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Donuts in Space
updated: Jan 05, 2013, 5:00 PM

By Chuck McPartlin

It's a cloudy weekend in Santa Barbara, so no stars for Edhat astrophiles. Feeling starved for distant photons, I shuffled through my old image files.

Here is a photo shot back in early November through a relatively small telescope, with 5 inches of aperture. It has about 350 eyeballs worth of light gathering area, compared to the average human pupil size of 7 millimeters. The camera is a low-light surveillance camera modified for astronomical use, allowing it to collect up to 8.5 seconds of data that are integrated (added) before being displayed.

The object in the picture is called the Ring Nebula, as you might suspect. Amateur astronomers with a penchant for junk food call it the powdered donut. It's a type of object called a planetary nebula. They really have nothing to do with planets, but observers in the 1700s saw them as small clouds (nebulae) that were often greenish and round, and thus appeared planet-like. It is actually one of the end phases in the life of a low-mass star like our Sun.

The more massive a star is, the higher the temperatures and pressures in its core, and the faster it consumes its fuel through nuclear fusion and dies. Massive stars, say greater than eight times the mass of the Sun, form iron in their cores within a couple of hundred million years. Iron is the most stably bound atomic nucleus, so its fusion does not release energy, but consumes it. This relatively sudden shutdown of the star's nuclear engine causes the core of the star to collapse, resulting in a supernova explosion that blows the outer layers of the star out into space. The core is compressed into a ball of neutrons, or winks out into a black hole for the most massive stars.

A star like our Sun fuses at a more sedate rate, converting about six billion tons a second of hydrogen to helium. Our Sun is roughly half way through its ten billion year life of fusing hydrogen. In about five billion years, the hydrogen concentration in the core will have dropped enough to cause the fusion reactions to start to sputter, resulting in episodes of swelling and shrinking that will puff off outer layers of its atmosphere. Eventually, the core will shut off, and collapse until it is dense enough to start fusing helium into carbon, nitrogen, and oxygen. This will cause a big burp that blows off the remaining outer layers, finally leaving behind a white dwarf star. The very hot surface temperatures of the white dwarf cause it to emit lots of ultraviolet light, which makes the elements it barfed off during earlier phases fluoresce, producing the visible nebula.

The Ring Nebula is about 2300 light years away, and the main donut shape is around a light year across. A light year is about 6 trillion (6,000,000,000,000) miles. By some estimates, the cloud has been expanding for 1600 years, which means that the gas is moving outward at about 430,000 miles per hour. Not bad for a mere burp. The white dwarf at its center, not visible in the image, has a surface temperature and luminosity about 200 times that of our Sun, but most of that output is in UV.

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