Curve Doppler Shift

Speed the galaxy is moving + contribution of gravity and halo

star clusters

Star clusters are groups of stars which are gravitationally bound. Two distinct types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are often very young

Stars evolution

Stars evolve over their lifetimes, they run out of hydrogen fuel and reconfigure themselves to star "burning" different elements to release energy. Their size and colour changes. This means that the same star appears at different places on a HR or CMD diagram at different stages of its life - Evolutionary track: up , as it runs out of hydrogen fuel to burn up into giant > across left> down to white dwarf (more red, less luminous) - Main sequence > giant star> white dwarf

Stellar nurseries and Nebula

Stellar nurseries - An area of outer space within a dense nebula in which gas and dust are contracting, resulting in the formation of new stars. Nebula - A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases in a galaxy

Ptolemy (85 - 165 C.E.)

Thought there was a epi cycle (Another circular orbit for Mars where it goes backwards at one point)

The Dark Universe: Is what we see all we have?

What we see is the lights coming from stars in the galaxy but even if the stars were removed the galaxy would still exist. Most of the skeleton of the galaxy is coming from dark matter.

Spectral lines and Atomic Transitions

When electron moves to lower energy level (shell) it releases energy in the form of a photon. This shows one emission line of the spectrum.

Neutron stars

These objects are what remains of the core of massive stars (10-30 solar masses) that have undergone stellar evolution, a core-collapse supernova and were massive enough after the supernova to contract beyond the white dwarf stage

Magnitude (m)

· 1 is brightest, 6 is dimmest (everything else falls in between). · Linear scale, each one magnitude is a factor of 2.512.

Mounting

· Azimuth mount: centered wherever the observer is on earth Equatorial mount: accounts for earth's axis

Refractors (glass lenses)

· Image is upside down. · Need to use convex lens as we try to use bigger lenses to capture more light. · Light rays go through the glass, different things are focussed at different wavelengths (chromatic aberration). · Only has one ray of light.

Degenerate matter

· Regular star: More mass = larger size. · Degenerate star: More mass = smaller size. - Because to hold up more mass you need more gravity to have more pressure against it. For degenerate stars you have to pack the particles closer together hence becomes smaller.

White dwarfs

· These stars are what remains of the core of low mass stars that have undergone evolution hence they are very hot. o These are compact objects with masses that of the sun but of sizes that of the Earth. o Nuclear reactions have stopped within them. o They are doomed to cool over billions of years. o Their cooling rates have given us independent estimates of the age of the universe. o Can explode in supernovae's.

Neutron star- conservation of angular momentum

· When a star shrinks down (from main sequence to neutron star) to a smaller size then it will spin up and faster. Low mass= high v, high mass= low v, angular momentum (p) conserved p=mv - angular momentum before (high mass, low speed) = angular momentum (low mass, high speed)

Two examples of explaining how things move in space without observing it

Uranus and Neptune - Uranus' orbit did not match calculations (assuming Uranus only talks to Sun) - Adams and Le Verrier guessed there might be another planet further from Sun (in same direction as Uranus) - They were right: Neptune exists (Uranus pulled towards Neptune so it slows down orbit) Mercury - Elliptical orbit - "Perihelion precession" ○ Out by 43" of arc/century - Tried everything explanation (vulcan) ○ Could be a smaller planet closer to the sun (vulcan) - New rules (solution): General Relativity 1915 ○ When closer to sun, it doesn't quite fit with Newton's law

Mercury

Very similar to Earth' moon in several ways: - Small, low mass so therefore, no atmosphere. (low mass = low escape velocity = cannot hold on to the atmosphere) - Lowlands flooded by ancient lava flows. Heavily cratered surfaces. - Closest planet to the sun, but has water ice. This water ice exists because there are some areas on - Mercury that are in permanent shadow and hence the water that has been brought upon the planet through comets have frozen and remained in those shadowed craters.

Reflectors (mirrors).

uses concave mirrors. light travels down the tube where it is reflected up to a secondary mirror near the top of the tube, which directs the light into the eyepiece.

Cosmic Distance Ladder

- each method forms a "rung" on the cosmic distance ladder, allowing us to calculate distances across the entire Universe!

Tycho Brahe (1546 - 1601 C.E.)

- Best observational astronomer before the invention of the telescope. - Could find location of planets to a few arc minutes. - Considered and understood that there would be observational bias hence used two observatories. - Had a mathematical model. He suggested that the Earth is still stationary and that the Sun was going around the Earth but the other planets were orbiting around the Sun. - Stars are 700 times farther away from Saturn than Saturn is from the Sun. (But it is actually 28500 times further).

Cepheid Variables

- Big pulsating stars ○ Two kinds of Cepheid: Population 1 and Population 2 ○ Different period-luminosity relationship ○ So Hubble was out by a factor of a few... - Key step in the Cosmic Distance Ladder ○ Need to know distance to local Cepheids ○ Recent (last decade) measures via parallax Cepheid star's period (how often it pulsates) is directly related to its luminosity or brightness.

Progress in Astronomy: Technology

- Biggest telescopes - Better detectors - More wavelengths (radio, ultraviolet, microwave) - Computing and data-processing

Microwave Background

- Billion (or so) CMB photons / hydrogen atom in the universe ○ These photons have redshifted as universe expands ○ Universe cools as it gets bigger ○ Temperature ~ 1/size - Universe now dominated by matter (and dark energy) ○ Early universe dominated by radiation / particle physics ○ So, in the very early universe, everything "melts"

Biological life in extremities

- Biological processes can be carried out in the absence of sunlight, at extreme temperatures and at extreme pressures. · E.g. Bacterial life found in sub-ice layer lake in Antarctica. · Black smoker in Atlantic ocean.

Copernicus (1473 - 1543 C.E.)

- Built on from Aristarchus' ideas where the Sun is in the centre and the planets orbit around it. - Problem was the parallax effect. The stars are too far for us to see the parallactic shift but people back then did not believe this and only believed that the Earth was stationary and hence thought this theory was wrong.

Solar Eclipse

- Can have solar eclipse when Moon comes between the Sun and the Earth (Sun gets blocked off)

What holds it together? Dark matter cannot it just be gas and dust

- Cannot be just ordinary matter that is not emitting light? ○ No otherwise we would still see it ○ Either in emission if gets heated up (dust) ○ Or via absorption (block the light, gas preferentially absorbs light) - Also constraints from the big bang (next week) - Black holes, or "MACHOS" - can't find enough of them - Plus, we don't how to make them So what is it? - Dark matter is likely to be a fundamental particle ○ But can't be one we have already know ○ We know dozens of particles, so room for more ○ Many reasonable theories can explain dark matter ○ Makes up about 23% of the universe - Direct detection experiments, particle accelerators (LHC), astrophysical evidence (to look for dark matter)

What are we looking at?

- Central object is invisible - But clearly VERY massive - Work out mass from orbital velocity ○ 4 million solar masses - S-02: closest approach 17 light hours - About twice the distance between Pluto and the sun

After Andromeda

- Cepheid discovered 1923, published 1925 ○ Although word got out beforehand - Hubble went looking for Cepheids in other bright galaxies ○ We can call them that now... ○ And he found them - Also looked at brightest stars, other "distance indicators"

Cepheid Distances

- Cepheids are bright and rare ○ Have to calibrate distance scale ○ Need distances to some Cepheids ○ Used to be indirect; now have parallaxes - If you are wrong about Cepheids distances, you are wrong about everything else

Examples of Dwarf planets

- Ceres (Asteroid belt) - Pluto (Kuiper belt) - At least five moons (Charon, Nix, Hydra etc.) - Haumea (Kuiper belt) - Has two moons (Hi'iaka and Namaka) - Makemake (Kuiper belt) - Eris (Scattered disk) - Most massive dwarf planet, but smaller than Pluto. - One moon (Dysnomia) - Sedna

Newton's Law of Gravity (EQUATION TESTED)

- Circular motion needs a force and Newton realised this and came up with this law. - G = gravitational constant, M & m = the mass of the two objects, d = distance between the objects - Fg will be the same for both objects. - The acceleration would however be different (F = ma) - Large objects with higher masses would have small acceleration, whereas small objects with low masses would have large acceleration (in order to be the same Fg)

Classes of spectral type (temperature scale)

- Classes from O (>3000K in temperature) to M (<3500 K in temperature) - Classes are further subdivided from 0 (hottest) to 9 (coolest) - The sun is a G2 star

Heat

- Colour changes. - Hotter stars = bluer stars = shorter wavelengths. - Becomes brighter with higher heat. - Really hot things are almost white light due to lots of wavelengths coming together. - Hotter objects are brighter and "bluer" than cooler objects

4 Million Solar Mass Black Hole

- Ensures centre of galaxy is a violent environment ○ Strong gravitational field ○ Huge accelerations - Radius of "event horizon" ○ ~60 million kilometres (well inside earth's orbit) ○ S0-2 does not come close enough to the black hole to be torn apart - Black hole never directly observed: but nothing else fits... Black holes and stars don't explain the radio or x-ray emission from the centre of the galaxy on their own... Galactic Centre also contains hot dense gas, emits radio and x-ray

Formation History of Neptune and Uranus

- Core accretion theory cannot be used to explain the formation of these ice giants because: - The escape velocity of when they were proto planets (9km/s) is comparable to their orbital velocity (5km/s). - This similarity in speeds makes it really easy for the stuff in the orbit to go over their escape velocities and hence end up flying away off into space (disappears). - The material out where Uranus and Neptune are today, is relatively easily ejected through interactions with Jupiter and Saturn. This leaves too little material to build these ice giants. - Could have migrated. - These planets may have formed closer to the centre (but outwards of the snowline) of the solar system and migrated out to their place after being formed. § Formed in a protoplanetary disc and the streams of material pulls the planets towards or away from the sun (outward migration for Uranus and Neptune). - Another theory is that these planets formed between Jupiter and Saturn and were then gravitationally scattered out of their present orbits.

Review of lecture 1

- Cosmology built around the big bang and expanding universe ○ New science: 20th century; some of it very new indeed. - The Milky Way is one of many galaxies in the universe - Galaxies interact, merge and evolve as the universe ages - Galaxies are grouped into clusters: "large scale structure" - Dark sector: dark matter and dark energy

Direct Detection

- Dark matter particles hitting earth ○ Interact weakly with regular matter ○ We are transparent to dark matter ○ And dark matter is transparent to us - Some dark matter particles interact ○ Detect recoiling atoms ○ Ongoing effort; many experiments

Of the inner planets, Mercury is the:

- Densest - Oldest surface - Massive core - Has a magnetic field, like Earth (Mars and Venus o not have an internal magnetic field).

How Big is the Universe

- Depends on what you mean - Mathematically, an open or flat universe is infinite ○ Wrong because we cannot see an infinite distance - Light travels at a finite speed: 300,000 km/s - Naive answer: Edge of visible universe 13.8 billion light years ○ But stretched by expansion ~40 billion years ○ Numbers modified by dark energy and inflation ○ Wrong because the source of the light has been moved away from us due to the expansion of the universe

General Relativity

- Discovered in 1915 ○ Changes ideas of space and time ○ And is a theory of gravity - "Matter tells space how to curve, and space tells matter how to move" - John Wheeler - Light moving in a straight line but the movement naturally becomes curved because of the gravity of a massive star

Dwarf and irregular galaxies

- Dwarf galaxies are small ○ 1% of the mass of the Milky Way (or less) ○ Fewer stars, so hard to see at great distances - Irregular galaxies are irregular ○ Can be rich in gas and dust ○ No fixed shape ○ Often disturbed by interactions with larger galaxies

Anti matter vs. matter

- Early universe has just a little bit more matter than antimatter ○ Electrons and positrons (and other, heavier pairs) annihilate ○ Wind up with a universe with more photons than electrons ○ And number of electrons matches number of protons (slightly more matter than anti matter) - Neutrons and protons: ratio depends on temperature ○ One neutron for every seven protons provided that the universe is hot enough (otherwise at "freeze out") ○ Depends on mass difference; photon density ○ Free neutrons: key difference to stellar interiors

Nucleosynthesis

- Early universe is too hot to form nuclei ○ Deuterium (proton + neutron) is fragile, first thing to be made ○ So universe must cool for nuclear burning to begin ○ Prediction: the universe is 25% helium BY MASS § 1/8 are neutrons straight after big bang, assume that all neutrons fuse into helium ○ Actually ~24% (some neutrons decay, plus other things)

The Atmosphere

- Earth had a primeval atmosphere from remaining gasses captured during formation of Earth. - Processes such as rainfall, water, temperature fluctuations, volcanism, plate tectonics and LIFE on the surface of Earth, has changed the atmosphere of the Earth dramatically from what it had been when it was first formed. - Atmospheric composition severely altered (secondary atmosphere) through a combination of two processes. 1. Outgassing: Release of gasses bound in compounds in the Earth's interior through volcanic activity. 2. Later bombardment with icy meteoroids and comets.

Problem that ancient Greeks faced

- Earth overtakes Mars in its orbit. - Mars appears to go backwards due to a projection effect due to Mars going slower in its orbit and Earth going past it. - They did not know that Mars and all the outer Earth planets does this loop.

Tectonic Plates

- Earth's crust is composed of several distinct tectonic plates, which are in constant motion with respect to each other (plate tectonics) - Evidence for plate tectonics can be found on the ocean floor and in geologically active regions all around the Pacific. - Bands of material come out from tectonic plates - Earthquake: due to convection currents from the Earth's mantle.

Einstein and Cosmology: 1920s

- Einstein tries to build an infinite, static universe ○ Realises it won't work - Even if the universe begins at rest, gravity sets things moving ○ Adds a "cosmological constant" term ○ Allowed by the math, but not otherwise required - "Greatest blunder of my life"

Emission Lines

- Electrons emit light at one specific wavelength. - This is what occurs in fluorescent light. - Can be used to determine what element you are looking at. - Physics is the same for emission and absorption spectrum.

Ancient Greek Astronomers

- Eudoxus (409 - 356 B.C.): Geocentric model of 27 nested spheres to explain planetary motion. ○ There were planets (wanderers) and the moon that was also in the sky moving (moving with respect to background stars, geocentric view where Earth was thought to be the centre of the universe and everything revolves around it). - Aristotle (384 - 322 B.C.): Universe can be divided into two parts ○ Followed the geocentric model, hence the initial assumptions were wrong so the logic ended up being wrong. ○ Imperfect, the changeable Earth, at the centre of the Universe. ○ Perfect Heavens (described by spheres). He expanded Eudoxus' Model to Use 55 Spheres - Aristarchus (310 - 230 B.C.): Got correct order of planets and a SUN centred solar system. Also believed that stars were other suns. (Idea rejected because of Aristotle's strong influence to the society).

Cosmological Motion

- Expansion of the universe is different from regular motion ○ New space "created" as the universe expands ○ Everyone thinks they are sitting still and at the "centre" ○ A homogeneous universe cannot have a "centre"

Supernovae

- Exploding stars: briefly outshine an entire galaxy ○ So bright enough to be seen across the universe - Several types of supernovae ○ Type II: Giant collapsing stars - "core collapse" ○ Type Ia: Accreting white dwarf in a binary system - Chandrasekhar limit ○ Maximal mass of a white dwarf

The Atmosphere of Venus

- Extremely inhospitable: (96% CO2, 3.5% Nitrogen and the rest is water, HCl and hydrofluoric acid. - Four thick cloud layers can be seen through UV image. - Has very efficient greenhouse not just because of the atmosphere being made up of carbon dioxide, but because it has a very dense atmosphere. - Has an extremely high surface temperature (745K = 472 degrees Celsius).

Numbers (Black hole merger)

- First merger observed: September 14, 2015 at 09:50:45 UTC - Distance from Earth: 420 Mpc; 1300ly (±30% in diastance) - 36+29 solar mass black holes progenitors that went into merge - 62 solar mass final state - 3 solar masses worth of energy radiated

The doppler effect

- Frequencies will change if something is moving. - Blue shift (to higher frequencies) : moving towards you - Red shift (to lower frequencies) : moving away from you

The Moon: The view from the Earth

- From Earth, we always see the same side of the moon. - Moon rotates around its axis in the same time that it takes to orbit around Earth. - Tidal coupling: - Earth's gravitation has produced tidal bulges on the moon. - Tidal forces have slowed rotation down to same period as orbital period. - Has an elliptical orbit and so may sometimes appear bigger and brighter.

Discoveries last year

- Further black hole mergers (Contribution from Virgo) - Nobel Prize (Rai Weiss, Kip Thorne, Barry Barish) for LIGO - Announcement of neutron star merger detection - Gravitational signal + optical and gamma ray counter parts

Conclusions for galaxies

- Galaxies (all galaxies) appear to be embedded in a dark halo ○ Halo extends well beyond the visible part of the galaxy ○ Dark fraction ~80% of total mass - And sometimes higher: ultra-faint galaxies ○ As few as 100s of stars ○ But a million solar masses - rest is dark matter ○ Gas (to make stars) can be stripped by interactions

Galaxy Clusters

- Galaxies are not distributed randomly in space ○ But form clusters and groups - Milky Way and M31: big galaxies in the local group - Clusters ○ e.g. Virgo cluster (~55 million light years) - Clusters combine to form superclusters ○ Local Group is part of the Virgo Supercluster

Active galaxies

- Galaxies contain millions, billions or trillions or stars ○ Intrinsically bright -- stars shine, after all. - But active galaxies generate energy at their cores ○ Remember the centre of the Milky Way

Local Group

- Galaxies continue to merge ○ Velocity of M31 measured in three dimensions ○ Will merge with Milky Way in ~4 billion years - Left with a big galaxy in an empty universe ○ Stars eventually grow old and die ○ Gas and dust consumed - Universe is "red and dead"

Galaxy clusters

- Galaxies live in clusters and superclusters ○ Measure their velocities - Assume all matter in is stars ○ Going too fast to hold together - Back to Zwicky (1930s) - If clusters not "bound", they must be "chance gatherings" - But there are far too many of them for that... - Look for "missing mass" ○ Hot gas (not enough to explain it to be bound) - Works nicely for dark matter -Needs a new modification to gravity (scale of galaxy too small)

What do we learn from LIGO detection?

- Gamma ray burst detected 1.8 seconds after gravitational wave signal ○ Speed of gravity = speed of light; ○ To within seconds in 140 million years - Strong evidence that gold (and other heavy metals) produced in neutron star interactions; not just supernovae! - Allows an independent measurement of Hubble's constant - Also kills lots of theories of "alternative gravity" that try to provide alternatives to dark matter

The Disk

- Gas and dust concentrated in the disk ○ Cannot make new stars without these - Star formation occurs in the disk (where we find young stars) - In open clusters ○ A few hundred stars, young, Population I ○ 10 to 40 light years across ○ Not evenly distributed in disk ○ Probe different lines of sight (e.g. blue stars, gas in 21cm)

Earth's interior

- Gets hotter towards the centre. - Earth's core is as hot as the sun's surface (6000 degrees Celsius). - Earth's core is solid despite being so hot. - Outer core is liquid. - Stiffer mantle.

Surface of Mars (geology, volcanism)

- Giant Volcanoes - Valleys - Impact craters - Has a massive trench (Marianna Trench) - Reddish deserts of broken rock, probably smashed by meteorite impacts. Volcanism Olympus Mons: - The highest and largest volcano in the solar system. - Youngest volcano on Mars. - Volcanoes on Mars are shield volcanoes (shallow slope 5~6 degrees). 21km above mean Mars surface.

Ripples (Black hole merger)

- Gravitational wave signals from two black holes orbiting each other (if the signal is constant then the ripples would be constant). - The gravitational waves take some energy out of the system ○ To pay for this the black holes move closer to each other (further down their potential wells) ○ This releases energy ○ Makes black holes speed up ○ Generate gravitational waves more efficiently ○ Radiate more quickly ○ Move even closer together ○ Will collapse into each other (merge) - Signal called a "Chirp" - Detect these signals with LIGO detectors ○ Bending and stretching of the signals ○ Rainer Weiss devised LIGO (1970) and built from 1990s - 2015

Mars

- Has a smaller diameter but also a much smaller mass so would have a much smaller escape velocity. - Axis tilted against orbital plane by 25 degrees, similar to Earth's inclination of 23.5 degrees. - Seasons similar to Earth: Has the growth and shrinking of polar ice cap. - Crust not broken into tectonic plates. There has been volcanic activity in the past (including highest volcano in the solar system) possibly ongoing.

Pluto surface

- Has craters. - Has ability to reform in some areas. - The area which has been reformed is mainly made of; nitrogen ice, carbon dioxide ice and these flow like glaciers.

Where we are now Lecture 7

- Have a complete list of basic ingredients of the universe ○ Dark matter, dark energy, radiation, atoms (baryons) ○ Plus neutrino background - Often called "Concordance Cosmology" or Lambda-CDM

Johaness Kepler (1571 - 1630 C.E.)

- Hired by Tycho to work out mathematical detail of his model (he however used Copernicus' model instead as he believed the Earth also orbited around the sun). - He could only model the solar system to fit Tycho's observations if: 1. Planetary orbits are ellipses with the Sun at one focus of the ellipse. 2. A line between the planet and the Sun sweeps out equal areas in equal times. - Areas taken up in 3 days will always be the same, whatever position of the elliptical orbit the planet is in at the time. (T1 / T2)^2 = (R1 / R2)^3 or comparing planets to Earth (Orbital Period / year)^2 = (Distance from Sun / A.U.)^3

1. Temperatures of stars (planck and wiens law)

- Hot= bright - Warm= dim Planck's law - Emission of EM radiation us described by Planck's law - The peak of the spectrum depends on the temperature of the object -Higher temperature, lower wavelength, intensity increases Wien's law - A star's temperature is related to its colour. - If we know its colour we can calculate its temperature T= (3 x 10^6) / λ(max), where peark wavelength is in nanometers (nm) and temperature is in degrees Kelvin (K) - Or if we want to find the peak wavelength λ(max)= (3 x 10^6)/T -1 nm= 10^-9 meters

Kilometres/second/megaparsec

- Hubble's Constant units: Kilometres / second / megaparsec ○ Current best value: 66.93 ± 0.62 km/s/Mpc - From CMB+other data, not a direct measurement [Planck] ○ Direct measurement: 73.00 ± 1.75 km/s/Mpc - A galaxy 100 megaparsecs away: Moving at 6,730 km/s. - A galaxy 1000 megaparsecs away: Moving at 67,300 km/s ○ 26% of the speed of light

Age of the Universe

- Hubble's constant is not really constant ○ Expansion rate slows down (at least without dark energy) ○ Measure Hubble's constant - compute age of the universe - Hubble got ~500 km/s/Mpc ○ Universe much younger than the sun. ~500 km/s/Mpc ○ Would be a problem if stars were older than the universe (cant exist that way) - Today: Hubble's constant ~70 km/s/Mpc (+ dark energy) ○ Universe ~13.8 billion years old Just a little older than oldest stars (which is good) ○ Would also be a problem if the universe was way older than all the stars because it is thought that the first stars formed were only a few hundred million years after the big bang

The evolution of solar type stars- Stars less than 8 solar masses (low and intermediate mass stars)

- Hydrogen burning, pp chain (proton-proton fusion chain process, Fusion into 2 hydrogen atoms into helium), happens in core of star 1. Proton + proton > deuterium (+ neutrino) 2. Dertrium +proton > helium-3 3. Helium-3 + helium-3 > helium-4 + 2 protons - Where does the energy come from? Some mass is converted into energy (E=mc^2) Evidence for nuclear fusion? Passage of neutrinos which exit the planet

The Scientific Method

- Hypothesis to explain an observation - Make a prediction - New Observations - Match? ○ Yes > Accepted theory ○ No > New hypothesis required. - A good theory will solve problems using the same pattern of reasoning or problem-solving strategy and opens up new areas of research. - Note it is different to using correlations (correlations may show higher chances but not 100% conclusive). - No matter how much evidence we have for a conclusion, the conclusion could still conceivably be false. Concept of falsifiability is key to science (Karl Popper). - Theories change but physical laws do not.

Cartoon Cluster

- If clusters not "bound", they must be "chance gatherings" - But there are far too many of them for that... - Look for "missing mass" ○ Hot gas (not enough!) - Works nicely for dark matter - Needs a new modification to gravity

Galaxy/cluster interactions

- If galaxies just made out of stars and gas, they can influence each other beyond gravitational forces - Two clouds of dark matter can just pass straight through each other (bullet cluster)

Impact Cratering

- Impact craters on the moon can be seen easily even with small telescopes - Ejecta from the impact can be seen as bright rays originating from young craters. History of Impact cratering - Rate of impacts due to interplanetary bombardment decreased rapidly after the formation of the solar system. - Most craters seen on the moon's (and Mercury's) surface were formed within the first ~ half billion years.

Constellations

- In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures. - The stars of a constellation only appear to be close to one another, this is usually only a projection effect. - A star may APPEAR to have the same brightness as another star but may be a lot further away. This means the star is actually a lot brighter to compensate for the distance.

Redshift & Distance

- In the 1920s, astronomers took spectra of galaxies ○ Milton Humason ○ Saw that galaxies were moving with large velocities ○ Some toward us, mostly away from us - Distant galaxies moving faster away from us than nearby ones ○ Seems as though the universe is expanding

Intermediate-mass stars (life line)

- Interstellar cloud (light-years across) - Collapses down to a proto-star - Starts main-sequence - Forms helium core, becomes red giant - Blue loop during helium burning - CO core, becomes AGB star - Dust driven mass-loss removes envelope - Forms a planetary nebula that slowly fades - Leaves a (carbon-oxygen) white dwarf remnant.

Magnitude scale

- Introduced by Hipparchus (160 - 127 B.C) - It is the other way around, the lower the magnitude means the brighter. Unlike how higher watts being more bright. - The more negative the magnitude, the brighter it is. - Brightest stars: ~1st Magnitude - Faintest stars (unaided eye): 6th Magnitude - 1st mag. Stars appear 100 times brighter than 6th mag. Stars. - 1 mag. Difference gives a factor of 2.512 in apparent brightness (Larger magnitude means fainter object).

Galilean Moons (Jupiter Moons)

- Io: closest to Jupiter - Highly volcanic - Hotspots on Io - The cause of the volcanism on Io is due to the intense gravitational field of Jupiter (differential gravitational field) - Europa - Looks almost void of any surface features except for the striations. - Very few impact craters (new surface) - Due to the gravitational pull, the striations are actually cracks and the material from inside come through these cracks and form the cold brittle surface ice. - Hence, Europa does not have many craters (they have been covered over/ reformed). - Plate tectonics seen due to tidal gravitational forces. - Ganymede - Solar system's largest moon. - Has an old surface, retains the impacts (very cratered). - Nothing gets rid of the craters (like how Io has volcanism and Europa has the ice). - Impact records of crater chains support the theory that comets are loose agglomerations of material. - Callisto

Dwarf Planets

- Is in orbit around the sun. - Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round (spherical) shape (due to hydrostatic equilibrium). - Vesta is a minor planet because it is not heavy enough to crush itself into a spherical shape (still quite lumpy and elliptical) - Has not cleared the neighbourhood of its orbit (it is not the biggest planet in its orbit). elliptical) - Is not a satellite of another planet.

Speed of gas particles

- It is a distribution (some fast, some slow). - The higher the temperature the higher the average speed, the cooler the temperature the lower the average speed. - Fast moving gas particles can escape from a planet's atmosphere such as Hydrogen. Also heavier molecules move slower at the same temperature than lighter molecules.

Other Evidence

- It is not just supernovae ○ Detailed appearance of the microwave background ○ Evolution of galaxy clusters (galaxy clusters stop growing) ○ "Baryon Acoustic Oscillations"

The Milky Way: Measuring its size

- Jacobus Kapteyn (1851-1922) ○ Parallax: Find the distances of nearby bright stars ○ Proper motion: stars appear to move relative to other stars ○ Closer stars appear to move more quickly than distant stars ○ Estimate the distance from spectral type § Use HR diagram and get the brightness and hence calculate distance ○ Estimate the size of Milky way by measuring distances of nearby stars and then compare to other fainter stars which are likely to be further away (measure size using this sort of method) - His conclusion ○ Milky Way 20,000 parsecs [65,000 ly] across § Wrong by a factor of 2 § But reasonably close ○ Fairly close to the centre. § Also ignored the effects of reddening and dust

Sorting out Great Debate

- Key work: Edwin Hubble ○ Hubble Space Telescope ○ Rhodes scholar & Anglophile - Discovered Cepheids in M31 (Andromeda galaxy) ○ Distance: ~million light years - 1923, Milky Way just one galaxy - He took images of M31 and looked at the outsides where there were individual stars

Oort and Lindblad

- Looked at proper motions and radial velocities ○ Used radial velocity from red (or blue) shift (could see what was moving towards and away from us) ○ Concluded that the Milky way galaxy rotates (1927) § If it did not rotate then the milky way would collapse into itself ○ Deduced the structure of the Milky Way ○ Milky way has; spiral arms, disk, bulge and halo... § Stars in plane are relatively old § Stars in spiral arms need to be continually refreshed, relatively young stars § Wave of star formation that propagates around the galaxy due to young stars going into supernova only after 1/100th of its orbit around the galaxy ○ 250 million years for stars (e.g. our sun) to orbit around the galaxy

Structure formation

- Lots of clumping as the universe evolves into the cosmic web structure - 80% dark matter - If it was just gas and atoms there would be more pressure so less collapse and hence clumping

Eccentricities of Ellipses

- Low eccentricities, the orbits are still very circular. - Centre of the ellipse becomes off centred of where the Sun is.

Lunar Eclipse

- Lunar eclipse occurs when the Earth passes between the Moon and the Sun. ○ The Earth casts the shadow on the moon. ○ The light gets bent when it goes through the atmosphere and this light is red. ○ Umbra is a full lunar eclipse (fully shadowed by Earth) Partial lunar eclipse called Penumbra.

Steady State Theory

- Main "competition" for the big bang until 1960s [Hoyle] ○ Universe still expanding in steady state cosmology ○ But new matter created to keep density constant [by magic] ○ Appearance of universe unchanging: no age problem - Hoyle coined the name "big bang", so he did invent the name ○ During a "radio lecture" in 1949 - Debate ends with discovery of microwave background (1965) ○ Except for Hoyle and a few others...

LHC "missing energy"

- Make dark matter, have a collision by two particles - Since dark matter doesn't interact with regular particles, with collision a lot of stuff moving in one direction but not the other ("missing energy")

Earth zones

- Mantle, outer core, inner core. - These zones all have different densities (the physical matter changes at a boundary). - Can measure the size of the zones in comparison to one another by watching how the seismic rays pass through the Earth. - Eg. Earthquakes give good signals for this.

The Milky Way: Globular Clusters (How we found out we are not at the centre)

- Many globular clusters visible in the sky (many in the milky way too) - Mostly 10s of 1000s of light years from earth - Harlow Shapley mapped distribution of globular clusters ○ Obtained distances from RR Lyrae variables § Have a characteristic relationship between luminosity and period § Although he initially thought they were Cepheids § [Cepheids discovered by Henrietta Swan Leavitt, 1908] § Globular clusters normally around the outsides of the Milky way as they define a bigger object such as the disk of the milky way itself (also more on one side of the galaxy) ○ Gets distances wrong, but shape right (of globular clusters) ○ Using clusters, we're not in the centre anymore (From Harlow Shapley) § We are not at the centre of the distribution of globular clusters § Globular clusters centred on the milky way rather than the sun ○ The Milky Way is about 100,000 light years in diameter

Three Possible Universes: Closed, Open and Flat (How to tell them apart)

- Measure average density AND Hubble's constant - For any value of Hubble's constant there is a critical density ○ Above critical density: closed, re-collapse ○ Below critical density: open, expand forever ○ At critical density: flat, expand forever - Note: this picture is modified by dark energy - Our universe is within 1% of being "flat" [Observations]

Hubble's Constant

- Measured: roughly ~70 km/s/Mpc ○ Mpc - mega-parsec, 1 million parsecs ○ If a galaxy is 100 Mpc away, 324 million light years ○ Receding at 70x100 = 7,000 km/s (kilometres/second) - Speed of light: 300,000 km/s - Hubble's constant tells us how fast the universe is expanding

Why inner core of Earth is solid

- Melting point is the temperature at which an element melts (transition from solid to liquid). - Melting point increases with increasing pressure toward the centre. - It gets to a point where the pressure is so great and the melting point is so great that it exceeds the temperature of the material in the core. - So it has an intense pressure but it is still not hot enough to melt the material at that high pressure. - Inner core becomes solid.

Microwave background

- Microwave background (to come) has hot and cold spots ○ Mix of spots matches predictions of dark matter - Ultra faint galaxies ○ As few a few hundred stars ○ But masses of hundreds of thousands of stars

Extrasolar Planets and the Habital zone

- Modern theory of planet formation linked to star's evolution o Many stars should have planets. o Planets orbiting around other stars are "extrasolar planets". - Cannot be imaged directly, look for 'wobbling' motion of the star around the center of mass HZ: Contains liquid water= supports life (0-100oc)

Distance: standard candles

- Parallax only works for nearby stars - However - if we know what type of star we are looking at then we know how bright it should really be. We can compare this value with how much dimmer it appears from Earth and use this to get a distance - How do we know what type of star it is? · We need to obtain a spectrum and compare it to other stars with similar spectra whose distances and intrinsic brightness we know · These types of objects are know as "standard candles" - Example: cepheid variables · Regular change in luminosity due to oscillations in size · Length of period is proportional to true brightness Remeber: if we know how bright they should be - we can infer their distance

CMB Discovery 1960's

- Penzias and Wilson: communications satellite technology (telephone company) ○ Sky "warm", same temperature wherever they "looked" ○ Assumed it was a problem with their electronics ○ Looking in microwave wavelengths (a few GHz) ○ Corresponds to 3Kelvin blackbody - Dicke & Peebles (Princeton) building receiver to look for CMB

Problems solved by "inflation"

- Period of accelerated expansion, just after the big bang ○ In fact, the universe could be "born" inflating ○ Universe grows (at least) 1030 times larger during inflation - Smooths out any initial bumps and lumps ○ Generates ripples via quantum fluctuations...

Dark Energy Discover

- Perlmutter, schmidt, Riess - Nobel prize physics 2011

"Dark Matter" in the Solar System

- Planets obey Kepler's Laws... ○ Kepler's laws are a consequence of Newton's laws - Plus small corrections ○ Planets pull on each other ○ Kepler's laws assume the planets only talk to the sun, and planets not talking to one another - Uranus discovered in 1781 - William Herschel

History of discovery of extra solar planets:

- Prior to 1992: only solar system planets 1995: first confirmed discovery of a planet around a normal star

Quasars and AGN

- QSO: Quasi-stellar radio source ○ Point-like; discovered in radio, late 50s + early 60s ○ No obvious "optical counterparts" ○ Associated with high redshift optical sources ○ 3C-273 -- occulted by the moon (1962) - Central engine of an AGN is an accreting black hole ○ 3C-273: energy output 2x1012 x sun (if omnidirectional)

Black Body Radiation

- Radiates over a range of wavelengths. - The light from a star is usually concentrated over a range of wavelengths. - The spectrum of a star's light is approximately a thermal spectrum called a black body spectrum. - A perfect black body emitter would not reflect any radiation. Black body: only ever emits or absorbs stuff at the given temperature.

Radio

- Radio waves generated by moving (accelerated) charged particles ○ Electromagnetic waves in general - Sag. A brightest source in radio sky ○ At long wavelengths ○ Something is moving electrons ○ Synchrotron radiation - Stronger source than the sun ○ Hard to explain Sag A with stars

What the Universe is Made of

- Relativistic astrophysics (black holes, pulsars, quasars...) ○ Late 1950s through to the present day... - Dark matter ○ Hints in the 1930s; confirmed (for most of us!) in the 1990s - Dark energy ○ Discovered in 1998 ○ 70% of the universe is dark energy

Neptune

- Rotational period 17 hours. - Has rings. - Position was predicted by Le Verrier and Adams. - Observed by Galileo during retrograde so he probably did not realise it was a planet. - Has massive dark spot (like Jupiter's big red spot) where there is a massive cyclonic storm occurring. - Has very strong (fastest) winds due to no turbulence which slow down the winds on other planets such as Jupiter.

Uranus

- Rotational period 17 hours. - Has rings. - Rotation axis in plane of orbit. - Uranus tilt 98 degrees. - Something hit it in the past which knocked it over. - Gives seasons due to this. - Has magnetic fields. - Hence can see an Aurora. - Discovered by William and Caroline Herschel - Moons: - Ariel - Umbriel - Oberon - Titania - Miranda - Puck

Alexander Friedmann

- Russian mathematician and meteorologist (1888-1925) ○ Pilot in World War I ○ World ballooning altitude record (7400 metres, 1925) ○ Expert in fluid dynamics - Developed cosmological solutions to general relativity ○ Homogeneous and isotropic universe

Spiral galaxies

- Same disk/bulge/halo structure as the Milky Way ○ Gas and dust in the arms ○ Continuous star formation - Spiral arm structure (no surprise there) ○ Formation mechanism unknown

Orbits and Spheres

- Satellites further from the earth or planets further from the sun ○ Move more slowly in their orbits - What if you could have an orbit inside the earth? ○ Only the fraction of the earth "inside" the orbit matters (contributes to the gravitational field) ○ Speed would be smaller than for an orbit at the surface ○ Assuming the earth has a constant density ○ Same with a galaxy... Closer to centre of sphere, the slower you would move (only applies to spheres)

Review of lect 2

- Shape and size of the Milky Way galaxy ○ What it is; how we figured it out ○ Disk, bulge and halo; Population 1 and 2 - Dust and gas [Pro-tip: dust lanes are not dark matter!] - Supermassive black hole in the centre (like all big galaxies) ○ Track stars in orbit around the black hole ○ Emission in radio and x-ray from hot gas - Quasars and "active galactic nuclei" or AGN (to come)

What we learn (lecture 7)

- Shape of the curve... ○ Expansion rate of the universe accelerating today ○ But was slowing down in the past (billions of years ago) ○ Predicted to be due to dark energy/matter - Consistent with a cosmological constant or "dark energy" ○ Idea of a cosmological constant is old (1920s) ○ Einstein's "biggest blunder" (if he really said it)

The Milky Way: What Shape is it?

- Sir William Herschel & Immanuel Kant - Disk shape collection of stars ○ This so how you can see the narrow band across the sky ○ See lots of stars when looking in directions lying in the disk ○ See fewer stars when looking in directions that lie out the disk ○ Sun is embedded inside the milky way - A couple of incorrect assumptions of Sun in centre of disk shaped milky way ○ No consideration of dust effects ○ Dust absorbs light (Milky way, especially the planes, have large amounts of dust) § Especially absorption of blue light compared to red light § Gives reddening to star colour § Cannot actually see parts of the milky way because the dust is thickest in those areas

Current Understanding of Galaxies

- Small galaxies form first ○ Big galaxies form from mergers of small galaxies ○ Galaxies can merge - Galaxies seen at a redshift of 8 ○ ie wavelength stretched by a factor of 8 ○ Seeing them ~hundreds of millions of years after big bang ○ Universe 10-20 times younger than it is today

Venus's Surface (radar map, craters, volcanism)

- Smooth lava flows. - Scattered impact craters (not as many as Mercury). - Surface features shown in artificial colours. - Volcanic regions Craters - Nearly 1000 impact craters on Venus (THIS IS NOT A LOT) - Surface not very old. - Craters appear sharp and fresh. - No water on the surface; thick, dense atmosphere. No erosion. Volcanism: - Has lava flow. - Can identify hotspots where the magma pushes the surface up ever so slightly (not actually bursting its way through) and then going back down again. - Actively going on. There is something going on underneath the surface.

Gaps in Saturn's ring system and the Moons

- Some of the gaps in Saturn's ring system are caused by orbital resonances with Saturn's inner moons. - Particles in the Cassini division orbit twice for every orbit of the moon Mimas. - This makes their orbit unstable, effectively causing the particles to take up a different orbit. - In the Enke gap, there is a tiny moon called Pan. - This moon sweeps material up as it orbits Saturn, especially in the equatorial regions of the moon. - The moon Titan has an extensive atmosphere. - Lakes on Titan. - First discovery of existing lakes other than on Earth. - Not quite water, actually comprised of hydrocarbons: methane, ethane. - Mimas - Very old (has many craters) - Iapetus - Also a very old surface. - Also has a seam. - Enceladus - Relatively new surface; some craters but not as many as Iapetus and Mimas. - Something must be reforming the surface since there are minimal amount of craters. - There is molecular hydrogen release coming out of the cold geysers. - Having geysers suggests that there is an internal heat source that liquefies the icy materials into water. - With this molecular hydrogen and the geysers shows that there could be a possibility of life being able to exist in Enceladus.

Seismology

- Soundwaves passing through the Earth - Seismic waves do not travel through Earth in straight lines or at constant speed. - They are bent by or bounce off transitions between different materials or different densities or temperatures (More dense towards centre). - Such information can be analysed to infer the structure of Earth's interior.

Newton's World

- Space and time ○ Label "where" and "when" - But are just co-ordinates ○ Not part of the "action" - Forces and rules of motion: tells us how objects will move ○ In particular: gravity

Types of Galaxies: differ mainly in shape

- Spiral galaxies - Elliptical galaxies - Irregular galaxies

The snowline

- This the point around the sun (or any star) where the radiation is not enough to vaporize volatile materials (materials such as; ammonia, methane). - Everything inwards of the snowline gets evaporated and blown away by the solar wind. - Inwards of the snowline, there is only refractory material left which is hard to vaporise (doesn't melt easily) due to everything else getting blown away. - Hence there is less material to make the planetesimals and therefore less material to make rocky proto planets. - This is why the planets inwards of the snow line (close to the sun) are relatively small compared to the outer planets which still had the volatile material available to them when forming the planetesimals and eventually the planet.

The hot big bang

- Timing of nucleosynthesis ○ Depends on "photon to baryon" ration ○ Baryons: neutrons and protons - Early universe: exactly the same amount of matter and antimatter ○ Complete annihilation; only photons left ○ But matter dominated by one part in a billion compared to antimatter - After a few minutes ○ Universe consists of: protons, helium nuclei, electrons, neutrinos, photons - But can't (yet) make atoms (an electron joined to a nucleus) ○ Photons so energetic they will ionize atoms they hit ○ Universe has to cool before atoms form ○ And a gas of protons and electrons is a plasma ○ Light cannot travel in a plasma (like the centre of a star)

Ancient Greek Astronomy

- Tried to make a model. - Models generally wrong because they were based on wrong "first principles". - They came up with a model that the sky was rotating, the Earth is spherical.

Lunar surface features

- Two dramatically different kinds of terrain can be seen. 1. Highlands: mountainous terrain, scarred by craters, older surfaces. 2. Lowlands: 3km lower than highlands; smooth surfaces (dark and called Maria). Would have been basins flooded by lava flows during the formation of the moon.

Proton-proton fusion chain process: + Neutrino properties

- Two hydrogen nuclei (protons) fused together in a fusion reaction > making detirm + positron (1 proton + 1 neutron) + neutrino + energy When does this reaction twice in 2 separate reactions, 4 protons > 1 He nucleus (2 protons + 2 neutrons), find that the He nucleus slightly lighter as some mass has been converted into energy (E=mc^2), this energy powers the sun Neutrino: - Neutrino doesn't react strongly with other materials So when neutrino gets produced in fusion reaction, it leaves the sun immediately

Using Type 1a Supernovae to observe

- Type Ia Supernovae are roughly similar to each other ○ Peak brightness varies by 20-30% ○ Can be "corrected" to ~10% - One supernova / big galaxy / 100 years ○ Look at 1000 galaxies now; again a month later ○ Find one supernova over ○ Follow supernova to reconstruct light curve ○ Needs big telescopes / space telescope: expensive

Why does the expansion accelerate?

- Under pressure - Negative pressure

Thermal history of the universe

- Universe cools as it expands ○ So: very early universe was very hot and very dense - Work out the thermal history of the universe ○ Learn how the universe evolves with time - in theory ○ Check against observations - Everything melts: ○ Ice to water to steam to atoms to plasma to protons and neutrons to quarks... ○ Theoretical physics can only go up to extent of quarks and electrons - 10^-6 seconds after big bang: quark-hadron phase transition ○ Around 10 Trillion Kelvins (give or take) ○ Protons and neutrons made out of quarks ○ Protons and neutrons "melt" above this temperature - Universe contains hydrogen nuclei (protons), free neutrons, electrons and positions (anti-electrons) ○ Plus neutrinos, photons and dark matter ○ Nuclei are not stable, so no helium atom can be formed (or anything else) § So even though we have all the materials to build the next most complex atom after hydrogen (helium), it is just not possible until the universe has cooled down further

Useful Numbers...

- Universe is roughly 13.8 billion years old ○ Sun is ~4.6 billion years old - Nearest big galaxies millions of light years away ○ Our galaxy ~100,000 light years across ○ Hundreds of billions or trillions of stars in a big galaxy

Relativity and Cosmology

- Up until 1915, gravity had been described by Newton's laws ○ No-one had talked about an "expanding universe" - After 1915, ○ de Sitter, first paper on cosmology [with hindsight] 1917 - Einstein, 1920s ○ Looked for a static universe in General Relativity ○ Had to work hard to do it

Do Distant parts of the Universe move faster than light?

- Yes, it seems so - but we can't see them - The light cannot reach us because the universe is expanding. ○ Even if photon is coming towards you at the speed of light, it cannot cover the expanding distance fast enough ○ If recession velocity is greater than light, light can't reach us - Does this conflict with relativity? ○ No (this is predicted by relativity, after all) ○ "Speed limit" is for actual motion; everyone "thinks" they are sitting still in an expanding universe

Are the distant stars also fainter?

- Yes, that is certainly true ○ The inverse square law... - But there are also more distant stars ○ And the two effects cancel out ○ So in an infinite universe, the night is as bright as the day...

Hubble telescope

- detect far away galaxies - 2.4 metre mirror telescope - Observe in the near ultra violet, visible and near infrared spectra wavelengths - Important= be very familiar with the EM spectrum

Newtons 3 Laws

1. A body at rest remains at rest. A body moving in a straight line maintains a constant speed and the same direction unless it is deflected by a force (Qualitative). 2. Or the acceleration of an object to which a force is applied is equal to the magnitude of the force divided by the mass of the object. Big force = Big acceleration (Quantitative). F=ma 3. Forces exist in pairs. Every force has an equal and opposite reaction. (Both Quantitative and Qualitative) E.g. Your weight pushes down on your chair which pushes you back up with exactly the same force hence you do not fly up to the ceiling or fall to the floor.

Kirchhoff's Laws of Radiation

1. A solid, liquid or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum. 2. A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum. 3. If light comprising a spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.

Why put telescopes high up?

1. Atmospheric absorption · The atmosphere absorbs light. Hence, it must have an effect on the light. · The atmosphere also absorbs gamma rays, x-rays, UV and Infrared spectrum · The atmospheric gases also absorb most of the infrared spectrum. · Long wavelenth radio waves blocked · Visible light is observable from Earth, with some atmospheric distortion 2. Atmospheric distortion - Atmosphere refracts starlight in random directions very quickly (which is why stars twinkle) and multiple images can be created. - On mountain tops, there is less atmosphere to look through= have less distortion.

The Solar Nebula Hypothesis

1. Big cloud of gas and dust collapses inwards due to gravity 2. Flattens into a disk, majority is mass in the gas cloud is in centre, becoming hotter and more dense, becoming the sun. outwards of the disk, planets will form. Sun and planets were formed from the same original materials 3. At some point the centre will get hot enough, the sun will 'turn on', fusion reactions will start and sun will start to produce energy, sun will push out solar wind, volatile materials will then evaporate. Therefore there is less stuff near the sun (smaller planets) and more stuff away (bigger planets, ice still remains) - Basis of modern theory of planet formation. - Planets form at the same time from the same cloud as the star - Planet formation sites seen today as dust disk of T tauri stars - Sun and solar system formed 5 billion years ago

Two ways of taking survey of stars in an astronomical survey

1. Brightness-limited - Only sees stars that are either very close, or are very big. 2. Distance/volume-limited - Gives a better deduction as it takes in all stars within that distance.

Why build big telescopes?

1. Collecting area 2. Resolution · Waves spread out. (light is a wave) · Bigger telescope= collect more spread

Low-mass star (life line) (more to the right of HR main sequence)

1. Interstellar cloud (light-years across) 2. Collapses down to a proto-star. 3. Starts main-sequence. 4. Forms helium core, becomes red giant. 5. Dust driven mass-loss removes envelope. 6. Forms planetary nebula that slowly fades. 7. Leaves a (helium) white dwarf remnant. -The universe is not old enough to have had this happen as of yet.

Blackhole: how to detect them

1. observing the accretion disk- x rays, materials falling into blackhole forming amount of energy 2. Gravitational microlensing - Gravity can distort background light, we can detect this distortion 3. Companion stars orbits 4. Gravitational waves- emitted from inspiral of two neutron stars

Types of telescopes

1. radio telescopes · Can detect magnetic fields basically. · A radio telescope reflects radio waves to a focus at the antenna. Because radio wavelengths are very large, the radio dish must be very large . · Interferometry (resolution trick) - Telescopes connected together to make an inferferometer can make even sharper images than a single large telescope if the interferometer is bigger than the large telescope. Images for same two stars are shown 2. Infra-red telescopes - is a telescope that uses infrared light to detect celestial bodies 3. X-ray telescopes - High energy photons reflect off a metal surface at shallow angles. A nested series of 2 or more cone-shaped reflectors focus the extreme- UV and X-ray light to make an image 4. Optical telescopes - is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum

Two Laws of Black Body Radiation (2)

2. The peak of the black body spectrum shifts toward shorter wavelengths when the temperature increases. Wien's Displacement Law: (Where T(k) is the temperature in Kelvin).

Asteroids, Comets and Meteors/meteorites

A: In addition to planets, small bodies (asteroids, comets, meteoroids) orbit the sun C: - Icy nucleus which evaporates and gets blown into space by solar wind pressure - Mostly objects in highly elliptical orbits, occasionally coming close to the sun - Gets heated up by solar sun radiation, vaporized and pushed away by solar wind M/M: - Small (μm - mm sized) dust grains throughout the solar system - If they collide with Earth, they evaporate in the atmosphere. - Visible as streaks of light: meteors.

Cosmology's Job

Cosmology studies the structure and evolution of the universe.... - Understand where the universe came from - Understand how the universe evolved ○ Understand how galaxies form ○ Understand how galaxies evolve and interact ○ Understand why stars (and planets) form ○ Understand when and where stars form ○ Explain what we see in the sky - Not just "what there is" but "how it got there" & "what it does"

Principle of Equivalence

Gravity = Acceleration 1. When there is gravity - Things move in the same way in a gravity field as those in a reference frame accelerating upward in with same magnitude. 2. Lack of gravity - You would feel weightless. Zero Gravity. 3. Time dilation - Time dilation gets worse when you accelerate.

Heezen-Tharp Map (Tectonic plates)

Heezen-Tharp Map (Tectonic plates) - Looked at ocean floor height (depth of ocean floor). - Mapped out sub-ocean ridges: these are regions where two tectonic areas are being forced apart and the hot mantle material is being forced up between the two gaps moving apart. - This forms a new surface on the ocean floor.

Water on Mars

Hidden water on Mars - No liquid water on the surface as it would evaporate due to the low pressure. - Water could be existent in frozen conditions (re-watch simulation). - Little atmosphere = low atmospheric pressure = immediately begins boil (boiled off) = turns immediately into solid (ice) - Many geological features when viewed up close by rovers appear the same as river features on the Earth Ice in the Polar Cap (Mars) - Contains mostly CO2, a bit of H2O - Brightest radar signal concluded to be the light coming from liquid water. - Melting point of water decreases under the pressure of an overlying glacier. - Water is special and increasing pressure = lower pressure. - Core of Earth is opposite where increasing pressure = increasing melt/freeze temperature. - Presence of salts on Mars further reduces the melting point of water liquid even at below freezing temperatures. - Water gets colder when salt is in the water (E.g. icicle of death) - Lowers freezing point (easier to freeze) Lake Vostok (Antarctica)

2. Composition of stars: spectral lines

Hotter H gas: emits photon, Cooler H gas: absorb photon

Cluster lensing

Light Bending - Orbits look Newtonian ○ But gravity really obeys Einstein's rules - Gravitational lensing ○ Light bent by gravity ○ Distant images distorted - Bent in Newtonian gravity too ○ More bending from Einstein ○ Prediction of general relativity Gravitational Lensing - Strong lensing ○ Double images, arcs - Weak lensing ○ Distant galaxies "squashed" We can rebuild a lensing map - Get "lensing map" - Find mass distribution - Most mass is dark matter ○ Independent test ○ Same rules (of gravity)

Electromagnetic Radiation

Light is seen as a wave (2 parts: magnetic field and electric field) or a particle - We can treat light as a wave (electromagnetic wave) ○ Components of electromagnetic waves: Electric field and magnetic field. ○ Electric fields and magnetic fields oscillate at 90 degrees (right angles) to each other.

Stellar lifetimes

More massive stars die sooner (shorter lives)

Primordial Nucleosynthesis

Primordial Nucleosynthesis - Most nucleosynthesis happens in "first three minutes" ○ Almost all neutrons now in helium-4 atoms ○ Small amounts of other atoms (helium-3, deuterium, lithium) ○ These amounts depend on "photon:baryon" ratio ○ Everything else made in stars (or via radioactive decay) - Know photon number-density today from temperature of CMB ○ Compute mass of atoms in universe: few % of total mass ○ More (indirect) evidence for dark matter

Two Kinds of planets: 1. Terrestrial 2. Gas/ice giants

Terrestrial (Earth like, rocky, solid surfaces, closer to sun) 1. Mercury 2. Venus 3. Earth 4. Mars - Four inner planets of the solar system - Relatively small in size and mass (Earth is the largest and most massive) with Rocky surface - Surface of Venus cannot be seen directly from Earth because of its dense cloud cover Gas/Ice Giants (Jovian/Jupiter-like) 1. Jupiter 2. Saturn 3. Uranus 4. Neptune - Craters are not seen on Jovian planets because they do not have a solid surface. - Much lower average density. - All have rings (Not only Saturn). - Mostly gas; no solid surface.

What the CMB look like

Two questions: 1. Why is the universe so smooth? a. Takes off the absolute value of the microwave background 2. Why isn't it completely smooth? a. If it was completely smooth then there would be no primordial ripples to build the universe off

The Evolution of Massive Stars- stars more than 8 solar masses

Type 2 Ø CNO cycle Ø Massive star > 10 solar masses= hydrogen > helium, helium > carbon, oxygen. CNO chain occurs only in massive stars temperature and pressure is high enough to allow fusion reactions to occur at the core Ø All produce energy due to loss in mass E-mc^2 Ø Heavier and heavier elements > Fe> supernova explosion Ø An alternative stellar lifecycle (is the same but the hydrogen envelope fades away Type 1b/c)

Spectrum

Type of spectrum seen depends on the temperature of the thin gas relative to the background. - Thin gas is hotter so emission lines are seen. - Thin gas is cooler so absorption lines are seen in the visible spectrum. - We cannot see the dark lines when looking at a rainbow because our eyes are not at a high enough resolution.

Motion of Stars

Types of Binary stars ( Stars that are far enough apart that they can be seen as separate stars through a telescope. ) - Apparent binaries: Stars seen close together in the sky but not physically connected. - Visual binaries: Stars seen close together in the sky and ARE physically connected (not many). - Spectroscopic binaries: Stars seen as one but are physically connected. Only detected because of the velocity of one or both stars is seen to change. Most times only one spectrum is observed. Can detect motion of the stars. · Eclipsing binaries: Seen as one in the sky but rotate about a common centre. Only detected because each star in turn partially blocks the light from the other (one goes in front of the other).

Degeneracy pressure - white dwarfs and neutron stars made of degenerate matter

Where does pressure come from for remnants? - If remnant mass (left over after star's evolution) <1.4 masses, form white dwarf o Force pushing back against gravity= electron degeneracy pressure - If remnant mass( left over after's life on main sequence and fusion reactions) between 1.4 and 3 solar masses, forms neutron star o Crushed to more dense state, neutron star holds against material from gravity by neutron degeneracy pressure

Three main types of stars of HR diagram

based on temperature and absolute magnitude), B-V= colour (smaller= bluer, larger=redder), longer wavelengths= redder colours, bluer= hotter, hotter= more massive (on main sequence) - Main sequence - White dwarfs - Giants/super giants - NEED TO KNOW AXIS AND ORDER OF THE VALUES, INDICATE ROUGHLY WHERE THE MAJOR GROUPS WHY DOES B-V EQUATE TO TEMPERATURE? - High mass stars= hotter, bluer - low mass= cooler, redder - Higher mass stars= shorter lifetime - Spectral class: O= very blue, hot massive, G= like the sun, M= red, small, cool - OBAFGEM Bigger mass= larger luminosity, brigher (absolute magnitude)

Recession velocity

the velocity of an external galaxy (or other object) away from our Galaxy due to the expansion of the Universe.

When the Sun is at 90 degrees to the moon (Half phase)

there are weak tides (leap tides).

Stellar remnants (corpses of stars)

· White dwarfs · Neutron stars Black holes - Star start at main sequence uses fuel available depends on temeprature and pressure at core determines what nuclear reactions possible to giant and end - Energy is being released when consuming fuels (nuclear reactions) and that energy is moving outwards pushing against materials pushing into the star from gravity, a balance energy from nuclear reactions and gravity pushing materials into star - When the star runs out of fuel, there is no longer energy available pushing outwards from core to counter the gravity force towards centre of star. Gravity collapses inwards the centre until it forms one of these forms of degenerate matter