You can’t spend much on Earth before noticing the Sun. This great big ball of atomic fire dominates the sky, and for good reason — it’s the largest body in the solar system. However, although it dwarfs anything in its close vicinity, the Sun isn’t that big for a star; actually, it’s pretty much average.
So, are there bigger stars out there? Yes, definitely; our Sun is technically a yellow dwarf, which makes it a bit below the average star size. They get much, much bigger. Here’s some of the largest, plumpest ones out there (that we’ve been able to spot).
But first, a word on stars
There are many different types of stars out there; some bigger, some smaller. Before going any further, however, you have to understand something: stars don’t have nice, tidy boundaries. They don’t have a rigid surface like a rocky planet or moon. Instead, these atomic fireballs have pretty diffuse surfaces as the super-heated mass of gas that makes them up slowly thins out into nothingness.
What astronomers use in lieu of a surface is a star’s photosphere — the level at which the star becomes transparent (i.e. where photons can escape the star). So, going forward, know that if I mention a star’s surface, I’m talking about its photosphere.
The second important thing to keep in mind is that we haven’t ever measured a star directly. Nobody went up to one with a ruler and started adding up distances. What we do have are estimations — reliable estimations, for the most part, but estimations nonetheless. Depending on a range of factors, such as distance or structures around stars or between them and Earth, these estimations can be more or less accurate, and fall within a smaller or larger area of confidence (i.e. “we know it’s between x and y miles/kilometers wide).
Bigger stars, biggest star
The largest known star in the universe so far is known as UY Scuti, a star 9,500 light-years away, close to the center of the Milky Way in the constellation Scutum (‘shield’). It’s a dust-enshrouded red supergiant (the largest class of stars out there) that’s around 1,700 times larger than our Sun in diameter. It was first spotted in 1860 by astronomers at the Bonn Observatory (Germany), who christened it BD -12 5055. Subsequent observations showed that BD -12 5055 grows brighter and dimmer over a 740-day period, so it was classified as a variable star. Variable stars regularly expand and shrink as their brightness changes.
Hypergiants are larger than supergiants, which themselves are larger than giant stars. Hypergiants are quite rare and shine brightly. They also lose more mass than smaller stars through stellar winds.
To give you an idea of just how huge UY Scuti is, if it replaced the Sun at the center of our solar system, its photosphere would extend past the orbit of Jupiter. The distance from the Sun to Jupiter is approximately 779 million km, or 484 million miles. Gas emanating from the star would form a nebula extending 400 AU (one astronomical unit, AU, is the distance between the Earth and the Sun). In effect, this would reach far beyond the orbit of Pluto (the average orbiting distance between Pluto and the Sun is 39.5 AU).
But here’s the rub. We don’t know for sure how big UY Scuti is. It also has a habit of changing in size. That variation in brightness we mentioned earlier comes hand-in-hand with variations in its radius, a variation with a margin of error of about 192 solar radii. If the lower-most value is correct, UY Scuti would be out-sized by other stars — around 30 known stars would out-size UY Scuti’s smallest estimated size.
Here’s a list of the largest contenders
WOH G64 (1,504 to 1,730 solar radii) — a red hypergiant star in the Large Magellanic Cloud in the constellation Dorado (in the southern hemisphere skies) located about 170,000 light-years away from Earth. This star’s brightness varies over time due, in part, to a torus-shaped cloud of dust that obscures its light. The torus was likely formed by the star during its death throes. WOH G64 was once more than 25 times the mass of the Sun, but as it neared exploding as a supernova, began to lose mass. Astronomers estimate that it has lost enough component material to make up between three and nine solar systems.
Mu Cephei (around 1,650 solar radii) — a red supergiant in the constellation Cepheus, 9,000 light-years from Earth. With more than 38,000 times the Sun’s luminosity, it’s also one of the brightest stars in the Milky Way.
V354 Cephei (1,520 solar radii) — a red hypergiant in the constellation Cepheus. V354 Cephei is an irregularly variable star, which means that it pulsates on an erratic schedule.
RW Cephei (1,535 solar radii) — an orange hypergiant in the constellation of Cepheus; also a variable star.
Westerlund 1-26 (1,530 to 2,550 solar radii). That’s quite a large estimate interval; if the upper estimate is correct, it would dwarf even UY Scuti, and its photosphere would reach farther than Saturn’s orbit. Westerlund 1-26 stands out as its temperature varies over time, but not its brightness.
KY Cygni (1,420 to 2,850 solar radii) — a red supergiant in the constellation Cygnus. The upper estimate is viewed with skepticism as a likely observational error, while the lower one is consistent with other stars from the same survey and with our understanding of stellar evolution.
VY Canis Majoris (1,300 to 1,540 solar radii) — a red hypergiant star that was previously estimated to be 1,800 to 2,200 solar radii, but that size put it outside the bounds of stellar evolutionary theory and were updated. Still, I have seen this star listed as the largest in some sources.
Betelgeuse (950 to 1,200 solar radii) — a red supergiant in the constellation Orion. Betelgeuse is one of the most well-known stars of its kind, as it’s the ninth-brightest star in the sky and can easily be seen with the naked eye between October through March on a clear night. It’s the closest star on this list and is expected to go supernova pretty much at any time.
Do note that stars, being balls of superhot plasma, don’t follow a linear weight-size relationship like you’d expect in, say, a cannonball, where the bigger shell is obviously heavier. UY Scuti, despite being one of the or the largest star we know, isn’t the most massive (heavy) one. That title comes to R136a1, a Wolf–Rayet star in the Tarantula Nebula some 163,000 light-years away. It has the highest mass and luminosity of any known star, and is also one of the hottest, at around 53,000 K.
Ironically enough, R136a1 weighs in at about 300 times the mass of the Sun but is only about 30 solar radii in size. UY Scuti is just 30 times more massive than the Sun despite being way larger. R136a1’s mass can be explained by its very high surface enhancement of heavy elements (which are dense) and depletion of hydrogen (which is light).