A stellar halo is the component of a galaxy's

galactic halo A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of a galaxy comprise its halo: * the stellar halo * the galactic corona (hot gas, i.e. a plas ...

that contains stars. The stellar halo extends far outside a galaxy's brightest regions and typically contains its oldest and most

metal-poor star In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-dark) matter in the universe is either hydrogen or helium, and astronomers ...

Observation history

Early studies, investigating the shape of the stellar halo of the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

, found some evidence that it may vary with increasing distance from the galaxy. These studies found halos with spherically shaped outer regions and flatter inner regions. Large surveys in the 21st century such as the

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...

have allowed the shape and distribution of the stellar halo to be investigated in much more detail; this data has been used to postulate a triaxial or

oblate In Christianity (specifically the Roman Catholic, Orthodox, Lutheran, Anglican and Methodist traditions), an oblate is a person associated with a Benedictine monastery or convent who is specifically dedicated to God and service. Oblates are i ...

halo. More recent studies have found the halo to be flattened with a broken power law radius dependence; evidence for triaxiality is unclear. As a result of their faint brightness, observations of stellar halos in distant galaxies have required very long

exposure time In photography, shutter speed or exposure time is the length of time that the film or digital sensor inside the camera is exposed to light (that is, when the camera's shutter is open) when taking a photograph. The amount of light that rea ...

s, the stacking of data from numerous galaxies to obtain averaged properties, or observing only the resolved

stellar populations In 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations. In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926. Baade observed that ...

. Individual resolved stars in stellar halos can only be measured in the Milky Way and Andromeda. The furthest stellar halos detected are at a

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...

distance of 1.

Structure and properties

In the

Lambda-CDM model The Lambda-CDM, Lambda cold dark matter, or ΛCDM model is a mathematical model of the Big Bang theory with three major components: # a cosmological constant, denoted by lambda (Λ), associated with dark energy; # the postulated cold dark mat ...

of the universe, galaxies grow by

mergers Mergers and acquisitions (M&A) are business transactions in which the ownership of a company, business organization, or one of their operating units is transferred to or consolidated with another entity. They may happen through direct absorpt ...

. Such mergers are the cause of substructure observed in the stellar halo of galaxies; streams of stars from disrupted

satellite galaxies A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and Luminosity, luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents ...

are detectable through their coherence in space or velocity; a number of these streams are observable around the Milky Way. As a result of the buildup from an assortment of satellite galaxies, variations in properties such as

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

are present across stellar populations in halos. Astrophysical simulations of galaxies have predicted that stellar halos should have two components: an inner region dominated by stars which formed within the galaxy, and an outer region primarily composed of stars accreted through merger events. Predictions for these components include different structure and rotation directions. Observational evidence for this dual halo in the Milky Way has been claimed but contested.

Milky Way

Studies of the

galaxy have found that approximately % of its total stellar mass is contained within the stellar halo, and that it extends to over 100

kiloparsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...

s from the galactic centre.

References

{{reflist, 25em Galactic astronomy Observational cosmology

Observation history

Structure and properties

Milky Way

See also

References