Cosmology

• One solar mass is the mass of the sun and is equal to 2 x10 ^30 kg.
• All stars start as a nebula, which is a dense ball of gas and dust which rotates and compresses under its own gravity to form a protostar surrounded by a disc of material called a circumstellar disc.
• When a protostar gets hot enough, it begins to fuse hydrogen, producing a strong stellar wind that blows away the circumstellar disc. When the inward force of gravity equals expansion force due to fusion, the star is considered main sequence.
• The greater the mass of the star, the shorter its main sequence period because it uses fuel quicker.
• Once the hydrogen runs out in a main sequence star, the temperature of the core increases and the outer layers of the star expand as helium is fused into heavier elements. The star is considered a red giant or supergiant depending on its mass.
• A red giant forms when the solar mass of the star is less than three, and a red supergiant forms when the solar mass is more than three.
• Red supergiants can fuse larger elements than red giants, up to iron.
• Once the red giant of a star less than 1.4 solar masses has used up all its fuel, fusion stops and the core contracts as gravity becomes the dominant force. The outer layers are thrown off, forming a planetary nebula, and a very dense white dwarf forms, which will eventually cool to form a black dwarf.
• Once the red giant/super giant of a star more than 1.4 solar masses has used up all its fuel, fusion stops and the core contracts as gravity becomes the dominant force, but suddenly stops and goes rigid as matter can no longer be forced closer. The outer layers then rebound off the core and are launched into space in a supernova releasing huge amounts of energy, fusing elements up to uranium.
• For a star between 1.4 and 3 solar masses, when the core collapses in a supernova, it forces protons and electrons to come together and form neutrons in a neutron star of density the same of that of nuclear matter.
• Pulsars are spinning neutron stars that emit beams of radiation from their magnetic poles.
• For a star of more than 3 solar masses, when the core collapses in a supernova, the gravity is so strong neutrons cannot withstand the gravitational force pulling them together, and light cannot escape.
• The event horizon of a black hole is the point at which escape velocity becomes more than the speed of light.
• The Schwarzchild radius is the radius of the event horizon and is R = 2 G M/c^2.
• A binary system is one where two stars orbit a common mass.
• Type 1 supernova occur when a star accumulates matter from its companion star in a binary system and explodes after reaching a critical mass.
• Type 2 supernova occur when the core of a high mass star collapses when it runs out of fuel.
• A type 1a supernova is a type 1 supernova with a white dwarf. When the companion star runs out of hydrogen it expands, allowing the white dwarf to accumulate some of its mass. After a critical mass, fusion begins and the white dwarf explodes in a supernova.
• Type 1a supernovae all occur at the same critical mass, meaning they have the same peak absolute magnitude (of about -19.3) and consistent light curves and so can be used as standard candles to calculate distances to far away galaxies.
• Type 1a supernova have been seen dimmer than they were predicted to be, meaning they are more distant than predicted, suggesting the expansion of the universe is accelerating.
• Dark energy, which is thought to have a constant, overall repulsive effect on the universe, is thought to be the reason the universe is accelerating as gravity follows an inverse square law so at large distances dark energy would be the dominant force.
• The doppler effect is the compression or spreading out of waves that are emitted or reflected from a moving source.
• The doppler effect causes distant objects to be shifted to the blue end of the spectrum when they move towards the earth (blue shift) or to the red end of the spectrum when they move away from the earth (red shift).
• Almost all distant objects are red shifted, providing evidence for an expanding universe.
• The red shift of an object in terms of receding velocity, frequency and wavelength can be calculated by z = v/c = $\delta$ f/f = - $\delta$ $\lambda$ / $\lambda$ respectively.
• The z value is positive for red shift and negative for blue shift.
• The doppler effect can be used to identify binary star systems as part of the spectra is red shifted when the other is blue shifted and visa versa.
• Spectroscopic binaries are binary star systems in which the stars are too close to be resolved by a telescope.
• Eclipsing binaries are when the plane of the orbit of the stars is in the line of sight from earth to the system, meaning they cross in front of each other as they orbit, producing periodic drops in light intensity, with two alternating minimums.
• Hubble's law states that the universe is expanding from a common starting point, and can be represented by the formula v = H d, where v is the recessional velocity and H is the Hubble constant, 65 km/s Mpc.
• The age of the universe can be found from Hubble's law and distance = velocity x time to show that the age of the universe, t = 1/H.
• The big bang theory suggests the universe began from a singularity that was infinitely small and hot and expanded, releasing lots of high energy gamma radiation.
• Cosmic Microwave Background Radiation (CMBR) is the gamma radiation leftover from the big bang that has been red shifted.
• Hydrogen was fused to Helium during the big bang in the early period where it was hot enough for fusion but cool enough for nucleons to exist, and the time period of this predicted by the big bang would produce a relative abundance ratio of H:He of 3:1, which is what we observe today, providing evidence for the big bang.
• A quasar is an active galactic nucleus which consists of a supermassive black hole surrounded by a disc of matter which, as it falls into the black hole, causes jets of radiation to be emitted from the poles.
• Quasars are characterised by having a very large red shift (being far away), being very bright (having a very large power output), and being not much larger than a star.
• Exoplanets are planets not within our solar system, ie they do not orbit the sun.
• Exoplanets are very difficult to detect directly as they tend to be obscured by the light of their host stars. There are two main methods of detecting them: the radial velocity and transit methods.
• The radial velocity method involves measuring a periodic doppler shift produced by the plant orbiting the star. It is most noticeable with high mass planets.
• The transit method involves measuring periodic intensity shifts in the light from a star. It only works if the planet orbits in our line of sight, and so is more likely to work for planets with small orbits.