# IB Physics – Astrophysics

Absolute magnitude
The apparent magnitude a star would have if viewed from a distance of 10 parsecs.
Apparent brightness
The received energy per second per unit area of detector.
Apparent magnitude
A relative logarithmic system classifying the brightness of stars seen from earth. An increase in apparent magnitude by 5 units implies a decrease in apparent brightness by a factor of 100.
Big Bang model
The theory according to which space, time, matter and energy were created at a singular point 13-14 billion years ago.
Binary star system
Two stars orbiting a common centre.
Cepheid variable
A star whose luminosity periodically changes due to expansions and contractions of its surface. The period of variation and the peak luminosity are related to each other.
Chandresekhar limit
The largest mass a white dwarf can have – 1.4 solar masses.
Electromagnetic radiation in the microwave region of the spectrum that fills the universe, with a black-body spectrum corresponding to about 2.7 K. The remnant of the high temperatures at the time of the Big Bang.
Critical density
The density of the universe at which the expansion of the universe continues at a slowing rate for ever, stopping after an infinite amount of time.
Dark matter
Matter that is too cold to radiate, and so cannot be seen.
Hertzsprung-Russell diagram
A plot of stars according to luminosity (vertical axis) and temperature (horizontal axis, higher temperatures on left).
Hubble constant
The slope of a graph of galaxy speed versus distance.
Hubble time
The inverse of the Hubble constant – giving an estimate for the age of the universe.
Hubble’s law
v = Hd
Galaxies are moving away from us with a speed which is proportional to their distance away from us.
Luminosity
The amount of energy radiated by a star per second, dependent on the surface temperature and the surface area of the star.
Magnitude-distance relation
m – M = 5log(d/10)
m is apparent magnitude, M is absolute magnitude and d is distance in parsecs.
Main sequence
Stars undergoing fusion of hydrogen to helium.
Mass-luminosity relation
L ∝ M^n
n is a number between 3 and 4.
Neutron star
A collapsed star composed almost entirely of neutrons whose degeneracy pressure balances the inward pressure due to gravity.
If there were an infinite number of stars in the universe and they lived forever, the night sky would be infinitely bright.
Oppenheimer-Volkoff limit
The largest mass a neutron star can have – 3-4 solar masses.
Parallax method
A method of measuring the distance to nearby stars (up to a few hundred parsecs) relying on the fact that a nearby star viewed from two different positions appears displaced relative to the background of distant stars.
Planetary nebula
The ejection of mass from an exploding red giant.
Pulsar
A rotating neutron star emitting radio waves.
Spectral class
A classification of stars according to surface temperature and colour.
Spectroscopic parallax
A method for determining distance to a main sequence star, using the Wien displacement law to determine surface temperature from the star’s spectrum, the Hertzsprung-Russell diagram to determine luminosity and apparent brightness measurements to determine distance.
Stellar evolution
(it’s complicated – learn it)
Supernova
The ejection of mass from an exploding supergiant star.
White dwarf
The final stage in the life cycle of a small star, where degeneracy pressure from electrons balances the inward pressure of gravity.
Wien displacement law
λ₀T = 2.90 × 10³ Km … λ₀ is the wavelength at which most energy from the star is emitted and T is the surface temperature.
Categories: Astrophysics