Black Holes: Gravity's Relentless Pull

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Black Holes: Gravity's Relentless Pull
OBJECT: NGC 7027 (Planetary nebula)
DISTANCE: 2,600 lightyears

This object in the constellation Cygnus (Swan) is number 7027 in the New General Catalog, a famous list of bright gas clouds, star clusters, and galaxies. The name planetary nebulae was given long ago to objects that appeared fuzzy and resembled planets through a small telescope. However, these objects have nothing to do with planets. Instead, they form when a star slowly sheds its outer gas layers towards the end of its life.

The visible light image of NGC 7027 shows the dying star as a dot, surrounded by the gas that it has shed. The radiation from the star causes atoms in the gas to glow in a brilliant display. The radio and X-ray images show different radiation from the same gas. The radio waves reveal the heat glow from relatively cool gas in the cloud, whereas the X-rays are produced only by the very hottest gas regions.

When a star nears the end of its life, it first becomes a red giant. What happens next depends on the mass of the star. Stars that are no more than eight times heavier than the Sun die relatively peacefully, lacking sufficient mass to become a black hole. They shed their outer layers and are visible as planetary nebulae for about 100,000 years. The stellar remnant is about as heavy as the Sun, but the mass is packed in a volume only about the size of the Earth. Because of its high temperature and small size it is called a white dwarf. It has no more nuclear fusion to support its temperature, so it cools slowly and eventually fades into obscurity.

X-Ray image of: NGC 7027 (Planetary nebula)
Regions with hot gas emit X-rays. This gas is distributed rather differently than the cooler gas visible in the radio image.