Why Do Stars Die?
Written by Fraser Cain
Question: Why Do Stars Die?
Answer: Stars are mostly balls of hydrogen gas that came together from a nebula of gas and dust. They generate their energy through the process of fusion. This is where atoms of hydrogen are combined together to form helium atoms. And in the process, the star generates a tremendous amount of energy in the form of radiation.
This radiation starts up being trapped inside the star, and it can take more than 100,000 years to work its way out. You might not realize it, but light can emit a force when it bumps up against something. So all the light inside the star emits a pressure that opposes the force of gravity pulling all the material inward.
A star can exist in relative stability in this way for billions of years. Eventually, though, the star runs out of hydrogen fuel. At this point, a new reaction takes over, as helium atoms are fused together into heavier and heavier elements, like carbon and oxygen.
Once the helium is used up, a medium-mass star like our Sun just runs out of fuel. It can no longer sustain a fusion reaction. And without the pressure of the light ballooning it out, the star contracts down into a white dwarf - made mostly out of carbon.
A white dwarf star shines because it's still very hot, but it slowly cools down over time. Eventually it will become cool enough that it's invisible. And if we could wait long enough, the star would become a black dwarf. The Universe hasn't existed long enough for us to have any black dwarfs, but there are plenty of white dwarfs.
We did a show on Astronomy Cast, where we tackled this topic. Check it out here: Where do stars go when they die.
Filed under: Astronomy, Questions
April 21st, 2008 at 8:53 pm
Hi Fraser
Photon pressure is minor in stars like our Sun. It's actually the gas pressure that matters. If the Sun suddenly lost its gravity the whole thing would explode violently because of its internal gas pressure being billions of atmospheres. Of course, being so hot the gas emits lots of photons as heat and that heat diffuses out to shine as we see it, but only in very large stars does the raw photon pressure get high enough to play a big role.
Some stars are burning so fast that their photons get too energetic and start converting into electron/positron pairs, causing a pressure *loss* that causes the star to collapse and explode. That's a pair instability supernova. Fortunately our Sun is much too cool for that.