Red supergiant star facts for kids

Hertzsprung–Russell diagram

Spectral type

Brown dwarfs

White dwarfs

Red dwarfs

Subdwarfs

Main sequence
("dwarfs")

Subgiants

Giants

Bright giants

Supergiants

Hypergiants

absolute
magni-
tude
(M_V)

Red supergiants (RSGs) are stars with a supergiant luminosity class (Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

Properties

Red supergiants are cool and large. They have surface temperatures below 4,100 K. They are typically several hundred to over a thousand times the radius of the Sun, although size is not the primary factor in a star being designated as a supergiant. A bright cool giant star can easily be larger than a hotter supergiant. For example, Alpha Herculis is classified as a giant star with a radius of between 264 to 303 R_☉ while Epsilon Pegasi is a K2 supergiant of only 185 R_☉.

Although red supergiants are much cooler than the Sun, they are so much larger. Red supergiants have masses between about 10 M_☉ and 40 M_☉. Their low surface gravities and high luminosities cause extreme mass loss, millions of times higher than the Sun, producing observable nebulae surrounding the star. By the end of their lives red supergiants may have lost a substantial fraction of their initial mass. The more massive supergiants lose mass much more rapidly and all red supergiants appear to reach a similar mass of the order of 10 M_☉ by the time their cores collapse. The exact value depends on the initial chemical makeup of the star and its rotation rate.

Most red supergiants show some degree of visual variability, but only rarely with a well-defined period or amplitude. Therefore, they are usually classified as irregular or semiregular variables. They even have their own sub-classes, SRC and LC for slow semi-regular and slow irregular supergiant variables respectively. Variations are typically slow and of small amplitude, but amplitudes up to four magnitudes are known.

In the latest stages of mass loss, before a star explodes, surface helium may become enriched to levels comparable with hydrogen.

Red supergiants are observed to rotate slowly or very slowly. Those red supergiants such as Betelgeuse that do have modest rates of rotation may have acquired it after reaching the red supergiant stage. The cores of red supergiants are still rotating and the differential rotation rate can be very large.

Examples

The Orion region showing the red supergiant Betelgeuse

Red supergiants are rare stars, but they are visible at great distance and are often variable so there are a number of well-known examples:

• Antares A
• Betelgeuse
• 119 Tauri
• Epsilon Pegasi
• Psi¹ Aurigae
• Lambda Velorum
• Mu Cephei
• NML Cygni
• S Persei
• Stephenson 2-18
• UY Scuti
• VY Canis Majoris
• Westerlund 1-26
• WOH G64

A survey expected to capture virtually all Magellanic Cloud red supergiants detected around a dozen M class stars M_v−7 and brighter, around a quarter of a million times more luminous than the Sun, and from about 1,000 times the radius of the Sun upwards.

See also

In Spanish: Supergigante roja para niños

• List of supernovae