Hypergiant

A hypergiant (luminosity class 0) is a star with a tremendous mass and luminosity, showing signs of a very high rate of mass loss. Because of its size and mass, it is a very interesting object for scientists studying the upper mass and luminosity limit for stars in general.

Characteristics

The word “hypergiant” is commonly used as a loose term for the most massive stars found, even though there are more precise definitions. In 1956, the astronomers Feast and Thackeray used the term super-supergiant (later changed into hypergiant) for stars with an absolute magnitude brighter than M_V = -7. In 1971, Keenan suggested that the term would only be used for supergiants showing at least one broad emission component in Hα, indicating an extended stellar atmosphere or a relatively large mass loss rate. The Keenan criterion is the one most commonly used by scientists today. ^[1] This means that a hypergiant doesn’t necessarily have to be more massive than a similar supergiant. Still, the most massive stars are considered to be hypergiants, and can have masses ranging up to 100-150 solar masses.

Hypergiants are very luminous stars, up to millions of solar luminosities, and have temperatures varying widely between 3,500 K and 35,000 K. Almost all hypergiants exhibit variations in luminosity over time due to instabilities within their interiors.

Because of their high masses, the lifetime of a hypergiant is very short in astronomical timescales, only a few million years compared to around 10 billion years for stars like the Sun. Because of this, hypergiants are extremely rare and only a handful are known today.

Hypergiants should not be confused with luminous blue variables. A hypergiant is classified as such because of its size and mass loss rate, whereas a luminous blue variable is thought to be a massive blue supergiant going through an evolutionary phase where it loses a lot of mass.

The stability of hypergiants

As luminosity of stars increases exponentially with mass, the luminosity of hypergiants often lies very close to the Eddington limit which, simply put, is the luminosity where the gravitational pressure inwards equals the continuum radiation pressure outwards. This means that the radiative flux passing through the photosphere of a hypergiant may be very close to how much the photosphere can handle. Above the Eddington limit, the star is supposed to generate so much radiation that parts of its outer layers are thrown off in massive outbursts, effectively restricting the star from shining at higher luminosities for longer periods.

A consequence of passing the Eddington limit is thought to be the initiation of a continuum driven wind ^[2] (from processes such as electron scattering, free-free and bound-free interaction), with extremely high mass loss rates up to 10 000 times stronger than the strongest line-driven stellar winds of sub-Eddington objects. As very few stars are thought to ever pass the Eddington limit, the continuum driven stellar winds are extremely rare and are mostly results of theoretical predictions.

A good candidate for hosting a continuum driven wind is η Carinae, one of the most massive and luminous stars ever observed. However, even with a mass of around 130 solar masses and a luminosity four million times higher than the Sun, η Carinae is thought to only occasionally reach super-Eddington luminosities. The last time might have been a series of outbursts in 1840-1860, reaching mass loss rates much higher than any of the more well known stellar winds can explain. ^[3]

As opposed to line driven stellar winds, continuum driving does not require the presence of metallic atoms in the photosphere. This is important, since most massive stars also are very metal poor, which means that we need an effect that works independently of the metallicity. In the same line of reasoning, the continuum driving may also contribute to an upper mass limit even for the first generation of stars right after the Big Bang, which did not contain any metals at all.

Another theory to explain the massive outbursts of for example η Carinae is the idea of a deeply situated hydrodynamic explosion, blasting off parts of the star’s outer layers. The idea is that the star even at luminosities below the Eddington limit would have insufficient heat convection in the inner layers, resulting in a density inversion potentially leading to a massive explosion. The theory has however not been explored very much, and it is uncertain whether this really can happen. ^[4]

Known hypergiants

Hypergiants are difficult to study due to their rarity. There appears to be an upper luminosity limit for the coolest hypergiants (those colored yellow and red): none of them are brighter than around bolometric magnitude -9.5, which corresponds to roughly 500,000 times Sun's luminosity. The reasons for that are currently unknown.

Luminous blue variables

Most luminous blue variables are classified as hypergiants, and indeed they are the most luminous stars known:

P Cygni, in the northern constellation of Cygnus.
S Doradus, in a nearby galaxy called the Large Magellanic Cloud, in the southern constellation of Dorado. This galaxy was also the location of Supernova 1987A.
Eta Carinae, inside the Keyhole Nebula (NGC 3372) in the southern constellation of Carina. Eta Carinae is extremely massive, possibly as much as 120 to 150 times the mass of the Sun, and is four to five million times as luminous.
The Pistol Star, near the center of the Milky Way, in the constellation of Sagittarius. The Pistol Star is possibly as much as 150 times more massive than the Sun, and is about 1.7 million times more luminous.
Several stars in the cluster 1806-20, on the other side of the Milky Way galaxy. One such star, LBV 1806-20, is the most luminous star known, from 2 to 40 million times as luminous as the Sun, and also one of the most massive.

Blue hypergiants

Zeta-1 Scorpii, the brightest star of the OB association Scorpius OB1 and a LBV candidate.
MWC 314, in the constellation of Aquila, another LBV candidate.
HD 169454, in Scutum
BD -14° 5037 near of the latter.
Cygnus OB2-12, which some authors consider an LBV.

Yellow hypergiants

Yellow hypergiants form an extremely rare class of stars, with only seven being known in our galaxy:

Rho Cassiopeiae, in the northern constellation of Cassiopeia, is about 500,000 times as luminous as the Sun.
HR 8752
IRC+10420
Also see stars in Westerlund 1.

Red hypergiants

RW Cephei
NML Cygni
VX Sagittarii
S Persei
VY Canis Majoris has the largest diameter of any known star, 1800 to 2100 times that of the Sun.

References

^ C. de Jager (1998). "The yellow hypergiants". Astronomy and Astrophysics Review. 8: 145–180.
^ A. J. van Marle (2008). "Continuum driven winds from super-Eddington stars. A tale of two limits". AIP Conference Proceedings. 990: 250–253. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ S. P. Owocki (2004). "A porosity-length formalism for photon-tiring limited mass losss from stars above the Eddington limit". Astrophysical Journal. 616: 525–541. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ N. Smith (2006). "On the role of continuum driven eruptions in the evolution of very massive stars and population III stars". Astrophysical Journal. 645: L45–L48. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[1] C. de Jager (1998). "The yellow hypergiants". Astronomy and Astrophysics Review. 8: 145–180.

[2] A. J. van Marle (2008). "Continuum driven winds from super-Eddington stars. A tale of two limits". AIP Conference Proceedings. 990: 250–253. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[3] S. P. Owocki (2004). "A porosity-length formalism for photon-tiring limited mass losss from stars above the Eddington limit". Astrophysical Journal. 616: 525–541. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[4] N. Smith (2006). "On the role of continuum driven eruptions in the evolution of very massive stars and population III stars". Astrophysical Journal. 645: L45–L48. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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