And An Answer to One of Them

Last week aboard the good ship Corinthian II, I was sitting out on the deck having a little lunch and chatting with some of fellow passengers about fascinating topics in astronomy. It’s always interesting to hear what fascinates people about space and astronomy and I’m always happy to answer questions about those topics.

One of the questions that comes up frequently (and did in the conversation I had that afternoon) is “What will happen to the Sun?” Most of the time, people really ARE interested in the science behind the Sun’s existence and I”m happy to oblige them with the executive summary of end-times astrophysics for our star.

And it IS (or will be) an astrophysical event. Each thing that will happen to the Sun can be figured out by applying the laws of physics, of gravity, gas laws, and other scientific knowledge.  No mysterious death rays or aliens figure into these, because those “actors” don’t usually follow the laws of physics (or of normality, as far as I can tell).  And yeah, there are all these crazy ideas out there floating around about how the Aztecs or Mayans or the Illuminati or the Pleiadians or some other alien race has predicted the Sun will go wonky next year, or that the death beam from the center of the Milky Way will cream us all at a predetermined time. However, nothing that anybody can dream up after a couple of beers (or surfing through weird Web sites) is as interesting as what will really happen to the Sun.

So, what WILL happen?  Take a gander at this image here to the left. It’s a planetary nebula — essentially what’s left over after a sun-like star loses most of its mass to space. The star doesn’t blow itself to smithereens — that’s what stars that are many times more massive than the Sun do when THEY die. No, stars like the Sun go to their fates more gently (for stars).  The short story is that it huffs off its outer atmosphere over long periods of time, and then what’s left collapses to become a white dwarf.  So, the Sun — in about 6-7 billion years, could look something like this.

This image actually shows what happens when two stars are involved in a planetary nebula. A pair of stars orbiting very close together are at the heart of this nebula (called PN G054.2-03.4). About 10,000 years ago one of the aging stars ballooned to the point where it enveloped its companion star. This caused the larger star to spin so fast that much of its gaseous envelope expanded into space. Due to centrifugal force, most of the gas escaped along the star’s equator, producing a dense ring. The embedded bright knots are the densest gas clumps in the ring.

The stars are furiously whirling around each other, completing an orbit in a little more than a day. (For comparison, Mercury, the closest planet to the Sun, takes 88 days to orbit the Sun.)

The Sun’s planetary nebula will be a glowing cloud of gas and dust, heated by radiation from the leftover white dwarf.  It will light up the clouds and highlight the clumpiness in the nebula.

What people are really wanting to know when they ask that question about the Sun dying is what will happen to Earth. Sad to say, the prognosis for our  little oasis in space isn’t good at that point. Life will have been crisped in the heat of the expanding outer atmosphere of the Sun–since it will swell up to become a red giant in the process of dying. The oceans will boil away. What ever is left could be a cinder. I say “could” because it’s possible that the Sun’s stellar wind will be very strong, which could cause the orbits of the planets to drift outwards. So, our planet might escape the fiery death part–at least for a while.

How does star death for the Sun happen? Look at what the Sun does. It goes about its daily business of turning hydrogen into helium in its core. It has been doing this for billions of years, like all stars do. The heat and pressure of the burning in this nuclear furnace is enough to keep the outer layers of the Sun from collapsing in. This is what’s happening now–the Sun we enjoy is in equilibrium–meaning the heat and pressures in the core balance the gravitational tendencies of the outer layers to want to fall in to the center.

But, in a few billion years, our star will start to run out of hydrogen in its core and lose the core pressure that holds up the other layers.  At that point, all they will collapse under the pull of gravity, and what’s left of the hydrogen will heat up. Fusion (the hydrogen-to-helium process) will resume. This time, however, the outer layers–particularly the outer atmosphere–will swell up a few hundred times larger and be cooler and redder than the Sun we know today.

At that point, our lovely yellow star will become a red giant. And, in that swelling, it will likely smother the inner planets. If the dying Sun has a strong-enough and mass-loaded stellar wind, that could push the planets out a bit, and that’s where astronomers speculate the Earth could escape being turned into a crispy planetary critter.  It’s hard to tell at this point what would really happen, but the ultimate fate of Earth and the inner planets isn’t going to be like we know it today.

So, that’s the answer to the question, “What will happen when the Sun dies?”  It’s a stellar process that occurs throughout the universe, and we understand more about it by studying the planetary nebulae whose remains chart the future of our own star.