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11/25/2015

What we have learned from VY Canis Majoris and how to Observe this Red Supergiant



Today a team of astronomers released a new paper on VY Canis Majoris. For those who don't know, VY Canis Majoris once held the title as largest star in the galaxy, but has been replaced by other stars that have been discovered and measured (Wikipedia LINK). I think one thing I am going to work on putting together is an observing program where you can go and take a look at these massive stars over the course of a year (the visible ones) and mark down which of these hypergriants you've seen. Anyway a paper, Large Dust Grains in the winds of VY Canis Majoris LINK released by P. Scicluna, R. Siebenmorgen, R Wesson, J.A.D.L. Blommaert, M. Kasper, N.V. Voshchinnikov and S. Wolf provides evidence and reason on why such massive stars like VY Canis Majoris lose so much mass in a given solar year (in the case of VY Canis Majoris it is losing 30 times the mass of the Earth each year). To quote the article in Astronomy Now:

But now, with the new SPHERE data, we have found large grains of dust around this hypergiant. These are big enough to be pushed away by the star’s intense radiation pressure, which explains the star’s rapid mass loss.”
The large grains of dust observed so close to the star mean that the cloud can effectively scatter the star’s visible light and be pushed by the radiation pressure from the star. The size of the dust grains also means much of it is likely to survive the radiation produced by VY Canis Majoris’ inevitable dramatic demise as a supernova. This dust then contributes to the surrounding interstellar medium, feeding future generations of stars and encouraging them to form planets.
Article on Astronomy Now: Ageing hypergiant star’s weight loss secret revealed LINK

Rather exciting news if you ask me! As that dust is pushed outward away from the star by its radiation, causing not only mass loss, but also perhaps signaling as the dust increases, how close the star may be to the end of its life (speculation on my part).  One idea that did come to me is that this gives further evidence to the notion that the supernova that caused the Cassiopeia A SNR was indeed a massive star that had shed sufficient mass in terms of large dust particles that when it went supernova, the light was indeed absorbed by these large dust particles that had been shed in significant mass loss prior to the end of the progenitor star's life. Cool!

It may also signal that as much as we anticipate viewing or observing a supernova explosion from a massive star, it may not be so tremendous as we may think. Sufficient absorption by the dust particles may lessen the view for us here on Earth. Then again, some of these like SN 1987A and others we observe from other galaxies are significantly bright enough to be seen visually. So the dust particles may explain mass loss, but the large dust particles may not hinder stars that may undergo mass loss, but not enough to hinder the light from the exploding supernova. More study will follow I am sure as we strive to find out more about these stars.  Guess we won't know for sure until we can really study a massive star that goes supernova. It could also explain why so many of the supernova's (not all, think Messier 1 in Taurus in 1054 CE) that have been seen from Earth have been from Type Ia explosions. A white dwarf exploding by colliding with another white dwarf, or by taking mass from its companion until it passes the Chandrasekhar limit of 1.44 solar masses do not generate the large dust particles prior to going supernova. It could be that is why they act more as a standard candle than a Type II SN.  Pure speculation on my ignorant part, but I welcome comments that clarify the points I have made.

So a few items. Here is a video based on the paper from ESO on the discovery. Take a look at it.









     What is interesting about that second video is this paper I have shared, explains more about how these massive stars like VY Canis Majoris are possibly viewed for losing so much mass via the large dust particles.


      Here are some pictures to give you a sense of how big a star VY Canis Majoris is (used from the public domain under Fair Use):









Yes, this is a very massive star yet in the night sky, well, if you want, use a telescope or binoculars and see what you can see and how large this star is in the sky. It is about 1.9 kiloparsecs or 3900 light years away from us here on Earth.  To help you star hop (and remember, there are more than one way to get there) I have made a few star charts that show how you may want to hop to this massive star. Again, I will be building a program of observing those most massive stars and sharing it here in the next couple of weeks, probably after December's new moon period so look for that.



1. Here is the first star chart to help you identify Canis Major if your not familiar with it. At the top of Canis Major is Sirius, the brightest star in our night sky of all the seasons.  Down from Sirius lays Wesson, at the dog's hind legs and tail. Go to Wessen to start our star hop.


2. Below is a series of hops you need to take to get in the general area to get serious if your using a telescope. 






3. Below is the way I use to get to VY Canis Majoris in my telescopes. To be honest, don't expect to see much more than a star but look to see if you can see color or anything around the star. If nothing else you can say you have seen one of the largest stars in the Milky Way.  This is a star that will end its life in a Supernova explosion, resulting in the creation most likely of a stellar black hole, or a pulsar (I learn to the black hole because of the mass of the star).  It will be interesting to see when this star does go supernova how much light is absorbed by the large dust particles and how much light blasts out.  





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