1. A congregation, or large grouping, of millions or billions of stars, held together by gravity or gravitational attraction and also known as an "island universe".
2. A collection of dust, gas, and stars measuring thousands of parsecs across.

A parsec comes from PARallax + SECond which is a unit of distance between stars. One parsec is about 3 1/4 light years.

1. A type of spiral galaxy first discovered by U.S. astronomer, Karl Seyfert (1911-1960), in the 1940s.

The central region of a Seyfert galaxy is distinguished by powerful radiation, much of it focused into narrow frequencies.

2. A galaxy with an unusually bright central nucleus, often emitting strongly in the infrared region of the electromagnetic spectrum, as a result of hot dust within it.

About two per cent of all galaxies are Seyferts.

3. A spiral galaxy with a small, compact, bright nucleus that exhibits variable light intensity and radio-wave emission.