Home > Galactic halo
The galactic halo is a region of space surrounding spiral galaxies, including our galaxy, the Milky Way. The galactic halo is believed to consist largely of stars, gas and dark matter.1 Halo stars
The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic disc ) in more or less conventional circular orbits around the center of the galaxy (the galactic core ), or in a spheroidal galactic bulge around the galactic core. However, some stars inhabit a spheroidal halo surrounding the galaxy. The orbital behaviour of these stars is as yet disputed, but they may describe retrograde and/or highly inclined orbits, or not to move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Elliptical Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.
Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metalThe English word metal is derived from the Latin word metallum in turn derived from the Greek metallon meaning a mine or quarry, as well as ore or metal. Thus, the word originally meant anything useful dug up from the ground, including stone, sand, clay o content than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusterA globular cluster is a spherical bundle of stars that orbits a galaxy as a satellite. Globular clusters are very tightly gravitationally bound, which gives them their spherical shape, and extremely dense (in relative terms) towards their core. Globular Cs.
The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarfThis article is about red dwarfs, the type of star. Red Dwarf is also the name of a British television series. According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spe stars close to the SunThe Sun (also called Sol is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally capt are thought to belong to the galactic halo, for example Kapteyn's StarKapteyn's Star (also known as GJ 191, HD 33793 or CD -45 1841) is a class M0 subdwarf discovered by Jacobus Kapteyn in 1897. 79 light years from Earth's Solar System, and is distinctive in a number of regards; it has a high radial velocity, orbits the Mil and Groombridge 1830Groombridge 1830 is a star. Also known as GJ 451, HD 103095, BD+38 2285, SAO 62738, it is a class G8 subdwarf catalogued by Stephen Groombridge in the 1830s. Its high proper motion was noted by Friedrich Wilhelm Argelander in 1842. 7 light years from Eart. Due to their irregular movement around the centre of the galaxy—if they do so at all—these stars often display unusually high proper motionProper motion is a component of the motion of stars. At first sight the stars seem to be in fixed positions with respect to each other, meaning they always form the same figures, and (for example) Ursa Major looks the same now as forty years ago. More car.
2 Halo dark matter
Beyond the visible, inner portion of the galactic halo lies a much larger region, known as the dark halo, extended halo or galactic corona, which contains large amounts of dark matter.
The presence of dark matter in the halo is demonstrated by its gravitational effect on a spiral galaxy's rotation curve. Without large amounts of mass in the extended halo, the rotational velocity of the galaxy should decrease at large distance from the galactic core. However, observations of spiral galaxies, particularly radio observations of line emission from neutral atomic hydrogen (known, in astronomical parlance, as HI), show that the rotation curve of most spiral galaxies remains flat far beyond the visible matter. The absence of any visible matter to account for these observations implies the presence of unobserved (i.e. dark) matter.
The nature of dark matter in the galactic halo of spiral galaxies is still undetermined, but the most popular theories are that the dark halo is home to vast numbers of small bodies known as MACHOs and/or weakly-interacting particles known as WIMPs. Observations of the halo of the Milky Way show that the number of MACHOs is not likely to be sufficient to account for the required mass.
