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Cat’s Eye Nebula

March 13, 2012

Red giants and white dwarfs are stellar opposites. A red giant is much larger, brighter, and cooler than the Sun, while a white dwarf is small and hot. Despite their huge contrast, though, a red giant ultimately becomes a white dwarf. In fact, we see that happening right now in the constellation Draco, the dragon.

Draco is home to the Cat’s Eye Nebula, which is a “planetary” nebula: a bubble of gas set aglow by a hot, dying star. It’s low in the north in early evening, but wheels around the North Star during the night, so it stands quite high in the sky at dawn. Through a small telescope it looks like a bit of fuzz among the myriad stars.

If you’d observed this object just a couple of thousand years ago, you wouldn’t have seen a planetary nebula at all. Instead, you would have seen a red giant nearing the end of its life. Then about 1300 years ago, the star cast off its outer layers, exposing its small but hot core -- a white-dwarf-to-be. The radiation from the core causes the surrounding gas to glow. So a planetary nebula represents a stellar metamorphosis: from a big, bright red giant to a small, hot white dwarf.

This same fate awaits the Sun. Billions of years from now, it’ll puff up to become a red giant, shining about a hundred times more brightly than it does now. Then it will cast off its outer layers, exposing its core and transforming itself into its own opposite: a tiny but hot white dwarf.

More about white dwarfs tomorrow.


Script by Ken Croswell, Copyright 2012


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