Extragalactic astronomy

The extragalactic astronomy ( " extragalactic " ) as a branch of astronomy concerned with celestial objects that are outside of our own galaxy, the Milky Way, are located. The term is also used for those of them incident radiation and associated methods.

At the beginning of the 20th century - such as in the Shapley -Curtis debate - was uncertain whether there is any observable objects outside the Milky Way. It was only in the 1920s, mainly by the discovery of Cepheids in ' spiral nebulae ' ( galaxies) by Edwin Hubble realized that galaxies far away objects are similar to the Milky Way. Until the Second World War the study of galaxies in the visible light was limited. The technical progress then allowed the observation of extragalactic objects on increasingly more regions of the electromagnetic spectrum, starting in radio and later in X-rays, gamma rays and ultraviolet and infrared. Through this expansion of the observation area and the increase in sensitivity of modern telescopes and instruments stood beside the stars and nebulae of the galaxies and more new phenomena such as active galactic nuclei, gas and dust in the interstellar medium of galaxies and finally the cosmic background radiation in the interest of astronomers.

The usual measure of extragalactic distances is the Megaparsec, abbreviated Mpc, which corresponds to a distance of one million parsecs, or 3.262 million light years. The nearest galaxies, the Magellanic Clouds, lie at a distance of about 0.05 Mpc, the farthest galaxies so far observed are thousands Mpc away.

The best known among the general extragalactic objects are the Andromeda Galaxy and some other nearby galaxies and quasars. A classification of extragalactic objects but includes more:

Extragalactic Objects

• Galaxies of different types, such as elliptical and spiral galaxies and their components such as stars, gas and dust. In the next few galaxies, individual stars can be spatially separated and thus examined individually.
• Active galaxies, where matter flows onto a black hole, such as quasars and radio galaxies and related phenomena such as radio jets.
• Groups of galaxies and galaxy clusters such as Coma and the Virgo cluster and the large-scale distribution of galaxies in the dark matter and dark energy filled universe.
• Intergalactic medium is hot ionized hydrogen gas and hydrogen plasma in the intergalactic space between galaxies in galaxy clusters.
• Luminous objects, such as supernovae and gamma-ray bursts.
• The cosmic background radiation from the time shortly after the Big Bang.