Stellar population

During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations. He noticed that bluer stars were strongly associated with the spiral arms and yellow stars dominated near the central galactic bulge and within globular star clusters.[1] Two main divisions were defined as Population I and Population II, with another newer division called Population III added in 1978,[2] which are often simply abbreviated as Pop I, II or III.

Between the population types, significant differences were found with their individual observed stellar spectra. These were later shown to be very important, and were possibly related to star formation, observed kinematics,[3] stellar age, and even galaxy evolution in both spiral or elliptical galaxies. These three simple population classes usefully divided stars by their chemical composition or metallicity,[a][b] whose small proportion of chemical abundance consists of heavier elements against the far more abundant hydrogen and helium.[4][3]

By definition, each population group shows the trend where decreasing metal content indicates increasing age of stars. Hence, the first stars in the universe (very low metal content) were deemed Population III, old stars (low metallicity) as Population II, and recent stars (high metallicity) as Population I.[5]

Artist's impression of the Milky Way (updated - annotated)
Artist's conception of the spiral structure of the Milky Way showing Baade's general population categories. The blue regions in the spiral arms comprise the younger Population I stars, while the yellow stars in the central bulge are the older Population II stars. In reality, many Population I stars are also found mixed in with the newer Population II stars.

Stellar populations

Observation of stellar spectra has revealed that stars older than the Sun have fewer heavy elements compared to the Sun.[3] This immediately suggests that metallicity has evolved through the generations of stars by the process of stellar evolution.[1]

Formation of the first stars

Under current cosmological models, all matter created in the Big Bang was mostly hydrogen (76%) and helium (24%), with only a very tiny fraction consisting of light elements. e.g. lithium and beryllium. When the universe had cooled sufficiently, the first stars were born as Population III stars without any contaminating heavier metals. This is postulated to have affected their structure so that their stellar masses became hundreds of times more than that of the Sun. In turn, these massive stars also evolved very quickly, and their nucleosynthetic processes created the first 26 elements (up to iron in the periodic table).[6]

Many theoretical stellar models show that most high-mass Population III stars rapidly exhausted their fuel and likely exploded in extremely energetic pair-instability supernovae. Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass Population III stars should be observable.[7] However, some Population III stars might be seen in high-redshift galaxies whose light originated during the earlier history of the universe. None have been discovered, however, scientists have found evidence of an extremely small Ultra Metal-poor star (slightly smaller than our sun) found in a binary system of the spiral arms in our Milky Way. It was discovered while investigating the "wobble" of its larger neighboring star expecting to find a black hole. This star is very likely going to further our knowledge of Population III stars.

Stars too massive to produce pair-instability supernovae would have likely collapsed into black holes through a process known as photodisintegration. Here some matter may have escaped during this process in the form of relativistic jets, and this could have distributed the first metals into the universe.[8][9][c]

Formation of the observable stars

The oldest observed stars,[7] known as Population II, have very low metallicities;[5][10] as subsequent generations of stars were born they became more metal-enriched, as the gaseous clouds from which they formed received the metal-rich dust manufactured by previous generations. As those stars died, they returned metal-enriched material to the interstellar medium via planetary nebulae and supernovae, enriching further the nebulae out of which the newer stars formed. These youngest stars, including the Sun, therefore have the highest metal content, and are known as Population I stars.

Population details

Population I stars

Treasures3
Population I star Rigel with reflection nebula IC 2118

Population I, or metal-rich, stars are young stars with the highest metallicity out of all three populations, and are more commonly found in the spiral arms of the Milky Way galaxy. The Earth's Sun is an example of a metal-rich star and is considered as an intermediate Population I star, while the solar-like Mu Arae is much richer in metals.[11]

Population I stars usually have regular elliptical orbits of the galactic centre, with a low relative velocity. It was earlier hypothesized that the high metallicity of Population I stars makes them more likely to possess planetary systems than the other two populations, because planets, particularly terrestrial planets, are thought to be formed by the accretion of metals.[12] However, observations of the Kepler data-set have found smaller planets around stars with a range of metallicities, while only larger, potential gas giant planets are concentrated around stars with relatively higher metallicity — a finding that has implications for theories of gas giant formation.[13] Between the intermediate Population I and the Population II stars comes the intermediary disc population.

Population II stars

Milky way profile
Schematic profile of the Milky Way. Population II stars appear in the galactic bulge and within the globular clusters

Population II, or metal-poor, stars are those with relatively little metal. The idea of a relatively small amount must be kept in perspective as even metal-rich astronomical objects contain low percentages of any element other than hydrogen or helium; metals constitute only a tiny percentage of the overall chemical makeup of the universe, even 13.8 billion years after the Big Bang. However, metal-poor objects are even more ancient. These objects were formed during an earlier time of the universe. Intermediate Population I stars are common in the bulge near the centre of our galaxy, whereas Population II stars found in the galactic halo are older and thus more metal-poor. Globular clusters also contain high numbers of population II stars.[14]

It is thought that population II stars created all the other elements in the periodic table, except the more unstable ones. An interesting characteristic of Population II stars is that despite their lower overall metallicity, they often have a higher ratio of alpha elements (O, Si, Ne, etc.) relative to Fe as compared to Population I stars; current theory suggests this is the result of Type II supernovae being more important contributors to the interstellar medium at the time of their formation, whereas Type Ia supernova metal enrichment came later in the universe's evolution.[15]

Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of Timothy C. Beers et al. and the Hamburg-ESO survey of Norbert Christlieb et al., originally started for faint quasars. Thus far, they have uncovered and studied in detail about ten ultra metal poor (UMP) stars (such as Sneden's Star, Cayrel's Star, BD +17° 3248) and three of the oldest stars known to date: HE0107-5240, HE1327-2326 and HE 1523-0901. Caffau's star was identified as the most metal-poor star yet when it was found in 2012 using Sloan Digital Sky Survey data. However, in February 2014 the discovery of an even lower metallicity star was announced, SMSS J031300.36-670839.3 located with the aid of SkyMapper astronomical survey data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are HD 122563 (a red giant) and HD 140283 (a subgiant).

Population III stars

Ssc2005-22a1
Possible glow of Population III stars imaged by NASA's Spitzer Space Telescope

Population III stars[16] are a hypothetical population of extremely massive and hot stars with virtually no metals, except possibly for intermixing ejecta from other nearby Population III supernovas. Their existence is inferred from physical cosmology, but they have not yet been observed directly. Indirect evidence for their existence has been found in a gravitationally lensed galaxy in a very distant part of the universe.[17] Their existence may account for the fact that heavy elements – which could not have been created in the Big Bang – are observed in quasar emission spectra.[6] They are also thought to be components of faint blue galaxies. These stars likely triggered the universe's period of reionization, a major phase transition of gases leading to the opacity observed today. Observations of the galaxy UDFy-38135539 suggest it may have played a role in this reionization process. The European Southern Observatory discovered a bright pocket of early population stars in the very bright galaxy Cosmos Redshift 7 from the reionization period around 800 million years after the Big Bang. The rest of the galaxy has some later redder population II stars.[18][19] Some theories hold that there were two generations of Population III stars.[20]

NASA-WMAP-first-stars
Artist's impression of the first stars, 400 million years after the Big Bang

Current theory is divided on whether the first stars were very massive or not; theories proposed in 2009 and 2011 suggest the first star groups might have consisted of a massive star surrounded by several smaller stars.[21][22][23] The smaller stars, if they remained in the birth cluster, would accumulate more gas and could not survive to the present day, but a 2017 study concluded that if a star of 0.8 solar masses or less was ejected from its birth cluster before it accumulated more mass, it could survive to the present day, possibly even in our Milky Way galaxy.[24]

One proposal, developed by computer models of star formation, is that with no heavy elements and a much warmer interstellar medium from the Big Bang, it was easy to form stars with much greater total mass than the stars commonly visible today. Typical masses for Population III stars are expected to be about several hundred solar masses, which is much larger than that of current stars. Analysis of data of extremely low-metallicity Population II stars such as HE0107-5240, which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 20 to 130 solar masses.[25] On the other hand, analysis of globular clusters associated with elliptical galaxies suggests pair-instability supernovae, which are typically associated with very massive stars, were responsible for their metallic composition.[26] This also explains why there have been no low-mass stars with zero metallicity observed, although models have been constructed for smaller Pop III stars.[27][28] Clusters containing zero-metallicity red dwarfs or brown dwarfs (possibly created by pair-instability supernovae[10]) have been proposed as dark matter candidates,[29][30] but searches for these types of MACHOs through gravitational microlensing have produced negative results.

Detection of Population III stars is a goal of NASA's James Webb Space Telescope.[31] New spectroscopic surveys, such as SEGUE or SDSS-II, may also locate Pop III stars. Stars observed in the Cosmos Redshift 7 galaxy at z = 6.60 may be Population III stars. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it.[19][32]

Further reading

  • Gibson, B.K.; et al. (2013). "Review: Galactic Chemical Evolution" (PDF). Publications of the Astronomical Society of Australia. Retrieved 17 April 2018.

Notes

  1. ^ Stellar metallicity is often defined using the ratio of total iron content of the star compared to hydrogen e.g. [Fe/H], which can be measured from many prominent absorption spectral lines found in most solar-like stars or red giant stars. The unit of measure often used for metallicity is the "dex", which is abbreviated from 'decimal exponent'. Stars with a higher metallicities than the Sun have positive logarithmic values, whereas those with a lower metallicities than the Sun have negative values. Hence, the measure of the Sun is 0.00 dex while Pop I or Type I Cepheid variable star Delta Cephei is +0.08 dex and Pop II or Type II Cepheid variable star W Virginis is about -1.0 dex. Young Population I stars have significantly higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have metallicities of less than −6.0, that is, less than a millionth of the abundance of iron in the Sun.
  2. ^ Stellar metallicity can also be expressed in total metals as 'Z', being the fraction of mass of a star that is not hydrogen (X) or helium (Y). That is, X+Y+Z=1.00. The Sun has Z=0.0122 to 0.0134. However, Z values in stars are difficult to determine and are usually only rough estimates.
  3. ^ It has been proposed that recent supernovae SN 2006gy and SN 2007bi may have been pair-instability supernovae where such super-massive Population III stars exploded. It has been speculated that these stars could have formed relatively recently in dwarf galaxies containing primordial metal-free interstellar matter; past supernovae in these galaxies could have ejected their metal-rich contents at speeds high enough for them to escape the galaxy, keeping the metal content of the galaxy very low. Stuart Clark (February 2010). "Primordial giant: The star that time forgot". New Scientist. Retrieved 1 February 2015.

References

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Boötes III (dwarf galaxy)

Boötes III is an overdensity in the Milky Way's halo, which may be a disrupted dwarf spheroidal galaxy. It is situated in the constellation Boötes and was discovered in 2009 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 46 kpc from the Sun and moves away the Sun with the speed of about 200 km/s. It has an elongated shape (axis ratio of 2:1) with the radius of about 0.5 kpc. The large size and an irregular shape may indicate that Bootes III in a transitional phase between a gravitationally bound galaxy and completely unbound system.Boötes III is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 18,000 times that of the Sun (absolute visible magnitude of about −5.8), which is much lower than the luminosity of many globular clusters. The mass of Bootes III is difficult to estimate because the galaxy is in process of being disrupted. In this case the velocity dispersion of its stars is not related to its mass.The stellar population of Bootes III consists mainly of moderately old stars formed more than 12 billion years ago. The metallicity of these old stars is low at [Fe/H]=−2.1 ± 0.2, which means that they contain 120 times less heavy elements than the Sun. Bootes III may the source of stars of the Styx stream in the galactic halo, which was discovered together with this galaxy.

Boötes II (dwarf galaxy)

Bootes II or Boo II is a dwarf spheroidal galaxy situated in the Bootes constellation and discovered in 2007 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 42 kpc from the Sun and moves towards the Sun with the speed of 120 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 51 pc.Bootes II is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 1,000 times that of the Sun (absolute visible magnitude of about −2.7), which is much lower than the luminosity of the majority of globular clusters. However the mass of the galaxy is substantial corresponding to the mass to light ratio of more than 100.The stellar population of Bootes II consists mainly of moderately old stars formed 10–12 billion years ago. The metallicity of these old stars is low at [Fe/H]=−1.8, which means that they contain 80 times less heavy elements than the Sun. Currently there is no star formation in Bootes II. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is only 86 solar masses.Bootes II is located only 1.5 degrees (~1.6 kpc) away from another dwarf galaxy—Boötes I, although they are unlikely to be physically associated because they move in opposite directions relative to the Milky Way. Their relative velocity—about 200 km/s is too high. It is more likely associated with the Sagittarius Stream and, therefore, with the Sagittarius Dwarf Elliptical Galaxy (SagDEG). Bootes II may be either a satellite galaxy of SagDEG or one of its star clusters torn from the main galaxy 4–7 billion years ago.

CH star

CH stars are particular type of carbon stars which are characterized by the presence of exceedingly strong absorption bands due to CH (methylidyne) in their spectra. They belong to the stellar population II, meaning they are metal poor and generally pretty middle-aged stars, and are under-luminous compared to the classical C–N carbon stars. The term 'CH star' was coined by Philip C. Keenan in 1942 as a sub-type of the C classification, which he used for carbon stars. The main molecular feature used in identifying the initial set of five CH stars lies in the Fraunhaufer G band.In 1975, Yasuho Yamashita noted that some higher temperature carbon stars displayed the typical spectral characteristics of a CH star, but did not have the same kinematic properties. That is, they did not have the higher space velocities characteristic of the older stellar population. These were dubbed CH-like stars. Many CH stars are known to be members of binary star systems, and it is reasonable to believe this is (or was) the case for all CH stars. Like Barium stars, they are probably the result of a mass transfer from a former classical carbon star companion, now a degenerate white dwarf, to the current CH-classed star.

Canes Venatici II (dwarf galaxy)

Canes Venatici II or CVn II is a dwarf spheroidal galaxy situated in the Canes Venatici constellation and discovered in 2006 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 150 kpc from the Sun and moves towards the Sun with the velocity of about 130 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an elliptical (ratio of axes ~ 2:1) shape with the half-light radius of about 74+14−10 pc.CVn II is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 8,000 times that of the Sun (absolute visible magnitude of about −4.9), which is much lower than the luminosity of a typical globular cluster. However, its mass is about 2.5 million solar masses, which means that galaxy's mass to light ratio is around 340. A high mass to light ratio implies that CVn II is dominated by the dark matter.The stellar population of CVn II consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at [Fe/H] ≈ −2.19±0.58, which means that they contain 150 times less heavy elements than the Sun. The stars of CVn II were probably among the first stars to form in the Universe. Currently there is no star formation in CVn II. The measurement have so far failed to detect neutral hydrogen in it—the upper limit is 14000 solar masses.

Dwarf spheroidal galaxy

A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are companions to the Andromeda Galaxy (M31). While similar to dwarf elliptical galaxies in appearance and properties such as little to no gas or dust or recent star formation, they are approximately spheroidal in shape and generally have lower luminosity.

Galactic disc

A galactic disc is a component of disc galaxies, such as spiral galaxies and lenticular galaxies. Galactic discs consist of a stellar component ( composed of most of the galaxy's stars) and a gaseous component (mostly composed of cool gas and dust). The stellar population of galactic discs tend to exhibit very little random motion with most of its stars undergoing nearly circular orbits about the galactic center. Discs can be fairly thin because the disc material's motion lies predominantly on the plane of the disc (very little vertical motion). The Milky Way's disc, for example is approximately 1 kpc thick but thickness can vary for discs in other galaxies.

IC 1613

IC 1613 (also known as Caldwell 51) is an irregular dwarf galaxy, visible in the constellation Cetus near the star 26 Ceti. It was discovered in 1906 by Max Wolf, and is approaching Earth at 234 km/s.

IC 1613 is a member of the Local Group. It has played an important role in the calibration of the Cepheid variable period luminosity relation for estimating distances. Other than the Magellanic Clouds, it is the only Local Group dwarf irregular galaxy where RR Lyrae-type variables have been observed; this factor, along with an unusually low abundance of interstellar dust both within IC 1613 and along the line of sight enable especially accurate distance estimates.,In 1999, Cole et al. used the Hubble Space Telescope to find that the dominant population of this galaxy has an age of ~7 Gyr. Using its Hess diagram, they found that its evolutionary history may be similar to that of the Pegasus Dwarf Irregular Galaxy. Both galaxies are classified as Ir V in the DDO system. Also in 1999, Antonello et al. found five cepheids of Population II in IC 1613, giving self-evident support for the existence of a very old stellar population component of IC 1613. In 1999, King, Modjaz, & Li discovered the first nova ever detected in IC 1613.IC 1613 contains a WO star known as DR1, the only one so far detected further away than the Magellanic Clouds. The galaxy also contains a Luminous Blue Variable candidate, and a rich population of OB-type stars and OB associations.

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.

Leo IV (dwarf galaxy)

Leo IV is a dwarf spheroidal galaxy situated in the Leo constellation, discovered in 2006 in the data obtained by the Sloan Digital Sky Survey. The galaxy is located at the distance of about 160 kpc from the Sun and moves away from the Sun with the velocity of about 130 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 130 pc.Leo IV is one of the smallest and faintest satellites of the Milky Way; its integrated luminosity is about 15000 times that of the Sun (absolute visible magnitude of −5.5±0.3), which is much lower than the luminosity of a typical globular cluster. However, its mass is about 1.5 million solar masses, which means that Leo's mass to light ratio is around 150. A high mass to light ratio implies that Leo IV is dominated by the dark matter.The stellar population of Leo IV consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at [Fe/H] ≈ −2.58 ± 0.75, which means that they contain 400 times less heavy elements than the Sun. The observed stars were primarily red giants, although a number of Horizontal branch stars including three RR Lyrae variable stars were also discovered. The stars of Leo IV were probably among the first stars to form in the Universe. Nevertheless, the detailed study of the stellar population revealed the presence of a small number of much younger stars with the age of about 2 billion years or less. This discovery points to a complicated star formation history of this galaxy. Currently there is no star formation in Leo IV. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is just 600 solar masses.In 2008, another galaxy called Leo V was discovered in the vicinity of Leo IV. The former is located 20 kpc further from the Milky Way than the latter and 3 degrees (~ 10 kpc) away from it. These two galaxies may be physically associated with each other.

Leo V (dwarf galaxy)

Leo V is a dwarf spheroidal galaxy situated in the Leo constellation and discovered in 2007 in the data obtained by the Sloan Digital Sky Survey. The galaxy is located at the distance of about 180 kpc from the Sun and moves away from the Sun with the velocity of about 173 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately spherical shape with the half-light radius of about 130 pc.Leo V is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 10,000 times that of the Sun (absolute visible magnitude of about −5.2 ± 0.4), which is much lower than the luminosity of a typical globular cluster. However, its mass is about 330 thousand solar masses, which means that Leo's V mass to light ratio is around 75. A relatively high mass to light ratio implies that Leo V is dominated by dark matter. The stellar population of Leo V consists mainly of old stars formed more than 12 billion years ago. The metallicity of these stars is also very low at [Fe/H] ≈ −2.0 ± 0.2, which means that they contain 100 times less heavy elements than the Sun.The galaxy is located only 3 degrees away from another Milky Way satellite, Leo IV. The latter is also closer to the Sun by 20 kpc. These two galaxies may be physically associated with each other. There is evidence that they are connected by a star bridge.

Messier 19

Messier 19 or M19 (also designated NGC 6273) is a globular cluster in the constellation Ophiuchus. It was discovered by Charles Messier on June 5, 1764 and added to his catalogue of comet-like objects that same year. It was resolved into individual stars by William Herschel in 1784. His son, John Herschel, described it as "a superb cluster resolvable into countless stars". The cluster is located 4.5° WSW of Theta Ophiuchi and is just visible as a fuzzy point of light using 50 mm (2.0 in) binoculars. Using a telescope with a 25.4 cm (10.0 in) aperture, the cluster shows an oval appearance with a 3′ × 4′ core and a 5′ × 7′ halo.M19 is one of the most oblate of the known globular clusters. This flattening may not accurately reflect the physical shape of the cluster because the emitted light is being strongly absorbed along the eastern edge. This is the result of extinction caused by intervening gas and dust. When viewed in the infrared, the cluster shows almost no flattening. It lies at a distance of about 28.7 kly (8.8 kpc) from the Solar System, and is quite near to the Galactic Center at only about 6.5 kly (2.0 kpc) away.This cluster contains an estimated 1,100,000 times the mass of the Sun and it is around 11.9 billion years old. The stellar population includes four Cepheids and RV Tauri variables, plus at least one RR Lyrae variable for which a period is known. Observations made during the ROSAT mission failed to reveal any low-intensity X-ray sources.

Messier 35

Messier 35 or M35, also known as NGC 2168, is an open cluster of stars in the northern constellation of Gemini. It was discovered by Philippe Loys de Chéseaux around 1745 and independently discovered by John Bevis before 1750. The cluster is scattered over an area of the sky almost the size of the full moon and is located 3,870 light-years (1,186 parsecs) from Earth. The compact open cluster NGC 2158 lies directly southwest of M35.

Leonard & Merritt (1989) computed the mass of M35 using a statistical technique based on proper motion velocities of its stars. The mass within the central 3.75 parsecs was found to be between 1600 and 3200 solar masses (95 percent confidence), consistent with the mass of a realistic stellar population within the same radius. Bouy et al. (2015) found a mass of around 1,600 M☉ within the central 27.5′ × 27.5′. There are 305 candidate members with a probability of 95% or higher, and up to 4,349 with a 50% membership probability. The cluster metallicity is given by [Fe/H] = −0.21±0.10.Of 418 probable cluster members, Leiner et al. (2015) found 64 that have variable radial velocities and thus are binary star systems. Four probable cluster members are chemically peculiar stars, while HD 41995, which lies within the cluster area, shows emission lines. Hu et al. (2005) found 13 variable stars in the cluster field, although at least three are suspect as cluster members.

Messier 52

Messier 52 or M52, also known as NGC 7654, is an open cluster of stars in the northern constellation of Cassiopeia. It was discovered by Charles Messier on September 7, 1774. M52 can be seen from Earth with binoculars. The brightness of the cluster is influenced by extinction, which is stronger in the southern half.R. J. Trumpler classified the cluster appearance as II2r, indicating a rich cluster with little central concentration and a medium range in the brightness of the stars. This was later revised to I2r, denoting a dense core. The cluster has a core radius of 2.97 ± 0.46 ly (0.91 ± 0.14 pc) and a tidal radius of 42.7 ± 7.2 ly (13.1 ± 2.2 pc). It has an estimated age of 158.5 million years and a mass of 1,200 M☉.The magnitude 8.3 supergiant star BD +60°2532 is a probable member of M52. The stellar population includes 18 candidate slowly pulsating B stars, one of which is a δ Scuti variable, and three candidate γ Dor variables. There may also be three Be stars. The core of the cluster shows a lack of interstellar matter, which may be the result of supernovae explosions early in the cluster's history.

Metallicity

In astronomy, metallicity is used to describe the abundance of elements present in an object that are heavier than hydrogen or helium. Most of the physical matter in the Universe is in the form of hydrogen and helium, so astronomers use the word "metals" as a convenient short term for "all elements except hydrogen and helium". This usage is distinct from the usual physical definition of a solid metal. For example, stars and nebulae with relatively high abundances of carbon, nitrogen, oxygen, and neon are called "metal-rich" in astrophysical terms, even though those elements are non-metals in chemistry.

The presence of heavier elements hails from stellar nucleosynthesis, the theory that the majority of elements heavier than hydrogen and helium in the Universe ("metals", hereafter) are formed in the cores of stars as they evolve. Over time, stellar winds and supernovae deposit the metals into the surrounding environment, enriching the interstellar medium and providing recycling materials for the birth of new stars. It follows that older generations of stars, which formed in the metal-poor early Universe, generally have lower metallicities than those of younger generations, which formed in a more metal-rich Universe.

Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars.

These became commonly known as Population I (metal-rich) and Population II (metal-poor) stars. A third stellar population was introduced in 1978, known as Population III stars. These extremely metal-poor stars were theorised to have been the "first-born" stars created in the Universe.

NGC 3169

NGC 3169 is a spiral galaxy about 75 million light years away in the constellation Sextans. It has the morphological classification SA(s)a pec, which indicates this is a pure, unbarred spiral galaxy with tightly-wound arms and peculiar features. There is an asymmetrical spiral arm and an extended halo around the galaxy.This is a LINER 2 galaxy that displays an extended emission of X-rays in the region of the nucleus. A hard X-ray source at the center most likely indicates an active galactic nucleus. The stellar population in the nucleus, and a ring at an angular radius of 6″, shows an age of only one billion years and is generally younger than the surrounding stellar population. This suggests that a burst of star formation took place in the nucleus roughly one billion years ago.In 1984, a Type II-L supernova was discovered in this galaxy. Designated 1984E, the spectrum of this event at maximum light showed prominent balmer lines that indicated the explosion occurred inside a dense shell of hydrogen surrounding the star. This shell was likely created by a strong stellar wind from the progenitor star. A second supernova was discovered in 2003; this time of type 1a. It was designated SN 2003 cg and reached peak magnitude 15.94.NGC 3169 is located in close physical proximity to NGC 3166, and the two have an estimated separation of around 160 kly (50 kpc). Their interaction is creating a gravitational distortion that has left the disk of NGC 3166 warped. Combined with NGC 3156, the three galaxies form a small group within the larger Leo 1 group. The three are embedded within an extended ring of neutral hydrogen that is centered on NGC 3169.

NGC 3201

NGC 3201 (also known as Caldwell 79) is a low galactic latitude globular cluster in the southern constellation of Vela. It has a very low central concentration of stars. This cluster was discovered by James Dunlop on May 28, 1826 and listed it in his 1827 catalogue. He described it as "a pretty large pretty bright round nebula, 4′ or 5′ diameter, very gradually condensed towards the centre, easily resolved into stars; the figure is rather irregular, and the stars are considerably scattered on the south".The radial velocity of this cluster is unusually high at 490 km/s, larger than any other cluster known. This corresponds to a peculiar velocity of 240 km/s. While high, this is lower than the escape velocity of the Milky Way galaxy. It is located at a distance of 16,300 light years from the Sun and has an estimated 254,000 times the mass of the Sun. This cluster is about 10.24 billion years old.The stellar population of this cluster is inhomogeneous, varying with distance from the core. The effective temperature of the stars shows an increase with greater distance, with the redder and cooler stars tending to be located closer to the core. As of 2010, is one of only two clusters (including Messier 4) that shows a definite inhomogeneous population.

Pisces II (dwarf galaxy)

Pisces II (Psc II) is a dwarf spheroidal galaxy situated in the Pisces constellation and discovered in 2010 in the data obtained by the Sloan Digital Sky Survey. The galaxy is located at the distance of about 180 kpc (kiloparsecs)

from the Sun. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an elongated shape with the half-light radius of about 60 pc and ratio of the axis of about 5:3.Pisces II is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 10,000 times that of the Sun (absolute magnitude of about −5), which corresponds to the luminosity of an average globular cluster. The stellar population of Pisces II consists mainly of moderately old stars formed 10–12 billion years ago. The metallicity of these old stars is low at −2.3 < [Fe/H] < −1.7, which means that the percentage of their mass that consists of "heavy metals" is no more than 1/80 of the corresponding percentage in the Sun.In 2016, follow-up work on Pegasus III highlighted that both it and Pisces II lie relatively close to each other (within approximately 43 kpc) and share similar radial velocities in the Galactic standard of rest frame (note: this is not the same as the LSR). This suggests that these two satellite galaxies may actually be associated with one another, although further spectroscopic measurements are required to confirm this.

Segue 2

Segue 2 is a dwarf spheroidal galaxy situated in the constellation Aries and discovered in 2009 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 35 kpc (35,000 parsecs (110,000 ly)) from the Sun and moves towards the Sun with the speed of 40 km/s. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 34 pc.The name is due to the fact that it was found by the SEGUE program, the Sloan Extension for Galactic Understanding and Exploration.

Segue 2 is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 800 times that of the Sun (absolute visible magnitude of about −2.5), which is much lower than the luminosity of the majority of globular clusters. However, the mass of the galaxy—about 550,000 solar masses—is substantial, corresponding to the mass to light ratio of about 650.The stellar population of Segue 2 consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at [Fe/H] < −2, which means that they contain at least 100 times less heavy elements than the Sun. The stars of Segue 2 were probably among the first stars to form in the Universe. Currently, there is no star formation in Segue 2.Segue 2 is located near the edge of Sagittarius Stream and at the same distance. It may once have been a satellite of Sagittarius Dwarf Elliptical Galaxy or its star cluster.In June 2013 The Astrophysical Journal reported that Segue 2 was bound together with dark matter.Circa 1,000 stars are supposed to exist within the galaxy.

Willman 1

Willman 1 is an ultra low-luminosity dwarf galaxy or a star cluster. Willman 1 was discovered in 2004. It is named after Beth Willman of Haverford College, the lead author of a study based on the Sloan Digital Sky Survey data. The object is a satellite of the Milky Way, at ~120,000 light-years away. Willman 1 has an elliptical shape with the half-light radius of about 25 pc. Its heliocentric velocity is approximately −13 km/s.As of 2007, it was declared the least massive galaxy known, opening up a new category of ultra-low-mass galaxies, lower than the then-theoretical minimum of 10 million solar masses thought to be needed to form a galaxy.As of 2016, it is the third dimmest likely galaxy known, after Segue 1 and Virgo I, and is less than one ten millionth the Milky Way's luminosity. It has an absolute magnitude of −2.7 ± 0.7. Observations indicate its mass is about 0.4 million solar masses, which means that Willman's 1 mass to light ratio is around 800. A high mass to light ratio implies that Willman 1 is dominated by dark matter. It is difficult, however, to estimate the mass of such faint objects because any mass estimate is based on an implicit assumption that an object is gravitationally bound, which may not be true if the object is in a process of disruption.The stellar population of Willman 1 consists mainly of old stars formed more than 10 billion years ago. The metallicity of these stars is also very low at [Fe/H] ≈ −2.1, which means that they contain 110 times less heavy elements than the Sun.

Formation
Evolution
Spectral
classification
Remnants
Hypothetical
Nucleosynthesis
Structure
Properties
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