Astrophysics

• One astronomical unit (AU) is defined as the mean distance between the earth and the sun
• A galaxy is a cluster of stars and planents
• A solar system is a collection of objects orbiting a star.
• Planetary satellites are object which orbit a planet. E.g. a moon or artificial satellite
• Comets are made of ice, dust and rock
• Planets are large objects which orbit stars
• Dwarf planets are planet-like objects which orbit stars but don't meet all the conditions to be a planet
• Asteroids are chunks of rock and metal that orbit the sun
• Distance to the sun (d) = radius of the earths orbit (r) / angle of paralax in radians (0)
• A star is exactly one parsec away if the angle of parallax, 0 = 1 arc second = (1/3600) of a degree
• angle of parallax in arc seconds (p) = 1 / distance to the star in parsecs (d)
• Clouds of dust and gas contract together very slowly to form a protostar. This protostar continues to contract and heat up until it can fuse hydrogen to helium and eventually forms a main sequence star
• When stars core can no longer fuse hydrogen they start to expand and form red giants. The outer shell now starts to fuse hydrogen and expand the star. The core contracts and heats up further until it becomes hot enough to fuse helium instead of hydrogen. When the stars core can no longer fuse helium it expands as the shall fuses helium
• In low-mass stars like our sun the carbon-oxygen core won't become hot enough to fuse and so will continue to collapse and form a white dwarf. When the core shrinks to about the size of our earth the pressure of the electrons stop it from collapsing any further
• The maximum mass for which the electron degeneracy pressure can counteract the gravitational force is called the Chandrasekhar limit. This is about a star core mass of 1.4 solar masses
• For stars below the Chandrasekhar limit the outer layer will continue to expand after the core has become a white dwarf and form a planetary nebula
• High mass stars have a shorter life. When are red giants, the core burning to shell burning can continue past carbon and oxygen. They become super red giants. When the core can't burn anymore fuel and begins to contract the outer layers fall in and rebound of the core causing huge shockwaves. This is a supernova
• A supernova can leave behind either a neutron star or a black hole. If a stars core is between 1.4 and 3 solar masses, it will leave behind a neutron star. Neutron stars are very dense and only around 20km across. They also rotate very fast.
• if a core is more than 3 solar masses then when it collapses it will form a black hole. Nothing can escape a black holes pull, not even light. The boundary of a black hole is called the event horizon. At the event horizon the escape velocity = c
• Wien's displacement law states: peak wavelength (y_max) is directly porportional to 1 / Temperature in kelvin (T)
• Luminosity in W (L) = 4pi*r^2*stefan constant*T^4
• The Hertzsprung-Russell diagram plots the luminosity of stars against their temperature. The higher temperatures are closer to the y axis. The luminosity of the stars are in solar luminosity units. Our sun is solar luminosity 1.0