Space

Name	Definition	Note
STELLAR PARALLAX
Intensity/ Flux		$I=frac{L}{4Pi d^{2}}$
Stellar Parallax	The apparent shift in the position of a nearby star, relative to more distant ones, due to the moment of the Earth around the Sun.	Parallax of nearby Star $d=frac{1AU}{Theta }$ $d(p.s.)=frac{1}{Theta$ (“)
Stellar Parallax	The star is viewed from two positions at 6- month intervals The change in angular position of the star against backdrop of fixed stars is measured Use trigonometry parallax to calculate the distance
Parsec	The distance that a star would be if it had a parallax of 1 arcsec
Elliptical orbit	Over the course of one year the stars will trace out an elliptical path on the sky. Stars have orbits not perpendicular to earth will appear to have elliptical orbits because only see the projection of the diameter.
Large distance	When d is large θ is small so the fractional uncertainty is large, therefore there is a large fractional uncertainty in the calculated value of d.
	Since Mars is farther away from the Sun than the Earth, for a given parallax we can calculate a larger value of stellar distance
STANDARD CANDLES
Standard candle	An astronomical object whose luminosity is know
Cepheid variables	Giant stars that become unstable and pulsate: their diameters oscillate and therefore they vary in luminosity Cepheid variables out ward pressure P and inward gravity compression are out of sync so the star and temperature pulsates
Cepheid variables	Determine distance to Cepheid Measuring period T $L=4Pi aR^{2}T^{4}$ give luminosity Light flux can be determined $I=frac{L}{4Pi d^{2}}$ Inverse square law gives the distance
Supernovae	The explosion of stars that have run out of fuel for nuclear fusion in their cores. Type 1a are standard candles Type 1a supernovae are extremely luminous they can be seen from a very large distance The light curve must be calibrated by using Cepheid variables to determine the distance to a galaxy that contains a type 1a supernovae
HR DIAGRAM
HR diagram	A Luminosity-Temperature diagram
Main sequence	Stars that convert Hydrogen into Helium via thermonuclear fusion in the core
Blue giants	Large mass, high temp and luminosity
Red giants	Low temp, high luminosity, converting He4 to C-12 and O-16
White dwarfs	Core of a red giant star Do not have fusion reaction Radius is very small $L=4Pi sigma R^{2}T^{4}$ so luminosity is low Surface temperature is high $lambda _{peak}$ is in UV spectrum Emits a lot of light in visible spectrum so appear white
Star	Stars are very good black bodies. The total radiations they emit per second only depend on the surface area and the absolute temperature. They obey Stefan’s law: $L=4Pi sigma R^{2}T^{4}$ And Wein’s law, $lambda _{peak}T=2.898times 10^{3}$ A star position on the HR depend on its mass and its age Stars are large ball of gas (mostly hydrogen, helium)
Life cycle of the stars	Gravity cause a large cloud of gas and dust to collapse & heat up When neutral temperature reach ≈ $10^{6}$ , nuclear reaction starts in the centre, H is converted to He A star is born, its life cycle of a star depends on its mass Young star groups have more red giant stars
Planetary nebula	Shell of gas ejected from RG star on its way to becoming a WD
Pulsar	Rotating neutron star with a very string magnetic field Pulsars beam radiation out along their magnetic axis
Gaseous nebula	Large cloud of gas & dust. They have very low temperature and density.

Name	Definition	Notes
Doppler’s effect
CMBR	Cosmic Microwave Background Radiation: Come from all part of the sky Its intensity is almost the same in every direction Black body radiation produced in the hot Big Bang Whose wavelength have been stretched by the cosmological expansion The peak wavelength Is now in the microwave part of the spectrum It implies that the temperature after Big Bang was very high If the temperature was exactly uniform across the sky, the density of the universe would be exactly uniform Gravity would not be able to form structures such as galaxy, stars and planets Low temperature region has higher density and will collapse first to form galaxy
Hubble’s Law	The recession velocity of a galaxy is directly proportional its distance from our galaxy It implies that in the past the universe was smaller By extrapolating backward far enough, everything in the universe was at the same location: a point of infinite density and temperature, The Big Bang	$v=H_{o}d$
Hubble’s parameter	The gradient of the Hubble’s law graph The present value is $H_{o}=71 km/s/Mpc$ Hubble’s constant not very accurate Because the distances to the galaxies are underestimated hence gradient is not as steep as in Hubble’s graph
Dark matter	Material that does not interact via the electromagnetic force. Its gravity may be responsible for explaining the rotation curves of galaxies and the stability of the galaxy clusters
Cosmological redshift	The increase in wavelength of radiation from distant galaxies due to the expansion of the universe
Redshift	The fractional increase in wavelength of light emitted by a source and detected by an observer due to the relative motion between them	$z=frac{lambda _{0}lambda _{e}}{lambda _{e}}=frac{v}{c}$
	Light from almost all galaxy are redshifted $lambda _{observed}> lambda _{lab}$ Due to Doppler effect galaxy are moving away from us Hubble’s law so distance between galaxy is increasing So, the universe is expanding
Big Bang Nucleosynthesis	The early universe was extremely hot and dense, the condition is suitable for thermonuclear fusion to occur
	In order to account for the measured shape of the graph there has to be more mass than can be accounted for by the visible matter. This extra mass is called dark matter Dark matter does not emit electromagnetic radiation, but it has gravitational effects The dark matter affects the gravity of the universe, which affect the rate at which the universe expands, so it affects whether the universe is open, closed or flat
	Because the total density of the universe is uncertain, the future of the universe is uncertain