Halo (astronomy)

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The halo (from ancient Greek ἅλως hálōs , atrium ') of a galaxy is an approximately spherical area that is larger than the galaxy itself and in the center of which the galaxy is embedded.

properties

In the halo there are globular clusters and a few, mostly old single stars that orbit the center of the galaxy outside of their plane of rotation, mostly on eccentric orbits. In addition, there are interstellar gas clouds and, according to the prevailing opinion, dark matter , whose gravity holds the galaxies together and whose composition is still unknown.

The halo of the Milky Way is approximately 165,000 light years (50 kpc ) in diameter , and that of the Andromeda Nebula is 1,000,000 light years (350 kpc) in diameter .

Age

For the Milky Way, spectroscopic observations from the Hubble Space Telescope were able to determine the mass of newly formed white dwarfs in the inner halo to be 0.551 solar masses and thus their age to be 11.4 ± 0.7 billion years. The age of the stars in the outer halo was independently calculated to be 13.5 billion years and is therefore equal to the age of the oldest globular clusters. These observations confirm the evolutionary scenarios of the Milky Way, which assume that our galaxy has grown from the outside in due to the incidence of matter. The halos of spiral galaxies are already fully developed at a redshift of z = 1, as studies in the Hubble Ultra Deep Field have shown. Because there is no star formation in the halos today, the halos were much brighter at that time and the younger stars emitted more strongly in the blue spectral range than they do today. These observations support the hypotheses that the halos formed predominantly from the accreted remains of dwarf galaxies . In the case of the Milky Way, the mass of the stars in the halo is approximately 10 8 solar masses .

Star currents

Several star currents could be detected in the halo of the Milky Way . It is a collection of stars of the same chemical composition and age, which orbit the center of the galaxy in an elongated elliptical orbit. Their origins lie in globular clusters and dwarf galaxies that have been captured by the Milky Way and that are disintegrated by its tidal forces . The most famous stellar current is the Magellanic Current , which orbits the Milky Way both before and after the Magellanic Clouds .

literature

  • Heather Morrison: Formation of the galactic halo. Astronomical Soc. of the Pacific, San Francisco 1996, ISBN 1-886733-13-9
  • Philip A. Davis, Saul J. Adelman: Hot stars in the galactic halo. Cambridge Univ. Press, Cambridge 1994, ISBN 0-521-46087-5

Web links

Individual evidence

  1. ↑ Jason S. Kalirai: The Age of the Milky Way Inner Halo . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.6802 .
  2. ^ Ignacio Trujillo, Judit Bakos: Stellar halos of disk galaxies at z ~ 1 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.7032v1 .
  3. ↑ Branimir Sesar et al .: Two Distant Halo Velocity Groups Discovered by the Palomar Transient Factory . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.0269 .
  4. ↑ Elizabeth Wylie-de Boer and Kenneth Freeman: Tracing the origin of the Aquarius stream: I . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.0784 .