Stellar association

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Main associations of the galactic plane in the night sky Sky Map 7 - Paper 29.png
Main associations of the galactic plane in the night sky

A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more visible stars. An association is primarily identified by commonalities in its member stars' movement vectors, ages, and chemical compositions. These shared features indicate that the members share a common origin. Nevertheless, they have become gravitationally unbound, unlike star clusters, and the member stars will drift apart over millions of years, becoming a moving group as they scatter throughout their neighborhood within the galaxy. [1]

Contents

Stellar associations were discovered by Victor Ambartsumian in 1947. [2] [3] [4] The conventional name for an association uses the names or abbreviations of the constellation (or constellations) in which they are located; the association type, and, sometimes, a numerical identifier.

Types

Victor Ambartsumian first categorized stellar associations into two groups, OB and T, based on the properties of their stars. [3] A third category, R, was later suggested by Sidney van den Bergh for associations that illuminate reflection nebulae. [5]

The OB, T, and R associations form a continuum of young stellar groupings. But it is currently uncertain whether they are an evolutionary sequence, or represent some other factor at work. [6] Some groups also display properties of both OB and T associations, so the categorization is not always clear-cut.

OB associations

Young associations will contain 10–100 massive stars of spectral class O and B, and are known as OB associations. These are believed to form within the same small volume inside a giant molecular cloud. Once the surrounding dust and gas is blown away, the remaining stars become unbound and begin to drift apart. [7] It is believed that the majority of all stars in the Milky Way were formed in OB associations. [7]

O class stars are short-lived, and will expire as supernovae after roughly one to fifteen million years, depending on the mass of the star. As a result, OB associations are generally only a few million years in age or less. The O-B stars in the association will have burned all their fuel within 10 million years. (Compare this to the current age of the Sun at about 5 billion years.)

The Hipparcos satellite provided measurements that located a dozen OB associations within 650 parsecs of the Sun. [8] The nearest OB association is the Scorpius–Centaurus association, located about 400 light-years from the Sun. [9]

OB associations have also been found in the Large Magellanic Cloud and the Andromeda Galaxy. These associations can be quite sparse, spanning 1,500 light-years in diameter. [10]

T associations

Young stellar groups can contain a number of infant T Tauri stars that are still in the process of entering the main sequence. These sparse populations of up to a thousand T Tauri stars are known as T associations. The nearest example is the Taurus-Auriga T association (Tau-Aur T association), located at a distance of 140 parsecs from the Sun. [11] Other examples of T associations include the R Corona Australis T association, the Lupus T association, the Chamaeleon T association and the Velorum T association. T associations are often found in the vicinity of the molecular cloud from which they formed. Some, but not all, include O-B class stars. To summarize the characteristics of Moving groups members: they have the same age and origin, the same chemical composition and they have the same amplitude and direction in their vector of velocity.

R associations

Associations of stars that illuminate reflection nebulae are called R associations, a name suggested by Sidney van den Bergh after he discovered that the stars in these nebulae had a non-uniform distribution. [5] These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed. [6] This allows the properties of the surrounding dark cloud to be examined by astronomers. Because R-associations are more plentiful than OB associations, they can be used to trace out the structure of the galactic spiral arms. [12] An example of an R-association is Monoceros R2, located 830 ± 50 parsecs from the Sun. [6]

Known associations

The Ursa Major Moving Group is one example of a stellar association. (Except for α Ursae Majoris and η Ursae Majoris, all the stars in the Plough/Big Dipper are part of that group.)

Other young moving groups include:

See also

Related Research Articles

<span class="mw-page-title-main">Open cluster</span> Large group of stars less bound than globular clusters

An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and many more are thought to exist. Each one is loosely bound by mutual gravitational attraction and becomes disrupted by close encounters with other clusters and clouds of gas as they orbit the Galactic Center. This can result in a loss of cluster members through internal close encounters and a dispersion into the main body of the galaxy. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.

<span class="mw-page-title-main">H II region</span> Large, low-density interstellar cloud of partially ionized gas

An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.

<span class="mw-page-title-main">NGC 604</span> H II region inside the Triangulum Galaxy

NGC 604 is an H II region inside the Triangulum Galaxy. It was discovered by William Herschel on September 11, 1784. It is among the largest H II regions in the Local Group of galaxies; at the galaxy's estimated distance of 2.7 million light-years, its longest diameter is roughly 1,520 light years (~460 parsecs), over 40 times the size of the visible portion of the Orion Nebula. It is over 6,300 times more luminous than the Orion Nebula, and if it were at the same distance it would outshine Venus. Its gas is ionized by a cluster of massive stars at its center with 200 stars of spectral type O and WR, a mass of 105 solar masses, and an age of 3.5 million years; however, unlike the Large Magellanic Cloud's Tarantula Nebula central cluster (R136), NGC 604's one is much less compact and more similar to a large stellar association.

<span class="mw-page-title-main">Herbig–Haro object</span> Small patches of nebulosity associated with newly born stars

Herbig–Haro (HH) objects are bright patches of nebulosity associated with newborn stars. They are formed when narrow jets of partially ionised gas ejected by stars collide with nearby clouds of gas and dust at several hundred kilometres per second. Herbig–Haro objects are commonly found in star-forming regions, and several are often seen around a single star, aligned with its rotational axis. Most of them lie within about one parsec of the source, although some have been observed several parsecs away. HH objects are transient phenomena that last around a few tens of thousands of years. They can change visibly over timescales of a few years as they move rapidly away from their parent star into the gas clouds of interstellar space. Hubble Space Telescope observations have revealed the complex evolution of HH objects over the period of a few years, as parts of the nebula fade while others brighten as they collide with the clumpy material of the interstellar medium.

Megrez, also called Delta Ursae Majoris, is a star in the northern constellation of Ursa Major. With an apparent magnitude of +3.3, it is the dimmest of the seven stars in the Big Dipper asterism. Parallax measurements yield a distance estimate of 80.5 light-years from the Sun.

Theta Ursae Majoris is a suspected spectroscopic binary star system in the northern circumpolar constellation of Ursa Major. It has an apparent visual magnitude of 3.17, placing it among the brighter members of this constellation. The distance to this star has been measured directly using the parallax method, yielding an estimated value of 43.96 light-years.

<span class="mw-page-title-main">Orion molecular cloud complex</span> Star-forming region in the constellation Orion

The Orion molecular cloud complex is a star-forming region with stellar ages ranging up to 12 Myr. Two giant molecular clouds are a part of it, Orion A and Orion B. The stars currently forming within the complex are located within these clouds. A number of other somewhat older stars no longer associated with the molecular gas are also part of the complex, most notably the Orion's Belt, as well as the dispersed population north of it. Near the head of Orion there is also a population of young stars that is centered on Meissa. The complex is between 1 000 and 1 400 light-years away, and hundreds of light-years across.

<span class="mw-page-title-main">Scorpius–Centaurus association</span> The nearest OB association to Earth

The Scorpius–Centaurus association is the nearest OB association to the Sun. This stellar association is composed of three subgroups and its distance is about 130 parsecs or 420 light-years. Using improved Hipparcos data, Rizzuto and colleagues analysed nearby stars more closely, bringing the number of known members to 436. They doubt the need to add a subclassification because they found a more continuous spread of stars.

36 Ursae Majoris is a double star in the northern constellation of Ursa Major. With an apparent visual magnitude of 4.8, it can be seen with the naked eye in suitable dark skies. Based upon parallax measurements, this binary lies at a distance of 42 light-years from Earth.

<span class="mw-page-title-main">NGC 6193</span> Open star cluster in the constellation Ara

NGC 6193 is open cluster containing 27 stars in the constellation Ara, visible to the unaided eye. NGC 6193 lies at the center of the Ara OB1 association, which extends over a square degree. The cluster is associated with neighboring regions of the nebulosity NGC 6188.

<span class="mw-page-title-main">Stellar kinematics</span> Study of the movement of stars

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

<span class="mw-page-title-main">Orion OB1</span> Hot giant star group

Orion OB1 is a contingent group of several dozen hot giant stars of spectral types O and B in Orion. Associated are thousands of lower-mass stars, and a number of protostars. It is part of the larger Orion molecular cloud complex. Owing to its relative closeness and complexity it is the most closely studied OB association.

<span class="mw-page-title-main">Cygnus OB2</span> Cluster of massive and luminous stars

Cygnus OB2 is an OB association that is home to some of the most massive and most luminous stars known, including suspected Luminous blue variable Cyg OB2 #12. It also includes one of the largest known stars, NML Cygni. The region is embedded within a wider one of star formation known as Cygnus X, which is one of the most luminous objects in the sky at radio wavelengths. The region is approximately 1,570 parsecs from Earth in the constellation of Cygnus.

<span class="mw-page-title-main">HD 165516</span> Star in the constellation Sagittarius

HD 165516 is a blue supergiant star in the constellation Sagittarius. It is part of the Sagittarius OB1 association and appears against a rich Milky Way starfield near the Triffid Nebula and Lagoon Nebula.

<span class="mw-page-title-main">Trumpler 16</span> Massive open cluster in the constellation Carina

Trumpler 16 is a massive open cluster that is home to some of the most luminous stars in the Milky Way galaxy. It is situated within the Carina Nebula complex in the Carina–Sagittarius Arm, located approximately 9,270 ly (2,842 pc) from Earth. The cluster has one star visible to the naked eye from the tropics southward, Eta Carinae.

<span class="mw-page-title-main">Carina OB1</span>

Carina OB1 is a giant OB association in the Carina Nebula, which is home to some of the most massive and luminous stars in the Milky Way Galaxy. It includes the young star clusters Collinder 228, NGC 3293, NGC 3324, IC 2581, Trumpler 14, Trumpler 15 and Trumpler 16, the last being the home of Eta Carinae. It also includes another massive and luminous star, HD 93129A. It is approximately 2,680 parsecs (8,700 ly) from Earth.

<span class="mw-page-title-main">Vela Molecular Ridge</span> Molecular cloud complex in the constellations Vela and Puppis

Vela Molecular Ridge is a molecular cloud complex in the constellations Vela and Puppis. Radio 12CO observations of the region showed the ridge to be composed of several clouds, each with masses 100,000–1,000,000 M☉. This cloud complex lies on the sky in the direction of the Gum Nebula (foreground) and the Carina–Sagittarius Spiral Arm (background). The most important clouds in the region are identified by the letters A, B, C and D, and in fact belong to two different complexes: the clouds A, C and D are located at an average distance of about 700-1000 parsecs and are related to the OB association Vela R2, while cloud B is located at a greater distance, up to 2000 parsecs away, and is physically connected to the extended Vela OB1 association.

<span class="mw-page-title-main">Vulpecula OB1</span>

Vulpecula OB1 is an OB association in which a batch of massive stars are being born. It was first identified by W. W. Morgan et al. (1953). The association is located in the Orion Arm about 7,500 light-years away from the Sun. Nebulae that are contained in this association include NGC 6820 and NGC 6823, plus Sharpless 2-88.

<span class="mw-page-title-main">Cepheus OB1</span> OB Association in the constellation Cepheus

Cepheus OB1 is an OB association around the cluster NGC 7380. The region is approximately 3,400 parsecs from Earth in the constellation of Cepheus, although many of its stars lie in neighboring constellations like Cassiopeia.

Camelopardalis OB1 is a group of young stars that share a common origin and a similar motion through space, but, as a whole, are no longer gravitationally bound. The name indicates this stellar association is located in the area of the Camelopardalis constellation which includes a number of massive, short-lived OB stars. The association is ~3,300 ly (1,000 pc) distant from the Sun, with members lying between 500 pc and 1,500 pc away. It is located on the edge of the local Orion Arm of the Milky Way galaxy and lies outside the traditional Gould Belt.

References

  1. ↑ "Discovery: New Moving Group in the Local Arm of the Milky Way". National Astronomical Observatories of China. Chinese Academy of Sciences. 13 May 2020. Retrieved 13 July 2024.
  2. ↑ Lankford, John, ed. (2011) [1997]. "Ambartsumian, Viktor Amazaspovich (b. 1908)". History of Astronomy: An Encyclopedia. Routledge. p.  10. ISBN   9781136508349.
  3. 1 2 Israelian, Garik (1997). "Obituary: Victor Amazaspovich Ambartsumian, 1912 [i.e. 1908] -1996". Bulletin of the American Astronomical Society. 29 (4): 1466–1467. Bibcode:1997BAAS...29.1466I.
  4. ↑ Saxon, Wolfgang (15 August 1996). "Viktor A. Ambartsumyan, 87, Expert on Formation of Stars". The New York Times . p. 22.
  5. 1 2 Herbst, W. (1976). "R associations. I - UBV photometry and MK spectroscopy of stars in southern reflection nebulae". Astronomical Journal. 80: 212–226. Bibcode:1975AJ.....80..212H. doi: 10.1086/111734 .
  6. 1 2 3 Herbst, W.; Racine, R. (1976). "R associations. V. MON R2". Astronomical Journal. 81: 840. Bibcode:1976AJ.....81..840H. doi: 10.1086/111963 .
  7. 1 2 "OB Associations". The GAIA Study Report: Executive Summary and Science Section. 2000-04-06. Retrieved 2006-06-08.
  8. ↑ de Zeeuw, P. T.; Hoogerwerf, R.; de Bruijne, J. H. J.; Brown, A. G. A.; Blaauw, A. (1999). "A HIPPARCOS Census of the Nearby OB Associations". The Astronomical Journal. 117 (1): 354–399. arXiv: astro-ph/9809227 . Bibcode:1999AJ....117..354D. doi:10.1086/300682. S2CID   16098861.
  9. ↑ Maíz-Apellániz, Jesús (2001). "The Origin of the Local Bubble". The Astrophysical Journal. 560 (1): L83–L86. arXiv: astro-ph/0108472 . Bibcode:2001ApJ...560L..83M. doi:10.1086/324016. S2CID   119338135.
  10. ↑ Elmegreen, B.; Efremov, Y. N. (1999). "The Formation of Star Clusters". American Scientist. 86 (3): 264. Bibcode:1998AmSci..86..264E. doi:10.1511/1998.3.264. S2CID   262334560 . Retrieved 2006-08-23.
  11. ↑ Frink, S.; Roeser, S.; Neuhaeuser, R.; Sterzik, M. K. (1999). "New proper motions of pre-main sequence stars in Taurus-Auriga". Astronomy and Astrophysics. 325: 613–622. arXiv: astro-ph/9704281 . Bibcode:1997A&A...325..613F.
  12. ↑ Herbst, W. (1975). "R-associations III. Local optical spiral structure". Astronomical Journal. 80: 503. Bibcode:1975AJ.....80..503H. doi:10.1086/111771.
  13. ↑ Lyder, David A. (November 2001). "The Stars in Camelopardalis OB1: Their Distance and Evolutionary History". The Astronomical Journal. 122 (5): 2634–2643. Bibcode:2001AJ....122.2634L. doi: 10.1086/323705 . S2CID   120758592.
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