Red Giant vs Blue Giant (How Are They Different?)

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The main difference between red giants and blue giants are that blue giants are far hotter, more luminous and fall under the bracket of a main sequence star whereas red giants are the phase that blue giants among other main sequence stars enter when they are unable to commit to nuclear fusion within the core, causing said star to bloat up.

These are only the main differences between the two but, if you want to learn more about the similarities and differences between a red giant and blue giant, continue reading.

What Is A Red Giant?

Red giant

A red giant is a star that is nearing the end of its lifespan, essentially entering the last stages of its evolution. The majority of stars in our Universe are known as main-sequence stars – or those which utilize nuclear fusion to convert hydrogen to helium.

During its ordinary life, the inward pressure of gravity is balanced by the outward pressure of nuclear fusion, which holds the mass together. But, when fusion ceases, the force of gravity exceeds the pressure of fusion, and the star begins to compress.

As this happens, it increases the temperature inside the star and ignites a burning hydrogen shell. The helium core continues to shrink while its temperature increases – and this gives energy to the hydrogen shell, which increases in luminosity as it continues to expand.

A red giant will grow 100 to 1000 times wider than the size of our sun, giving it a total width of somewhere between 100 million to 1 billion kilometers. And it’s this vast surface area that keeps the temperature relatively cool – 2,200 to 3,200 degrees Celsius.

Our sun will turn into a red giant in around five billion years. At this point, the sun will engulf the inner planets as it expands.

Some stars can be bigger than your average red giants, such as a red supergiant, and even those that are amongst the brightest stars around called hypergiants. These are formed from dying stars are that far bigger in mass than the Sun and even those that fall under the supergiant moniker.

What Is A Blue Giant?

Blue stars

A blue giant star is a stellar body that is in the process of using its last remaining supplies of hydrogen. Thanks to its vast mass, a blue giant generates significantly more energy than other stars – R136a1, for example, generates more energy over four seconds than our Sun does in an entire year.

In essence a blue giant star is the phase just before bloating up where they start burning their remaining hydrogen at a rate so fast (whilst staying within the stars Eddington limit), it goes through more energy per second than other smaller yellow dwarf stars like our Sun does in a year.

These giant stars are categorized into 2 different types, a Type O or a Type B star.

A Type O star is a bluish-white star that possesses a surface temperature of around 30,000 to 50,000 degrees Celsius. This type of star has a strong absorption line composed of ionized helium.

A Type B star is blue and highly luminous, with surface temperatures between 10,000 and 30,000 degrees Celsius. This is a main-sequence star, which is between 2 and 16 times the mass of the Sun.

Due to the excess amount of energy required as well as the specific circumstances of a blue giant needing to be on the boundary of the Eddington limit to even form, these stars are a very rare phenomenon within our universe.

Blue giants also have a shorter lifespan due to their excessive energy consumption. This typically falls within the 10 – 20 million year range.

Similarities Between Red Giants And Blue Giants

Both are made made up of hydrogen and helium, both convert hydrogen to helium via nuclear fusion and both can eventually go supernova once the balance between the inwards and outwards forces cannot be maintained.

Differences Between Red Giants And Blue Giants

As for the differences between the two, they include the following:

  • Blue giants are far hotter than red giants where they can range anywhere from 10,000 – 50,000 degrees Celsius depending on the type whereas red giants are 2,200 – 3,200 degrees Celsius.
  • Your typical red giant is much larger than a blue giant.
  • Blue giants only shine for around 10 million years whilst red giants can shine anywhere from 20 million – 2 billion years.
  • Blue giants convert hydrogen at a much faster rate than a red giant
  • Blue giants are more scarce than red giants.
  • Blue giants are much brighter than your average red giant.
  • Blue giants are main sequence stars whereas red giants are not.
  • All high mass main sequence stars will go through a supernova whilst not all red giants will experience this explosion.


Blue giants are amongst the most violent, luminous and powerful stars in the universe which is why they are able to use more energy is seconds than our Sun does in a year and are extremely rare however, the red giant phase is what most stars will go through, including smaller blue giants.

Most of these stars will eventually go through a supernova explosion leaving behind neutron stars or black holes as a result whilst smaller red giants will only leave behind white dwarf stars in the end.

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