Astrophysics

• One astronomical unit is defined as the mean distance between the Earth and the Sun. (Approximately equal to 150 million km).
• A light year is the distance light travels in a year.
• The greater the angle of parallax the closer the object. At different points in the earth's orbit nearby stars appear to move relative to very distant stars. By measuring angle we can find out how far the star is.

  
• A star is exactly one parsec (pc) away from the earth if the angle of parallax (θ) as the earth moves through 1 AU is 1 second of arc (1/3600 degrees)
• distance=1/Parallax(arcseconds) parallax(arcseconds)=1/distance
• 1. Cloud of dust and gas contract due to force of gravity
  2. When clumps get dense enough, the cloud fragments into regions called protostars that continue to contract and heat up
  3. Eventually reach hot enough temp ( few million degrees) and hydrogen start to fuse together to form helium.
  4. As star temp increases its volume decreases, gas pressure increases.
  5. Radiation pressure from EM radiation produced from fusion combined with gas pressure counteract force of gravity
  6. Star has now reached main sequence and will stay there relatively unchanged while it fuses hydrogen into helium.
• When the hydrogen in the core runs out, nuclear fusion stops and the core contracts and heats up under the weight of the star while the outer layers expand and cool, turning it into a red giant
• The material surrounding the core still has plenty of hydrogen, and the heat from the contracting core raises the temperature of this material enough for the hydrogen to fuse (hydrogen shell burning)
• The core continues to contract until it gets hot enough for helium to fuse into carbon and oxygen (core helium burning), releasing huge amounts of energy and pushing the outer layers of the star outwards
• When the helium runs out, the carbon-oxygen core contracts again and heats a shell around it so that helium can fuse in this region (shell helium burning)
• In low-mass stars the carbon-oxygen core isn't hot enough for any further fusion so it continues to contract under its own weight until the electron degeneracy pressure is equal to the force of gravity.

  Max mass for which the electron degeneracy pressure can counteract gravitational force is called Chandrasekhar limit (1.4 times the mass of the sun)
• For stars below the Chandrasekhar limit the helium shell becomes increasingly unstable as the core contracts. The star pulsates and ejects its outer layers into space as a planetary nebula, leaving behind a dense core.

  Star is now very hot dense solid called a white dwarf which will simply cool down and fade away.
• 1. Stars with large mass have a lot of fuel but use it up more quickly.
  2. When they are super red giants (core burning to shell burning) can continue beyond helium fusion.
  3. Can go all the way up to iron
  4. When core runs out of fuel it contracts and forms white dwarf core. If core is larger than Chandrasekhar limit , core of star starts to contact, the outerlayers fall in and rebound off the core, setting up huge shockwaves
  5. Huge shockwaves cause stars to explode in supernova leaving a neutron star or black hole.
• If a white dwarf core is 1.4 to 3 times mass of sun, it is a neutron star. Electrons get squashed onto the atomic nuclei combining with protons to form neutrons and neutrinos. They emit radio waves are about 20 km wide. They also spin up to 600 times per second.
• If the core of a star is more than 3 times the suns mass the neutrons can't withstand the gravitational forces and star contiues to collapse into a black hole.

  Collapse into infinitely dense point called a singularity.

  Not even light can escape. The boundary of this region is called the event horizon.
• The Doppler effect is that the Observed frequency is different to source frequency when source moves relative to observer.
• Temp goes from right to left.

  
• You don't see any stars in any transitional period on the H-R diagram because they are unstable and the transitions happen quickly (compared with the life of the star).
• Luminosity is a measure of how bright an object is.
• 1. If you heat a gas to a high temp, many of its electrons move to higher energy levels
  2. As they fall back to ground state photons are emitted
  3. If you split the light from hot gas with diffraction grating you get a line spectrum
  4. Each line corresponds to a particular wavelength.
  5. Different atoms have different electron energy l; levels and so different sets of emission spectra. This means you can identify a gas from its emission spectrum.
• 1. You get a line of absorption when light with a continuous spectrum energy (white light) passes through a cool gas.
  2. At low temps, most of the electrons in the gas atoms will be in their ground states.
  3. Photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels
  4. These wavelengths are missing from the continuous spectrum when it comes out on the other side.
  5. You see a continuous spectrum with black lines in it corresponding to the absorbed wavelengths.
• Emission vs Absorption spectrum

  
• Stars can be assumed to emit radiation in a continuous spectrum. This radiation has to pass through a large amount of gas at the surface of the star before travelling to Earth. Gas absorbs particular wavelengths of light depending on the element it consists of. Comparing the absorption spectra of stars to sets of emission spectral lines from the lab allows you to identify elements within a star.
• The luminosity of a star is the total energy it emits per second and is dependent on its temperature and surface area.

  
• The relationship between intensity and wavelength varies with temp.

  The most common wavelength beocmes shorter as the surface temp of the star increases. This is called the peak wavelength. Can use peak wavelength to estimate a stars peak surface temp using weins displacement law (max wavelength inversely proportional to Temp(kelvin)).

  
• COSMOLOGICAL PRINCIPLE: On a large scale the universe is homogeneous (every part is the same as every other part) and isotropic (it looks the same in every direction) and the laws of physics are universal (the same everywhere).
• Recessional velocity is directly proportional to distance where the constant is Hubbles constant.
• The Big Bang Theory: The universe started off very hot and very dense (perhaps as an infinitely hot, infinitely dense singularity) and has been expanding ever since.
• The Big Bang model predicts that loads of gamma radiation was produced in the very early universe. Therefore this radiation should still be observed today

  However, because the universe has expanded the wavelengths of this cosmic radiation have been stretched and are now in the microwave region.
• 1. 10^-43 seconds: the infinitely hot, infinitely small, infinitely dense point
  2. 10^-43 - 10^-4 seconds: Universe expands and cools and unified force splits into gravity, strong nuclear, weak nuclear and EM forces. The universe is a sea of quarks, antiquarks, leptons and photons.
  3. 10^-4 seconds: Universe is cool enough for quarks to join up to form particles like protons and neutrons.
  4. About 100 seconds: Protons are cool enough to fuse to form helium.
  5. about 300,000 years: electrons combine with helium and hydrogen nuclei to form atoms.
• In the 1970's the Vera Rubin observed that stars at the edges of galaxies were moving faster than they should given the mass and distribution of stars in the galaxy. For Newtons laws to hold there needed to be extra matter in the galaxies that hadn't been acounted for. We call this dark matter.
• One explnation is that dark matter is made up of MACHO's (Massive Compact Halo Objects) e.g black holes
• Another idea is that dark matter is made up of WIMP's (weakly interacting massive particles). They do not interact with EM forces and gravity.
• The expansion of the universe appears to be accelerating. Astronomers are trying to explain this acceleration using dark energy.
• Based on current observations dark energy makes up about 70% of the universe. As dark matter makes up another 25% this means that only about 5% of the universe is made up of ordinary matter. Or to put it another way, we have very little idea what 95% of the universe is made up of.