Astro 7N Unit 4 Review Notes

Milky Way Galaxy

- Contains about 400 billion stars - the vast majority are red, M-types stars, but the relatively few blue stars are vastly more luminous, so the blue ones can outshine all the red ones - the empty space between stars is vast; galaxy is mostly empty space between stars

Andromeda and the Local Group

- the "Large Magellanic Cloud" and "Small Magellanic Cloud" are satellite dwarf galaxies of the Milky Way (visible at night in the southern hemisphere) - dwarf galaxies contain tens of millions to 1 billion stars - Andromeda Galaxy is similar to the Milky Way, about 2.5 million light years away - the Andromeda Galaxy also has dwarf satellite galaxies - the "Local Group" contains about 40 dwarf galaxies along with the larger Andromeda and Milky Way —like two major cities, each with their suburbs - Andromeda and Milky Way will collide in 5 billion years and form a single galaxy

Particle Physics

- Conventional matter" (everything we can see on Earth, or in the Universe) is made of atoms, but atoms are not fundamental particles. - Fundamental particles are quarks, leptons, and bosons. These particles cannot be broken up into smaller units, as far as we know. - "Flavors," or types, of quarks: "up," "down," "charm," "strange," "top," and "bottom." •For each quark there is also a corresponding, oppositely-charged, anti-quark: anti-up, anti-down, anti-charm, anti-strange, anti-top, and anti-bottom. •There are 6 types of quarks and 6 types of anti-quarks. - Leptons are lighter fundamental particles and include electrons and neutrinos. •In total, there are 6 types of leptons and 6 types of anti-leptons. - The nucleus of an atom contains neutrons and protons; a complete atom of a chemical element contains a nucleus with electrons around it. •a proton contains 2 up quarks and 1 down quark (or, " p = uud " ) •a neutron contains 2 down quarks and an up quark (or, " n = udd " ) •The 3 quarks in either of these particles must have different "colors." - There are 30 types of fundamental particles — 12 quarks/anti-quarks, 12 leptons/anti-leptons, and 6 bosons. - Almost everything we observe in nature is made of up quarks, down quarks, and electrons. The other particles are either not very stable and/or do not interact much with other matter.

Dark Energy

- Dark energy comprises about 71% of the overall energy density of the Universe - Type la supernovae (powerful explosions produced when a lot of new matter falls onto a white dwarf, from a companion star) are good "standard candles." - Knowing the luminosity of a standard candle, we can measure its apparent brightness and calculate its distance from the Inverse-Square Law. - The relationship between the apparent brightnesses and redshifts for Type Ia supernovae can be used to measure the geometry of space. •If the geometry is "flat," parallel light rays will continue along parallel paths. •If the geometry is "open," parallel light rays diverge. •If the geometry is "closed," parallel light rays converge. - Observations of Type Ia supernovae in distant galaxies show that the universe is not just expanding, but that the expansion is accelerating—so not only are the spaces between galaxies getting larger over time, but now the rate at which those spaces are getting larger is itself increasing. - Dark energy, a sort of "anti-gravity," causes the acceleration of the universe. We really do not know what it is yet. - Dark energy is everywhere in space, but there is only a small amount per unit volume; only because of the hugeness of space does it add up. - Dark energy may have to do with Einstein's "cosmological constant" (which he invented and then abandoned, thinking it was wrong). A cosmological constant is the energy density that may fill empty space.

Dark Matter

- Dark matter comprises about 24% of the overall energy density of the universe - It is known to exist through the observed large rotation velocities in galaxy disks, motions of galaxies in clusters, and by the way that light is bent by galaxies and galaxy clusters. - Dark matter emits no radiation of any form we can observe — no visible light, no heat, no X-rays, etc. We know about it through the gravitational influence of its mass on ordinary matter.

Supernovae

- Supernovae are more luminous than novae, but only happen once (the star is basically destroyed); novae can be a repeating process -different colors from different chemicals -the supernova which eventually formed the Crab nebula was seen on Earth in the year 1054; the Cassiopeia A supernova was about 340 years ago • -we generally expect a supernova in the Milky Way once every couple hundred years

The Big Bang Expansion of the Universe

- The universe is expanding in the sense that space is being added everywhere in between the galaxies. - All galaxies throughout the universe would measure redshifts from galaxies distant from them. - There is no center to the universe, nor is there an edge. It is likely that the universe is infinite. - The Big Bang expansion began about 13.8 billion years ago. At that time the universe was very hot and dense. All matter and energy in our observable universe was then compressed into a single point of space.

The Galactic Center

- Towards the constellation of Sagittarius, called "Sag A*" - A supermassive black hole at very center leads to faster stellar motion there - The mass of this black hole (as determined using Kepler's third law, from observations of the orbits of the nearby stars) ≈ 4 million solar masses! - the radius of the black hole is only about 100 Astronomical units - Supermassive black hole has grown by accreting mass from objects over time

Novae

- a nova (plural novae) is caused by a binary star, wherein one star of the two evolves faster than the other - the faster-evolving star, perhaps already a white dwarf, can be close enough to still be partly surrounded by a common envelope of gas from its nearby companion star (which evolves more slowly, but still eventually reaches its own red-giant phase) - mass transfer of hydrogen from the companion star onto a carbon white dwarf temporarily causes extra burning on the white dwarf's surface - the white dwarf plus the new material added suddenly brightens; this can happen periodically, repeatedly - an expanding "light echo" pattern around the nova "V838 Monocerotis"

Open Star Clusters

- have hundreds, up to thousands of stars - the stars formed at about the same time, from the same initial gas & dust cloud - cluster only stars bound by gravity for a few million years - tend to have lots of blue stars visible, because of relatively young ages (and because the blue ones vastly outshine the redder ones) - the "Pleiades" is a notable example

Globular Star Clusters

- hundreds of thousands, to millions, of stars - tend to be yellow in color, with a number of red giants - many have ages around 10 billion years, overall considered old clusters - an example is "47 Tucanae"

Star Forming Regions

- stars form in giant molecular clouds, which typically contain several million times the Sun's mass worth of gas and dust - these clouds are dark/dusty and cool, only about 10 degrees Kelvin - the "Jeans mass" tells you if a cloud will collapse to form stars: •if the cloud's mass is greater than the Jeans mass, gravity dominates over internal gas pressure and it will collapse •if the mass is smaller than Jeans mass, internal pressure dominates and it will not collapse -sections of the cloud collapse into clumps, in which individual stars form in with protoplanetary disks The famous "Orion Nebula" and the "Eagle Nebula" are examples

Comparisons between spiral and elliptical galaxies

-Spiral galaxies have more gas, more dust, and more new star formation than ellipticals do • - Spiral galaxies have very noticeable disks (again, disks have bluer color on account of younger stars), while elliptical galaxies do not (more older stars, yellower colors) - The largest galaxies are ellipticals, but there are many small ellipticals as well

Estimated Number of Galaxies in Observable Universe

50 billion to 100 billion galaxies in the observable universe (5 x 1010 to 1011) ... each with 1 billion to 100 billion stars per galaxy (109 to 1011) So, in total, as many as 1011 x 1011 = 1022 stars in the observable universe —and most stars probably have planets! • the "Hubble Deep Field" was used to obtain this result; it is a tiny patch of sky that the Hubble Space Telescope imaged for 10 full days

What are the three major parts of the Milky Way Galaxy (and other spirals)?

A bugle, disk, and a halo

How do we know that dark matter exists, and what do we know about its nature?

Dark matter is known to exist through the observed large rotation velocities in galaxy disks, motions of galaxies in clusters, and by the way that light is bent by galaxies and galaxy clusters. Dark matter has gravitational influence of its mass on ordinary matter.

Early Galaxies

Deep images of the sky, obtained with the Hubble Space Telescope, allow us to study the types of galaxies that exist at different distances from our galaxy. • Looking into the distance means looking back in time because the speed of light determines how long the light takes to reach us; 1 light year is how far light travels in 1 year. • In the nearby/recent universe spiral and elliptical galaxies are common; there are also dwarf galaxies grouped around the larger galaxies. • In the earlier universe, irregular galaxies are much more common. Galaxies were still in the process of being assembled, e.g. many galaxies similar to the "Tadpole Galaxy".

Hubble's Law

Distances to galaxies can be estimated using "standard candles," like Cepheid variables or Type Ia supernovae seen in other galaxies Distances to galaxies can also be crudely estimated by taking the assumed real size in kiloparsecs divided by the apparent angular size measured in milliradians. The answer comes out in Megaparsecs (millions of parsecs). The redshifts of galaxies are found to be larger the farther away the galaxies are. This relationship is known as Hubble's Law,

History of the Universe

Events in Order 1. Before the Big Bang (actually before 10−36 seconds, during the "Planck time") we do not know what happened. The current laws of physics are not able to draw conclusions, but matter was dense like the singularity in a black hole, and energies of photons were very high. 2. Quarks form — photons turn into particle/anti-particle pairs, and vice versa. 3. Quarks bind together into protons and neutrons (with three different colors joining) 4. Protons and neutrons bind together to form helium nuclei. This is called nucleosynthesis. 25 percent of the mass was converted to helium nuclei at that time. The universe is still a hot, dense plasma at this time (about 100 seconds), like as in the core of a star. It is still "opaque," so that light cannot freely go through it. 5. Electrons join with nuclei to form complete atoms. The universe becomes transparent, so radiation could hereafter pass more freely through space. We see the microwave background radiation — equivalent to 2.73 degrees Kelvin, now — coming from this time, about 300,000 years after the Big Bang began. A map of small temperature differences in the cosmic microwave background can be used to measure many fundamental properties of our Universe, like content and expansion rate.) 6. Stars and galaxies start to form via gravity. 7. The first large stars die, and spread elements heavier than hydrogen and helium (formed through fusion in their cores) throughout the universe. 8. Quasars light up at the centers of many forming galaxies, as nearby fuel feeds the supermassive black holes. Galaxies form from many smaller clumps merging together. 9. Our Sun and Earth form about 4.5 billion years ago. 10. First mammals and dinosaurs appeared on Earth 225 million years ago, more than 13 billion years after the Big Bang expansion began. 11. Humans began to roam the Earth just over 100,000 years ago.

Type II Supernovae

Occur when massive stars die - the kinds that lead to neutron stars or black holes - the observed spectra of type II supernovae contain prominent Hydrogen lines

What different kinds of objects are present in the Milky Way Galaxy?

Open star clusters, globular star clusters, planetary nebulae, novae, supernovae, star-forming regions, and the galactic center.

How were galaxies different in the past than at present?

Past galaxies were more active, formed stars at a higher rate, and galaxy shape.

Classification of Galaxies

Spiral galaxies can have a central bar, in which case they are "SB" galaxies — or not, in which case they are "S" galaxies. • Spirals — barred or not — are classified as "a", "b", or "c" type, with "c" type having loose, lumpy arms and a smaller bulge than "a" •example: an "SBa" galaxy is a barred spiral with tight, smooth arms and a relatively large bulge. Elliptical galaxies range from E0 (circular) to E7 (elongated) depending on their shape on the sky; •you might remember this by the number "0" being rounder than a "7" Some galaxies have a more irregular structure and cannot be classified as spiral or elliptical; these are called Irregular galaxies.

Forces

The four forces: gravity, electromagnetism, and "strong" and "weak" nuclear forces - "Gravitons" have not been discovered yet, but are thought to be the bosons that carry the force of gravity, and act on mass. - Photons are the bosons that carry the electromagnetic force, and act on electric charge. - "Gluons" are bosons that carry the "strong" force, that holds atomic nuclei together. - W+, W-, and Z bosons carry the "weak" force. The weak force is responsible for radioactive decay. - Bosons are exchanged between particles in order to make the forces work.

Three Components of the Milky Way Galaxy

Three Components: Bulge, Disk, Halo disk is 100,000 light years in diameter — spiral pattern; young, blue stars dominate • our Sun is 26,000 light years out from the galaxy center, in the disk • stars form in "density waves" in molecular clouds strewn about the disk's spiral arms; open clusters tend to be found in the disk bulge is a spherical region in the center of the disk; stars are older, so yellow colors halo is larger, spherical region around whole galaxy; old stars, few heavy elements • globular star clusters live in the halo

Galaxy Collisions

Tidal debris is left behind because of the gravitational pull of one colliding galaxy on another. • New star clusters are formed from gas in galaxy collisions, so we often see young, massive blue stars. • Collisions can cause "starburst galaxies", which have a fast rate of star formation. • When galaxies collide, the odds are extremely low that stars/planets would collide, given their relatively small sizes in comparison to the spaces between them. • A merger of two spiral galaxies may give rise to an elliptical galaxy or an irregular galaxy.

"Powers of Ten"

Using a base number of 10 with an exponent. Our number system is based on the powers of 10. •Neutron star is 10,000 times bigger than a person (you might also remember neutron stars are "city-sized") •Earth is about 10 million times bigger than a person • Stars are tens to hundreds of times bigger in diameter than Earth • Nebulae can be millions of times bigger than individual stars. • Galaxies are billions or trillions of times bigger than individual stars. • Nearest galaxy (Andromeda) is tens of times the diameter of a galaxy away. • Galaxy cluster is hundreds of times bigger than an individual galaxy. • Observable Universe is a million times bigger than the Milky Way Galaxy.

How do we know that dark energy exists, and what do we know about its nature?

We know that dark energy exists through how it affects the universe's expansion. Dark energy makes up about 71% of the overall density of the Universe.

What is matter composed of?

atoms

What is the universe made of?

matter and energy

Type la supernovae

occur when very large amounts of material are suddenly added to a white dwarf from a binary companion; the resulting burst destroys the white dwarf - Type la spectra do not contain hydrogen, since they come from white dwarfs primarily made from carbon

Redshift

the shifting of the lines in the spectrum of an object to longer wavelengths (to redder colors), due to its motion away from the observer. (alternatively, "blueshift" would be seen for an object moving towards the observer) Specifically, redshift ( "Z" ) is ... Z = (𝜆 observed - 𝜆 rest ) /( 𝜆 rest )

Ages of Star Clusters

• ages of star clusters can be determined by seeing what spectral class of star has most recently "turned off" of the main sequence in the cluster H-R diagram • many stars formed in clusters that later dispersed, leaving stars more isolated

Planetary Nebula

•a low-mass star runs out of core nuclear fuel, blows off outer layers (recall Unit 3) • stars can pulsate and eject gas in layers (can lead to a sort of shell- or ring-like appearance) • bipolar jets often lead to symmetry in the resulting nebula • lasts tens of thousands of years • can be a few light years in size •the core of the former star, a white dwarf, is left behind at the center of the nebula • this will be part of the end state of the Sun, in about 5 billion years