Intergalactic star

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

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, before being expelled as the result of either galaxies colliding or of a multiple-star system traveling 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 hypothesis 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. [2]


Collisions between galaxies are commonly thought to be a source of intergalactic stars. NGC4676.jpg
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 HE 0437-5439 mechanism.jpg
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. [3] However, when large galaxies collide, some of the gravitational disturbances might also expel stars. 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. [4]

Another hypothesis, that is not mutually exclusive to the galactic collisions hypothesis, is that intergalactic stars were 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. [5] In this respect, model calculations (from 1988) predict the supermassive black hole in the center of our Milky Way galaxy to expel one star every 100,000 years on average. [6]

Observation history

In 1997, the Hubble Space 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 by 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." [7]

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. [8] [9]

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. [10]

Some recently discovered supernovae have been confirmed to have exploded hundreds of thousands of light-years from the nearest star or galaxy. [11] [4] Most intergalactic star candidates found in the neighborhood of the Milky Way seem not to have an origin in the Galactic Center but in the Milky Way disk or elsewhere. [12] [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 how for the first time this diffuse radiation might originate from intergalactic stars. If that is the case, they 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. [8] [9] [14] [15]

Known locations

The first intergalactic stars were discovered in the Virgo cluster of galaxies. These are largely approximately 300,000 light years away from the nearest galaxy. 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 (and most likely, this total outweighs any of its 2500 galaxies). [16]

As of 2015, approximately 675 rogue stars have been discovered at the edge of the Milky Way, towards the Andromeda Galaxy. [10]

See also

Related Research Articles

Galaxy formation and evolution from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time

The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure.

Galaxy Gravitationally bonnd astronomical structure

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.

Globular cluster Spherical collection of stars

A globular cluster is a spherical collection of stars. Globular clusters are very tightly bound by gravity, with a high concentration of stars towards their centers. Their name is derived from Latin globulus—a small sphere. Globular clusters are occasionally known simply as globulars.

Quasar Active galactic nucleus containing a supermassive black hole

A quasar is an extremely luminous active galactic nucleus (AGN), powered by a supermassive black hole, with mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up because of friction and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous; the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way. Usually, quasars are categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.

Elliptical galaxy Galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile

An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.

Spiral galaxy Class of galaxy having a number of arms of younger stars

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Virgo Cluster galaxy cluster in the constellation Virgo

The Virgo Cluster is a large cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1,300 member galaxies, the cluster forms the heart of the larger Virgo Supercluster, of which the Local Group is a member. The Local Group actually experiences the mass of the Virgo Supercluster as the Virgocentric flow. It is estimated that the Virgo Cluster's mass is 1.2×1015M out to 8 degrees of the cluster's center or a radius of about 2.2 Mpc.

Messier 87 Elliptical galaxy in the Virgo Galaxy Cluster

Messier 87 is a supergiant elliptical galaxy with several trillion stars in the constellation Virgo. One of the most massive galaxies in the local universe, it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.

Supermassive black hole Largest type of black hole; usually found at the center of galaxies

A supermassive black hole is the largest type of black hole, with mass on the order of millions to billions of times the mass of the Sun (M). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at the galaxy's center. The Milky Way has a supermassive black hole in its Galactic Center, which corresponds to the location of Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei and quasars.

Messier 61 Intermediate barred spiral galaxy in the constellation Virgo

Messier 61 is an intermediate barred spiral galaxy in the Virgo Cluster of galaxies. It was first discovered by Barnaba Oriani on May 5, 1779, six days before Charles Messier discovered the same galaxy. Messier had observed it on the same night as Oriani but had mistaken it for a comet. Its distance has been estimated to be 45.61 million light years from the Milky Way Galaxy.

Galactic Center Rotational center of the Milky Way galaxy

The Galactic Center is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which powers the compact radio source Sagittarius A*, which is almost exactly at the galactic rotational center. The Galactic Center is approximately 8 kiloparsecs (26,000 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula.

Intermediate-mass black hole Class of black holes with a mass range of 100 to 100000 solar masses

An intermediate-mass black hole (IMBH) is a class of black hole with mass in the range 102–105 solar masses: significantly more than stellar black holes but less than the 105–109 solar mass supermassive black holes. Several IMBH candidate objects have been discovered in our galaxy and others nearby, based on indirect gas cloud velocity and accretion disk spectra observations of various evidentiary strength.

Sagittarius A* The supermassive black hole at the center of the Milky Way

Sagittarius A* is a bright and very compact astronomical radio source at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, visually close to the Butterfly Cluster (M6) and Shaula. Sagittarius A* is the location of a supermassive black hole, similar to massive objects at the centers of most, if not all, spiral and elliptical galaxies.

Messier 60 Elliptical galaxy in the constellation Virgo

Messier 60 or M60, also known as NGC 4649, is an elliptical galaxy approximately 57 million light-years away in the equatorial constellation of Virgo. Together with NGC 4647, it forms a pair known as Arp 116. Messier 60 and nearby elliptical galaxy Messier 59 were discovered by Johann Gottfried Koehler in April 1779, observing a comet in the same part of the sky. Charles Messier added both to his catalogue about three days after this.

Dwarf galaxy Small galaxy composed of up to several billion stars

A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

A dark galaxy is a hypothesized galaxy with no, or very few, stars. They received their name because they have no visible stars, but may be detectable if they contain significant amounts of gas. Astronomers have long theorized the existence of dark galaxies, but there are no confirmed examples to date. Dark galaxies are distinct from intergalactic gas clouds caused by galactic tidal interactions, since these gas clouds do not contain dark matter, so they do not technically qualify as galaxies. Distinguishing between intergalactic gas clouds and galaxies is difficult; most candidate dark galaxies turn out to be tidal gas clouds. The best candidate dark galaxies to date include HI1225+01, AGC229385, and numerous gas clouds detected in studies of quasars.

HE 0437-5439 Hypervelocity star in the constellation Dorado

HE 0437-5439 is a massive, unbound hypervelocity star (HVS), also called HVS3. It is a main sequence B-type star located in the Dorado constellation. It was discovered in 2005 with the Kueyen 8.2-metre (320 in) telescope, which is part of the European Southern Observatory's Very Large Telescope array. HE 0437-5439 is a young star, with an age of around 30 million years. The mass of the star is almost nine times greater than the mass of the Sun and the star is located 200,000 light years away in the direction of the Dorado constellation, 16 degrees northwest of the Large Magellanic Cloud (LMC) and farther away than the LMC. The star appears to be receding at an extremely high velocity of 723 kilometres per second (449 mi/s), or 2,600,000 kilometres per hour (1,600,000 mph). At this speed, the star is no longer gravitationally bound and will leave the Milky Way galaxy system and escape into intergalactic space. It was thought to have originated in the LMC and been ejected from it soon after birth. This could happen if it originally was one of a pair of stars and if there is a supermassive black hole in the LMC.

NGC 4639 Barred spiral galaxy in the constellation Virgo

NGC 4639 is a barred spiral galaxy located in the equatorial constellation of Virgo. It was discovered by German-born astronomer William Herschel on April 12, 1784. John L. E. Dreyer described it as "pretty bright, small, extended, mottled but not resolved, 12th magnitude star 1 arcmin to southeast". This is a relatively nearby galaxy, lying approximately 72 million light-years away from the Milky Way. It is a companion to NGC 4654, and the two appear to have interacted roughly 500 million years ago. NGC 4639 is a member of the Virgo Cluster.

In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space.

Laura Ferrarese Italian astrophysicist

Laura Ferrarese FRSC is a researcher in space science at the National Research Council of Canada. Her primary work has been performed using data from the Hubble Space Telescope and the Canada-France-Hawaii Telescope.


  1. M. L. Graham; D. J. Sand; D. Zaritsky; C. J. Pritchet (2 July 2015). "Confirmation of Hostless Type Ia Supernovae Using Hubble Space Telescope Imaging". the Astrophysical Journal . 807 (1): 83. arXiv: 1505.03407 . doi:10.1088/0004-637X/807/1/83.
  2. 1 2 "NewsCenter – Hubble Finds Intergalactic Stars (01/14/1997)". HubbleSite. 14 January 1997. Retrieved 9 December 2010.
  3. Witze, Alexandra (2014). "Half of Stars Lurk Outside Galaxies". Nature. Nature Publishing Group. doi:10.1038/nature.2014.16288. S2CID   124585348 . Retrieved 3 January 2015.
  4. 1 2 Nola Taylor Redd (27 August 2015). "Lonely Supernova Likely Exiled by Merging Black Holes". Future US. Retrieved 7 May 2021.
  5. Britt, Robert Roy. "Exiled stars: Milky Way Boots Members". USATODAY. . Retrieved 4 January 2015.
  6. NASA: Hyperfast Star Was Booted From Milky Way (22 July 2010)
  7. Britt, Robert Roy. "Exiled Stars: Milky Way Boots Members". USATODAY. Retrieved 4 January 2015.
  8. 1 2 Cooray; et al. (22 October 2012). "A measurement of the intrahalo light fraction with near-infrared background anisotropies". Nature. arXiv: 1210.6031v1 .
  9. 1 2 Zemcov; et al. (5 November 2014). "On the Origin of Near-Infrared Extragalactic Background Light Anisotropy". Nature. arXiv: 1411.1411 .
  10. 1 2 Salisbury, David (30 April 2012). "Rogue stars ejected from the galaxy found in intergalactic space". Research News. Vanderbilt University. Retrieved 3 January 2015.
  11. Stallard, Brian (7 June 2015). "Between Galaxies: Lonely Supernovae Identified". Nature World News.
  12. Bromley, Benjamin C.; Kenyon, Scott J.; Brown, Warren R.; Geller, Margaret J. (1 November 2018). "Nearby High-speed Stars in Gaia DR2". The Astrophysical Journal . 868 (1): 25 (15 pp.). arXiv: 1808.02620 . Bibcode:2018ApJ...868...25B. doi:10.3847/1538-4357/aae83e. S2CID   119241516.
  13. de la Fuente Marcos, R.; de la Fuente Marcos, C. (8 July 2019). "Flying far and fast: the distribution of distant hypervelocity star candidates from Gaia DR2 data". Astronomy and Astrophysics . 627: A104 (17 pp.). arXiv: 1906.05227 . Bibcode:2019A&A...627A.104D. doi:10.1051/0004-6361/201935008. S2CID   186207054.
  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.
  16. Henry C. Ferguson; et al. (1998), "Detection of intergalactic red-giant-branch stars in the Virgo cluster", Nature, 391 (6666): 461–463, arXiv: astro-ph/9801228 , Bibcode:1998Natur.391..461F, doi:10.1038/35087, S2CID   4390832