The Universe’s Largest Known Star

by thistinyplanet 0

VY Canis Majoris

 

In the video, it states that VY Canis Majoris is 2,800,000,000 km in diameter and that it would take 1100 years traveling in jet aircraft at 900km/hr to circle it once. The diameter of VY Canis Majoris originates from a study and paper submitted by Professor Roberta M. Humphreys from the University of Minnesota.

In her document Professor Humphreys concludes that VY Canis Majoris is between 1800 and 2100 solar radii. (The solar radius is approximately 695,500 kilometers (432,450 miles) or about 110 times the radius of the Earth, or 10 times the average radius of Jupiter.)

Using the 2100 solar radii as a base, the circumference of VY Canis Majors works out to be 8,796,459,430 km and it is this distance which is used in the determination of it taking 1100 years to circumnavigate the star in a jet traveling at 900 km/hr.

Other authors have creatively expressed that VY Canis Majoris is so gigantic that:

– a rocket traveling at the speed of light would take eight hours (+/- minutes either side) to travel around its circumference.

– if it were possible to  drive on the surface of Canis Majoris, at 300 mph non stop 24/7, it would take 2113½ years to drive around the entire circumference and the speedometer’s trip  would show 5 billion 553 million 107 thousand 960 miles  compared with 3½ days to do the same thing on the Earth, with its equatorial circumference of 24,901.5 miles.

– if for some reason you wanted to walk the circumference of VY Canis Majoris, assuming a walking speed of 3 mph/8 hours a day and a suit somehow capable of protecting you from temperatures reaching five thousand degrees Fahrenheit, it would take you approximately six hundred and fifty thousand years to get back to where you started. Compare that to the paltry 310 years required to walk around Sol, our sun, or the downright pedestrian and very reasonable 2 years 7 months to complete a circuit around the Earth.

– it would take over 7,000,000,000,000,000 (7 Quadrillion) Earths or 7,000,000,000 (7 Billion) Suns to fill VY Canis Majoris.

But what if VY Canis Majoris isn’t such a behemoth after all?

There is no argument that VY Canis Majoris is a monster of a star, but as with a lot of things in astronomy there is disagreement. Part of the controversy about VY Canis Majoris’ size is that the various larger estimates of the size and luminosity fall outside the bounds of current stellar theory, both beyond the maximum predicted size of any star and far cooler than a star of its luminosity can become.

One of the inherent problems apparently in determining the size of very large stars such VY Canis Majoris is of defining the “surface” (and radius) of the stars. It’s not as if there is a definite line drawn somewhere so scientists attempt to determine the boundary using a measurement called the Rosseland Radius which is based on opacity (the degree to which light is not allowed to travel through an object or space).

In 2012 a more recent examination of VY Canis Majoris using different methodology established its radius as 1420 ± 120 solar radii.

Using this revised solar radii, how does it change our travel plans? With a new diameter of 1,975,220,000km and circumference of 6,205,336,641km, it will now take us only 787 years to travel around VY Canis Majoris once in our jet traveling at 900km/hr. We have knocked 313 years off our travel schedule!.

But this new radius figure means that VY Canis Majoris isn’t the grand-daddy of stars any more. That honor now goes to NML_Cygni, part of the Cygnus constellation.

Coming in at a whopping 1650 solar radii, NML_Cygni is about 5300 light-years away. Also known as V1489 Cyg, it passes high overhead at Charlottetown, Prince Edward Island. It is best viewed from early September through early December, with the best evening viewing in late October.

NML_Cygni has a diameter or 2,295,150,000km and a circumference of 7,210,426,379km, about one billion kilometers larger than VY Canis Majoris and more than 110 solar radii larger than the next largest star. Our jet aircraft will take us about 914.5 years to travel around it. (Remember we can travel around the Earth in about 44.5 hours!)

At these dimensions it is really hard to imagine the size of these stars. While the video in this post is helpful, this image really helps to establish perspective.

VY Canis Majoris

And to add more perspective, now compare the Sun, Jupiter and our little home.

While we are pondering the immense size of these stars we must remember that these red hypergiants live relatively short lives. A typical red hyper giant phase lasts about half a million years. A massive star becomes a red hyper giant near the end of its life, as it burns up all the hydrogen fuel at its very core.

“As the core contracts under its own gravitational pull, the outer layers of the star will expand one hundred times larger than its original size, and it rapidly begins to lose mass.  It has been assessed that VY Canis Majoris has already exhausted about half of its mass, and at a tremendous rate due to its brightness too.

It will eventually explode as a supernova estimated around the year 3200, when it does, the fragments will all become the building blocks for new stars in the ever changing, ever active, all wonderful, truly majestic and mystifying Universe.

Simply though, VY Canis Majoris is a dying star, shedding off huge amounts of matter into the surrounding nebula that makes it hard for Hubble to clearly see with any great definition.”(http://www.vincelewis.net/stars.html)

So in thinking about the largest stars, perhaps the final word should be left to Professor Humphreys. She was asked in this article the question: “How big can stars get?”. Her reply:

“She noted that the largest stars are the coolest. So even though Eta Carinae is the most luminous star we know of, it’s extremely hot – 25,000 Kelvin – and so only a mere 400 solar radii.

The largest stars will be the cool supergiants. For example, VY Canis Majoris is only 3,500 Kelvin. A really big star would be even cooler. At 3,000 Kelvin, a cool supergiant would be 2,600 times the size of the Sun.

That, she believes, is the largest possible star.”

I have no doubt that sometime NML_Cygni will be dethroned as the largest star, but for me, there is something majestic about VY Canis Majoris, the first of the hypergiants to reign.

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