HOW DO STARS DIE AND HOW LONG DO STARS LIVE??
Blog#38 Saturday, November 21st, 2020
A star’s life expectancy depends on its mass. Generally, the more massive the star, the faster it burns up its fuel supply, and the shorter its life. The most massive stars can burn out and explode in a supernova after only a few million years of fusion. A star with a mass like the sun, on the other hand, can continue fusing hydrogen for about 10 billion years. And if the star is very small, with a mass only a tenth that of the sun, it can keep fusing hydrogen for up to a trillion years, longer than the current age of the universe.
As stars get smaller, they live longer. Our own sun has been around for 4.3 billion years, slowly turning hydrogen into helium at its core. The sun will run out of this hydrogen fuel in another 5 billion year or so, and it will turn into a red giant. It will expand to many times its original size and then eject its outer layers and shrink down to a tiny white dwarf star, a dense object the size of the Earth. So the total lifespan of a star with the mass of the Sun is about 10 billion years.
The smallest stars are the red dwarfs, these start at 50% the mass of the Sun, and can be as small as 7.5% the mass of the Sun. A red dwarf with only 10% the mass of the Sun will emit 1/10,000th the amount of energy given off by the Sun. Furthermore, red dwarfs lack radiative zones around their cores. Instead, the convective zone of the star comes right down to the core. This means that the core of the star is continuously mixed up, and the helium ash is carried away to prevent it from building up. Red dwarf stars use up all their hydrogen, not just the stuff in the core. It’s believed that the smaller red dwarf stars will live for 10 trillion years or more.
How long do stars last? The biggest stars last only millions, the medium-sized stars last billions, and the smallest stars can last trillions of years.
Answering the question, “How do stars die?” also depends on its mass. The most massive stars quickly exhaust their fuel supply and explode in core-collapse supernovae, some of the most energetic explosions in the universe. A supernova’s radiation can easily (if only briefly) outshine the rest of its host galaxy.
The remnant stellar core will form a neutron star or a black hole, depending on how much mass remains. If the core contains between 1.44 and 3 solar masses, that mass will crush into a volume just 10 to 15 miles wide before a quantum mechanical effect known as neutron degeneracy pressure prevents total collapse. The exact upper limit on a neutron star mass isn’t known, but around 3 solar masses, not even neutron degeneracy pressure can combat gravity’s inward crush, and the core collapses to form a black hole.
Average stars with up to 1.44 solar masses, such as the Sun, face only a slightly less exotic fate. As they run out of hydrogen to fuse in their cores, they swell into red giant stars before shedding their outer layers. The remnant left behind in these planetary nebulae is a white dwarf star. Like neutron stars, white dwarfs no longer fuse hydrogen into helium, instead depending on degeneracy pressure for support — this time, the electrons are degenerate, packed together and forced into higher energy states, rather than the neutrons.
(Wednesday, November 25th,2020)
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