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Galaxies

March 28, 2013 by  


tufailEvery galaxy in the universe has on average one hundred billion stars in it. The vast majority of these stars are small and difficult to see: cool red dwarfs and dim white dwarfs.

Others stars, like our Sun, are yellow and temperate.

But there are some stars, between 20 to 40 more massive than our Sun, that roam these galaxies like Titans.

Known as Hypergiants, these stars are 3,600 times the diameter of the Sun and are among the most luminous stars in the heavens.

They are so luminous because they are devouring themselves, consuming their fuel at fantastic rates. Hypergiants release as much energy in six seconds as our Sun does in an entire year.

Because they are so active, their lifetimes are measured only in millions of years which is a blink of an eye on cosmic timescales.

There are not many hypergiants known. In our galaxy, the Milky Way, among the hundreds of billions circling the heavens, there are only about 10 known stars like these.

Among them, a star named VY Canis Majoris, named for the constellation in which it resides.

VY Canis Majoris is fitful and restless, it’s entire lifespan only a few million years. It is a very bright red giant star, its luminosity due to enormous mass loss. This star is losing billions of tons of material every day which surrounds it like a death shroud.

The reason for this mass loss is poorly understood, but it is believed to be due instabilities in the interior and exterior layers of the star. These instabilities are usually the progenitor of a supernova.
VY Canis Majoris has already shed over half of its original mass. It is in the final throes of death and could explode at literally, any time.

If it exploded today, some 4,900 light years away, the blast wave would slam into the surrounding envelope of material with a velocity of a few thousand kilometers per second and produce what is known as a supernova.

Also known as a core collapse supernova, these explosions result when the interior of the star can no longer support it and it collapses at rates of up to 70,000 kilometers per second.

This rapid collapse heats the interior of the star, producing high energy gamma rays that can decompose iron nuclei created during the lifetime of the star into helium and free neutrons and neutrinos. As the core collapses, electrons and protons are squeezed together, producing neutrons and more neutrinos.

Since neutrinos rarely interact with other forms of matter, they escape, accelerating the core collapse and slams into the outer layers of the star, triggering the supernova explosion.

Milliseconds after the collapse began, the core has detached itself from the from the outer layers of the star. With most Type II supernovae, this crush is finally halted by the strong nuclear force, preventing the neutrons from packing together any tighter.

A neutron star is born.

But VY Canis Majoris is so large, many astronomers believe its fate may be even more spectacular. It may die in what is called a hypernova. These outbursts contain more energy than 100 supernovae, emitting enormous quantities of gamma rays when it occurs.

These gamma rays can actually pose a threat to nearby stars and planets, and depending on their distance, are strong enough to destroy any life that may reside there. Fortunately for us, if this is in fact VY Canis Majoris fate, it is so far away that Earth will not be affected.

Finally, the core from this hypergiant is so massive that its collapse crushes even the neutrons together, the collapse is so inexorable nothing can stop it, leaving behind a black hole.

The spectacle of exploding stars peppers our history – ancient peoples have recorded many of them. There is roughly one supernova in our galaxy every fifty years.

The first report of the variability of VY Canis Majoris appears in the star catalogue of Lalande, which lists VY CMa at a visual magnitude of 7 on March 7, 1801. VY CMa was a relatively well observed star throughout the 19th century, thanks in part to its brightness and redness (which makes changes in magnitude more noticeable, but difficult to estimate accurately).

Most of these observations were made with instruments of 6 inches of aperture or less. It was pretty much believed to be a single red star during this time period. Between 1900 and 1917 no known observations of this star exist. However, in the 20th century larger-aperture telescopes became more available for observing. “In 1917 Guerin was using the new 7.5-inch meridian circle at Cordoba Observatory… when he noticed ‘three nuclei’” (Robinson, IBVS 599) and so VY CMa was recognized as a multiple-companion system.

During most of this period the unique variability of the star went largely unnoticed. On September 1, 1970, L.J. Robinson released the preliminary results of a study he made using 2,000 astronomical photographic plates in the collection of the Harvard College Observatory (IBVS, Number 465). VY CMa was thrust into the spotlight and ever since has been a popular and challenging variable star.

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