VY Canis Majoris (VY CMa) stands out as one of the most extreme stars in the Milky Way. VY CMa is found in the constellation Canis Major and lies some 3,900 to 4,900 light-years away from the Solar System (1.2 to 1.5 kiloparsecs). It is a massive and oxygen-rich star, but whether it is a red hypergiant or red supergiant is still open to debate. Regardless, VY CMa has one of the largest radii of any known star, is one of the most massive, and is one of the most luminous stars in the galaxy.
Can I See VY CMa?
VY CMa is a pulsating variable star. That is actually evident in the lettering “VY” prefixing the star’s name. VY CMa’s magnitude fluctuates, but it is usually over magnitude 7. This tells us that despite being so monstrous in size and mass, and despite the star’s brilliant luminosity, VY CMa cannot be seen with the unaided eye. A large nebulous cloud engulfing the star absorbs a good deal of its light and obscures it from the naked-eye view. However, it’s quite an easy target using a small telescope.
Starting at Orion’s belt, trace a line eastward until you reach the brightest star in the night sky, Sirius, which also lies in the same constellation Canis Major. There is a small triangle of stars lying about an outstretched fist below Sirius. From these stars, you can easily find your way to VY CMa using a detailed star map.
Viewing VY CMa through your telescope reveals more than just a star. Tune your telescope to a medium or high magnification on a steady night, and you will see a small red nebula enveloping the star. What you are seeing here is stellar material expelled by VY CMa as it evolved to a red supergiant or red hypergiant star.
VY Canis Majoris History
The earliest recorded sighting of VY Canis Majoris was in March 1801. The French astronomer Jérôme Lalande noted VY CMa in his star catalog. Lalande’s recording shows that VY CMa was brighter at the time of his observations, so it is evident that VY CMa has steadily faded since the mid-19th century.
It was around the same time that the star was noted as crimson in color. Astronomers of the era initially believed that VY CMa may have been a multiple star, due to what appeared to be different areas of brightness. Further observations showed that these bright areas are variations in the star’s surrounding nebula. These findings were confirmed with a visual study in the late 1950s, and high-resolution recordings in 1998. Both studies showed that VY CMa does not have any companion stars.
The constellation Canis Major is full of variable stars. VY CMa’s own variable brightness was first noted in Germany in 1931. The star was designated ‘VY Canis Majoris’ 8 years later.
VY Canis Majoris: Physical Properties
VY CMa: One of The Largest Stars
Scientists have known that VY Canis Majoris is an extraordinary star by nature since at least the mid-20th century; acknowledging it as one of the largest known stars. However, VY CMa’s true nature remained uncertain. Astronomers still considered VY CMa a post-main sequence red giant until the late 20th century. Still, VY CMa has always been of particular interest to professional and amateur astronomers alike due to its many extremes.
Imagine moving at the speed of light to as you traveled around the Sun’s circumference. It would take you only 14.5 seconds to complete your journey moving at this speed. VY CMa’s diameter is so large that it would take you 6 hours to complete the same journey at the speed.
You can also get an idea of its large size by imagining that VY CMa was at the center of the Solar System instead of the Sun’s place. VY CMa’s surface would certainly reach past Jupiter’s orbit, and possibly even extend beyond the orbit of Saturn.
The first size estimates of real significance showed that the star was far larger than even most red supergiants, and possibly had a radius over 1,000 R☉. A study in 2006 suggested that the star’s radius may be only 600 R☉.
The same study also placed VY CMa’s luminosity at 60,000 the Sun’s Luminosity (60,000 L☉) and showed an initial mass estimate of 15 M☉. Other conclusions from the 2006 study included a temperature of 3,650 K and a distance of 1.5 kpc or around 4,892 light-years.
Very soon after these findings, another paper was published and determined that VY CMa’s actual size was closer to 1,800–2,100 R☉. The conclusion drawn in this paper was that VY CMa is indeed a true hypergiant. The results worked on effective temperatures of 3,450–3,535 K, and luminosity of 430,000 L☉ (but still based on a distance of 1.5 kpc).
More accurate calculations in recent times determined that VY CMa’s radius could be somewhat lower, possibly under 1,500 R☉. And yet another recent estimate derived an average size of 2,000 R☉. As you can see, studying the size of an object so distant can be very tricky, but methods do keep improving and astronomers have a good drive for chasing increasing accuracy!
VY Canis Majoris Mass
One way to calculate a star’s mass is to directly measure the effects of its gravity on a companion body. But since VY CMa does not have any companion star, its mass cannot be measured directly. Astronomers have to be innovative in determining such a star’s mass. By comparing VY CMa’s temperature and luminosity to the evolutionary paths of most massive stars, scientists can estimate that VY CMa may have had an initial mass of 25 M☉ and a current mass of 15 M☉, or an initial mass of 32 M☉ and a current mass of 19 M☉ depending on different factors. These calculations also infer that VY CMa is about 8.2 million years old.
Scientists have recorded that VY CMa produces strong stellar winds. The star is losing a lot of material as it ages, and therefore quite a bit of mass. VY CMa loses approximately 6×10−4 M☉ every year. This is an unusually high rate even for a star of its kind. As a result, VY CMa is regarded as one of the most important stars for understanding high-mass loss close to the end of a massive star’s life.
VY Canis Majoris Temperature
VY CMa’s temperature is in the range of 3,450 to 3,535 K. Like all of the star’s physical characteristics, an accurate measure of temperature is difficult to determine. The Sun’s effective temperature is 5,777 K, so VY CMa is much cooler. This is expected as it no longer fuses hydrogen, and stars cool as they age past the main sequence.
Some of the first estimates of VY CMa’s luminosity were based on a distance of 1.5 kpc/ 4,892 light-years and yielded luminosities of between 200,000 and over 500,000 times the Sun’s luminosity (L☉). One study even placed VY CMa at 2.1 kpc/ 6,800 light-years and yielded a luminosity a million times that of the Sun! Most recent calculations arrive at 350,000 L☉, and that is working on distances below 1.2 kpc. Regardless, VY CMa is certainly one of the most luminous stars in the galaxy.
Despite being one of the most luminous stars in the Milky Way, most of its light is absorbed by the surrounding envelope and so it is obscured. It would be a naked-eye star if none of its light was absorbed.
VY CMa Variability
VY Canis Majoris is a long period variable star. Its apparent magnitude at minimum brightness is around 9.6. Its maximum brightness is generally magnitude no brighter than magnitude 7. VY CMa has a period of around 956 days (that is, the time it takes to go from its brightest to its dimmest and then back again). Other calculations suggest periods of 1,600 and 2,200 days.
VY Canis Majoris Distance
Calculating the distances to stars is a tricky business even when they are close by. Astronomers prefer to use parallax to determine the distances to stars. However, it is hard to get accurate measurements of VY CMa’s distance from the Solar System because it is too far away for the parallax method to be used with any accuracy. Generally, scientists estimate that is anywhere between 3,900 and 4,900 light-years from the Earth. Again, this is just a rough estimate due to VY Canis Majoris’ great distance from the Solar System
VY CMa’s Surrounding Nebula
The large star is surrounded by a red reflection nebula which is so bright that astronomers once thought it was a companion star. It fact, it was nebula’s brightness which led to its discovery in 1917 using a 7 inch telescope, and that’s quite a medium-small device by today’s standards! The shell wasn’t the only object mistaken as a star, as it contains dense patches which were also mistaken for companion stars. The shell is quite extensive and dense in comparison to similar nebula, and was formed by material ejected from the star. The nebula has an estimated temperature of 800 K.
Scientists have studied VY CMa’s nebula in detail using the Hubble Space Telescope. The extensive observations revealed intricate structures within the nebula which suggest past stellar eruptions.
VY Canis Majoris Evolution
VY Canis Majoris’ evolutionary path is completely different from the Sun’s. The Sun is only about half-way through its life cycle and is around 4.6 billion years old. VY CMa, on the other hand, is an evolved star which has already moved off the main sequence but has an age less than 10 million years old.
VY CMa has evolved so quickly because of how massive it is. Astronomers theorize that VY CMa left its main sequence over a million years ago already. Like everything else about the star, VY Canis Majoris’ future is uncertain. In all probability, VY CMa’s fate will parallel the majority of other cool supergiant stars. It will likely explode as a supernova, possibly in the next 100,000 years or so.
Observations show that the star CMa is quite unstable and has a significant mass-loss rate. This, along with other factors, suggest that VY CMa will produce a long-lasting type II supernova of moderate luminosity.
These supernovae events produce gamma-ray bursts and create a shock wave so powerful it travels at thousands of kilometers per second. The shock wave will interact with the star’s surrounding envelope for a decade or more after the supernova.
The remnants of a massive star that died in a supernova event can either be a neutron star (or a pulsar if it ejects streams of radiation as it rotates) or a black hole. VY CM will probably leave behind a black hole due to its extreme mass – a fitting end for a star this spectacular.