All we know about distant stars is obtained by observations, for which we are restricted to the spectrum of EMR (electromagnetic radiation) which reaches us after many years from the star.
Although this sounds limiting, a lot of information is contained in this radiation. Not only can we measure the star's surface temperature, we can determine its chemical composition from its absorption spectrum.
The spectrum of light that we receive can be plotted as wavelength against intensity. Using Wien's Law, from this we can determine the surface temperature. This same spectrum displays dark lines. These mark where all the light of a certain wavelength has been absorbed by a specific chemical. This is like a finger print, giving us a profile of the chemical composition of the star!
As the temperature of a star changes, so do the energy states of the electrons. Hot stars have hydrogen atoms which have lost their electrons. Absorption of light occurs when electrons absorb the photon and change energy levels (transition states). Without the electron, a hydrogen atom is just a proton, so cannot absorb the light, and there is no absorption line in the stellar spectrum. Cooler stars, on the other hand, do display dark absorption lines for hydrogen in their spectra. The hotter the star, the higher the states to which electrons will transition, resulting in different absorption spectra.
Stars are classified into seven spectral classes according to their temperatures.
Other information stellar spectra provide are: radial velocity, rotation speed, and magnetic fields.
Stars exist under a state of equilibrium between gravitation and radiation pressure. These are called 'Main Sequence' stars, and make up 90% of all known stars of our galaxy.
Nuclear fusion provides the energy which stars have: powerful and continuous radiation and heat. Depending on its mass, every star has its own characteristics of temperature, radiation and luminosity. After its initial nuclear radiation energy is depleted beyond a critical percentage, stars change their state. How this happens, again depends on their mass.
The Hertzsprung-Russel diagram is a graphical demonstration of the relationship between surface temperature and luminosity. It maps the Main Sequence stars, showing that as the temperature increases, from right to left, the luminosity increases. In the top right-hand corner are the Red Giants. In the bottom left-hand corner are the White Dwarfs.
Be careful not to interpret the graph as meaning that a star will 'evolve' along the line of main sequence. The position of the star on the main sequence is fixed by its size. When the main sequence stars have exhausted a critical percentage (this depends on their size) of the hydrogen fuel they are born with, if they are large enough they will become Red Giants, and then White Dwarfs.
These stars are stars which were in the Main Sequence, but whose hydrogen fuel has been largely converted to helium. As their name suggests, they are reddish in colour, large and relatively cool. about 1% of stars are Red Giants.
Although the luminosity is proportional to the fourth power of the temperature, and only the first order of the surface area, the enormous size of red giants results in a net luminosity greater than that of Main Sequence stars. In fact, Red Giants may have radii of the order of 1000 times greater than Main Sequence stars!
Their masses may also be a thousand times that of Main Sequence stars, like our Sun, but since volume increases with the cube of the radius, this means Red Giants have much lower densities. In fact, Red Giants have a central hot core, which is surrounded by an envelope of tenuous gas.
As the name suggests, these stars are small. They are dense and hot, but nevertheless difficult to detect because of their size. About 9% of stars are White Dwarfs.
A white dwarf called Sirius B has the mass of our Sun but is the size of the Earth! This means it has a density one million times that of the Earth.
White dwarfs are a state of equilibrium after a star has collapsed due to gravitational force. Under the extreme pressure that results from this gravitational collapse, the electrons generate huge energies, and prevent the collapse from continuing beyond a certain point.
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