As you stargaze over the next few weeks, keep in mind that most of those tiny points of light scattered across the sky are burning infernos of gas. These stars are very much like the Sun. Some are bigger and more powerful, and some smaller. But they are not constant. Stars change over time, and evolve into different states. Understanding this process of “stellar evolution” is my primary passion in astronomy, and was the focus of a meeting we just held at the Space Telescope Science Institute, “The Mass Loss Return from Stars to Galaxies.”
Stars are sort of like humans … They are energetic when young, “cool” when old, and kind of boring in the middle years. The most important property of a star that defines how it will evolve over time is its mass. A low-mass star, like our Sun, will slowly burn its hydrogen into helium, and remain in a state of equilibrium for billions of years. This is great for us on Earth, since it provides us with a stable environment. But in about 4 billion years, the Sun will expand and begin to lose its outer layers. During this stage, called the red giant phase, the Sun will be so large that it will encompass the Earth’s orbit around it, crisping our planet!
Unlike our Sun, more massive stars – about 10 times the Sun’s mass – will suffer a very different fate. These stars burn through their gas very quickly, like sports cars, and then blow up as supernova explosions. In doing so, the star experiences a very energetic death and sprays 90% of its material into its surroundings. Why does this matter to us if our Sun will never meet this fate? Because this spewed-out material from exploding stars is very important in the cosmic cycle of star and planet formation. All the elements heavier than hydrogen and helium are produced in the cores of these massive exploding stars. That includes everything you see around you, from the computer you’re reading this on to the skin on your body. Yes, you are made of “star stuff.” Our Sun and its planets formed in a region of space that had already been polluted by previous supernovae, and so these heavy elements exist here.
So what about the death of our own star? During one of the breaks at the meeting, I spoke with a colleague of mine about the end fate of the Sun. When we look at the nearby galaxy, we see beautiful stars in the “planetary nebula” phase of their life cycle. This phase only lasts for a short amount of time, during which the outer layers shed by the star – the “mass loss” of the meeting’s title – are illuminated by the hot and exposed core of the dying star, the white dwarf. The resulting pictures of these objects are among the most beautiful sights in the universe. My colleague and I asked ourselves whether the Sun would end its life in one of these states, but we concluded that it would be unlikely.
The Sun has a couple things working against it. First, it doesn’t have as much mass as some of the other stars that become planetary nebulae, so the stellar ejecta will be less dense. Second, because it has less mass, it will evolve more slowly than larger stars. By the time the core of the dying Sun is ready to light up the material around it, that material will be more dispersed. Both of these effects lead to an unlikely case for a bright illumination of the gas.
Eventually, after the outer layers have been shed, the remnant star of the Sun – the stellar cinder – will cool and dim as time passes. This type of white dwarf star is the final resting state of 98% of all stars. These dead stars are littered all across our galaxy, and they have incredible properties. First, having no nuclear fuel, they are extremely faint and hard to detect. Powerful telescopes like Hubble have, however, revealed large populations of these stars in the nearby galaxy. Second, these stars are very dense. Although the progenitor lost half (or more) of its mass, the core is very small – about the size of the Earth. The density of the star is therefore about a million times higher than the density of ordinary matter on Earth. A tablespoon of material from a white dwarf would “weigh” as much as a school bus. Finally, the composition of that core is largely carbon, an end product of helium burning in the progenitor star. So, a white dwarf is essentially highly compressed carbon. In other words, our Sun will end its life as a giant natural diamond!