Everything is relative. By that, I mean: Jupiter, when compared to Earth, is large. Yet Jupiter, when compared to the Sun, is small. By virtue, the Sun in comparison with hypergiants is basically microscopic. Our local star actually sits right in the middle, between big and small, by star classification. There are many stars in our galaxy alone that fall on one end of the spectrum or the other. Yet, none has quite captured the imagination exactly like the "nearby" star known as VY Canis Majoris (otherwise known as HD 58061 or HIP 35793) can.
Located approximately between 3,800 and 5,000 light-years from Earth in the constellation of Canis Major, VY Canis Majoris is technically classified as a red hypergiant, which means it is among the largest of stars known to exist in our galaxy. How large is it, you might ask? Well, VY Canis Major is estimated to be larger than between 1800 and 2100 Suns, with between 15 to 25 times more mass. At its peak, it may be even weighed as much as 40 solar masses (one solar mass is equivalent to one of our suns, or 1.989 × 1030 kg), but astronomers believe the star has moved beyond "main sequence" and is reaching the end of its stellar life span. Therefore, a significant amount of its mass has already been blown away by solar winds.
What is Going on on Canis Majoris?
Canis Majoris was once thought to be the largest star in the known universe, but it has since been dethroned by UY Scuti — a hypergiant located near the center of the Milky Way (in the Scutum constellation) measuring in with an astonishing radius of at least 1,420 times that of the Sun (489 trillion Earths could fit into it volume-wise). It's a bit difficult to determine the exact size of either star, as VY Canis Majoris and UY Scuti are both variable. Their apparent size and luminosity often change, meaning they are technically classed as pulsating variable stars.
There are two main categories of variable stars. Intrinsic variables are stars whose luminosity physically changes due to pulsations, eruptions, swelling, and shrinking. Extrinsic variables are stars whose luminosity appears to change due to stellar rotation or being eclipsed by another star or planet.
There are also different subtypes of variable stars. For instance, we have intrinsic Cepheus variable stars, and they have proven themselves to be extremely important in helping us determine exactly how far away certain objects are. How, you might wonder? Well, with a Cepheus variable star, the absolute luminosity of the object is directly proportional to the time in which it takes for the star to change in brightness. This means that they expand and shrink following a specific pattern.
We used to have to guestimate how far away certain objects were, but thanks to the work of astronomer Henrietta Swan Leavitt, we no longer have to make educated guesses when a Cepheid is nearby. According to NASA, in 1912, Leavitt was, "recording and cataloging variable stars, whose brightness changes from time to time. She noticed that a certain type of variable star, called a Cepheid variable, changes from bright to dim and back fairly regularly. The bright star on the front of this lithograph is an example of a Cepheid variable. Called RS Puppis, this star rhythmically brightens and dims over a six-week period. The star is about 10 times more massive than our Sun and around 200 times larger."
"Leavitt went on to discover a direct relationship between the time it took a Cepheid variable to go from bright to dim to how bright it actually was. Knowing this relationship allowed astronomers to measure the distance to any place where they could see a Cepheid. These special stars are space’s version of a milepost marker, helping astronomers determine how far away star clusters, nebulae, and even nearby galaxies are from Earth."
The reason some of these stars periodically dim can sometimes be as a result of external factors, such as when they cross behind a dense cloud of dust relative to our telescopes and tools, but it can be internal too — especially when it comes to hypergiants like UY Scuti and VY Canis Majoris. For its part, VY Canis Majoris dims in brightness every 2,200 days or so. It's believed that the cause of its "pulsations" can be attributed to actual changes within the outer layers of gas on the surface. Additionally, in 2021, Hubble was able to discern that the star has spit out large filaments of hot gas, apparently coinciding with the changes in brightness. Some were expelled hundreds of years ago, while some are much newer. Regardless, in the two hundred some years since its discovery, it now only shines a 6th as brightly as it once did.
In March, 2021, astronomers using the Hubble announced they believe the cause is that the huge clouds of gases being released are forming dust "clouds" that is gradually blocking more and more of its light.
What Will Happen When VY Canis Majoris Dies?
Believe it or not, VY Canis Majoris is less than 10 million years old. Compared to our middle-aged, 4.6 billion-year-old Sun, it's extremely young, but it's already in the midst of dying. It likely has already shed off over half its mass. Generally though, The more massive the star, the faster it burns through its gas content.
Once it finally runs out of fuel in its core, the star will significantly compress and become a core-collapse supernova (by some estimates, it might even spawn a hypernova. which can generate 100,000 times more energy than a typical supernova, not to mention its powerful expulsion of gamma-ray bursts) This is where the central region of the star becomes a neutron star or black hole, and the outer regions of gas are ejected into space — forming a supernova remnant. For a few weeks or months, the wreckage of the explosion will outshine the rest of the galaxy, and be easily visible from here on Earth during the daytime.
Earth is in no danger though, given the distance between our planet and VY Canis Majoris. Even if it were much closer, the star could go up in smoke any time in the next 100,000 years, and who even knows whether humanity will survive that long.
Fast Facts About VY Canis Majoris:
- VY Canis Majoris is one of the largest stars in the known universe
- It's so large, if it were replaced by the Sun, it would extend for hundreds of millions of miles, perhaps even extending beyond the orbits of Jupiter and Saturn.
- It is a red hypergiant and a pulsating variable star.
- It is cooler than the Sun, but about 300,000 times brighter
- Its apparent magnitude varies from 6.5 to - 9.6, and its absolute magnitude is roughly – 9.4.
- Its radius is at least 1,420 times that of our Sun
- It has a surface temperature of about 3,500 kelvin, which is relatively cool for a star.
- It loses 30 times the mass of Earth per year.