Star life and death

Star Life & Death


Honestly, the life of a star is fairly boring. It fights gravitational collapse for several million to several billion years (depending on the mass) by fusing atoms together. The energy the star gains by fusing these atoms keeps it from collapsing. If a star is massive enough, it will fuse heavier and heavier atoms -- hydrogen to heluim, heluim to carbon, carbon to ... until ... elements are fused into iron. Fusing iron to form heavier elements actually requires energy, so the star would not gain anything by continuing fusion of iron atoms.

Most of the star's life is spent fusing hydrogen into helium. Our sun has been doing this for some five billion years, and is expected to continue doing it for another five billion or so years. This hydrogen burning starts from the very center of the star, and moves its way out, leaving a core of helium behind.





Low Mass Stars

If the star is small enough (much less than the mass of our Sun), it never gets beyond hydrogen burning. This is because its central temperature never gets high enough to start fusing helium into carbon. Once such a star has used up most of its hydrogen, it will begin to cool and collapse into a "brown dwarf".


Intermediate Mass Stars

Stars with masses close to that of our Sun (up to about five times the mass of our Sun) will experience helium-to-carbon burning in their cores. Outside the helium core, hydrogen will continue burning into helium.





At this point, the outer layers of the star will expand to conserve energy -- the star swells, becoming brighter and cooler. This is called the red giant phase of the star. The red giant loses many of its outer layers because of the radiation coming from the core blows it away. Eventually the star will cool down so much that the carbon burning stops. Such a star will collapse into a white dwarf.


High Mass Stars

High mass stars end their lives spectacularly. They, too, go through a stage where they swell up, though they swell even more than their lower-mass counterparts. This stage is called the red supergiant phase. These stars are so large that their central temperature becomes high enough that further burning in their core will occur. Eventually, they have so many layers, that they may look like an onion -- see figure below.
Image depicting 'onion' layers of nuclear burning in a high mass star









This process necessarily ends when the core has been fused into iron. Once this occurs, the core no longer has any resistance to gravity -- the core collapses. During this core collapse, the outer layers of the star are blown off in a supernova explosion. The core collapses either into a neutron star or into a black hole.


Neutron Stars

During the core collapse of the stars with masses between 15 and 30 times that of our Sun, the electrons and neutrons in the core combine into neutrons. Usually neutrons will decay into a proton and electron quickly; however, when the density of protons and electrons is high enough, it becomes less adventageous for a neutron to decay. This mass of neutrons will collapse as much as they can without violating the "no two objects can occupy the same space" law of physics (the Pauli exclusion principle).

Neutron stars are about 10 km (6 miles!) in diameter with a mass of about one and a half times that of our Sun. This makes for a huge density!


Black Holes

Exactly how black holes form is still a bit of a mystery. A neutron star can not hold itself up against gravity if it has more mass than three of our Suns. So, if the core of a massive star at the end of its life has more mass than three Suns, the core collapses into a singularity, called a black hole. Mathematically, a singularity is a point of infinite density -- a point where an finite amount of mass is squished into zero volume. Astrophysicists and physicists don't like infinities, so are still debating what exactly this means in the "real Universe".

The Sources section deals more with black holes and properties that we can infer about them from high energy observations.