# Physics – Option E – Astrophysics Roy Johnson
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Absolute magnitude

The apparent magnitude a star would have if observed from a distance of 10 pc.
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Apparent brightness

The received energy per second per unit area of detector. It equals b = L/4(pi)d^2. Its units are Wm^-2.
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Apparent magnitude

A measure of brightness of a star as seen form earth in a relative system of classification. The higher the numerical value of apparent magnitude, the dimmer the star. An increase in apparent magnitude by 1 unit implies a decrease in apparent brightness by a factor of (5root)100 approximately equal to 2.51.
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Big Bang model

The theory according to which space, time, matter and energy were all created at a singular point some 13 to 14 billion years ago.
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Binary star system

Two stars orbiting a common center.
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Cepheid variable

A star whose luminosity changes periodically due to contractions and expansions of its surface. There is a definite relationship between the period of variation of the luminosity and the peak luminosity. Thus, knowledge of the period gives the peak luminosity L which, together with the known peak apparent brightness b, gives the distance d through b = L/4(pi)d^2.
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Chandrasekhar limit

The largest mass a white dwarf can have. It is about 1.4 solar masses.
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The CMB is electromagnetic radiation in the microwave region that fills the universe. It has a blackbody spectrum corresponding to a temperature of about 2.7K. It is the remnant of the high temperatures at the time of the Big Bang and provided one of the strongest pieces of evidence in favor of the Big Band model.
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Critical density

In classical cosmology, the density p(subscript c) of the universe for which the expansion continues forever at a slowing rate and stops after an infinite amount of time. It separates a universe that will expand forever (an open universe, p less than p(subscript c)) from one that will re-collapse (a closed universe, p greater than p(subscript c)). A universe with a density equal to the critical density is call flat.
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Dark matter

Matter that is too cold to radiate, and so cannot be seen. It has been invoked to solve the puzzle of the missing mass of the universe. As much as 90% of the mass of the universe may be in the form of dark matter.
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HR (Hertzsprung-Russell) diagram

A plot of stars according to luminosity (vertical axis) vs. temperature (horizontal axis, temperature increasing to the left), or absolute magnitude vs. spectral class.
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Hubble constant

The slope of a graph of galaxy speed vs. time.
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Hubble time

The inverse of the Hubble constant, giving an estimate for the age of the universe.
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Hubble’s law

Distant galaxies are moving away from earth with a speed v that is proportional to their distance d from the earth; that is, v = Hd, where H is the Hubble constant.
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Luminosity

The amount of energy radiated by a star per second, i.e. the power radiated by the star. Luminosity depends on the surface temperature T and the surface area A of the star, and is given by L=o’AT^4. The constant o’ us the Stefan-Boltzmann constant (o’ = 5.67×10^-8 Wm^-2 K^-4).
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Magnitude-distance relation

The equation relating a star’s apparent magnitude m to its absolute magnitude M and distance d (in parsecs); m-M=5log(d/10).
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Main sequence

Stars undergoing nuclear fusion of hydrogen into helium. They lie on a strip on the HR diagram from top left to bottom right.
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Mass-luminosity relation

The relation between the luminosity and the mass of a main sequence star, L proportional to M^n, where n is between 3 and 4. It can be used to explain why massive stars spend little time on the main sequence.
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Neutron star

An end stage in the evolution of high-mass stars. A collapsed star composed almost entirely of neutrons whose degeneracy pressure balances the inward pressure due to gravity. It is very dense and often has a very strong magnetic field and rotates.
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The night sky would be bright if there were an infinite number of stars in an eternal universe. In fact; 1) There is a finite number of stars 2) They would not live forever So the night sky is dark.
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Oppenheimer-Volkoff limtit

The largest mass a neutron star can have. It is about 2-3 solar masses. The uncertainty in this limit comes from the face that the equation of state of the matter inside a neutron star is not precisely known.
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Parallax method

A method for measuring the distances to nearby stars that relies on the face that a star appears displaced relative to the background of distant stars when viewed from two different positions in space. Satellites in orbit outside the earth’s atmosphere can measure distances up to almost 1000 pc in this way.
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Planetary nebula

The ejection of mass from an exploding red giant star.
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Pulsars

Rotating neutron stars emitting radio waves.
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Spectral class

A classification of stars according to surface temperature and color. The classes are OBAFGKM, with O being hot and blue, and M cool and red. Our sun is a class-G star (yellow-orange at 6000K).
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Spectroscopic parallax

A method for measuring the distance to a main-sequence star. It consists of determining the star’s surface temperature (or spectral class) from its spectrum using the Wien displacement law. Using this, it’s luminosity L (or absolute magnitude M) can be estimated form the HR diagram. Its apparent brightness b can be measured, allowing the determination of the distance d, through b = L/4(pi)d^2.
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Stellar evolution

The evolution of a star from its birth to its life on the main sequence, then to its life as a red giant or super giant, and finally its death. The way the star dies is determined by its mass. If the star is not too massive (under 10 solar masses), a planetary nebula ejects most of the mass of the star and leaves behind a dense, hot core (a white dwarf) of maximum mass 1.4 solar masses (the Chandrasekhar limit). If the star is more massive, a supernova ejects most of the star’s mass, leaving behind a neutron star of maximum mass about 3 solar masses (the Oppenheimer-Volkoff limit). If the star is even more massive, it ends up as a black hole.
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Supernova

The ejection of mass form an exploding super giant star.
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White warf