Non MainSequence Stars

Many of the brightest stars visible in our sky are not main sequence however. Marking the shoulder of Orion is the red supergiant Betelgeuse. This is one of the largest stars in all the heavens. If it were put at the center of the solar system in place of the Sun, it would swallow everything out to Mars and continue well into the asteroid belt. It shines in our sky at apparent magnitude +0.5 from a distance of 430 light years. For all its size though, Betelgeuse has only about 12 to 17 times the mass of the Sun. The star is also surrounded by a massive shell of dust of its own making caused by a powerful stellar wind that is blowing out the stars outer layers. Betelgeuse is a highly evolved star that is dying, likely fusing helium in its core into carbon and oxygen. A spectacular fate awaits it in the not too distant celestial future. Stay tuned.

Stars that are big are not always red. At the other end of Orion from Betelgeuse is Rigel. This is a massive blue supergiant that makes a beautiful contrast with red

Betelgeuse and nearby white Sirius. Rigel is 775 light years away and shines at a visual magnitude of +0.2. It is spectral type B8, which gives it a surface temperature of some 11,000K, more than twice as hot as the Sun and among the hotter stars in the heavens. Rigel shines with 40,000 times the luminosity of the Sun. Rigel burns hotter than Betelgeuse does because it is a bit smaller and more massive. More mass requires more fusion to prevent the mass from collapsing itself. Rigel is even more evolved than is Betelgeuse and is dying. Its supply of hydrogen is exhausted and it is now fusing helium into carbon, beryllium and oxygen. Rigel will eventually share the same spectacular fate as awaits Betelgeuse.

Both Rigel and Betelgeuse are very massive stars that quickly exhausted their supplies of hydrogen, leaving a helium ash core behind that was spurned into fusion when gravitational energy spurred by core collapse caused helium fusion to begin, while residual hydrogen is fused in an outer shell driving the stars outer layers into space. These stars are massive enough that eventually they will progress to fusing carbon into oxygen and eventually oxygen into iron.

At the bottom end of the Hertzsprung-Russell diagram are the white dwarfs. Sirius has a tiny faint companion that shines faintly at magnitude +8.0. Sirius B is spectral type A2 on the H-R diagram, just slightly to the right of where Sirius A is. Despite the fact that it is smaller in size than Earth (11,200 kilometers) it appears to have about the same mass as the Sun. This means that one cubic centimeter of matter from Sirius B would weigh approximately 15 tons! Sirius B is a foreshadowing of what awaits our Sun at the end of its life, crushed by its own weight into a degenerate mass of heavy neutrons that emit heat of over 20,000 K. Sirius B is dead from the point of view that it no longer supports any nuclear fusion. The heat from this "star" is a result of energy given off through gravitational compression, a runaway of the same phenomenon that cause Jupiter and Saturn to emit more heat than they receive from the Sun. It's light is created by the crushing of the atoms under the amazing pressure created by stellar collapse.

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