Bubbles and jets of gas and dust erupt in the Cat's Eye Nebula, the last gasp of a dying star. The star is expelling its outer layers into space, forming a beautiful structure known as a planetary nebula. Thousands of years from now, the nebula will fade, leaving only the star's hot, dense, dead core, known as a white dwarf. This view, from Hubble Space Telescope, also shows light from the star illuminating shells of gas and dust that were expelled into space before the star entered the planetary nebula phase. [NASA/ESA]
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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