The Nature of the Universe
What elements do interstellar dust and gas content?
Hydrogen, helium, iron, silicone, and carbon.
While an interstellar gas/dust cloud collapses, what is happening at the centre of it?
At the centre of this collapsing dust and gas cloud, the material becomes so hot and dense that fusion reactions start. Hydrogen burning takes place: 4 1,1H -> 4,2He + 2 0,+1e +2 ν.
Redshift = in what direction is it moving?
Away from us.
What is H0?
The Hubble constant, ≈ 70kms-1Mpc-1.
What is the astronomical unit (AU)?
The average distance from the Earth to the Sun ≈ 1.5x10^11m.
What is the cosmological principle?
The cosmological principle is the idea that the universe has the same large-scale structure when observed from any point within it.
What is a light year?
The distance travelled by light through a vacuum in one year ≈ 9.46x10^15m.
The faster the source is receding..?
The greater the redshift.
What is the implication of Hubble's law?
The implication of Hubble's law is that the universe is expanding, as the galaxies are moving away from each other.
What is the parsec (word)?
The parsec is the distance that gives a parallax angle of 1 arc second.
When does a star reach a steady temperature?
When power released in fusion = power radiated away from the star.
How are planets, planetary satellites, and other objects formed?
By the gravitational attraction of materials orbiting the star pulling matter together.
What are the units of the Hubble constant H0?
s^-1.
What four things does the Big Bang model of the universe suggest?
1) The universe is not static - it is expanding. 2) The universe is not infinite. 3) The finite age of the universe and the finite speed of light suggest light from the most distant galaxies has yet to reach us. 4) As distant galaxies recede, their light is red-shifted, so it is less energetic and dimmer.
What assumptions do we make if we use the equation for the age of the universe?
1) Universe has been expanding at a steady rate over time. 2) Calculation of H0 is accurate - as nothing can move faster than c, the upper limit of the observable size of the universe is only 14x10^9ly from us.
What is the value of the parsec in metres and light years?
1pc ≈ 3.1x10^16m ≈ 3.3ly
What is the equation for hydrogen burning?
4 1,1H -> 4,2He + 2 0,+1e +2 ν.
What is the equation for Hubble's law?
v = H0x, where H0 is the Hubble constant, ≈ 70kms-1Mpc-1.
What is the equation for redshift?
Δλ/λ = v/c.
While hydrogen burning occurs, what happens?
Fusion reactions release energy as mass is converted into energy (∆E= ∆mc^2). Temperature of the stellar material increases more rapidly, and the star will glow for millions/billions of years.
If the gradient of vαx is steep (H0 = large), what does this suggest?
Galaxies are moving fast and the universe is young.
What is Hubble's graph evidence of and why?
Hubble's graph is evidence of a Big Bang; as v α x, at some point, all of the galaxies must have been concentrated together in a very small space.
How can redshifts be explained?
In terms of the Doppler Effect: a) Source emits 1 complete wave in time λ/c. b) As it does so, the source moves distance v * λ/c. c) The λ is, therefore, stretched by v * λ/c. d) The change in λ, Δλ = v/c * λ.
What happens when an interstellar dust and gas cloud undergoes gravitational collapse?
It heats up due to losing gravitational potential energy and gaining kinetic energy. The particles collide with each other, sharing out their energy, and getting hotter in the process. They form a gas whose internal energy is increasing.
What happens where interstellar gas and dust clouds are denser?
Its own gravity causes material to pull together and contract to form a denser mass, which has a stronger gravitational pull, so more matter is pulled in - and so on. This process is known as gravitational collapse.
For a star heavier than 3 masses, what are the two ultimate fates it can have?
Lighter stars turn into a neutron star. Heavier stars' supernovae form a neutron star so massive, it continues to collapse inwards under its own gravity to form a black hole.
How can we interpret Hubble's law and what is this an example of?
No matter where you are positioned in the galaxy, you will observe that all galaxies are receding, and the further away the galaxy, the faster it is receding. This is an example of the cosmological principle.
Describe what the image demonstrating the parsec looks like.
Right-angled triangle. 1 side = 1Au Other side = pc Linking 1Au and pc is the Sun (with right angle) Angle = 1 arc second
What is Hubble's Law?
Speed of recession of the galaxy is proportional to the distance of the galaxy: v α x.
What are most galaxies' line spectra wavelengths shifted towards?
The red end of the spectrum - redshift.
If the gradient of vαx is less steep (H0= small), what does this suggest?
The universe is older.
What is 1 arc second?
1 arc second = 1/3600 degrees. There are 60 arc seconds in one minute of arc, and 60 arc minutes in a degree.
How do we convert H0 into SI units?
1) 1pc ≈ 3.1x10^16m; 1Mpc ≈ 3.1x10^22m. 2) 70kms-1Mpc-1 = 70x10^3ms-1/3.1x10^22m = 2.26x10^-18s-1 ≈ 2.3x10^-18s-1.
Describe the evolution of a star heavier than 3 solar masses.
1) Approaching the end of its life, it swells to become a super red giant. 2) When it collapses to form a white dwarf, if its mass is still more than 1.4 solar masses, its gravity is strong enough for it to collapse further and overcome the Fermi pressure. 3) Electrons combine with protons to form neutrons and neutrinos. 4) Neutrinos escape and the central core of the star is made entirely of closely packed neutrons. 5) Outer shells of the neutron core collapse violently and rebound against the neutron core, generating a shockwave, which explodes the surface layers of the star as a supernova, blasting off heavier elements (e.g. carbon, oxygen, and iron), into the galaxy. 6) Depending on the star's mass: a) Lighter stars - the core is made entirely up of neutrons with a density of 10^18kgm-3. It is a neutron star. b) Heavier stars - supernova forms a neutron star so massive, it continues to collapse inwards under its own gravity to form a black hole.
Describe the evolution of a star like our Sun whose mass is less than 3 solar masses after its hydrogen fuel runs low.
1) As temperature and pressure of the innermost parts of a star increase, more complex fusion reactions can occur, producing other elements e.g. carbon, silicone, and iron. 2) As fusion reaction rate slows down, the core of the star collapses under gravitational attraction. 3) The thin shell of the nuclei surrounding the core of the star fuse to produce beryllium, carbon, and oxygen nuclei. 4) A thin layer of the hydrogen shell surrounding the helium core becomes hot enough to fuse hydrogen nuclei together again. 5) Increased power production from the helium shell causes the outer shell of the star to expand due to radiation pressure. 6) The size of the star increases and its surface temperature drops; it has become a red giant and will engulf nearby planets. 7) The core continues to collapse, its density and temperature increasing. 8) The helium nuclei in the outer shell reach 10^8K and fuse together at a phenomenal rate. 9) Helium flash occurs, wherein the material surrounding the core is ejected away to form a planetary nebula. 10) A bright, central core - a white dwarf - is left behind to cool and dim over millions of years.
What are the 8 principle contents of the universe?
1) Galaxies 2) EM radiation 3) Fast-moving particles 4) Clouds of dust 5) Stars 6) Planets (including minor planets) 7) Natural planetary satellites (e.g. moon, asteroids) 8) Comets
What are the two arguments against Olber's paradox?
1) If we lived in an infinite, uniform, static universe, the sky would always be brightly lit because in no matter what direction you looked, your line of sight would always eventually reach a star. Even if they are dim, their large numbers would compensate. 2) In an infinite universe with an infinite number of stars, the number of stars increases with the square of the distance. The intensity of light from these stars decreases according to the inverse square law with distance. These two effects cancel each other out and, hence, the night sky ought to be bright.
What are the five characteristics of a white dwarf?
1) No hydrogen fusion occurs. The white dwarf glows because oh photons from past fusion reactions are still leaking away from it. 2) It is very dense. 3) Electrons move freely throughout the star as plasma, no longer attached to individual atoms, as the star becomes more compressed. 4) Electron degeneracy pressure (aka Fermi pressure) prevents further gravitational collapse, due to Pauli's exclusion principle (no two electrons can exist in the same quantum state). Further collapse would violate this principle. 5) The maximum mass of a white dwarf is about 1.4 solar masses, the upper limit being known as the Chandrasekhar limit.
Describe the process in which a star is born.
1) Stars form from clouds of interstellar dust and gas, whose elements include hydrogen, helium, iron, silicone, and carbon. These materials are thinly scattered throughout the galaxy. Where the clouds are denser, its own gravity causes material to pull together and contract to form a denser mass, which has a stronger gravitational pull, so more matter is pulled in - and so on. This process is known as gravitational collapse. 2) As the interstellar dust and gas collapses, it heats up due to losing gravitational potential energy and gaining kinetic energy. The particles collide with each other, sharing out their energy, and getting hotter in the process. They form a gas whose internal energy is increasing. 3) At the centre of this collapsing dust and gas cloud, the material becomes so hot and dense that fusion reactions start. Hydrogen burning takes place: 4 1,1H -> 4,2He + 2 0,+1e +2 ν. 4) Fusion reactions release energy as mass is converted into energy (∆E= ∆mc^2). Temperature of the stellar material increases more rapidly, and the star will glow for millions/billions of years. 5) The star reaches a steady temperature when power released in fusion = power radiated away from the star. 6) The star has a stable size because of equilibrium between attractive gravitational forces and outward forces due to radiation pressure from photons released by fusion.
What are the four principle contents of our solar system?
1) The Sun 2) Planets (Mercury, Venus, Earth, Mars, Neptune, Jupiter, Saturn, Uranus, Neptune) 3) Planetary satellites 4) Comets
What are the three main points of the cosmological priniciple?
1) The universe is homogenous: on a large scale, the universe is the same at all places; its density is the same everywhere. 2) The universe is isotropic: the universe is the same in all directions. Strong support for this comes from the cosmic microwave background radiation, which has the same intensity in all directions. 3) The laws of physics are universal: laws of physics on Earth can be applied to other places in the universe.
When does a black hole form?
A black hole forms when matter collapses almost to a point - a singularity. Its gravitational field is so strong within a few km of this singularity that not even light can escape it.
What equation can we use if the rate of expansion of the universe has been constant?
Age of the universe ≈ 1/H0 (but convert H0 into SI units first, which are s-1).
What is the parsec (formula)?
Distance (pc) = 1/parallax (arc seconds)
In order to resolve Olber's paradox, what must be true?
Either the universe is actually finite, non-uniform, or not static - or some combination of these.
What is Olber's paradox?
For an infinite, uniform, and static (neither expanding nor contracting), the night sky should be bright because of light received in all directions from stars.
How can lines in spectra of light from individual stars or galaxies be interpreted?
They can be interpreted to identify the elements present in a star, as every atom of a particular element produces a line spectrum whose wavelengths are characteristic of the element.
What are the spectra of other galaxies like?
They share the same pattern of lines but are slightly shifted out of position.
What two things does Δλ/λ = v/c only apply to?
This only applies to the component of v along the line joining the galaxy to the observer; the galaxy may have another component of velocity at right-angles to this. This also only applies to galaxies travelling slowly compared to c.
Blueshift = in what direction is it moving?
Towards us.
How are interstellar gas and dust clouds scattered throughout the universe?
Very thinly.
When does a star have a stable size?
When there is equilibrium between attractive gravitational forces and outward forces due to radiation pressure from photons released by fusion.
What is gravitational collapse?
Where a gas/dust cloud's own gravity causes material to pull together and contract to form a denser mass, which has a stronger gravitational pull, so more matter is pulled in - and so on.
