Core and Nebula

A star may last in this second red giant phase for a mere 100,000 years before its carbon core shrinks to an incredibly dense inert mass only about M000 the current radius of the sun—or about the size of the earth. The surrounding shells continue to fuse helium and carbon, and the outer layers of the star continue to expand and cool.

The outer layers of the star are now so far from the core, that they are able to lift off and move out into interstellar space, often in several distinct shells. These outer layers are puffed off from the star like spherical smoke rings, leaving behind the bare, hot core of the star. The cast-off outer layer of the star (which can contain 10 to 20 percent of the mass of the star) is misleadingly called a planetary nebula. Planetary nebulae are the ejected gaseous envelopes of red giant stars. These shells of gas are lit up by the ultraviolet photons that escape from the hot, white dwarf star that remains. In truth, a planetary nebula has nothing to do with planets. The source of the misnomer is that, in early observations, these objects were thought to resemble planetary disks.

A Hubble Space Telescope portrait of a young planetary nebula, the Stingray Nebula, located in the direction of the southern constellation Ara. The bright central star is visible within the great envelope of ionized gas.

(Image from NASA)

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