VY CANIS MAJORIS: DISCOVERY, CHARACTERISTICS, STRUCTURE, FORMATION AND EVOLUTION - ARTE - 2025

VY Canis Majoris is a star in the constellation Canis Majoris or Can Mayor, where Sirius is also found. VY Canis Majoris is about 4900 light-years distant from Earth and is visible with binoculars and telescopes, showing a distinctive red color.

The first observations of VY Canis Majoris (VY CMa) date from the early 19th century. They are due to the French astronomer Joseph DeLalande in 1801, who classified it as a magnitude 7 star.

Figure 1. VY Canis Majoris in the constellation of Orion, is a star with a radius thousands of times greater than that of the Sun, it is surrounded by a nebula consisting of material that the star itself continually casts. Source: Wikimedia Commons. Judy Schmidt.

With improvements in telescope design, astronomers in the early 20th century quickly realized how unique VY CMa is, thanks to its variable brightness and shrouded in a complex nebula, full of clumps and condensations.

For this reason for some time it was thought that it was rather a star system. This idea is currently out of the question, although some astronomers argue that there is at least one companion.

Observations indicate that VY CMa is extremely luminous and of exceptional size, thousands of times larger than the Sun. To such an extent that, if it took its place, the star would extend as far as the orbit of Saturn.

VY CMa is definitely in a highly unstable stage, which precedes the end of its life, because the star is rapidly shedding its outer layers and throwing them into space, where they spread out like a nebula around it.

That is why astronomers do not rule out the possibility that VY CMa will undergo a supernova outbreak in a short time.

characteristics

Astronomers are very interested in studying a star as unique as VY CMa, because its information is decisive in the study of stellar evolution.

VY CMa is characterized by being among the stars with the greatest radius and is also one of the most luminous. It is also among the most massive of the red supergiant stars, stars that have already traveled a long way in their stellar life.

VY CMa is also fascinating because its days are expected to end suddenly, in a large supernova explosion. Let's look at some of the more interesting details:

Location

VY CMa is visible from Earth in the constellation Canis Major, close to Sirius and the constellation Orion. It is between 3,900 and 4,900 light years from Earth.

It is not easy to pinpoint the distance, firstly because the star is not close and secondly because it is continuously spewing material. Therefore it is enveloped in a nebula (see figure 1) that makes it difficult to see the atmosphere of the star and makes it difficult to make accurate estimates.

Figure 2. The constellation Can Major and VY CMa circled in red, near NGC 2362, an open cluster filled with young stars and stars in formation. Source: Wikimedia Commons. Canis_major_constellation_map.png: Torsten Bronger.derivative work: Kxx.

Variability

By 1931 it was already a fact that VY CMa was experiencing notable variations in its brightness, such that it was described as a long-period variable star.

Although it is very bright, its absolute magnitude ranges between -9.5 and -11.5. Compare Sirius, which has magnitude -1.6, and the Sun, the brightest object seen from Earth, at -26.7.

To identify variable stars, astronomers assign them a name consisting of one or a pair of capital letters, followed by the name of the constellation in which they are found.

The first variable discovered is assigned the letter R, the next one the S, and so on. When the letters are finished, a sequence begins with RR, RS and so on, so that VY CMa is number 43 among the variable stars of Can Major.

And why do VY CMa or other stars experience changes in their brightness? It may be because the star changes its luminosity, due to contractions and expansions. Another reason may be the presence of another object that temporarily eclipse it.

Radio

Some astronomers estimate the radius of VY CMa at up to 3,000 times the radius of the Sun. Other more conservative estimates point to a size of 600 solar radii, although the most recent measurements place it at 1,420 solar radii.

The fact that VY CMa is enveloped in a nebula of matter ejected by the same star is responsible for the variable radius of the star. A figure that until now is still under discussion.

For a time VY CMa was the largest known star. Today it is surpassed by UY Scuti (1708 solar radii) in the constellation of the Shield and by Westerlund 1-26 (2544 solar radii according to some, 1500 according to others) in the constellation of Ara.

Mass

Not necessarily because it is a large star, it is the most massive star of all. From the temperature and the magnitude (bolometric) it is estimated that the current mass of VY CMa is 17 ± 8 solar masses (the mass of the Sun is 1.989 × 10 ^ 30 kg).

VY CMa loses mass at the rate of 6 × 10 ^ −4 solar masses every year, not counting violent mass ejections that occur frequently. In this way the nebula that surrounds the star is formed.

Temperature and luminosity

The temperature of VY Canis Majoris is estimated at 4000 K and a luminosity between 200,000 and 560,000 times that of the Sun. The luminosity is equivalent to the power (energy per unit time) emitted by the star into space.

The luminosity of the Sun is used as a reference and unit to measure the power of astronomical objects. One (1) solar luminosity equals 3,828 × 10 ^ 26 watts.

The temperature and luminosity of VY Canis Majoris place it in the supergiant region of the HR stellar classification diagram.

Figure 3. HR diagram of the stars. Red supergiants and hypergiants like VY Canis Majoris are at the top right. Source: Wikimedia Commons.

The HR or Hertzsprung-Russell diagram is a graph of the luminosity of stars as a function of their temperature. The position of a star in this diagram indicates its evolutionary state and depends on its initial mass.

The stars that consume hydrogen to form helium in their nucleus are those that are in the main sequence (main sequence), the diagonal of the diagram. Our Sun is there, while Proxima Centauri is at the bottom right, because it is colder and smaller.

Instead Betelgeuse, Antares and VY CMa left the main sequence, because they have already run out of hydrogen. They then migrated towards the evolutionary line of the red supergiant and hypergiant stars, in the upper right of the diagram.

Over time (astronomical, of course) stars like the Sun become white dwarfs, moving down the HR diagram. And red supergiants end their days as supernovae.

Structure

Stars are basically huge spheres of gas made up of hydrogen and helium for the most part, accompanied by traces of the other known elements.

The structure of stars is more or less the same for all: a nucleus where fusion reactions take place, an intermediate layer called the mantle or envelope, and the outer layer or stellar atmosphere. Over time, the thickness and characteristics of these layers change.

There are two forces that keep the star cohesive: on the one hand, the gravitational attraction that tends to compress it, and on the other, the pressure generated from the nucleus by fusion reactions, which expands it.

Figure 4. A star is in hydrostatic equilibrium when the gravity that tends to compress it is balanced by the fusion pressure that expands it. Source: F. Zapata.

When an imbalance occurs, such as hydrogen depletion, gravity prevails and the star's core begins to collapse, generating large amounts of heat.

This heat is transmitted to the adjacent layers and give rise to new fusion reactions that temporarily restore balance to the star. But in the process, the outermost layers expand violently and the star swells, turning into a red giant.

And if the initial mass of the star was greater than 8 solar masses, then it becomes a supergiant or a hypergiant, like VY Canis Majoris.

Hypergiant stars are rare in the universe, unless we know of. There are blue, white, yellow, red… The difference in color is due to temperature, the blue ones are hotter and the red ones are colder.

As stars approach the end of their evolution, they acquire a layered structure of onion, because as it burns heavier elements, an outer layer of the less dense element burned before remains, as seen in the figure.

That is why in VY Canis Majoris chemical compounds of the most diverse nature have been detected.

Figure 5. "Onion" layered structure of a star in its final stage of evolution. Source: European Southern Observatory.

Formation and evolution

Like all stars, VY Canis Majoris must have formed thanks to gravity taking care of compacting the gas and cosmic dust into a huge cloud.

As it happens, the temperature increases until the star's nuclear reactor starts. Then the hydrostatic equilibrium arises between the forces mentioned above: the compacting gravity and the pressure from the core wants to expand the star.

At this point and always according to its mass, the star is located in the main sequence. For VY Canis Majoris it should have been to the left of the diagram, in the region of the blue giant stars, but once the hydrogen was exhausted, it passed to the evolutionary line of the hypergiants.

Such massive stars often end their days in a supernova explosion, as we have said. But they can also experience mass losses and become a blue giant, at least for a short time, ending their days as a neutron star or black hole.

Comparison with the Sun

The following image shows a comparison between the sizes of VY Canis Majoris and the Sun. Not only do they differ in size, mass and temperature, but the evolutionary lines of both are very different.

Figure 6. Comparative size between the Sun, including the Earth's orbit (in the rectangle) and VY Canis Majoris. Source: Wikimedia Commons.

The Sun will eventually break out of the main sequence and become a red giant, spreading in size beyond Earth. But there is still a long way to go, since the Sun is barely half its life as a stable star. It has been in existence for about 4.603 billion years.

It still has as many, but by its mass, the Sun will end its days as a white dwarf, while VY Canis Majoris may do so in a much more spectacular way.

References

1. American Association of variable Star Observers. VY Canis Majoris. Recovered from: aavso.org.
2. Carroll, B. An Introduction to Modern Astrophysics. 2nd. Edition. Pearson.
3. Martínez, D. The stellar evolution. Vaeliada. Recovered from: Google Books.
4. Paolantonio, S. The remarkable variable star VY Canis Majoris. Recovered from: historiadelaastronomia.files.wordpress.com.
5. Rebusco, P. Fusion in the Universe: where your jewelery comes from. Recovered from: scienceinschool.org.
6. Wikipedia. Red supergiant. Recovered from: es.wikipedia.org.
7. Wikipedia. VY Canis Majoris. Recovered from: en.wikipedia.org.