Galactic halo

The halo (from the Greek ἅλως halos, halo ') of a galaxy is a nearly spherical region that is larger than the galaxy itself, and in the center of the galaxy is embedded.

Properties

In halo are globular clusters and a few, mostly old single stars, outside their plane of rotation, mostly on eccentric orbits, circling the center of the galaxy. We can also see interstellar gas clouds and prevailing opinion is also dark matter, whose gravitational hold the galaxies and their composition is still unknown.

The halo of the Milky Way has a diameter of about 165,000 light years.

Age

On the basis of spectroscopic observations from the Hubble Space Telescope was the mass of newly formed white dwarfs in the inner halo to 0.551 solar masses and thus their age is determined at 11.4 / -0.7 billion years. The age of the stars in the outer halo was calculated independently at 13.5 billion years and is thus identical with the age of the oldest globular clusters. These observations confirm the development scenarios of the Milky Way, which assume that our galaxy has grown through the accretion of matter from outside to inside. The halos of spiral galaxies are fully developed, as shown by studies in the Hubble Ultra Deep Field have shown already at a redshift of z = 1. Because there is no star formation in the halo today, the halos were at this time much brighter and younger stars emitted more in the blue spectral range than today. These observations support the hypothesis according to which halos have formed primarily from the remains of accreted dwarf galaxies. In the case of the Milky Way, the mass of the star is about 108 solar masses in the halo.

Stellar streams

In the halo of the Milky Way several stellar streams could be detected. This is a collection of stars of the same chemical composition and age, which rotate the center of the galaxy on an elongated elliptical orbit. They originated in globular clusters and dwarf galaxies that have been captured by the Milky Way and dissolve by its tidal forces. The most prominent star power is the Magellanic Stream, which even after the Magellanic Clouds orbiting the Milky Way, both before.