Red supergiant
Cool, large stars with supergiant luminosity class K or M.
Red supergiants are a type of star classified as luminosity class I with a K or M spectral type. While they are the largest stars by volume, they are neither the most massive nor the most luminous. The brightest and most famous examples are Betelgeuse and Antares A, which are also the only red supergiants visible at first magnitude.
Quick Facts
- Spectral types
- K and M
- Surface temperature
- below 4,100 K
- Typical radius
- several hundred to over a thousand times the Sun
- Typical luminosity
- highly luminous, typically tens of thousands of times the Sun
- Mass range
- between about 8 and 30 or 40 solar masses
- Age range
- no more than about 25 million years
Facts from the source article.
Classification
Stars are classified as supergiants based on spectral luminosity class, which uses diagnostic spectral lines to estimate surface gravity and thus size relative to mass. Larger stars are more luminous at a given temperature and are grouped into luminosity bands. The Yerkes or Morgan-Keenan (MK) classification system groups stars into five main luminosity groups: I supergiant, II bright giant, III giant, IV subgiant, and V dwarf. Supergiants are further divided into normal supergiants of class Ib and brightest supergiants of class Ia, with intermediate class Iab also used. Exceptionally bright stars with strong mass loss may be designated luminosity class 0, though this is rare; more often Ia-0 or Ia+ is used. These hypergiant classifications are rarely applied to red supergiants, though the term red hypergiant is used for the most extended and unstable ones like VY Canis Majoris and NML Cygni. Red supergiants are the coolest supergiants, M-type and at least some K-type, with no precise cutoff. K-type supergiants are uncommon and sometimes described as orange or yellow supergiants.
Properties
Red supergiants are cool and large, with spectral types K and M and surface temperatures below 4,100 K. They are typically several hundred to over a thousand times the Sun's radius, though size is not the primary factor in supergiant designation; a bright cool giant can be larger than a hotter supergiant. Despite being much cooler than the Sun, they are highly luminous, typically tens of thousands of times the Sun's luminosity. Red supergiants have masses between about 8 and 30 or 40 solar masses; main-sequence stars more massive than about 40 solar masses do not become red supergiants. Their low surface gravities and high luminosities cause extreme mass loss, millions of times higher than the Sun, producing observable nebulae. Most red supergiants show visual variability, usually classified as irregular or semiregular variables, with sub-classes SRC and LC for slow semi-regular and slow irregular supergiant variables.
Evolution
Red supergiants develop from main-sequence stars with masses between about 8 and 30 or 40 solar masses; higher-mass stars never cool enough to become red supergiants. Lower-mass stars become AGB stars and produce white dwarfs. Main-sequence stars in this mass range have temperatures between about 25,000 K and 32,000 K, spectral types early B or late O, and are already luminous due to CNO cycle fusion. They exhaust core hydrogen after 5–20 million years, then burn hydrogen in a shell around a helium core, expanding and cooling into supergiants with luminosity increasing by a factor of about three. The supergiants rapidly pass through the Cepheid instability strip, with the most massive spending a brief period as yellow hypergiants, then reach late K or M class as red supergiants. Helium fusion in the core begins smoothly during expansion or once a red supergiant, producing little immediate surface change. Red supergiants develop deep convection zones reaching from the surface over halfway to the core, causing strong nitrogen enrichment at the surface.
Clusters
Red supergiants are young stars, no older than 25 million years, and they only arise in sizeable star clusters, so they tend to be found near large groupings of stars. Because this phase is brief and the stars themselves are rare, each cluster usually contains only a few at any given time. The Hodge 301 cluster in the Tarantula Nebula has three. Before the 2000s, the record for the most red supergiants in one cluster was five, in NGC 7419. Most are seen alone, like Betelgeuse in the Orion OB1 association and Antares in the Scorpius–Centaurus association. Starting in 2006, several massive clusters were discovered near the base of the Crux-Scutum Arm, each holding many red supergiants: RSGC1 has at least 12, RSGC2 (Stephenson 2) at least 26, RSGC3 at least 8, and RSGC4 (Alicante 8) at least 8. In total, 80 confirmed red supergiants have been found in a small patch of sky toward these clusters, which are part of a burst of star formation that happened 10 to 20 million years ago near the inner end of the galaxy’s central bar.
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