In its infinite scope, the universe is full of awe-inspiring wonders. We’re blessed with nebulae, cosmic clouds, planets, moons, and more stars than we can count. Some stars stand out more than others, as is the case with UY Scuti and VY Canis Majoris.
The Biggest Stars in the Known Universe
UY Scuti and VY Canis Majoris have each been viewed at one point as the largest star in the known universe. Interestingly, there was some controversy until recently over which one was actually the biggest. This confusion stems from the fact that VY Canis Majoris is quite a bit closer to Earth and that both are actually variable stars.
What Is a Variable Star?
Variable stars are so named because their brightness changes at regular or irregular intervals. Most stars have a slight variation, but some can swing upwards of twenty magnitudes. There are over 2 million variable stars that have been discovered to date.
Although scientists are still discovering the reasons behind this phenomenon, there are a few obvious reasons. Some stars pulse, constricting and swelling due to forces within. In other instances, stars appear to vary because they are part of a binary system with another, less bright star.
When it comes to cataloging variable stars, scientists and astronomers look to the Argelander Naming Scheme. Each variable star in a constellation is given a letter designation, starting with R through the letter Z. After the first nine discoveries, scientists move to a double letter convention (RR, RS, RT, etc.) and continue from there.
As you can see from the names of these two stars, they both exist on constellations with dozens of other stars of variable brightness. UY Scuti is the 38th variable in the constellation Scutum, and VY Canis Majoris is the 43rd of Canis Major.
UY Scuti vs VY Canis Majoris: A Comparison
Each of these stars has some impressive and unique qualities about them. Let’s compare the two.
UY Scuti is situated in the constellation Scutum, the shield. The star itself exists out near the center of the Milky Way, some 9,500 light-years from us out on the fringe. At such an extreme distance, there’s still some uncertainty surrounding the details of the star.
VY Canis Majoris is hidden away in Canis Major, the big dog. It is much closer to us than UY Scuti, just 3,900 light-years away. Even so, there is still much we’re forced to estimate about its properties.
To measure the size of these two behemoth stars, we use our Sun as a baseline. At 432,690 miles (696,340 kilometers) from center to surface, the Sun makes up one Solar radius.
UY Scuti is a pulsing variable star that changes its size over an approximate two-year window. The star can range anywhere from 1500 solar radii to 1900 solar radii at a given time. On average, UY Scuti’s radius alone is 735,564,500 miles (1,183,776,300 kilometers). Its diameter is nearly a billion and a half miles from side to side.
VY Canis Majoris pulses in the same way UY Scuti does. However, VY Canis Majoris ranges from 1300 to 1540 solar radii. It has an average radius of 613,850,000 miles (987,895,800 kilometers).
Based on these numbers, VY Canis Majoris can be larger than UY Scuti, but UY Scuti is the larger star on average.
In terms of mass, UY Scuti is somewhere between 7 and 10 times more massive than our Sun. VY Canis Majoris wins out here, with an estimated mass at least 17 times greater than the Sun. These numbers are impressive considering that both stars have already shed a substantial amount of mass to fast-moving stellar winds.
Both UY Scuti and VY Canis Majoris are hypergiant red stars. Hypergiants are incredibly rare stars much larger than even supergiants.
UY Scuti changes its luminosity slightly as expected of a variable star, completing one cycle of dimming and brightening every 740 days. However, it has an average brightness 340,000 times that of our Sun.
VY Canis Majoris also varies in luminosity, but over a longer period of time. It’s currently thought that VY Canis Majoris takes 2,000 days for one complete cycle. Its average brightness is 270,000 times our Sun’s at current estimates.
For as intense as these two stars are, neither one is bright enough to be seen from Earth with the naked eye. The best we can see is around a magnitude 6 star, and these don’t quite make the cut.
As UY Scuti’s brightness does change, so does its apparent magnitude from here on Earth. Ranging from 8.29 to 10.56, it takes a considerable telescope to be able to see. Although bright, this low magnitude is due to its distance and the fact it’s located in the Zone of Avoidance, a very hazy area of the Milky Way.
Located some 6,000 light-years closer to Earth, VY Canis Majoris has a considerably higher apparent magnitude between 6.5 to 9.6. That being said, these numbers are still too low to make VY Canis Majoris visible from Earth without a telescope.
What Would Happen if UY Scuti Exploded?
Star explosions in space are known as supernovae, and they can be downright impressive. Whether a star goes supernova or not depends solely on its mass, and UY Scuti doesn’t have a ton of it. What it does have left is eroding away by stellar winds.
There’s a chance that UY Scuti can lose enough mass to fade into a white dwarf, but it will likely still explode some day. Even if it does, it won’t be anything more spectacular than an “ordinary” supernova. Given its distance, we may never even see it from Earth.
With the information we have today, these two stars are listed as the largest in the known universe. They are well over a thousand times larger than our Sun, with brightnesses that are off the charts. Unfortunately, they are too far away to appreciate in a photo from here on Earth. Scientists are already discovering new stars that may just take these titles away.