Image Credit & Copyright: Robert Gendler and Jim Misti (http://www.robgendlerastropics.com)
The Ring Nebula (known as Messier 57 and NGC 6720) is a bright planetary nebula of about 2.5 light-years across (its bright central part about 0.9 light-year across), located some 2,300 light-years away in the northern constellation of Lyra, south of the bright star Vega. It is approaching us at approximately 19.2 kilometers per second, and is estimated to be expanding since 6,000 to 8,000 years at 20 to 30 kilometers per second.
Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through early telescopes.
A planetary nebula represents the final stage in the evolution of a star similar to our Sun. Only a few thousand years ago, the star at the center of the Ring Nebula was a red giant, but then ejected its outer layers into space to form this expanding nebula. The stellar remnant at the center is merely its hot core. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.
The Ring Nebula is, most probably, actually a ring of bright light-emitting material (with a mass of about 0.2 solar masses) surrounding its central star, and not a spherical (or ellipsoidal) shell. This symmetric bipolar nebula has thick equatorial rings and strong concentrations of material along its equator. Looping structures extend beyond the Ring’s central regions.
Its central star is becoming a compact white dwarf star and consists now primarily of carbon and oxygen with a thin outer envelope composed of lighter elements. Its mass is about 0.615 solar mass, with a surface temperature of 120,000 degrees Celsius, but will soon start to cool down, shine as a white dwarf for several billions of years, and then eventually end as a cold black dwarf. It has now an intrinsic brightness of about 50 to 100 times that of our Sun.
The interior parts of the Ring Nebula have a blue tinge that is caused by doubly ionized oxygen emission. In the outer region of the nebula, the reddish hue is caused by hydrogen and ionized nitrogen emission.
In this image we can also see the spiral galaxy IC 1296 (below left), which is much bigger and hence farther away … about 200 million light-years distant.