# Intergalactic star

An intergalactic star, also known as an intracluster star or a rogue star, is a star not gravitationally bound to any galaxy. Although a source of much discussion in the scientific community during the late 1990s, intergalactic stars are now generally thought to have originated in galaxies, like other stars, but later expelled as the result of either colliding galaxies or of a multiple star system travelling too close to a supermassive black hole, which are found at the center of many galaxies.

Collectively, intergalactic stars are referred to as the intracluster stellar population, or IC population for short, in the scientific literature.[1]

The Virgo cluster of galaxies, where the phenomenon known as intergalactic stars was discovered.

## Discovery

The common belief that stars exist only in galaxies was disproven in 1997 with the discovery of intergalactic stars.[2] The first to be discovered were in the Virgo cluster of galaxies, where some one trillion are now surmised to exist.[3]

## Formation

Collisions between galaxies are commonly thought to be a source of intergalactic stars.
Proposed mechanisms for the ejection of intergalactic stars by supermassive black holes.

The way these stars arise is still a mystery, but several scientifically credible hypotheses have been suggested and published by astrophysicists.

The most common hypothesis is that the collision of two or more galaxies can toss some stars out into the vast empty regions of intergalactic space. Although stars normally reside within galaxies, they can be expelled by gravitational forces when galaxies collide. It is commonly believed that intergalactic stars may primarily have originated from extremely small galaxies, since it is easier for stars to escape a smaller galaxy's gravitational pull, than that of a large galaxy.[4] However, when large galaxies collide, some of the gravitational disturbances might also produce intergalactic stars in theory. In 2015, a study of supernovae in intergalactic space suggested that the progenitor stars had been expelled from their host galaxies during a galactic collision between two giant ellipticals, as their supermassive black hole centres merged.[5]

Another hypothesis, that is not mutually exclusive to the galactic collisions hypothesis, is that intergalactic stars could have been ejected from their galaxy of origin by a close encounter with the supermassive black hole in the galaxy center, should there be one. In such a scenario, it is likely that the intergalactic star(s) was originally part of a multiple star system where the other stars were pulled into the supermassive black hole and the soon-to-be intergalactic star was accelerated and ejected away at very high speeds. Such an event could theoretically accelerate a star to such high speeds that it becomes a hypervelocity star, thereby escaping the gravitational well of the entire galaxy.[6] In this respect, model calculations (from 1988) predicts the supermassive black hole in the center of our Milky Way galaxy to expel one star every 100,000 years on average.[7]

## Observation history

In 1997, the Hubble telescope discovered a large number of intergalactic stars in the Virgo cluster of galaxies. Later in the 1990s, scientists discovered another group of intergalactic stars in the Fornax cluster of galaxies.

In 2005, at the Smithsonian Center for Astrophysics, Warren Brown and his team attempted to measure the speeds of hypervelocity stars using the Doppler Technique, by which light is observed for the similar changes that occur in sound when an object is moving away or toward something. But the speeds found are only estimated minimums, as in reality their speeds may be larger than the speeds found by the researchers. "One of the newfound exiles is moving in the direction of the constellation Ursa Major at about 1.25 million mph with respect to the galaxy. It is 240,000 light-years away. The other is headed toward the constellation Cancer, outbound at 1.43 million miles per hour and 180,000 light-years away."[8]

In the late 2000s, a diffuse glow from the intergalactic medium, but of unknown origin, was discovered. In 2012, it was suggested and shown that it might originate from intergalactic stars. Subsequent observations and studies have elaborated on the issue and described the diffuse extragalactic background radiation in more detail.[9][10]

Some Vanderbilt astronomers report that they have identified more than 675 stars at the edge of the Milky Way, between the Andromeda Galaxy and the Milky Way. They argue that these stars are hypervelocity (intergalactic) stars that were ejected from the Milky Way's galactic center. These stars are red giants with a high metallicity (a measure of the proportion of chemical elements other than hydrogen and helium within a star) indicating an inner galactic origin, since stars outside the disks of galaxies tend to have low metallicity and are older.[11]

Some recently discovered supernovae have been confirmed to have exploded hundreds of thousands of light years from the nearest star or galaxy.[12][5]

## Mass

Although the precise mass of the intergalactic star population cannot be known exactly, it is estimated that locally they make up 10 percent of the mass of the Virgo cluster of galaxies. This means that, most likely, intergalactic stars in the Virgo cluster would collectively have a larger mass than any particular one of the 2500 galaxies in the cluster.[13]

In 2005, the Spitzer Space Telescope revealed a hitherto unknown infrared component in the background from the cosmos. Since then, several other anisotropies at other wavelengths - including blue and x-ray - have been detected with other space telescopes and they are now collectively described as the diffuse extragalactic background radiation. Several explanations have been discussed by scientists, but in 2012, it was suggested and shown for the first time, that this diffuse radiation might in fact originate from intergalactic stars. If that is the case, intergalactic stars might collectively comprise as much mass as that found in the galaxies. A population of such magnitude was at one point thought to explain the photon underproduction crisis, and may explain a significant part of the dark matter problem.[9][10][14][15]

## Known locations

The first intergalactic stars were discovered in the Virgo cluster of galaxies. These stars form a massive group approximately 300,000 light years away from the nearest galaxy.

As of 2015, approximately 675 rogue stars have been discovered at the edge of the Milky Way, towards the Andromeda galaxy.[11]

## References

1. ^ See "Confirmation of Hostless Type Ia Supernovae Using Hubble Space Telescope Imaging" for example
2. ^ "NewsCenter – Hubble Finds Intergalactic Stars (01/14/1997) – Introduction". HubbleSite. 1997-01-14. Retrieved 2010-12-09.
3. ^ "NewsCenter – Hubble Finds Intergalactic Stars (01/14/1997) – Release Text". HubbleSite. 1997-01-14. Retrieved 2010-12-09.
4. ^ Witze, Alexandra. "Half of Stars Lurk Outside Galaxies". Nature. Nature Publishing Group. Retrieved 3 January 2015.
5. ^ a b Space.com: [ https://www.space.com/30380-lonely-supernova-exiled-by-black-holes.html Lonely Supernova Likely Exiled by Merging Black Holes] (27 August 2015)
6. ^ Britt, Robert Roy. "Exiled stars: Milky Way Boots Members". USATODAY. Space.com. Retrieved 4 January 2015.
7. ^ NASA: Hyperfast Star Was Booted From Milky Way (22 July 2010)
8. ^ Britt, Robert Roy. "Exiled Stars: Milky Way Boots Members". USATODAY. Space.com. Retrieved 4 January 2015.
9. ^ a b Cooray et.al. (22 October 2012). "A measurement of the intrahalo light fraction with near-infrared background anisotropies" (PDF). Nature.
10. ^ a b Zemcov et.al. (5 November 2014). "On the Origin of Near-Infrared Extragalactic Background Light Anisotropy" (PDF). Nature.
11. ^ a b Salisbury, David. "Rogue Stars Ejected From the Galaxy Found in Intergalactic Space". Vanderbilt University. Vanderbilt University. Retrieved 3 January 2015.
12. ^ Stallard, Brian. "Between Galaxies: Lonely Supernovae Identified". Nature World News.
13. ^ Henry C. Ferguson; et al. (1998), "Detection of intergalactic red-giant-branch stars in the Virgo cluster", Nature, 391: 461–463, arXiv:astro-ph/9801228, Bibcode:1998Natur.391..461F, doi:10.1038/35087
14. ^ "Colossal Gas Cloud Discovered Around Milky Way". Space. Retrieved 3 January 2015.
15. ^ Choi, Charles Q. "Lost in Space: Half of All Stars Drifting Free of Galaxies". Space. Purch. Retrieved 3 January 2015.

## Sources

Blitzar

Blitzars are a hypothetical type of astronomical object in which a spinning pulsar rapidly collapses into a black hole. They are proposed as an explanation for fast radio bursts (FRBs). The idea was proposed in 2013 by Heino Falcke and Luciano Rezzolla.

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

CN star

A CN star is a star with strong cyanogen bands in its spectrum. Cyanogen is a simple molecule of one carbon atom and one nitrogen atom, with absorption bands around 388.9 and 421.6 nm. This group of stars was first noticed by Nancy G. Roman who called them 4150 stars.

Frozen star (hypothetical star)

In astronomy, a frozen star, besides a disused term for a black hole, is a type of hypothetical star that, according to the astronomers Fred Adams and Gregory P. Laughlin, may appear in the future of the Universe when the metallicity of the interstellar medium is several times the solar value.

Infrared dark cloud

An infrared dark cloud (IRDC) is a cold, dense region of a giant molecular cloud. They can be seen in silhouette against the bright diffuse mid-infrared emission from the galactic plane.

Intergalactic dust

Intergalactic dust is cosmic dust in between galaxies in intergalactic space. Evidence for intergalactic dust has been suggested as early as 1949, and study of it grew throughout the late 20th century. There are large variations in the distribution of intergalactic dust. The dust may affect intergalactic distance measurements, such as to supernova and quasars in other galaxies.Intergalactic dust can form intergalactic dust clouds, known to exist around some galaxies since the 1960s. By the 1980s, at least four intergalactic dust clouds had been discovered within several megaparsec (Mpc) of the Milky Way galaxy, exemplified by the Okroy cloud.In February 2014, NASA announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed as early as two billion years after the big bang, are widespread throughout the universe, and are associated with new stars and exoplanets.

Iron star

In astronomy, an iron star is a hypothetical type of compact star that could occur in the universe in the extremely far future, after perhaps 101500 years.

The premise behind iron stars states that cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into iron-56 nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting stellar-mass objects to cold spheres of iron. The formation of these stars is only a possibility if protons do not decay. Though the surface of a neutron star may be iron, according to some predictions, it is distinct from an iron star.

Unrelatedly, the term is also used for blue supergiants which have a forest of forbidden FeII lines in their spectra. They are potentially quiescent hot luminous blue variables. Eta Carinae has been described as a prototypical example.

Lambda Boötis star

A Lambda Boötis star is a type of peculiar star which has an unusually low abundance of iron peak elements in its surface layers. One possible explanation for this is that it is the result of accretion of metal-poor gas from a circumstellar disc, a second possibility is the accretion of material from a hot Jupiter suffering from mass loss. The prototype is Lambda Boötis.

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

NGC 5291

NGC 5291 is a system of interacting galaxies in the constellation Centaurus. It is surrounded by a collisional ring, containing young and star-forming tidal dwarf galaxy, where dark matter has been detected.

Photosphere

The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to it being a spherical surface that is perceived to emit light. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately 2/3, or equivalently, a depth from which 50% of light will escape without being scattered.

In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Starfield (astronomy)

A starfield refers to a set of stars visible in an arbitrarily-sized field of view, usually in the context of some region of interest within the celestial sphere. For example: the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

Stellar-wind bubble

A stellar-wind bubble is a cavity light years across filled with hot gas blown into the interstellar medium by the high-velocity (several thousand km/s) stellar wind from a single massive star of type O or B. Weaker stellar winds also blow bubble structures, which are also called astrospheres. The heliosphere blown by the solar wind, within which all the major planets of the Solar System are embedded, is a small example of a stellar-wind bubble.

Stellar-wind bubbles have a two-shock structure. The freely-expanding stellar wind hits an inner termination shock, where its kinetic energy is thermalized, producing 106 K, X-ray emitting plasma. The hot, high-pressure, shocked wind expands, driving a shock into the surrounding interstellar gas. If the surrounding gas is dense enough (number densities ${\displaystyle n>0.1{\mbox{ cm}}^{-3}}$ or so), the swept-up gas radiatively cools far faster than the hot interior, forming a thin, relatively dense shell around the hot, shocked wind.

Stellar atmosphere

The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.

Stellar mass

Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (M☉). Hence, the bright star Sirius has around 2.02 M☉. A star's mass will vary over its lifetime as additional mass becomes accreted, such as from a companion star, or mass is ejected with the stellar wind or pulsational behavior.

Supernova impostor

Supernova impostors are stellar explosions that appear at first to be a supernova but do not destroy their progenitor stars. As such, they are a class of extra-powerful novae. They are also known as Type V supernovae, Eta Carinae analogs, and giant eruptions of luminous blue variables (LBV).

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

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