Category Archives: stellar winds

astronomy, astronomy news, planetary nebulae, star death, stellar evolution, stellar winds, sun

The End of Sun-like Stars

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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.

This intriguing new picture from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 surrounding a dim and dying star located about 3300 light-years away in the constellation of Scutum (The Shield). This is the most detailed picture of this object ever taken.
This intriguing new picture from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 surrounding a dim and dying star located about 3300 light-years away in the constellation of Scutum (The Shield). This is the most detailed picture of this object ever taken.

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.

astronomy, planetary nebulae, star death, stellar winds, sun

Questions Astronomers Get

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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.

The Necklace Nebula, a recently discovered planetary nebula in the constellation Sagitta. This is a Hubble Space Telescope image. Courtesy NASA, ESA, and the Hubble Heritage Team (STScI/AURA).

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.)

An artist's-eye view of what the Sun and solar system will look like in a few billion years as the Sun ages and dies. Courtesy ESA.

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.