By Julianne Dalcanton | August 17, 2007 4:16 pm

I’m normally not one to jump on the latest press release, but this image is awfully purty.

The blue blob is not a comet, but is actually hot gas thrown off by an aging, rather famous star named Mira. (I realize this is sounding very “Sunset Boulevard”, but stay with me.)

Different types of stars are often named after the first one discovered, and, as Mira was one of the first stars of its type discovered, there is a whole class of pulsating red stars known as “Mira variables” in its honor. These “asymptotic giant branch” stars form when a moderately massive star begins to run out of fuel. After losing its most stable source of energy generation (hydrogen fusion in the very center), the star procedes to go through a series of conniption fits as it attempts to find alternate sources of fuel. During these fits, the star becomes quite creative about the ways in which it fuses hydrogen and helium into more massive elements. Fusion normally happens in the very center of a star, but at the end of its life, the star has already used up all the hydrogen & helium in the center. Instead, the star is forced to fuse these elements in shells surrounding the core (forming an onion of inert carbon ash, surrounded by a layer of helium fusing to carbon, another layer of hydrogen fusing to helium, and then an outer layer of inert hydrogen). At this point, the going gets rough for the star. First, because all the star’s energy is being generated much further out, the outer layers get pushed outwards, and the star expands to hundreds of times its initial size. Due to this swelling, the outer layers become only weakly bound to the star, making them easy to strip off. Then, to make things worse, the star begins to pulsate due to its inability to find a stable state of shell burning. During these pulsations, the star usually pushes off a good fraction of these outer layers in a “stellar wind”, depositing much of the star’s mass into the gas that pervades the galaxy.

What’s most striking to me is not just that you can see this hot expanding gas in the ultraviolet image above, but that you can see the bow shock (on the right hand side) where the stellar wind goes plowing into the gas that was already drifting through the galaxy. Like a person running into the ocean too fast, the hot gas from the wind stops when it hits the dense cool galactic gas, and shock heats to a high temperature, making a characteristic arc of very hot, luminous gas. This interaction with the surrounding gas shaped the highly asymmetric structure. There either was more dense galactic gas on the right side of the image than on the left, or more likely, the star was moving rapidly to the right, increasing the chances of forming a shock front in that direction. The shock confined the stellar wind on one side, but left it free to expand on the other. Moreover, if the star was racing to the right, material that got blown off would be left behind, leaving the long trail drifting off to the left, like a contrail behind an airplane.

You can actually see these shocks all over the place in astronomy. The most famous is probably the Bullet Cluster, which Sean discussed here.

See that red cone on the right? Another bow shock, formed as the hot gas from one galaxy cluster rammed into the gas from another galaxy cluster, as the two were pulled together by their gravitational attraction.

Another great example can be seen in the Orion nebula, a young region of star formation that appears as one of the “stars” in Orion’s sword:

The difference is that here, instead of the star ramming into some more slowly moving gas, some fast moving gas is probably ramming into the gas around the star. (To bring this full circle, the fast moving gas is probably due to stellar winds and radiation from dying stars off to the right of the image.)

These sorts of shocks happen on earth was well, most notably around the noses of planes moving at supersonic speeds. However, the shocks produced on earth almost never get hot enough to light up, leaving astronomy to provide the most dramatic visualizations of the phenomena.


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