Monday, February 19, 2007

A "new star" in the sky

As I mentioned last week, this Friday marks the 20th anniversary of Supernova 1987A, the explosion of a star in the Milky Way's neighbor galaxy the Large Magellanic Cloud. This is the closest visible supernova to go off since the 1600s, and so is one of the most important objects for studying these stellar explosions.

As if to honor this anniversary, a nova ("new star") has appeared in the constellation Scorpius, and has been given the completely boring name of V1280 Scorpii. This past weekend it was visible to the naked eye, but it is fading rapidly. If you want to try and find it, you'll need to wake up early in the morning. Here is a sky map from Sky & Telescope magazine. Today's Astronomy Picture of the Day has a picture of the nova taken from the desert of Iran.

What is a nova? Most people have heard of supernovae, which is the explosion of an entire star. A nova is also a thermonuclear explosion, but it only happens on the surface of a specific type of star, a white dwarf, and the star lives to tell about it.

White dwarfs are the exposed nuclear furnaces of dead stars. Most stars become white dwarfs. They run out of nuclear fuel, but the nuclear furnace is too small to explode spectacularly. The outer layers of the star float out into space as a planetary nebula, leaving the white dwarf behind to slowly cool and fade away.

Some white dwarfs, though, have a companion star. This star all but ignores the death throes of its sister star, and continues to burn its own fuel. Eventually, though, it begins to run out of fuel, too, and it starts to swell up into a red giant star. But as it swells up, the outer parts of the dying star come close enough to the white dwarf that they are pulled on to the white dwarf by gravity. An artist's rendition of what this might look like can be found here.

This gas is mostly hydrogen, the typical fuel used by stars. At first, the hydrogen just sits on top of the white dwarf star, minding its own business. But as more gas continues to pour on, the hydrogen gets compressed and starts to heat up. Eventually some part of the hydrogen gets hot enough to start a nuclear reaction, and the surface of the star ignites in a huge explosion of hydrogen. The explosion pushes most of the accumulated matter, now burned into helium, off into space.

Over time, the system calms down, and more gas from the companion star starts to settle on the white dwarf, and the cycle begins anew. For some white dwarfs, the time between novae is only a few tens of years, but for most stars (such as the white dwarf in Scorpio that just went nova), the interlude can be tens of thousands of years. We astronomers have no warning of a nova -- when an amateur astronomer sees it is often the first we know about it. How many novae happen in our own galaxy that we don't know about? That I don't know.

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