Death of Stars

When observing a star, the lager it is the shorter its life is going to be. The smaller it is the longer the life. Shorter life is not saying much though, as the most massive stars live for billions of years. When a star reaches about middle age, it starts fusing hydrogen into helium. Once it has run out of usable hydrogen that it can convert into helium, it can then take on one of several paths. It can lead to white dwarfs, novae, supernovae, neutron stars or even black holes.

II. When a star begins the phase of dying, it is deprived of the energy that is needed to prevent it from collapsing in on its core. The size of the star depends on what happens next in this vicious cycle.

III. We still don’t know exactly what happens to low mass stars when they die as the universe has not been around long enough for a low mass star to stop fusing hydrogen into helum. It is believed that a star with a solar mass of less than 0.5 will never be able to fuse helum even after the core stops hydrogen fusion. The star is not massive enough to exert pressure on its core. These are called red dwarfs. An example of a red dwarf is Proxima Centauri, which is shown below.

Red dwarfs with a mass of less than 0.1 scientist believe will stay on the main sequence for over 6 trillion years before they start to die. Lower mass stars are thought to be very convective which means these stars will not have the outer layers of hydrogen. If the star does have these hydrogen layers, then it is more likely to be a mid sized star that will form into a red giant.

IV. In mid size solar mass stars, hydrogen that was fusing in the core is still fusing outside the core into a shell. The much hotter core, starts to push out the layers of the star, and causes them to expand and cool and turns the star into a red giant. As the hydrogen around the core is consumed, the core absorbs the remaining helium, w hich causes the star to contract futher which causes the remaining hydrogen to fuse faster than it would if it was a main sequence star.

This leads to heli8um fusion and the triple-alpha process in the core. The energy energy released by helum fusion causes the core to expand, so that hydrogen fusion in the overlying layers slows down and total energy generation decreases. This causes the star to contract and it starts to slowly skrink in radius and the surface temperature starts to increase. Changes in the enrgy output causes the star to change in size and termperature. Huge pulsations build up and the outer layers of the star have enough kinetic energy for the layers to be ejected. This can form a planetary nebula where the core will start to cool down and become a white dwarf.

A red giant is a star in the process of fusing helum to form carbon and oxygen. If there is insuffiecient energy to make this happen, the outer shell of the star will shed leaving behing an inert core of oxygen and carbon. If enough energy is involved in the casting off of stellar casings, a nebula can form.

V. Very massive stars, more than 40 solar masses, there are very rapid stellar winds, therefore they lose mass very rapidly due to the radiation pressure. Stars cannot be more than 120 solar masses because the outer layers would be expelled by the extreme radiation. These type of stars are unlikely to survive as red supergiants, and instead they will destroy themselves as a supernovae. There are different types of supernovaes, Type I and Type II. A type I supernovae is when a star accumulates matter froma nearby neighbor until a runaway nuclear reaction ignites. Type II supernovaes, which are more common, occurs when a star runs out of nuclear fuel and collapses under its own gravity.

In a Type II supernovae, once the star’s core surpasses a certain mass (The Chandrasekhar limit) the star begins to impolode. The core then heats up and becomes denser. Eventually the implosion bounces back off the core, expelling the stellar material into space. What’s left is a neutron star. Type I supernovaes lack a hydrogen signature in their light spectra. These type of supernovae are generally thought to originate from white dwarf stars in a close binary system. As the gas of the copanion star accumulates onto the white dwarf, the white dwarf is progressively compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmi supernova outburst.

Reference

http://www.space.com/6638-supernova.html

http://en.wikipedia.org/wiki/Stellar_evolution