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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Cosmos

Galaxies

Andromeda

At first glance, galaxies consist of stars, planetary systems, gas nebulae and clouds of dust, all bound by gravitational attraction.  But the mass of galaxies is actually dominated by Dark Matter.  According to our current knowledge the universe includes more than 1010 galaxies which in turn contain between 109 and 1012 stars each.  Compared to their own diameters, the distance between galaxies is rather large (actually they were called island universes until the first decades of the XX century).  For example, our neighbour galaxy, the Andromeda Nebula, is about 2.5 million light years away, whereas the diameter of our galaxy is about a hundred thousand light years.  Galaxies often form bigger structures, called galaxy clusters, that could consist of up to 2,500 individual galaxies. Galaxies are categorized as elliptic, lenticular, spiral and irregular.  Spiral galaxies are subdivided into normal and barred.

Elliptic galaxies mostly contain old stars which were born 10 billion years ago.  These galaxies do not have enough interstellar gas and dust to build new stars.  The luminosity of these galaxies is mainly caused by red giants.  Spiral galaxies, like the Milky Way, show a central structure called the bulge which has a similar composition as an elliptic galaxy.  Their spiral arms contain big clouds of dust and gas in which new stars are born.  Barred galaxies are a particular form of spiral galaxies that show a central bar-shaped structure, composed of stars, showing a sharp bend in the outer part.  Galaxies without any distinctive centre are called irregular galaxies.

Whereas in the centre of the Milky Way there is a black hole of ca. 3 million times the Sun's mass, the central region of some galaxies is dominated by super-massive black holes of more than 1 billion solar masses.  Furthermore, such systems have an accretion disk and, perpendicular to the plane of this disk, two huge symmetric structures called jets can be seen.  These jets are probably collimated, relativistic flows of matter whose length exceeds the diameter of their host galaxy.  The central region and the jets of these galaxies can emit very variable electromagnetic radiation with huge changes in the emitted flux on timescales that can range from minutes to years.  By exploring these phenomena scientists seek to understand those central regions as well as their underlying particle populations. The very high energy gamma-ray emission (energies about several GeV) of some of these sources is examined today by Imaging Atmospheric Cherenkov telescopes like the H.E.S.S. Experiment.  The multi-wavelength study of these active galaxies is crucial in revealing whether such galaxies are the long wanted sources of the extragalactic component of the cosmic-ray flux.


Cosmos | Supernovae | Pulsars | Galaxies | Cosmic Rays | γ-Astronomy