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Red Stars are the Coolest

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Astronomical image of Betelgeuse courtesy of NASA.
Twinkle, twinkle, little star,
How I wonder what you are.
Up above the world so high,
Like a diamond in the sky.

– English lullaby adapted from the 1806 poem 'The Star' by Jane Taylor (1783 - 1824).

Not all stars sparkle like diamonds. This Entry will tell you about the kinds of stars which are as red as the blood in your veins.

'Cool' Stars

Here on Earth we talk about things which are 'red-hot' but in space red stars are the ones with the coolest surface temperatures when compared to the much-hotter blue stars. An average temperature for a red star would be 3-4,000°C. Red stars are classified 'M' on the Hertzsprung-Russell Diagram. They can be any size or mass, ranging from supergiant Betelgeuse (alpha Orionis), to the smaller types known as red dwarfs.

Red Dwarf Stars

Astronomers believe that red dwarfs are the most common stars in the Universe. In fact, 19 of the top 30 nearby stars are red dwarfs. They last much longer than the more massive, brighter stars because they burn1 their fuel much more slowly. The closest star to our own Sun, lying just 4.2 light years distant, is a red dwarf called Proxima Centauri2. This and all the other red dwarfs are too dim to be seen by the naked eye. Other red dwarfs in our galaxy include the 20 light year distant local neighbour Gliese 581 which has a triple planetary system. GJ 1061 in the southern constellation Horologium is just 12 light years away – at only 10% of our Sun's mass, this is one of the smallest stars known.

Sci-fi Stars

Local red dwarf stars are a staple in the science fiction universe, for example:

  • At just under six light years distance lies Barnard's Star. You may remember this as the 'roundabout' mentioned by Ford Prefect to Arthur Dent in Douglas Adams' essential read The Hitchhiker's Guide to the Galaxy.

  • Wolf 359 is less than eight light years away. It is well-known to Star Trek fans as the location of a 24th Century battle between the United Federation of Planets and the Borg.

Note: the above stars are real, but unfortunately you would need specialist equipment to see them.

  • The Red Dwarf is the Jupiter Mining Corporation ship containing the last human, Dave Lister, a humanoid feline called Cat, the annoying hologram Arnold Judas Rimmer, a humanoid robotic servant called Kryten, and Holly, the ship's computer in the popular sci-fi comedy Red Dwarf.

Red Giant Stars

Our own Sun is a yellow star about halfway through its life cycle. When it has finished turning all its hydrogen into helium, its outer shell will swell and as it cools so its colour will redden. Eventually this 'new' red giant star will reach critical point when it will simply burst, creating a beautiful planetary nebula. We won't be around to witness this, unfortunately. By that time the dying Sun will have swallowed Mercury and Venus, and the Earth will be toast. Don't panic though, this won't happen for another five billion years or so. The inner, white-hot core of ultra-dense material is known as a white dwarf and after all the helium gas has blown away, this is all that will remain.

Antares (alpha Scorpii) is a red supergiant of the first magnitude. It is 400 times larger and 10,000 times brighter than our own Sun. Antares is one of the four 'Royal Stars' in Mesopotamian culture, the others being Aldebaran (alpha Tauri), Regulus (alpha Leonis) and Fomalhaut (alpha Piscis Austrini). The Romans knew Antares as Cor Scorpionis (the heart of the scorpion), and in some cultures this red star represents the Archangel Oriel.

Kappa Crucis is a red supergiant nestling in NGC 4755, an open cluster containing over 100 stars more than 6,400 light years distant. NGC 4755 contains mostly blue stars so kappa is quite outstanding amongst them. For this reason NGC 4755 is also known as the Kappa Crucis Cluster. John Herschel, (son of William), once described the cluster as 'a casket of variously coloured precious stones' so NGC 4755 is colloquially known as 'Herschel's Jewel Box' in his honour. Sir Patrick Moore also took a fancy to this gorgeous cluster, featuring it as number 94 in his Caldwell Catalogue.

Mu Cephei, also known as Herschel's Garnet Star, is well-known to amateur astronomers in the Northern Hemisphere. Its red colour is so strong that it looks completely different to all the other stars around it. As well as being one of the largest stars in our galaxy, it is one of the most luminous known and is the prototype for the class of Mu Cephei variables.

Omicron Ceti has the common name Mira, meaning 'the amazing one', which certainly lives up to its name. It is a red giant star which varies between +2 and +10 in magnitude. As Mira travels through interstellar space at 130km/s it leaves behind a trail of stellar material almost 13 light years3 long. Mira has a white dwarf binary companion, VZ Ceti, at 65 AU (astronomical units) distance, which is more than twice the distance between our Sun and its most distant planet Neptune. Mira's partner used to be a red star, but that's another story.

Carbon Stars

Red giants with an abundance of carbon in their make-up are called carbon stars. A carbon star has such a cool surface that simple carbon molecules can exist in the star's outer atmosphere. These filter out the bluer colours from the star's spectrum, making these stars appear an even deeper red than normal red stars. They are classified as C-type stars.

Examples of Carbon Stars

StarDesignation or
Common Name
ConstellationComments/Images
Y Canum VenaticorumLa SuperbaCanes VenaticiLa Superba at APOD
X Trianguli AustralisX TrATriangulum AustraleSemi-regular variable4
R LeporisHind's Crimson StarLepusDiscovered in 1845 by English astronomer JR Hind
CW LeonisCW LeoLeoCW Leonis at APOD
R SculptorisR SclSculptorR Sculptoris at APOD
W OrionisW OriOrionPulsating semi-regular variable
TT CygniTT CygCygnusTT Cygni at APOD

Recycled Stardust

Depending on the mass of the star, when it runs out of nuclear fuel it will either cast off its outer gaseous layer and create a nebula, or go supernova. The exploding star ejects chemical elements into space but none of it goes to waste. Some of the material helps create new stars. Other elements are utilised elsewhere and for human beings, beneficially so. The carbon in your body was manufactured in the heart of such a star.

Red Star Trivia

Here on Earth there are red stars aplenty, featuring in art, business and culture. Examples include:

  • Charles Darwin's coat of arms boasts two red stars. The flag of New Zealand features four red stars in the shape of the constellation Crux.
  • There's a red star on the label of a Heineken beer bottle, and also one on San Pellegrino, Italian mineral water. Macy's department stores of New York have a red star on their company logo.
  • A red star is one of the symbols of the Communist movement. The Order of the Red Star was awarded for bravery in the Russian military between 1930 and 1991. Almost four million medals were issued both during war and in peacetime.
  • Red Star Animal Emergency Services is part of the American Humane Association (AHA). It has been providing emergency help for animals since 1916.
  • The Red Star Flowerhorn Cichlid is a popular aquarium fish. Red Star is a long-flowering (all summer) bulbous plant suitable for a rock garden. It has earned an Award of Garden Merit from the Royal Horticultural Society.
  • In the world of football there is Red Star FC of Saint-Ouen, France; FC Red Star Zürich, Switzerland; the Chicago Red Stars, USA; Seaham Red Star FC of County Durham, England; and the Red Star Belgrade team of Serbia.
  • Ancient Egyptians decorated tomb walls and ceilings, with stars representing the souls of deceased pharaohs living on in the afterlife with their many gods. The stars were mostly painted white, but exceptions were made for Aldebaran5, Antares and Betelgeuse, which were depicted in red ochre.
Image credit: NASA
1This is a nuclear reaction rather than a chemical one.2Proxima meaning 'close'.3A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.4A star whose luminosity changes, which could be due to its being transited or eclipsed by a companion, or because of changes in its mass over short periods of time.5Although Aldebaran is a first magnitude orange giant star, it has a reddish hue. Interestingly Aldebaran has a red dwarf binary companion, alpha Tauri B, which only scientific instruments can detect.

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