Rho Cassiopeiae

Rho Cassiopeiae (/ˌroʊ kæsiəˈpiːaɪ, -sioʊ-, -iː/; ρ Cas, ρ Cassiopeiae) is a yellow hypergiant star in the constellation Cassiopeia. It is about 3,400 light-years (1,000 pc) from Earth, yet can still be seen by the naked eye as it is 500,000 times brighter than the Sun. On average it has an absolute magnitude of −9.5, making it visually one of the most luminous stars known. Its diameter measures between 400 and 500 times that of the Sun, approximately 627,000,000 kilometers, or about twice the size of the Earth's orbit.

Rho Cassiopeiae is a single star, and is categorized as a semiregular variable. As a yellow hypergiant, it is one of the rarest types of stars. Only around a dozen are known in the Milky Way, but it is not the only one in its constellation which also contains V509 Cassiopeiae.[9]

Rho Cassiopeiae
Location of Rho Cassiopeiae

Location of Rho Cassiopeiae in the Cassiopeia constellation.
Observation data
Epoch J2000      Equinox J2000
Constellation Cassiopeia
Right ascension  23h 54m 23.0s[1]
Declination +57° 29′ 58″[1]
Apparent magnitude (V) 4.1 to 6.2[2]
Spectral type G2Iae[3] (F8pIa-K0pIa-0)[2]
U−B color index 1.15[4]
B−V color index 1.26[4]
Variable type SRd[2]
Radial velocity (Rv)−47[3] km/s
Proper motion (μ) RA: −4.54[1] mas/yr
Dec.: −3.45[1] mas/yr
Parallax (π)0.9470 ± 0.2021[5] mas
Distanceapprox. 3,400 ly
(approx. 1,100 pc)
Absolute magnitude (MV)–9.5[6]
Mass40[6] M
Radius400–500[6] R
Luminosity~500,000[6] L
Surface gravity (log g)0.1[3] cgs
Temperature5,777–7,200[3][6] K
Metallicity [Fe/H]0.3[7] dex
Rotational velocity (v sin i)25[8] km/s
Age4-6[6] Myr
Other designations
7 Cassiopeiae, HR 9045, BD+56°3111, HD 224014, SAO 35879, FK5 899, HIP 117863, GC 33160
Database references


Rhocas lightcurve
Visual light curve for Rho Cas from 1933 to 2015

The Bayer designation for this star was established in 1603 as part of the Uranometria, a star catalog produced by Johann Bayer. The star catalog by John Flamsteed published in 1712, which orders the stars in each constellation by their right ascension, gave this star the Flamsteed designation 7 Cassiopeiae.

Rho Cas was first described as variable in 1901. It was classified only as "pec." with a small but definite range of variation.[10] Its nature continued to be unclear during the deep visual minimum in 1946, although it was presumed to be related to the detection of an expanding shell around the star. The spectrum developed lower excitation features described as typical of an M star rather than the previous F8 class.[11] The nature of Rho Cas was eventually clarified as a massive luminous unstable star, pulsating and losing mass, and occasionally becoming obscured by strong bouts of mass loss.[12]

Rho Cas usually has an apparent magnitude near 4.5, but in 1946 it unexpectedly dimmed to 6th magnitude and cooled by over 3,000 Kelvin, before returning to its previous brightness. A similar eruption was recorded in 1893, suggesting that it undergoes these eruptions approximately once every 50 years. This happened again in 2000–2001, when it was observed by the William Herschel Telescope.[13]

In 2013, a shell ejection produced dramatic spectral changes and a drop of about half a magnitude at visual wavelengths.[14] Weak emission lines of metals and doubled H-α absorption lines were detected in late 2014, and unusual tripled absorption lines in 2017.[15] The brightness peaked at magnitude 4.3 before fading to 5th magnitude. In 2018 it brightened again to magnitude 4.2.[16]


Rho Cassiopeiae Sol VY Canis Majoris - 2019-05-14
From left to right: the Sun (small, faint dot on the far left representing 1 R – too small to be visible in this thumbnail), the Pistol Star, Rho Cassiopeiae, Betelgeuse, and VY Canis Majoris. The orbits of Jupiter ( 5.23 AU) and Neptune ( 30.01 AU) are included for comparison.

Rho Cassiopeiae is one of the most luminous yellow stars known. It is close to the Eddington luminosity limit and normally loses mass at around 10−6 M/yr, hundreds of millions of times the rate of the solar wind. Much of the time it has a temperature over 7,000 K, a radius around 400 R, and pulsates irregularly producing small changes in brightness. Approximately every 50 years it undergoes a larger outburst and blows off a substantial fraction of its atmosphere, causing the temperature to drop around 1,500 K and the brightness to drop by up to 1.5 magnitudes. In 2000–2001 the mass loss rate jumped to 5×10−2 M/yr, ejecting in total approximately 3% of a solar mass or 10,000 Earth masses.[8][13] The luminosity remains roughly constant during the outbursts at half a million L, but the radiation output shifts towards the infra red.

Surface abundances of most heavy elements on Rho Cas are enhanced relative to the Sun, but carbon and oxygen are depleted. This is expected for a massive star where hydrogen fusion takes place predominantly via the CNO cycle. In addition to the expected helium and nitrogen convected to the surface, sodium is strongly enhanced, indicating that the star had experienced a dredge-up while in a red supergiant stage. Therefore, it is expected that Rho Cas is now evolving towards hotter temperatures. It is currently core helium burning through the triple alpha process.[7]

The relatively low mass and high luminosity of a post-red supergiant star is a source of instability, pushing it close to the Eddington Limit. However, yellow hypergiants lie in a temperature range where opacity variations in zones of partial ionisation of hydrogen and helium cause pulsations, similar to the cause of Cepheid variable pulsations. In hypergiants, these pulsations are generally irregular and small, but combined with the overall instability of the outer layers of the star they can result in larger outbursts. This may all be part of an evolutionary trend towards hotter temperatures through the loss of the star's atmosphere.[7]


ρ Cassiopeiae is a member of the Chinese constellation Flying Serpent 螣蛇 (Téng Shé), in the Encampment mansion. In order, the 22 member stars are α and 4 Lacertae, π2 and π1 Cygni, stars 5 and 6, HD 206267, 13 and ε Cephei, β Lacertae, σ, ρ, τ, and AR Cassiopeiae, 9 Lacertae, 3, 7, 8, λ, ψ, κ, and ι Andromedae. Consequently, the Chinese name for ρ Cassiopeiae is 螣蛇十二 (Téng Shé shíèr, English: the Twelfth Star of Flying Serpent)[17]


  1. ^ a b c d Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. ^ a b c "GCVS Query=Rho Cas". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. Retrieved 2010-11-24.
  3. ^ a b c d Klochkova, V. G.; Panchuk, V. E.; Tavolganskaya, N. S.; Usenko, I. A. (2013). "Instability of the kinematic state in the atmosphere of the hypergiant Rho Cas outside outburst". Astronomy Reports. 58 (2): 101–111. arXiv:1312.6922v1. Bibcode:2014ARep...58..101K. doi:10.1134/S1063772913120044.
  4. ^ a b Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
  5. ^ Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051.
  6. ^ a b c d e f Gorlova, N.; Lobel, A.; Burgasser, A. J.; Rieke, G. H.; Ilyin, I.; Stauffer, J. R. (2006). "On the CO Near‐Infrared Band and the Line‐splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae". The Astrophysical Journal. 651 (2): 1130–1150. arXiv:astro-ph/0607158. Bibcode:2006ApJ...651.1130G. doi:10.1086/507590.
  7. ^ a b c Israelian, G.; Lobel, A.; De Jager, C.; Musaev, F. (1998). "The Variable Spectrum of the Yellow Hypergiant rho Cassiopeiae". ASP Conf. Ser. 154: 1601. Bibcode:1998ASPC..154.1601I.
  8. ^ a b Lobel, A.; Israelian, G.; De Jager, C.; Musaev, F.; Parker, J. Wm.; Mavrogiorgou, A. (1998). "The spectral variability of the cool hypergiant rho Cassiopeiae". Astronomy and Astrophysics. 330: 659. Bibcode:1998A&A...330..659L.
  9. ^ Israelian, G.; Lobel, A.; Schmidt, M. R. (1999). "The Yellow Hypergiants HR 8752 and ρ Cassiopeiae near the Evolutionary Border of Instability". The Astrophysical Journal. 523 (2): L145. arXiv:astro-ph/9908308. Bibcode:1999ApJ...523L.145I. doi:10.1086/312283.
  10. ^ Pickering, E. C.; Colson, H. R.; Fleming, W. P.; Wells, L. D. (1901). "Sixty-four new variable stars". Astrophysical Journal. 13: 226. Bibcode:1901ApJ....13..226P. doi:10.1086/140808.
  11. ^ Beardsley, Wallace R. (1953). "The spectrum of ρ Cassiopeiae". Astronomical Journal. 58: 34. Bibcode:1953AJ.....58...34B. doi:10.1086/106807.
  12. ^ Sheffer, Yaron; Lambert, David L. (1986). "Spectroscopic observations of yellow supergiants. I – Radial pulsations of Rho Cassiopeiae". Publications of the Astronomical Society of the Pacific. 98: 914. Bibcode:1986PASP...98..914S. doi:10.1086/131844.
  13. ^ a b Lobel, A.; Dupree, A. K.; Stefanik, R. P.; Torres, G.; Israelian, G.; Morrison, N.; De Jager, C.; Nieuwenhuijzen, H.; Ilyin, I.; Musaev, F. (2003). "High‐Resolution Spectroscopy of the Yellow Hypergiant ρ Cassiopeiae from 1993 through the Outburst of 2000–2001". The Astrophysical Journal. 583 (2): 923–954. arXiv:astro-ph/0301238. Bibcode:2003ApJ...583..923L. doi:10.1086/345503.
  14. ^ Kraus, M.; Kolka, I.; Aret, A.; Nickeler, D. H.; Maravelias, G.; Eenmäe, T.; Lobel, A.; Klochkova, V. G. (2019). "A new outburst of the yellow hypergiant star ρ Cas". Monthly Notices of the Royal Astronomical Society. 483 (3): 3792–3809. arXiv:1812.03065. Bibcode:2019MNRAS.483.3792K. doi:10.1093/mnras/sty3375.
  15. ^ Klochkova, V. G.; Panchuk, V. E.; Tavolzhanskaya, N. S. (2018). "Changes of the Optical Spectrum of the Hypergiant ρ Cas due to a Shell Ejection in 2013". Astronomy Reports. 62 (9): 623–635. arXiv:1808.00220. Bibcode:2018ARep...62..623K. doi:10.1134/S1063772918090068.
  16. ^ Loughney, D. (2018). "Rho Cassiopeiae - an Update (Abstract)". Journal of the American Association of Variable Star Observers (Jaavso). 46 (2): 192. Bibcode:2018JAVSO..46R.192L.
  17. ^ 陳輝樺 (Chen Huihua), ed. (7 July 2006). "Activities of Exhibition and Education in Astronomy" 天文教育資訊網 (in Chinese). Retrieved 10 June 2015.

External links

Cassiopeia (constellation)

Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivaled beauty. Cassiopeia was one of the 48 constellations listed by the 2nd-century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today. It is easily recognizable due to its distinctive 'W' shape, formed by five bright stars. It is opposite Ursa Major.

In northern locations above latitude 34ºN it is visible year-round and in the (sub)tropics it can be seen at its clearest from September to early November. Even in low southern latitudes below 25ºS it can be seen low in the North.

At magnitude 2.2, Alpha Cassiopeiae, or Schedar, is generally the brightest star in Cassiopeia, though is often shaded by Gamma Cassiopeiae, which has brightened to magnitude 1.6 on occasion. The constellation hosts some of the most luminous stars known, including the yellow hypergiants Rho Cassiopeiae and V509 Cassiopeiae and white hypergiant 6 Cassiopeiae. The semiregular variable PZ Cassiopeiae is one of the largest known stars. In 1572, Tycho Brahe's supernova flared brightly in Cassiopeia. Cassiopeia A is a supernova remnant and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. Fourteen star systems have been found to have exoplanets, one of which—HR 8832—is thought to host seven planets. A rich section of the Milky Way runs through Cassiopeia, containing a number of open clusters, young luminous galactic disc stars, and nebulae. IC 10 is an irregular galaxy that is the closest known starburst galaxy and the only one in the Local Group of galaxies.

HR 5171

HR 5171, also known as V766 Centauri, is a triple star system in the constellation Centaurus, around 12,000 light years from Earth. It is either a red supergiant or recent post-red supergiant yellow hypergiant, and one of the largest known stars. Its diameter is uncertain but likely to be around either 1300 or 1500 times that of the Sun. It is a contact binary, sharing a common envelope of material with a smaller yellow star, the two orbiting each other every 1,304 ± 6 days.


A hypergiant (luminosity class 0 or Ia+) is among the very rare kinds of stars that typically show tremendous luminosities and very high rates of mass loss by stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in the literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants are classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially with star formation, stability, and their expected demise as supernovae.

List of brightest stars and other record stars

This is a list of the brightest stars together with record holders of other categories with many details in compact form that can be compared. The brightest stars are completely listed until apparent magnitude of 2 including Polaris. Some record holders, like the nearest star, the largest star, the most luminous star in the Milky Way etc. are added to the list.

The main purpose for this list is the possibility to compare stars of different categories, like to compare the most luminous known star R136a1 with the brightest star of our sky Sirius, which is not possible with the existing lists in Lists of stars.

For multiple values from different sources the average value is displayed. From binary star systems the brighter (A) star is considered except for magnitude and luminosity, where it is combined. More Properties (e.g. Temperature, Age) will be added. Data is yet to be added, verified and corrected.

List of largest stars

Below is an ordered list of the largest stars currently known by radius. The unit of measurement used is the radius of the Sun (approximately 695,700 km; 432,288 mi).

The exact order of this list is very incomplete, as great uncertainties currently remain, especially when deriving various important parameters used in calculations, such as stellar luminosity and effective temperature. Often stellar radii can only be expressed as an average or within a large range of values. Values for stellar radii vary significantly in sources and throughout the literature, mostly as the boundary of the very tenuous atmosphere (opacity) greatly differs depending on the wavelength of light in which the star is observed.

Radii of several stars can be directly obtained by stellar interferometry. Other methods can use lunar occultations or from eclipsing binaries, which can be used to test other indirect methods of finding true stellar size. Only a few useful supergiant stars can be occulted by the Moon, including Antares and Aldebaran. Examples of eclipsing binaries include Epsilon Aurigae, VV Cephei, and HR 5171.

List of supernova candidates

This is a list of supernova candidates, or stars that astronomers have suggested are supernova progenitors. Type II supernova progenitors include stars with at least 10 solar masses that are in the final stages of their evolution. Prominent examples of stars in this mass range include Antares, Spica, Gamma Velorum, Mu Cephei, and members of the Quintuplet Cluster. Type Ia supernova progenitors are white dwarf stars that are close to the Chandrasekhar limit of about 1.44 solar masses and are accreting matter from a binary companion star. The list includes massive Wolf–Rayet stars, which may become Type Ib/Ic supernovae.

Luminous blue variable

Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in both their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Large Magellanic Cloud. They are extraordinarily rare with just 20 objects listed in the General Catalogue of Variable Stars as SDor, and a number of these are no longer considered to be LBVs.

Phi Cassiopeiae

Phi Cassiopeiae (φ Cas, φ Cassiopeiae) is a star in the constellation Cassiopeia. φ Cassiopeiae is a multiple star with a combined apparent magnitude of +4.95. The two brightest components are A and C, sometimes called φ1 and φ2 Cas. φ Cas A is an F0 bright supergiant of magnitude 4.95 and φ Cas C is a 7.08 magnitude B6 supergiant at 134".

Pi1 Cygni

Pi¹ Cygni (π¹ Cygni, abbreviated Pi¹ Cyg, π¹ Cyg) is a binary star in the northern constellation of Cygnus. It is visible to the naked eye, having a combined apparent visual magnitude of 4.66. The distance to this system can be roughly gauged by its annual parallax shift of 1.89 mas, which yields a separation of around 1,700 light years from the Sun, give or take a hundred light years.

The two components are designated Pi¹ Cygni A (officially named Azelfafage , the traditional name for the system) and B.

Solar mass

The solar mass (M) is a standard unit of mass in astronomy, equal to approximately 2×1030 kg. It is used to indicate the masses of other stars, as well as clusters, nebulae, and galaxies. It is equal to the mass of the Sun (denoted by the solar symbol ⊙︎). This equates to about two nonillion (two quintillion in the long scale) kilograms:

M = (1.98847±0.00007)×1030 kg

The above mass is about 332946 times the mass of Earth (M), or 1047 times the mass of Jupiter (MJ).

Because Earth follows an elliptical orbit around the Sun, the solar mass can be computed from the equation for the orbital period of a small body orbiting a central mass. Based upon the length of the year, the distance from Earth to the Sun (an astronomical unit or AU), and the gravitational constant (G), the mass of the Sun is given by:

The value of G is difficult to measure and is only known with limited accuracy in SI units (see Cavendish experiment). The value of G times the mass of an object, called the standard gravitational parameter, is known for the Sun and several planets to much higher accuracy than G alone. As a result, the solar mass is used as the standard mass in the astronomical system of units.

Supergiant star

Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,450 K to over 20,000 K.


A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a transient astronomical event that occurs during the last stellar evolutionary stages of the life of a massive star, whose dramatic and catastrophic destruction is marked by one final, titanic explosion. This causes the sudden appearance of a "new" bright star, before slowly fading from sight over several weeks or months or years.

Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by Walter Baade and Fritz Zwicky in 1931.Only three Milky Way, naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies. The most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but two more recent supernova remnants have also been found. Statistical observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, and that any galactic supernova would almost certainly be observable with modern astronomical telescopes.

Supernovae may expel much, if not all, of the material away from a star at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, and in turn, sweeping up an expanding shell of gas and dust, which is observed as a supernova remnant. Supernovae create, fuse and eject the bulk of the chemical elements produced by nucleosynthesis. Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements. Furthermore, the expanding shock waves from supernovae can trigger the formation of new stars. Supernova remnants are expected to accelerate a large fraction of galactic primary cosmic rays, but direct evidence for cosmic ray production was found only in a few of them so far. They are also potentially strong galactic sources of gravitational waves.Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion, completely disrupting the star. In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time.

Owing to the wide range of astrophysical consequences of these events, astronomers now deem supernova research, across the fields of stellar and galactic evolution, as an especially important area for investigation.

V382 Carinae

V382 Carinae, also known as x Carinae (x Car), is a yellow hypergiant in the constellation Carina. It is a G-type star with a mean apparent magnitude of +3.93, and a variable star of low amplitude.

V509 Cassiopeiae

V509 Cassiopeiae (V509 Cas or HR 8752) is one of two yellow hypergiant stars found in the constellation Cassiopeia, which also contains Rho Cassiopeiae.

HR 8752 is around 4,500 light-years from Earth. It has an apparent magnitude that has varied from below +6 in historical times to a peak of +4.6 and now around +5.3 and is classified as a semiregular variable star of type SRd. It is undergoing strong mass loss as part of its rapid evolution and has recently passed partway through the yellow evolutionary void by ejecting around a solar mass of material in 20 years.A hot main sequence companion (B1V) was described in 1978 on the basis of a colour excess in the ultraviolet.

VY Canis Majoris

VY Canis Majoris (abbreviated to VY CMa) is an extreme oxygen-rich (O-rich) red hypergiant (RHG) or red supergiant (RSG) and pulsating variable star located at 1.2 kiloparsecs (3,900 ly) away from Earth in the constellation of Canis Major. It is one of the largest known stars by radius, and is also one of the most luminous and massive red supergiants, as well as one of the most luminous stars in the Milky Way.

VY CMa is a single star with a large infrared (IR) excess, making it one of the brightest objects in the sky at wavelengths of between 5 and 20 microns (µm) and indicating a dust shell or disk heated by the star. It is about 17±8 times the mass of the Sun (M☉). It is also surrounded by a complex asymmetric circumstellar envelope (CSE) caused by mass loss from the star itself. It produces strong molecular maser emission and was one of the first radio masers discovered. VY CMa is embedded within the large molecular cloud Sh2-310, one of the largest star-forming H II regions with a diameter of 480 arcminutes (') or 681 ly (209 pc).The radius of VY CMa is about 1,420 times that of the Sun (R☉), which is close to the Hayashi limit and corresponds to a volume about 3 billion times bigger than the Sun. A hypothetical object travelling at the speed of light would take 6 hours to travel around the star's circumference, compared to 14.5 seconds for the Sun. If placed at the center of the Solar System, VY CMa's surface would extend beyond the orbit of Jupiter, although there is still considerable variation in estimates of the radius, with some making it larger than the orbit of Saturn.

Variable star

A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.

This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.

Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

Yellow hypergiant

A yellow hypergiant is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (MV) around −9, but also one of the rarest with just 15 known in the Milky Way and six of those in just a single cluster. They are sometimes referred to as cool hypergiants in comparison to O- and B-type stars, and sometimes as warm hypergiants in comparison to red supergiants.

Star clusters
Molecular clouds

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