General Astronomy/The Death of Low Mass Stars

Basically, the greater a star's mass, the greater the pressure in its core (caused by gravity) and the higher the internal temperature. High temperatures force a greater consumption of the available nuclear fuel. The greater the consumption of fuel, the hotter and brighter the star, and the shorter its lifetime.

Stars that are mid-range in mass, such as our Sun, typically live a few billion years (about 10 for the Sun). Smaller stars live much longer, perhaps several hundred billion years. The universe is still too young to have any of the oldest of the smallest stars to have consumed all their fuel. Their lives are longer than that of the universe to date.

But a middle-sized star such as the Sun cannot last as long as the universe has existed. Our sun has existed less than a half the time since the Big Bang (about 13-14 billion years ago). Since the Sun is thought to be typical, we can use it as an example for studying similar stars.

A star like the sun first fuses hydrogen (as it does now) into helium thereby releasing energy, that ultimately leaves the Sun as sunlight. When the Sun is about twice as old as it now is, it will no longer have enough hydrogen in its core to sustain stable hydrogen fusion. The resulting slow down in fusion will result in less outward pressure, to withstand the inward pull of gravity. As a result, the star will contract, compressing the core, and causing its temperature to rise.

The higher temperatures in the sun's core will allow it to fuse helium into heavier elements, but at the same time, the sun's outer layers will expand and cool. As a result it will turn red, making it a red giant.

For our Sun, this stage lasts only a few tens to a few hundreds of millions of years, and another collapse ensues. This time the core is compressed and heated further, but it has run out of hydrogen and helium fuel in its core and is in essence, a hot, dying ember composed largely of carbon called a white dwarf.

A white dwarf is a strange beast, having the mass about the same as the Sun, yet is only about as large as the Earth. At this point, the white dwarf is so tightly packed that a teaspoon of its matter would weigh about 2 tons on Earth! In fact, it is no longer even normal matter that obeys the laws of physics we are used to. Instead, it obeys quantum mechanical laws that prevent it from collapsing any further.

At the same time the white dwarf is formed from the leftover core of the star, the outer layers of the red giant are blown off into space to form a planetary nebula. The white dwarf remnant simply cools, slowly changing from white hot through the range of colors eventually ending, after tens of billions of years, as a cold black dwarf.

The upper limit for the mass of a white dwarf is about 1.44 times the mass of the Sun. So any star whose core ends up with a mass of more than 1.44 solar masses at the end of its life (after the outer layers have been blown off into a planetary nebula) cannot form a white dwarf, but ends up in an even more bizarre state. This is called the Chandrasekhar Limit.

More massive stars have even wilder lives.