The sun may appear to be the biggest star in the sky, but that is simply due to its proximity. On a stellar scale, it’s fairly typical — roughly half of the known stars are larger, and the other half are smaller. UY Scuti, a hypergiant with a radius about 1,700 times that of the sun, is the largest known star in the universe. It’s not the only one dwarfing Earth’s main star.
The biggest of them all
UY Scuti was first catalogued in 1860 by German astronomers at the Bonn Observatory, who called it BD -12 5055. During a second detection, astronomers discovered that it dims and brightens over a 740-day cycle, prompting them to identify it as a variable star. The star is about 9,500 light-years away from the Milky Way’s core.
UY Scuti is a hypergiant, a designation that comes after supergiant, which comes after giant. It is located in the constellation Scutum. Hypergiants are extremely bright stars that are extremely rare. Fast-moving stellar winds cause them to lose a lot of mass.
Both stellar proportions, of course, are estimates dependent on measurements taken from afar.
“The complication with stars is that they have diffuse edges,” wrote University of Sussex astronomer Jillian Scudder. “Most stars lack a solid surface where the gas stops and the vacuum starts, which would have acted as a sharp dividing line and a simple marker of the star’s end.”
Astronomers instead depend on a star’s photosphere, which is where the star becomes transparent to light and light particles, or photons, may escape.
“From the perspective of an astrophysicist, this is the star’s surface, and this is where photons will escape the star,” Scudder said.
UY Scuti’s photosphere would stretch well beyond Jupiter’s orbit if it replaced the sun in the solar system’s core. The nebula of gas ejected from the star stretches much farther out, past Pluto’s orbit and 400 times the Earth-Sun radius.
UY Scuti, on the other hand, isn’t a one-trick pony. The star’s brightness varies as its radius varies, with a margin of error of around 192 solar radii, according to Scudder. Other stars will be able to overtake UY Scuti in the fight for size as a result of these mistakes. There are as many as 30 stars with radii that suit within UY Scuti’s smallest estimated scale, so it shouldn’t be too comfortable on its throne.
The broad radius of UY Scuti does not make it the most massive star. R136a1, which has a mass of about 300 times that of the sun but just 30 solar radii, is the recipient of this award. In contrast, UY Scuti is just around 30 times the mass of the sun.
contenders for the title
So, if UY Scuti weren’t exactly 1,708 solar radii away, which star would take his place? Here are a few of the stars who might take centre stage:
- WOH G64, with solar radii ranging from 1,504 to 1,730. In the Large Magellanic Cloud, it is a red hypergiant star (a satellite galaxy to the Milky Way). Its brightness varies, much like UY Scuti. According to some figures, its radius may be as large as 3,000 solar radii. The presence of dust, which influences the brightness of the star and its associated radius, contributes to the variations.
- At 1,535 solar radii, RW Cephei. This star is a variable star and an orange hypergiant in the constellation Cepheus.
- Westerlund 1-26, with solar radii ranging from 1,530 to 2,550. If the upper estimate is right, the star’s photosphere would surround Saturn’s orbit if it were located in the solar system’s middle. The temperature of the star varies, but not its brightness.
- At 1,420 to 2,850 solar radii, KY Cygni can be found. In the constellation Cygnus, it’s a red supergiant. Astronomers believe the upper estimate is suspect due to an observational error, while the lower estimate is compatible with other stars from the same survey as well as theoretical stellar evolution models.
- VY Canis Majoris, with solar radii ranging from 1,300 to 1,540. This red hypergiant star was previously estimated to be between 1,800 and 2,200 solar radii in diameter, but its size placed it beyond the scope of stellar evolution theory. It was resized after new measurements were taken. (It is still mentioned as the largest star in some sources.)