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How Big Are Stars Compared to the Sun? From Tiny Red Dwarfs to Betelgeuse

Published · Updated · Space Simulator

Look up on a clear night and every star is the same thing: a point of light. But those points hide an enormous range of sizes. Some stars are smaller than Earth. Others are so large that, if you put one where the Sun is, its surface would swallow the orbits of Mercury, Venus, Earth and Mars.

In this guide we walk up the size ladder, from the smallest stars to some of the largest ever measured, using published measurements. If you would rather see it than read it, open our star size comparison, which draws each of these objects to scale, side by side.

Start with the Sun: our measuring stick

Astronomers usually give star sizes in solar radii, so it helps to know the Sun first.

NASA calls the Sun a medium-sized star, and it notes that many stars are much larger. That is true, but plenty of stars are also much smaller. Let's start at the bottom.

The small end: stars smaller than you'd expect

Sirius B: a star the size of a planet

Sirius, the brightest star in our night sky, has a faint companion called Sirius B. It is a white dwarf, the leftover core of a star that has used up its fuel. According to NASA and ESA/Hubble, Sirius B is about 12,000 km across, slightly smaller than Earth, yet it holds about 98% of the Sun's mass. Its surface gravity is about 350,000 times Earth's.

So "star" does not always mean "big". Sirius B is a stellar mass squeezed into a planet-sized ball.

Red dwarfs: barely bigger than Jupiter

Red dwarfs are the most common stars in our galaxy, and they are small, cool and dim.

Both of these systems have known planets: you can fly to them in our 3D Milky Way and exoplanets view, or place Proxima Centauri, TRAPPIST-1, the Sun and Sirius into our interactive 3D Solar System & Star System Builder to see how star size and luminosity reshape the habitable zone and planetary orbits.

The middle: stars a bit like the Sun

Sirius A: about 1.7 Suns

Sirius A, the bright star itself, has a radius of about 1.7 solar radii (Liebert et al. 2005). It is hotter and bluer than the Sun, with a surface near 10,000 K, but in size it is still in the Sun's league. Many of the stars you can see with the naked eye are like this: bigger and brighter than the Sun, but not monsters.

The giants: when stars swell up

When a star like the Sun runs low on hydrogen in its core, its outer layers expand and cool. It becomes a red giant.

Even these giants would fit inside Mercury's orbit if you put them where the Sun is. Mercury orbits at about 0.39 AU, and 1 AU is about 215 solar radii.

Rigel: a blue supergiant

Rigel, the bright blue-white star at Orion's foot, is a blue supergiant with a radius of about 79 solar radii (Moravveji et al. 2012). At the Sun's position, its surface would reach about 0.37 AU, almost out to Mercury's orbit. It is also very hot, about 12,000 K at the surface.

The supergiants: the biggest stars we can measure

This is where the numbers get hard to picture.

Antares: past the orbit of Mars

Antares, the red heart of Scorpius, has a radius of about 680 solar radii (Ohnaka et al. 2013). Put it in the Sun's place and its surface would lie about 3.2 AU out, beyond the orbit of Mars.

Betelgeuse: two-thirds of the way to Jupiter

Betelgeuse, Orion's red shoulder, is the most famous supergiant. A 2020 study led by Meridith Joyce combined brightness data with computer models of the star's pulsations and found a radius of about 764 solar radii. The likely range runs from about 700 to 880 (Joyce et al. 2020). That is smaller than some older estimates. As Sky & Telescope put it, at the Sun's position Betelgeuse would extend about two-thirds of the way to Jupiter instead of all the way.

Some numbers to help you picture it:

VY Canis Majoris: one of the largest known stars

VY Canis Majoris is a red hypergiant. Measurements with ESO's Very Large Telescope Interferometer give a radius of about 1,420 solar radii, give or take about 120 (Wittkowski et al. 2012). At the Sun's position, its surface would reach about 6.6 AU, beyond the orbit of Jupiter (5.2 AU). Light would need nearly two hours to cross it.

Quick comparison table

ObjectTypeRadius (Sun = 1)At the Sun's position, it would reach…
Sirius BWhite dwarf~0.008slightly smaller than Earth
TRAPPIST-1Red dwarf~0.12a little bigger than Jupiter
Proxima CentauriRed dwarf~0.14about 1.4 Jupiters
SunYellow dwarf1—
Sirius AA-type star~1.7—
ArcturusRed giant~25well inside Mercury's orbit
AldebaranRed giant~45inside Mercury's orbit
RigelBlue supergiant~79almost to Mercury's orbit
AntaresRed supergiant~680beyond Mars
BetelgeuseRed supergiant~764two-thirds of the way to Jupiter
VY Canis MajorisRed hypergiant~1,420beyond Jupiter

The radii in this table match the values used in our star size comparison, so you can check every row visually.

Why do star sizes differ so much between sources?

If you search for Betelgeuse's size, you'll find numbers from about 500 to nearly 1,300 solar radii. Joyce et al. (2020) note this spread in older studies. Several things cause it:

  1. Supergiants have fuzzy edges. Their outer layers are thin, extended and full of molecules and dust. Where you put the "surface" depends on the wavelength you observe at. For VY CMa, Wittkowski et al. found the star looks up to about 50% larger in some infrared bands than in a cleaner continuum band.
  2. Distance matters. Telescopes measure a star's angular size. Turning that into kilometres needs the distance, and distances to supergiants are uncertain.
  3. Some stars pulsate. Betelgeuse swells and shrinks on a cycle of about 400 days (Joyce et al. 2020).

So treat any supergiant size as "about", with an uncertainty of 10–20% or more.

Size isn't everything: distance and light

Big stars can look faint, and small ones can look bright, because they sit at very different distances. Light from Proxima Centauri takes about 4.24 years to reach us. Joyce et al. (2020) estimate Betelgeuse at about 168 parsecs, roughly 550 light-years. So the Betelgeuse you see tonight is how it looked centuries ago.

That delay is the idea behind our Light-Year Birthday tool. Enter the month you were born to find stars whose light set out toward Earth around then and is arriving now. And to feel how small even the biggest star is next to the galaxy around it, try the Journey Through the Universe. It's one continuous zoom from Earth to the edge of the observable universe, with each step at its real scale.

FAQ

Is the Sun a big star or a small star? Somewhere in the middle. It is much larger than red dwarfs like Proxima Centauri, which are the most common type of star. But it is tiny next to giants and supergiants. NASA describes it as a medium-sized star.

What is the biggest star in the universe? Nobody knows for sure. We can only measure stars that are bright and close enough, and supergiant sizes carry large uncertainties. VY Canis Majoris, at roughly 1,420 solar radii (Wittkowski et al. 2012), is among the largest stars measured so far.

How many Earths fit inside the Sun? About 1.3 million by volume, according to NASA. About 109 Earths would fit across its diameter.

How many Suns fit inside Betelgeuse? With a radius of about 764 solar radii, Betelgeuse has a volume of roughly 446 million Suns. Because its size is uncertain, the true figure could be noticeably lower or higher.

What is the smallest kind of star? Among stars that still fuse hydrogen, the smallest are red dwarfs only a little bigger than Jupiter, like TRAPPIST-1. White dwarfs such as Sirius B are even smaller, about Earth-sized, but they are stellar remnants that no longer fuse hydrogen.

Explore it yourself: Star size comparison · Build a Solar & Star System (3D Orbit Sandbox) · Exoplanets and the Milky Way in 3D · Light-Year Birthday · Journey Through the Universe

Sources

Derived figures (light-crossing times, volumes in Suns, how far each star would reach in AU) are simple calculations from the radii above, using 1 AU ≈ 215 R☉ and c = 299,792 km/s.