Planetary Nebulae

Several times a year I go out and give public talks about astronomy and one of the questions I get a lot is, “What will happen to the Sun?”  Sometimes people have this idea that the Sun will blow up in a huge explosion and overtake Earth. Others worry about something hitting the Sun and causing it to do something.  Actually, things DO hit the Sun— comets do this, for example. But so far, none has made a difference in how the Sun behaves.

What DOES make a difference in how the Sun (and other stars) acts are age and mass. Stars with masses ranging from one solar mass to about 8 solar masses have fairly quiet deaths — that is, they don’t blow up in titanic explosions so much as they just “puff out” their outer atmospheres to space and then fade away.

The Sun is the one we care the most about. It is about 4.6 billion years old and it will likely live another four billion years before it starts to age and die. That aging process is of great interest to astronomers and so they study other stars as they die to see how the Sun will do it. The Sun and stars like it (similar in mass and luminosity) shine for billions of years before they hit retirement age and start to swell up.

As they do this, their atmospheres get “huffed off” by a stellar wind similar to our solar wind. It’s almost as if the star is gently sneezing its outer layers to space. This takes a while — and all that material eventually ends up in a cloud of gas and dust that surrounds the cloud. That cloud (with the dying star at the center) is what’s called a “planetary nebula”. The name was bestowed by William Herschel, who thought they looked similar to a distant gas giant planet.  There’s nothing planetary about these things — they’re really stars like the Sun moving through an important step in the aging and death process. 

Planetary nebulae come in many different shapes.  This image comes from the European Southern Observatory’s Very Large Telescope. It’s of a nebula called IC 1295, and since the image is such high resolution, you can actually make out multiple shells of material surrounding the dying star. This implies the atmosphere blew out in episodes as the star’s faltering core emitted sudden bursts of energy.

The gas surrounding the dying star (which is the small blue-white spot in the heart of the nebula next to a reddish spot) is bathed in strong ultraviolet radiation from the aging star, which makes the gas glow. Different chemical elements glow with different colors, and the green color you see here comes from ionized oxygen (that is, oxygen gas heated by radiation from the central star and is now emitting greenish light).

This cloud won’t last forever. In a few tens of thousands of years, the clouds will slowly dissipate. Eventually only the remains of the star will be left behind as a white dwarf.  It will continue to shrink a bit longer, but eventually that will stop and the white dwarf will continue to cool for billions of years. I read somewhere that in the entire history of the universe, not one white dwarf has yet cooled to completion. There hasn’t been time in the 13.8-billion-year age of the cosmos for them do that.

So, that’s the fate of the Sun in general. It won’t blow up as a supernova (because it doesn’t have the mass to do so). It will gently (for a star) sigh its life away. Hopefully by that time, humanity will have found other worlds to live on.