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M Type Stars - Red Dwarf and Giant Stars

Red stars are at two different ends of the spectrum in terms of size, they can be amongst the very smallest stars and they can the largest stars in the universe. The red refers to the heat/luminosity of the star that it generates. Red stars are the second coolest of all stars with brown stars being the first.

Red Star Temperatures

Red stars are cool stars compared to the blue-hot stars such as Spica which is about ten times the temperature of Proxima Centauri. A Red Star would have a temperature of about 3,500K which is nearly half that of the Sun's temperature of 6,000 K.

Red Giants, Supergiants and Hypergiants

When a star is nearing the end of its life, it will expand in size. The expansion is caused by the outward pressure overpowering the inward containment caused by gravity. As the star grows in size, it will get colder.

Our Star, the Sun will one day become a Red Giant star before eventually collapsing and becoming a Planetary nebula before finishing as a White Dwarf or a Neutron Star. The Planetary Nebula phase won't begin for another billion or so years so enough time for us to finally and hopefully make it off this planet to another world.

The largest stars we know of in the Universe are Red Hypergiants. UY Scuti is widely recognised as the largest known star with a radius 1,708 times the size of the sun. This star is probably going to go out with a bang in the form of a supernova based on diagram at Schools Observatory.

Betelgeuse in the constellation of Orion is a red supergiant star, one that has been identified as being most likely to go supernova in the next million or so years. The explosion should be seen in the night sky.


There are no known as far as this site is aware of any exoplanet orbiting a Red Giant+ stars. If you know of any, do let me know. Detecting an exoplanet around a giant+ star is harder because the effect a planet will have a star is neglibile.

When a planet orbits a star, it can cause the star to wobble, our Earth causes the Sun to wobble. That is one way of detecting a planet by detecting the wobble the star makes. A planet orbiting a supergiant probably won't cause much of a wobble.

The transit method would detect whether a planet has passed in front of the star. An exoplanet orbiting a Red Giant+ star would be hard to spot. Planets have been found in orbit round Orange Giant Stars such as Hamal but that star is only double digit sizes bigger.

List of Red Giant Stars Examples

Red Dwarf Star

In addition to being the name of the space ship in the television series of the same name, a Red Dwarf is the name for some of the smallest stars out there in the universe. They get their name from their colour and because of their size. Red dwarf stars will have a spectral type starting with an 'M' but beware UY Scuti, the largest star also has a M or K spectral type, the key is the rest of the spectral type. Their temperatures are roughly about 2,500 - 4,000K which might seem hot to you and me but compared to other stars out there, that's pretty cool.

brown dwarf stars have also been referred to as Red Dwarf Stars because they are amongst the coolest stars out there. Brown Dwarves are normally referred to as failed stars as nuclear fusion, the process of turning hydrogen into helium has not started.

Red Dwarf stars are created in the same way that any other star is created from a dust cloud, its just that the cloud that formed it wasn't as large as other clouds that formed larger stars.

Whilst difficult to see from the Earth, they are very common in the universe and are said to make up at least three quarters of the stars in the milky way. The nearest and most well known is Proxima Centauri, which is also the nearest star to our own.


Red Dwarfs are known to have Extrasolar Planets (Exoplanets) orbiting them such as Gliese 581b and Proxima Centauri. Compared to other types of stars, it is easier to spot planets round Red Dwarf stars as they exert more of an influence on the star that a planet does to a larger star. Proxima Centauri has been known to flame its planet meaning any home of life exisitng on it is very small, negligible if anything. We have spotted planets around giant stars as well but not as prevalent.

Trappist-1 is a red dwarf star in the constellation of Aquarius. The star is most famous for having seven planets in orbit round it. Some of those planets fall within the Goldilocks zone, the area which isn't too hot and too cold for life to exist. The Goldilocks Zone around a red dwarf is closer to the star than it would be for a star like our Sun.

A problem with living on a red dwarf's orbiting planet is that the planet could be tidally locked, meaning that only one side of the planet would ever have access to the stars ligh. The planet would have to be nearer to the Sun proportionally compared to what our planet is to our star.

The nearest star to us excluding the Sun is a Red Dwarf star which was only discovered in 1915 by Robert T.A. Innes according to Google. The star although being our nearest neighbour, is not bright enough to be seen here on Earth.

Lifespan of a Red Dwarf

Red Dwarf stars are extremely energy efficient compared to the giant stars which burn through their fuels quickly. A giant star could expect to live billions of years, a dwarf star can expect to live trillons of years. The Sun has an expected five billion years left whereas Proxima is trillions. When the Sun goes down on Earth, our species will have hopefully moved off and colonised planets elsewhere, maybe round a red dwarf star. Dwarf stars are cooler stars so they convert their fuels slower. Red Dwarf stars are convective meaning that heat from the core is carried to the surface, cools and then returns to the core. Swinbourne,

When a Red Dwarf has exhausted its fuel, it will hypothetically turn into a blue dwarf star before cooling again and becoming White Dwarf Star

Red stars are at two different ends of the spectrum in terms of size, they can be amongst the very smallest stars and they can the largest stars in the universe. The red refers to the heat/luminosity of the star that it generates. Red stars are the second coolest of all stars with brown stars being the first.

List of Red Dwarf Stars Examples

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