A monster version of our sun has been found, the largest known member of the family of yellow stars to which our sun belongs.

The whopper sun emits light in similar wavelengths as our sun but its diameter is over 1300 times larger. That means it would engulf all the planets between Mercury and Jupiter if placed at the centre of our solar system. The star’s size also means it is touching its smaller, companion star (see diagram, below).

(Source: ESO)

Dubbed HR 5171 A, the star is located in the constellation Centaurus around 12,000 light years from Earth. It was already known to be a hypergiant, the largest class of stars, but its exact size hadn’t been well studied.


Now a team led by Oliver Chesneau of the Côte d’Azur Observatory in Nice, France, has taken a closer look with the Very Large Telescope in Atacama, Chile. They report that it is twice as large as expected.

It still isn’t the largest star we know about – that crown goes to UY Scuti, which is around 1700 times larger than our sun. But UY Scuti is in a different stage of stellar evolution and so belongs to a different family of stars called red stars.

Peanut sun

Both red and yellow stars can be hypergiants, but yellow hypergiants were previously thought to be at most 700 times the size of the sun. The new measurement of HR 5171 A shows they can be much bigger. HR 5171 A is 50 per cent larger than the red hypergiant Betelgeuse, which is located in the constellation Orion and is the ninth brightest star in the night sky.

Another surprise for Chesneau’s team was the discovery that HR 5171 A has a little brother. Previous observations suggested the star varied in brightness. Now the team has shown that this is due to a companion star that is around one third its size.

The two stars orbit each other, forming a binary system. However, though their centres are separated by more than the distance between our sun and Saturn, HR 5171 A is so large that the two are touching, forming a continuous peanut-shaped structure. Guess this star system ain’t big enough for two.

Reference: arxiv.org/abs/1401.2628v2