On March 7, 1801, a French astronomer by the name of Jérôme Lalande noticed a star he labeled as a 7th magnitude star (a reference to its brightness). This dated star catalogue entry was the first sighting of a star that has fascinated and puzzled astronomers ever since.
Now called the VY Canis Majoris, or VY CMa, it is often considered to be the largest star currently known. Estimates place its size at more than 1,500 times the size of the Sun, with a radius of around 1,400 times that of the Sun. To get a clearer picture of just how big that is, it means that if VY CMa were to take the Sun’s place, it would extend past Jupiter or Saturn. In addition, VY CMa is a very bright, red star, with a luminosity (a measurement of star brightness) that makes it one of the top 50 brightest stars. The light from the star also changes periodically. Thanks to a degree of predictability in these changes, astronomers have determined that the light changes from the star about every 2,200 days.
Despite its almost incomprehensible size and impressive luminosity, VY Canis Majoris presents a bit of a puzzle for astronomers. At one time, the star was thought to consist of multiple stars, due to the fact that six distinct bright spots could be sighted around the star. Today, it has been determined through more careful observation (in part by the Hubble Space Telescope) that these bright spots are only bright areas within the nebulae around VY Cma, and are not multiple stars. VY CMa’s size and brightness are due to a single, spectacular star.
However, the star’s exact size and brightness still elude astronomers, with current numbers exhibiting an estimate instead of precise measurements. For instance, the star’s radius has been estimated at anywhere between 1,400 and 1,800 times that of the sun, with the most recent estimates placing it on the smaller end of that range. The difficulty in achieving a more certain estimate of its size is due in part to the fact that its properties do not fully fall within expected limits (for temperature, density, and even radius) for a star of its kind. In addition, its rapid loss of mass and ever-changing limits make it difficult to accurately measure its radius.
Even its exact location in the sky, at about 4,900 light years from Earth, is an estimate. Due to VY CMa’s distance, the tools generally used to determine the exact position of a star are less than accurate.
What is certain, however, is that VY Canis Majoris is an enormous star that, because of its huge mass and its luminosity, can safely be categorized as a hypergiant star. One of only about 10 such stars within the billions of stars of the Milky Way galaxy, VY CMa is a rare astronomical phenomenon. Hypergiants are known not only for their size and luminosity but also for their active nature and brief life cycles. These stars achieve such intense brightness because they consume their own mass at such an astounding rate that they burn themselves out within a few million years (as opposed to the billions of years other stars take to burn).
Astronomers now believe that VY Canis Majoris has burned more than half of its mass, and continues to lose mass at an extraordinary rate. Because of its volatility and rapid consumption of mass, astronomers also believe that the star will explode into a hypernova in the very near future. A hypernova is an intense explosion of a star that results when the core of the star can no longer sustain the outer mass of the star. The explosion from a hypernova often results in a black hole due to the extraordinary forces that demolish even the tiniest elements of matter. Hypernovas are extreme forms of the supernovas, and occur only rarely, but with the force of 1,000 supernovas combined. As a result, the hypernova of the VY CMa will be a breathtaking and once-in-a-lifetime event for astronomers. While the event itself will be shatteringly large and astronomically significant, there is no cause for concern: The star is far enough away from Earth that even a hypernova will fail to affect us.