The Wonderful

Bad Astronomy
The entire universe in blog form
Aug. 15 2007 11:32 AM

The Wonderful

The image above is substantially cool. But it'll take a moment to explain why. Stick with this; you'll like it.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


In the constellation of Cetus, the whale, is what appears to be a run of the mill red star. At a distance of about 400 light years, the fact that you can see it with your unaided eye at all means it's an intrinsically luminous star: at that distance the Sun would be completely invisible.

The star is a red giant, a star that was once much like the Sun but is now terminally ill. Stars make energy in their core through the fusion of light elements into heavier ones; the Sun is currently fusing hydrogen into helium. Eventually it will run out of hydrogen, and will begin to fuse helium into carbon and oxygen. In 7 billion years or so the helium in the core will run out as well. The carbon and oxygen ash from the process will form a ball about the size of the Earth. It will contract and get incredibly hot. Helium outside the core, previously unavailable for fusion (like having a spare can of gasoline in the trunk of your car) will start to fuse in a thin shell surrounding the core. This will dump vast amounts of heat into the outer part of the Sun, which will respond like any gas will when heated: it will expand and cool.

The Sun will become a red giant. But thin shell helium fusion is unstable, and so a red giant can expand and contract, sometimes almost in a spasm, ejecting material off its surface, and briefly becoming very luminous before settling down again. This will happen three or four times for the Sun, and it will totally eject its outer layers, exposing the hot core to space*. When this is all done, the Sun will be a white dwarf, and will slowly cool for the next few hundred billion years.

The image at the top shows a star that is undergoing this process right now. Called Mira -- "wonderful" -- it's slightly more massive than the Sun, and far older. It has only a short time left -- maybe only hundreds of thousands of years, maybe less -- before its paroxysms slough off that last bit of outer layer, and it becomes a white dwarf. These spasms change its luminosity, and we see this as a brightening and dimming of the star; it's sometimes too faint to see with the unaided eye, and other times can brighten considerably.

Mira has long been studied by astronomers to give us insight on what will happen when the Sun dies. Observations have revealed the star isn't round: that makes sense, since it is ejecting huge amounts of material in expanding clouds. It has a small companion, a more normal star that appears to be collecting some of the ejected material and forming it into a disk around itself.

Mira is definitely wonderful, in the sense of evoking wonder. And now we have found out it's even more amazing than we thought. Most stars near the Sun orbit the center of the Galaxy at roughly the same speed, but some are faster than others. Mira, it so happens, is plowing through this local region of space at about 130 kilometers per second (about 80 miles per second). There is gas and dust out there, a thin haze floating among the stars. As Mira screams through this fog, the gas it is ejecting as it convulses is blown backwards, leaving a long tail behind it -- imagine running down the street with a smoke bomb in your hand and you'll get the idea.

Now take another look at the image at the top of this page. Mira is on the right hand side, and is moving left to right. The long tail of ejected material is incredible -- it's 13 light years long! It has taken Mira 30,000 years to move this distance, which means that the material in the left hand side of the tail was ejected 30 millennia ago. If you look at the location of the star itself, you'll see a parabolic arc in front of it; that's the bow shock, where Mira's ejected material is slamming into the material between stars (called the interstellar medium or ISM).

The images are in the ultraviolet, which means the gas is emitting UV radiation. This indicates that the material is being heated by the collision with the ISM, and is slowly losing that energy by glowing in the UV. The images were taken by the Galaxy Explorer (GalEx) mission. In a routine survey, an astronomer noticed that Mira looked fuzzy, so they took deeper images. The tail near the star was revealed, so they scheduled even more observations to trace it out... I can just imagine how surprised they were when they realized what they had found! The image is actually a mosaic of the images GalEx took.

The material blown off will eventually merge with the ISM and form new stars. The elements created in the fusion forge deep inside of Mira will eventually find themselves in new stars, some of which will be like the Sun, or like Mira once was. They too will age, step through the fusion process in their cores, and eventually become red giants... and the cycle starts again. It's quite possible that some of the heavier elements we see in the Sun itself were seeded into the Galaxy by some anonymous star like Mira more than 5 billion years ago.

So when you look at this image of Mira with its comet-like tail, think on this: you are seeing a star's way of making new stars. Like life itself, in death is renewal and the foundation for future generations.

*Needless to say, the Earth doesn't fare well in all this.



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