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Solar system

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Mercury, venus, earth, mars, jupiter, saturn, uranus and neptune.

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Stellar clusters

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Groupings of a large number of stars bound by gravity. 1. Globular clusters: large number of mainly old stars. 2. Open clusters: small number of young stars that are farther apart.

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Galaxy

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A huge assembly of stars bound together by gravity.

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Cluster of galaxies

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Galaxies close to one another and affecting one another gravitationally, behaving as one unit.

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Supercluster of galaxies

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A cluster or collection of clusters of galaxies.

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Nebula

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Clouds of dust, i.e. Compounds of carbon, oxygen, silicon and metals, as well as molecular hydrogen, in the space between stars.

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Planetary nebula

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The ejected envelope of a red giant star.

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Comet

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A small body (mainly ice and dust) orbiting around the sun in an elliptical orbit. Their tails lights up when close to the sun.

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Dark matter

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Generic name given to matter that is too cold to radiate. Is said to exist around galaxies for them to be kept bound together and for their rotational velocities to be constant throughout.

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Interstellar medium

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Gases (mainly hydrogen and helium) and dust grains (silicates, carbon and iron) filling the space between stars. The density is very low. The temperature of the gas is 100K.

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Main-sequence star

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Normal star that is undergoing nuclear fusion of hydrogen into helium.

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Constellation

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A group of stars in a recognizable pattern that appear to be next to each other in space.

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Binary star

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Two stars orbiting a common center.

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Black hole

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Singularity in space and time; the end result of the evolution of a very massive star.

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Black dwarf

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The remnant of a white dwarf after it has cooled down.

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Brown dwarf

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Gas and dust that did not reach a high enough temperature to initiate fusion of hydrogen into helium.

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Supernova (type 1a)

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The explosion of a white dwarf that has accreted mass for my companion star (binary system) exceeding the Chandrasekhar limit. It doesn't have a hydrogen lines and its spectrum and its luminosity falls quickly.

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White dwarf

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The end result of A red giant whose core doesn't exceed the Chandrasekhar limit. It is the small dents star with a very low luminosity.

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Neutron star

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The end result of the explosion of a red supergiant; a very small star that is very dense. These form when the core of the star exceeds the Chandrasekhar limit but does not exceed the Oppenheimer-Volkoff limit.

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Supernova (type II)

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Explosion a massive red supergiant. There are hydrogen lines in the spectrum and its luminosity falls gently.

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Red giant and supergiants

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The main sequence star evolves into a red giant when the hydrogen has been used up. Red giants fuse helium into carbon. And more massive stars helium fuses with carbon to produce oxygen. And even more massive stars neon, sodium and magnesium are produced. Silicon is then produced by the fusion of oxygen and the process ends with iron. The stars are very large, cool and a reddish in appearance.

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Proton proton cycle

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Cycle in which four hydrogen atoms fuse into helium.

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CNO cycle

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And stars more massive than our sun, there is the second way to fuse hydrogen into helium which involves carbon, oxygen and nitrogen.

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Radiation pressure

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The pressure caused by photons radiating outwards colliding with surrounding material. As long as the star is fusing hydrogen into helium in the main sequence, it will have the radiation pressure necessary to maintain a constant size.

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Apparent brightness

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The received power per unit area.

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Luminosity

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Total power radiated by a star.

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Wein's displacement law

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Product of wavelength and temperature is equal to a constant (0.0029). This implies that the higher the temperature the lower the wavelength at which most of the energy is radiated.

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Spectral classes

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O, B, A, F, G, K, M (oh be a fine guy kiss me).

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Mass-luminosity relation

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This relation only applies to main sequence stars. L is proportional to M^3.5

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Cepheid variable stars

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These stars periodically expand and contract as a result of helium ionization. As a result of the expansion and contraction their brightness also vary. The period of cepheid variable stars is proportional to their luminosity. They act as standard candles.

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Chandrasekhar limit

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1.4 solar masses (mass of the core only). If this limit is surpassed, then the star will experience further collapse into a neutron star (electrons will be driven into protons to produce neutrons). If not, it will stay as a stable white dwarf.

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Hubbles law

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Velocity of receding galaxies is proportional to their distance from us. This was discovered when observing the wavelengths from the emission spectrum of distant stars. They found red-shift.

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Oppenheimer-Volkoff limit

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Equal to 2-3 solar masses. If this limit is exceeded then the neutron star will collapse into a black hole.

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Electron degeneracy pressure

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Electron degeneracy pressure prevents for further collapse of the core and this is what maintains a stable white dwarf. This electron degeneracy pressure is the pressure that prevents two electrons from occupying the same space.

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Neutron degeneracy pressure

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The pressure that prevents further collapse from a neutron star. This pressure prevents two neutrons from occupying the same space.

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Big bang model

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The discovery of the expanding universe by Hubble implies a definite beginning and this model describes a singularity that began expanding indefinitely.

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The age of the universe

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The inverse of the Hubble constant gives an upper bound on the age of the universe-that is, the actual age is less. This is because the estimate is based on a constant rate of expansion equal to the present rate.

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Cosmic microwave background radiation

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Remnant of the Big Bang radiation. It is said to be isotropic, yet it has a few fluctuations. It's anistropies in the CMB are crucial in understanding of the formation of structures (if it didn't have fluctuations, then atoms would not have moved to form structures).

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Standard candle

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A star of known luminosity.

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Jeans criterion

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A criterion that involves mass of a gas cloud, it's temperature, it's radius and the number of particles, for the gas cloud to collapse into a protostar.

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Protostar

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A very young star that is still accreting mass.

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Nucleosynthesis

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An onion-like layered structure in the star, with the heaviest element in the core, surrounded by progressively lighter elements. Iron, silicon, magnesium, neon, oxygen, carbon, helium, hydrogen.

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Neutron capture

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Heavier elements than iron are produced by neutron capture (the nuclei absorb neutrons). S-process: slow process. The isotope does have time to decay because the number of neutrons present is small. The isotopes will undergo a series of decays, producing new elements. R-process: the nuclei keep absorbing neutrons one by one, forming very heavy neutron-rich isotopes.

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Cosmological principle

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Isotropy principle: whichever direction you look at, the universe is the same. Homogeneity principle: on a large enough scale, the universe looks uniform.

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Critical density

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The density at which the universe will expand at a constant rate, and halt after an indefinite amount of time.

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Types of universes

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1. Closed universe: If the density of matter is greater than the critical density, the universe will stop expanding and begin contracting. 2. Flat universe: If the density of matter is equal to the critical density, then the universe will expand at a constant rate indefinitely 3. Open universe: If the density of matter is less than the critical density, then the universe will expand at an accelerated rate indefinitely.

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Dark energy

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The presence of this energy creates a repulsive force that counteracts and dominates over the effects on gravity, causing an accelerated expansion.

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