Glossary for Deep Sky Objects

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arms

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A pinwheel structure, composed of dust, gas and young stars, that winds its way out from the core of a spiral galaxy.

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barred spiral galaxy

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A spiral galaxy with a 'bar' of stars and interstellar matter, such as dust and gas, slicing across its center. The Milky Way is thought to be a barred spiral galaxy.

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black dwarf

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The presumed final state of evolution of a low/medium mass star in which no radiation is emitted. There are none of these in the universe yet – it’s not old enough.

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black hole

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An invisible region in space, formed from the death of a massive star during a supernova explosion, that has a huge amount of mass compacted into an extremely small volume. A black hole’s gravitational influence is so strong that nothing, not even light, can escape its grasp.

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brown dwarf

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A 'failed star'; a giant ball of gas that is too massive to be a planet but not massive enough to be a star. During formation, the protostar didn’t reach the critical mass required to ignite nuclear fusion. It glows briefly, then its energy dies out and emits only infrared radiation.

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central bulge

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The spherical structure at the center of a spiral galaxy that is made up primarily of old stars, gas, and dust.

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dark nebula (or absorption nebula)

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Dark cloud of dust which is visible only because it absorbs the light of stars behind it.

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disk

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A flattened region of gas and young stars in a galaxy that surrounds the bulge in a spiral galaxy.

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doppler effect

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The apparent change in wavelength of sound or light caused by the motion of the source, observer or both.

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elliptical galaxy

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A galaxy that appears spherical or football-shaped. Elliptical galaxies are comprised mostly of old stars and contain very little dust and 'cool' gas that can form new stars.