User:Kyu261/sandbox/helium star

A helium star is a family of stars, which have strong lines for He on the absorption spectrum while those for H are weaker than normal. The extreme helium stars can totally lack hydrogen lines on their spectrum, which is now believed to be caused by the remove of thin hydrogen layer in the mass loss procedure.

These helium stars can either form at their early stages due to binary effects or after massive stars loss hydrogen envelopes due to stellar winds. Pure helium stars lie on or near a helium main sequence, analogous to the main sequence formed by the more common hydrogen stars. The helium cores will lose weight due to continuous "burning" instead of gaining weight from hydrogen fusion. After they run out of their fuels, most of them would expand and form blue supernovae or become white dwarfs. Black holes will not be formed due to a small weight.

Helium stars are also an ideal model of WR stars (those who lack H) and are often used to study Wolf-Rayet Evolution.

Mass Loss
Generally speaking, all hydrogen lacking stars are observed to experience mass loss. It occurs when a star ejects a large portion of its own mass. This process is crucial for helium star formation. After they loss the whole hydrogen envelopes, they can then become helium stars. There are two ways in which they become helium stars.

Binary System
Binary system is a stellar system in which two stars come close to each other and affect each other. When two stars are too close to each other, the gravitational force between them will distort their outer stellar atmosphere. Eventually, after their hydrogen layers are completely destroyed, they become helium stars in a binary system.

Mass exchange is another important factor in the formation and life stage of helium stars in binary system. The two stars can exchange their components due to strong gravitational pulls when they are close enough. This mass flow causes the hydrogen layer to escape at a higher rate. Thus, they can become helium stars soon after their birth. As their helium cores are burning, the mass exchange will not stop, which means that the product of helium fusion will appear on the surface very early.

Stellar Winds
For more massive single stars (eg: type O stars), they have much stronger stellar winds than regular stars. Stellar wind is the ejection of gas from the upper atmosphere of a star. Usually, stellar wind will not affect a star's life process. If the wind is strong enough, like those in type O stars, it will bring a large mass of the star away. As the hydrogen atmosphere is blown away, the helium core is revealed and the star becomes a helium star. The result is a complete change of the star's evolution process. The star, which is meant to become a supernova, can finally become a white dwarf at the end of its life.

Late Stage Evolution
While a normal hydrogen star with a small mass runs out of its fuels, it would expand and become a red giant. Then the hydrogen atmosphere and the helium core will separate. The heavier helium core would collapse to become the predecessor of white dwarf. The remaining hydrogen envelop will escape and become stellar materials. For the star to survive and keep burning, it has to burn the helium core. The remaining core will become R CrB stars, through (extreme) helium stage. It will contract to a smaller volume but with higher temperature. As nuclear fusion keeps happening, the helium components will finally become a C/O core. The fusion process will eventually bring the remnant to white dwarfs, the dead bodies of stars.

Wolf–Rayet Stars
Wolf–Rayet stars are massive stars that have lost most or all of their hydrogen via powerful stellar winds. However, the WR stars are formed at the very end stage of a star. Therefore, the time available to research is quite short. Helium stars are the key to understand the evolution process of WR stars. The helium stars almost have no hydrogen component and can be seen as WR stars throughout their lifetime. Therefore, they are also used as an ideal model to study Wolf-Rayet evolution process.

Final Products
As WR stars, helium stars will also undergo the continuous fusion of the core. Depending on the mass of the core, the final component will contain various elements and their isotopes like C, O, 18O, and 19F.


 * <1.6–1.8 M⊙, degenerate cores of carbon and oxygen
 * 1.9–2.4 M⊙, neon, oxygen, and magnesium
 * 2.5–3.2 M⊙, silicon