Stars are plasmas mostly hydrogen, with some helium and a very small percentage of other elements. Their surface temperatures range from 2000 K to 50,000 K and their masses range from 0.04 to 75 solar masses, most being in the 0.1 to 5 range. Their size range from white dwarfs (12,800 km = 8000 miles) to supergiants such as Antares, Betelgeuse, and Rigel, which are millions of miles in diameter. Most stars form a binary system.
Billions of stars are visible in the night sky. Stars like the Sun are are nuclear reactors and burn Hydrogen to radiate light. Hence they are visible even over vast distances. There are two ways to measure their brightness.
1. Apparent Brightness, or magnitude, refers to the brightness of the star as observed from Earth.
2. The energy output of a star is measured by absolute magnitude, which is the apparent magnitude a star would have if it was 10 parsecs (32.6 light years ) from Earth.
The brightest stars were listed as stars of the first magnitude, those
not quite as bright as stars of the second magnitude, and so on to six
magnitudes by Hipparchus
of
Nicaea, a Greek astronomer and mathematician in 129 B.C. A modified version
of his scale is used today. When a comparison was made between stars
of first and sixth magnitude, the former gave off about 100 times as much
radiant energy. From this observation, a definition of the
magnitude scale was made in which each magnitude was made equal to the
fifth root of 100, which equals 2.512. So a star of first magnitude
was 2.512 times as bright as a star of second magnitude, 2.512 times 2.512
= 6.31 times as bright as one of third magnitude and so on. When
giving the magnitude, the greater the negative number, the brighter the
star, the greater the positive number, the dimmer the star.
On this scale, the apparent magnitude of the Sun, the brightest object
in the sky is -26.7, the full moon is -12.7, the planet Venus is -4.2,
and the star Sirius is -1.43. The absolute magnitude of the Sun is
+4.83. When the absolute magnitude is plotted against the surface
temperatures we get an H-R diagram, named after Ejnar Hertzsprung, a Danish
astronomer, and Henry Russell, an American astronomer. 
Note that the temperature axis (x-axis) is reversed, that is, the temperatures increase to the left instead of to the right. Most stars on the H-R diagram get brighter as they get hotter. These stars form the main sequence, a narrow band, going from upper left to lower right. Stars above the main sequence that are cool yet very bright must be unusually large and so are called red giants or super giants. Stars below the main sequence that are hot yet very dim must be small and so are called white dwarfs.
Stars are placed in seven different spectral classes that range from type O (50,000 K) to type M (2000 K) The Sun is a type G star. The majority of the stars (at lower right) are small, cool, red type M stars called red dwarfs.
For details and a sketch of the diagram click below.
The closest star that can be observed from the United states is Sirius, which is 8.7 light years away. But there a group of stars closer that are visible from the Southern Hemisphere. Alpha Centauri A (apparent magnitude -0.01) and its close companion, Alpha Centauri B (apparent magnitude +1.33) revolve around each other and to the unaided eye, appear as a single star. Nearby is a faint red dwarf, Proxima Centauri, which is slightly closer to Earth. At 4.3 light years, they are the the closest stars to Earth.
Stars have a life cycle. They are born, radiate energy, expand, possibly explode, and then die. The greater the mass of the star, the faster it moves through its life cycle. The birth of a star begins with the condensation of interstellar material (mostly hydrogen) in a nebula, because of gravitational attraction between the interstellar material, radiation pressure from nearby stars, and supernova shock waves. Its size depends upon the total mass available. This determines the rate of contraction. As the interstellar mass condenses, and loses gravitational potential energy, the temperature rises, and the material gains thermal energy. As the star continues to decrease in size, the temperature continues to increase until a thermonuclear reaction begins, and hydrogen is converted to helium. This process is called hydrogen burning, but it is nuclear burning, or fusion, not a chemical fire. This could last 1 million to 200 billion years. (About 10 billion years for our Sun).
As the hydrogen in the core is converted into helium, the core begins to contract and heat up, heating the surrounding shell of hydrogen and causing the fusion of the hydrogen in the shell to proceed rate. This rapid release of energy causes the star to expand and enter the first red giant phase of its evolution. In this phase, the star varies in temperature and brightness. The star becomes unstable and the outer layers expand forming a beautiful planetary nebula. Nucleosynthesis occurs when the core gets so hot that helium can fuse into carbon, and other nuclear reactions occur. The outer shell eventually diffuses into interstellar space, and the star's core becomes a white dwarf, then cooling into a black dwarf.
Large mass stars (10 to 25 times Sun) have more gravitational attraction,
and they collapse to 20 km in diameter. The electrons and protons
in this super dense star combine to form neutrons and this forms a neutron
star composed of 99% neutrons. The rapid spinning of this star
give out radio waves in pulses and so are called pulsars.
The pulse is 0.03 to 4 seconds, and are detected by large radio telescopes.
At the center of the Crab Nebula is a rapidly spinning Pulsar, the remains
of the 1054 A.D. supernova. The period is increasing, indicating
that the rotating neutron star is gradually slowing down.
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