VY Canis Majoris is only a magnitude 8.7 star as seen from Earth [1], which means that its about 3 magnitudes fainter than what can be seen with the naked eye. However, VY Canis Majoris is actually a red supergiant that is much, much larger than the Sun. That actually seems to understate its size.
So, to begin with, VY Canis Majoris is often described as one of the largest stars ever discovered in terms of its diameter. It's 1420 times the diameter of the Sun [2], which is equivalent to a diameter of 13.2 AU or radius of 6.6 AU. For context, if VY CMa was placed in the Solar System, it would extend beyond the orbit of Jupiter. Alternately, if the Sun was the size of a golf ball [3], VY Canis Majoris would be about the size of an Airbus A350-800 [4], or if the Sun was the size of a person, VY Canis Majoris would be roughly the size of the center of Manchester, England, and a person would be the size of a low density lipoprotein (which is also known as a cholesterol molecule) [5].
However, this is not the only way in which VY Canis Majoris is extreme. The star is also 270 thousand times brighter than the Sun [2]. For comparison, if the Sun was a single light bulb, VY Canis Majoris would be equivalent to 270 thousand lightbulbs, or, alternately, VY Canis Majoris would be equivalent in brightness to roughly the amount of light produced by the floodlights in five modern sports stadiums [6]. VY Canis Majoris is also currently about 17 times the mass of the Sun [2], which means that if the Sun was the mass of a golden retriever [7], VY Canis Majoris would be equivalent in mass to a moose [8].
Aside from these various comparisons to the Sun that show that VY Canis Majoris is large, the other important thing to keep in mind is that the star is approaching the end of its lifespan. VY Canis Majoris would have initially formed as a very hot blue star about 25 times the mass of the Sun [2] that, in my previous comparison, would have been equivalent in mass to a large cow [9], but, more importantly, would have been a powered by the fusion of hydrogen into helium in its core. Like other massive hot blue stars, VY Canis Majoris would have used up the hydrogen in its core in just a few million years, which is very fast in astronomical terms. After this, it would have expanded to become a red supergiant, the fusion of hydrogen into helium would have continued in a shell around the star's core of helium, and eventually, the fusion of helium into carbon and oxygen would have be triggered in the star's core, followed by the fusion of those elements into even heavier elements later on. At some point any day now in astronomical terms, which means sometime in the next 100000 years or so, the core of VY Canis Majoris will fill up with iron, and iron cannot be fused to produce energy, so the star will first implode and then a reverse shock will create an explosion in the form of a supernova.
So, VY Canis Majoris is approaching the end of its lifespan. One current estimate places its age at 8.2 million years [10]. For context, the Sun has an age of somewhere around 4.5 billion years, so if the Sun was a person who had just reached retirement, VY Canis Majoris would be a baby about six weeks old, although that six week old baby would still be a giant compared to the retiree and would explode in a giant ball of ionized gas and electromagnetic radiation any second now, and you would not want to clean up that baby's diaper.
Anyhow, while you and I and most other people typically picture a supernovae explosion as the point in time in which all of the gas within a star will be ejected into space and return to the interstellar medium, VY Canis Majoris is an example of a situation where gas is currently being expelled into space before the star has reached the supernova stage. Thinking a little about this, it really isn't unexpected. After all, we are talking about a star larger than Jupiter's orbit, and while VY Canis Majoris is also more massive than the Sun, it's not that much more massive, so the star's surface gravity is going to be really weak. All of this gas flies away from the star in really really large blobs. Both the Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array have been able to image these blobs [11, 12, 13, 14], which, as seen from Earth, are located at angular distances from VY Canis Majoris equivalent to tenths of an arcsecond. For comparison, a tenth of an arcsecond is equivalent to the apparent size of a Lego minifigure as seen from 82.5 km away. The amount of gas in each of these blobs is equivalent to a couple of percent of the mass of the Sun [11, 12]. These blobs of gas appear to have ejected from the star in various events within the past 250 years [12, 13], and enough observational data is available that astronomers have even been able to see these things move over time [12].
The other interesting thing about this gas flowing away from VY Canis Majoris is that astronomers have seen a lot of molecules and dust particles that have formed within the gas [15, 16, 17, 18, 19, 20, 21]. This also may not make sense at first to some people, because we expect stars to be hot and to basically destroy things like molecules specifically, but these molecules and dust grains are forming as the ejected gas cools. Smaller dying stars are known to eject lots of molecules and dust into interstellar space, but I don't think big stars like VY Canis Majoris necessarily get enough attention in terms of how much of this stuff they produce before they explode as supernovae. Additionally, dying stars are known to be a major source of interstellar dust, so VY Canis Majoris provides some great data on how much interstellar dust can be produced by one really large star before it reaches the supernova stage.
What's extra exotic about VY Canis Majoris is that some of the molecules around the star produce what are called masers [22, 23, 24, 25]. A maser is like a laser except that, instead of visible light, a maser produces microwaves or radio waves, and the microwaves are what the m in masers stands for. The way masers form is rather complex, but it involves a lot of gas where the right type of molecule is in an excited state. Although masers can appear around a few different types of objects, including around supermassive black holes in other galaxies, they are also very rare. The first masers around VY Canis Majoris were discovered way back in 1969 [22], and the star has been an exciting object for astronomers to observe to understand this phenomenon in more detail.