Astronomy Magazine

Dwarf Galaxies

Posted on the 09 May 2011 by Gabe12logan
In the immediate neighborhood of our Galaxy system a few more small satellite galaxies was found, the so-called elliptical spheroids that can be considered as the companions of the Milky Way. Although they were close, due to their low surface brightness, they were difficult to detect. The first dwarf galaxy was discovered in 1937th from Harvard's Observatory in Boyden (South Africa). It was detected in constellation Sculptor, and because of that is called Sculptor dwarf galaxy. The first observations showed that this is a dwarf galaxy composed of a very weak stars. Brightest stars were red and somewhat similar to the stars which form globular clusters. However, unlike globular clusters, density of this star system is very small compared to its size. System in the Sculptor is 260 000 ly away from us.
Later it was discovered a few of these spheroids, such as a system in Fornax located 450 000 light years away, dwarf galaxies in the constellations Draco and Ursa Minor constellations, known for Barnard's galaxy, and the farthest satellite spheroids Leo I and Leo II, and many others. Stellar populations that we observe in these strange dwarf galaxies are similar to those in elliptical galaxies. However, they have very few stars, between 100 000 and several millions. This means that their masses are not much larger than the masses of globular clusters in the Milky Way. Most of these galaxies is dead, all the stars in them are very old, and there is no indication of the creation of new stars. The explanation can be found in a small mass and density, in the Milky Way stars are born from gas discharging older stars, and in these dwarfs there is no enough mass, and there is no enough gravity which would maintained the gas.
At a distance of only 78 000 light years away, hidden behind the center of our galaxy, are the remains of tiny dwarf galaxies called Sagittarius Dwarf, which in the past collides with our galaxy. Tidal forces of the Milky Way spread the dwarf galaxy, and today only its compact center can be recognize. The diameter of the remains of this galaxy is, after the longest axis, a quarter of the diameter of the Milky Way. But the mass of this galaxy is only one thousandth of the mass of our galaxy. It is estimated that the stars of the Sagittarius dwarf galaxy will merge in the Milky Way within the next 100 million years. Some astronomers believe that the halo of our galaxy originated due to a number of such collisions with dwarf galaxies and many globular clusters are actually newcomers from other galaxies.

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