Étoile hypergéante

Comparaison de taille entre le Soleil et VY Canis Majoris, une hypergéante connue comme étant la plus grande étoile observée.

Une hypergéante (de classe lumineuse 0) est une étoile de masse et de luminosité très élevés, qui possède un indice de perte de masse élevé. A cause de sa taille et de sa masse, une hypergéante est un objet particulièrement intéressant pour les scientifiques : il leur permet d'étudier les limites de formation des étoiles, tant en terme de taille que de luminosité.

Caractéristiques

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. 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.

Temps de vie d'une hypergéante

Une hypergéante vit au moins entre 3 à 6 millions d'années, mais aussi, tout dépend de leur taille.

Hypergéantes connues

Les hypergéantes sont difficiles à étudier du fait de leur rareté. Elles semblent être la limite de luminosité pour les hypergéantes les plus "froides" (celles qui sont jaunes ou rouges) : aucune d'entre elles ne dépasse la magnitude bolométrique de -9.5, qui correspond environ à 500.000 fois la luminosité du Soleil. On ne connait pas la raison de ce phénomène.

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 de Cassiopée, est environ 500.000 fois plus lumineuse que le Soleil.
• 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.

Voir aussi

• Supergéante
• Hypernova
• Limite d'Eddington