Extragalactic Astronomy
The extragalactic astronomy (“extra-galactic”) as a branch of astronomy deals with celestial objects that are outside the Milky Way . The term is also used for the radiation they enter and for associated methods.
At the beginning of the 20th century - as in the Shapley-Curtis debate , for example - it was uncertain whether there were any observable objects outside the Milky Way. It was only in the 1920s that it became clear , mainly through the discovery of Cepheids in 'spiral nebulae' (galaxies) by Edwin Hubble , that galaxies are distant objects similar to the Milky Way. Until World War II, the study of galaxies was limited to visible light. Technical progress then made it possible to observe extragalactic objects over increasingly wider areas of the electromagnetic spectrum, beginning in the radio range and later also in X-rays , gamma rays and in the ultraviolet and infrared ranges . As a result of this expansion of the observation area and the increase in sensitivity of modern telescopes and instruments, in addition to the stars and gas nebulae of the galaxies, new phenomena such as active galactic nuclei , gas and dust in the interstellar medium of the galaxies and finally the cosmic background radiation came into the interest of astronomers. From 2020, the Euclid space telescope, which is currently under construction, should provide further insights.
The usual extragalactic unit of measure for distance is the megaparsec , or Mpc for short , which corresponds to a distance of 1 million parsecs or 3.262 million light years . The nearest galaxies, the Magellanic Clouds , are at a distance of about 0.05 Mpc, the most distant galaxies observed so far are thousands of Mpc away.
The most popular extragalactic objects are the Andromeda Nebula and several other nearby galaxies, as well as the quasars . However, a system of extragalactic objects includes more:
Extragalactic objects
- Galaxies of various types, e.g. B. elliptical and spiral galaxies , and their components such as stars, gas and dust. In the next galaxies, individual stars can also be spatially separated and thus examined individually.
- Active galaxies in which matter flows into a black hole , such as quasars and radio galaxies, and the associated phenomena such as radio jets .
- Galaxy groups and galaxy clusters such as B. Coma and Virgo clusters and the large-scale distribution of galaxies in the universe filled with dark matter and dark energy .
- Intergalactic medium is hot ionized hydrogen gas and hydrogen plasma in the intergalactic space between the galaxies in galaxy clusters.
- Bright individual objects such as supernovae and gamma-ray flashes .
- The cosmic background radiation from the time shortly after the Big Bang .
- Intergalactic stars , these are stars that, according to today's view, formed in galaxies, but were able to leave them as a result of various, discussed processes (see hyperspeed )