The Largest Star

The Largest Star

When we look into the night sky, the stars are tiny pinpoints of light.

This, of course, is because stars are immense distances away from us.

If we could get close up, we would see that stars are huge, many bigger than our Sun.

Star: SDO


But which is the largest star?

That’s a question we’re often asked in our Wonderdome shows.

So let’s find the answer.


Star Size

For normal, main sequence stars, their size depends on the size of the gas cloud from which they were made.

Orion nebula

The Orion nebula, a ‘star factory’.

The more hydrogen gas in the nebula, the bigger the star.


Small stars

The smallest stars are red dwarfs. They glow dimly, burning up their hydrogen fuel slowly.

They are much smaller than our Sun.

Their diameters are around 10% that of the Sun, maybe only 150,000km across.


Red dwarfs are the most common type of star in our galaxy, making up 75% of the stellar population.

But because they glow so feebly, they are difficult to see even with powerful telescopes.

The nearest star to the Sun, Proxima Centauri, is a red dwarf.


Medium size stars

Moving up the scale, we have Sun-type yellow stars.

These stars are around 1,500,000 km across and have a surface temperatures of 5,000 to 6,000 C.



White stars are hotter, with a surface temperature of between 7,500 and 10,000 C

They are around twice the diameter of our Sun, up to 3,000,000 km.



Blue stars are super stars. They weigh in at 16 times the mass of the Sun.

Their surface temperature ranges from 20,000 C to 33,000 C.

Rigel, blue suerstar.


A blue star’s diameter may reach six times that of the Sun, some 9,000,000 km across.

The light output from these massive stars is prodigious, 60,000 to 100,000 times greater than our Sun.


Red Giants

Blue superstars are big. But they are not the largest stars.

When yellow and white stars die, they swell up to become red giant stars.

illustration of a Red Giant star

Red Giant star: NASA’s Goddard Space Flight Center/Chris Smith (KBRwyle)


In 4 or 5 billion years, when our Sun dies, it will grow from 1.5 million km across to 30 million km in diameter.

Bad news then for the planets!



When blue stars die, they grow into red supergiants.

Supergiant stars may be 700 times wider than the Sun, with diameters around 1 billion km.


To see this for yourself, look at the constellation Orion in the winter.

Bottom right, the blue superstar is Rigel.

It is  10 million km across and  may be 100,000 times brighter than the Sun.


Constellation Orion: Stellarium


When Rigel dies, it will become like Betelgeuse, at top left of Orion.

Betelgeuse is a famous red supergiant.

It’s one of the biggest stars we can see with the naked eye.

This supergiant is 640 times larger than the Sun, with a diameter of 960 million km.


Betelgeuse is approaching the end of it’s life.

Some time in the next 100,000 years it will explode as a supernova.

When this happens, it will blaze as bright as full Moon in our skies, though the Earth will be unharmed.

It will leave a supernova remnant – a neutron star or black hole.



Athough Betelgeuse is an enormous star, their are even bigger ones out in our galaxy.

To denote the difference in scale, these are called hypergiants.

Artist's impression showing how big a hypergiant star is in comparison to our Sun. Credit: M. Weiss/CfA


Two hypergiants have competed for the title ‘Largest star’.

Because these stars are far away, measurements of their diameters is difficult.

A large margin of error is built in to the estimates.


UY Scuti

UY Scuti lies in the constellation Scutum, the Shield.

The star is around 5,900 light years away and is invisible to the naked eye.

It is over 900 times wider than the sun, a diameter of 1,260 million km.

Around 750 million Suns could fit inside it!

UY Scuti: ESO Digitized Sky Survey 2


VY Canis Majoris

An even larger star lies in the winter constellation of the big dog, Canis Major.

VY Canis Majoris (VY CMa) is around 3,900 light years away.

It is a monster, some 1,400 times the diameter of our Sun.

That makes it just over two billion kilometres across.

If VY CMa replaced our Sun, its surface would be beyond the orbit of Jupiter.

More than 5 billion Suns could fit inside it.

File:Wide-field view of the sky around VY Canis Majoris.jpg

Starfield around VY Canis Majoris: ESO DSS2

In our ESO image, VY Canis Majoris is at the centre, giving no clue as to its enormous size.


Illustration showing how big VY Canis Majoris is compared to our Sun.

The diagram above shows the true scale.

The sun is a tiny dot at the centre.

The orbits of all the planets out to Jupiter are inside VY CMa


As I said earlier, measuringthe size of distant stars is difficult.

There will always be some argument as to which is the biggest.

But, for me, the evidence is solid.

VY Canis Majoris is the largest star we’ve found – so far.


Stephenson 2-18

However there is another contender: Stephenson 2-18

This hypergiant lies in the constellation Scutum again, around 19,000 light years away.

The star is at the centre of the image below.

File:Stephenson 2 DFK 1 seen by PanSTARRS DR1.png

Stephenson 2-18 :  Credit: Université de Strasbourg


Estimates put it at 2,150 times the size of the Sun, around 1,500,000,000 km or 930,000,000 miles.

And it shines about 440,000 times brighter!


If these numbers are correct, then Stephenson 2-18 is larger than current theory holds possible.

In truth, the measurements are uncertain and Stephenson’s place at the top of the pile is controversial.


And that odd name? Stephenson 2-18 doesn’t have a simple name.

It was named for its discoverer, Charles Bruce Stephenson.

The star is also known as RSGC2-18 and Stephenson 2 DFK 1.


It is in the same direction as a star cluster, Stephenson 2, discovered by Charles Stephenson in 1990.

It isn’t clear whether Stephenson 2-18 is part of the cluster.

And it isn’t certain that the figures hold true, that it really is the largest star.

As so often in astronomy, more evidence is needed.


Videos about Star Size

Youtube has several videos that show the sizes of stars to scale, from dwarf stars to hypergiants.

This one from Global Data includes Stephenson 2-18.


Dennis Ashton, blog author

The author: Dennis Ashton is a Fellow of the Royal Astronomical Society and a Wonderdome presenter.

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