Largest Star in the Universe
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  JanL

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Posted: 2012-October-28 at 12:49pm | IP Logged Quote JanL

Hi Karl, it is extremely possible that you are very right on this.  I know there is a limit to how large a star can be, BUT I don't how large that is.  Same with how hot a star is.  After all the universe is a very, very big place.
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  nkalanaga

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Posted: 2012-October-28 at 10:10pm | IP Logged Quote nkalanaga

Blue supergiants can explode.  The supernova in the LMC a few years back was a blue star.  Apparently some of them are large enough that they don't form red giants.  The radiation pressure blows the outer layers off as they expand, leaving the blue core exposed until it explodes.

N Kalanaga
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  karl.garnham1

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Posted: 2012-October-29 at 2:16pm | IP Logged Quote karl.garnham1

Hi Jan

I reckon we will find one soon one that Dwarfs VY Canis
Majoris at least I think the trouble is the fact we don't
know how big the limit is on how large a Star can grow.

Hi nkalanaga

Oh Blue Supergiants can explode I didn't know that does
that mean it applies to Blue Hypergiants also? What
about Yellow Hypergiants can they explode as well as I
know virtually nothing about them. You explain how
Stars work very Well its almost like a cuckoo clock if you
look at it in a certain way the moment the movement
reaches the hour the Cuckoo comes out and air is shot in
between two bellows that make the sound of the
Cuckoo. So a stars radiation pressurises the core to
collapse and explode. Thank you for the info.

Karl


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  nkalanaga

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Posted: 2012-November-03 at 1:35am | IP Logged Quote nkalanaga

I would imagine that hypergiants of any type would be even more likely to explode than supergiants.  I had to look up the hypergiant class, and the sources agreed that many astronomers prefer to consider them a subgroup of supergiants, although they do have their own luminosity class.  From Wikipedia:
" There is an MKK luminosity class 0 (zero) for hypergiants, but this is rarely seen in published spectral classifications. More commonly hypergiants will be classed as Ia-0, Ia+, or even just Iae based solely on the observed spectra. As noted, red supergiants rarely receive these extra spectral classifications."

So, yes, blue hypergiants would also tend to lose their outer layers rather than become red giants.  Further comments in the article include:
"Stars with an initial mass above about 25Ms quickly move away from the main sequence and increase somewhat in luminosity to become blue hypergiants. They cool and enlarge at approximately constant luminosity to become a red hypergiant, then contract and increase in temperature as the outer layers are blown away. They may "bounce" backwards and forwards executing one or more "blue loops", still at a fairly steady luminosity, until they explode as a supernova or completely shed their outer layers to become a Wolf-Rayet star. This means that yellow hypergiants may be either high mass cooling stars or low mass stars losing the last of their atmospheres, although it appears that cooling stars only remain as yellow hypergiants for a very short time. Stars with an initial masses above about 40Ms are simply too luminous to develop a stable extended atmosphere and so they never cool sufficiently to become a yellow or red hypergiant, which means that the cool hypergiants have a maximum luminosity about half a million times that of the sun. Blue hypergiants can be much more luminous, up to several million times the sun."

N Kalanaga
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  karl.garnham1

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Posted: 2012-November-03 at 9:28am | IP Logged Quote karl.garnham1

Thanks nkalanaga

That has helped loads with my research on Hypergiant
Stars. Have you seen a film called Sunshine? its highly
inaccurate but it has some cool ideas the sun is dying in
the film(without expanding to become a red giant star.)
and all life on earth faces extinction so a group of
scientists make an artificial sun and go all the way to the
sun to reignite the dying star. Personally I think the Sun
would expand not just dim away but that film had some
good ideas and fantastic Soundtrack. I don't think we
could ever reignite the sun as its huge and a man made
star would be nothing due to the huge mass of the Sun.

Could be wrong though but its a interesting film.

Karl


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  nkalanaga

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Posted: 2012-November-03 at 11:38pm | IP Logged Quote nkalanaga

No, I've never heard of the film, which isn't surprising, as we don't watch many movies.  Someone, many years ago, had a short story along those lines.  The Sun simply "went out", and humans put all of the nuclear weapons we could find on a ship and used them to restart it.  The story was set far in the future, and humans had explored and settled the Solar System, but when the Sun started dimming, they had retreated to Earth and dug in.  This was supposedly the last ship and trained crew available, and they succeeded, although the ship itself was lost, because the engines weren't strong enough to escape the Sun's gravity.  Apparently the drive, which seemed to be gravity-based, made the difference, by interacting with the Sun's gravity field.  I wish I could find the story so I could give the author's name!

And, yes, the idea of the Sun simply "going out" is unlikely, as the lack of radiation pressure would cause it to shrink, increasing the density, and reigniting the core.  On the other hand, that's exactly how red dwarfs age.  They're fully convective, so when the core runs out of hydrogen, the outer layers are also depleted.  They don't have enough mass or gravity to raise the core pressure high enough to ignite helium, so they simply cool off.  On the other hand, they live so long that none have ever died, so any race on a planet orbiting one would probably not be worried.

N Kalanaga
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  karl.garnham1

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Posted: 2012-November-04 at 6:13am | IP Logged Quote karl.garnham1

Your like me I can't sit still long enough to watch many
movies anymore so I am constantly on computers and
soon I will be fixing clocks mainly cuckoo clocks. Their is
a theory that says when a red Dwarf star Ages it
becomes a Blue Dwarf and then a white dwarf is left
after that. The trouble is the universe is currently not old
enough to support that theory as Red Dwarf stars can
live for Trillions of years and the Universe is 13.75 Billion
years old. My guess is though you are right they would
just wink out of existence. There is one quote from that
film that is 100% true nothing suvives not you parents
or your children not even the Stars.

Thanks for the info

Karl


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  nkalanaga

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Posted: 2012-November-05 at 12:16am | IP Logged Quote nkalanaga

It's doubtful that one could get a blue or white dwarf from a red dwarf simply because they never get that hot.  The white dwarf is actually the core of a star after the cooler outer layers have been blown off, and the outer layers of a red dwarf don't go anywhere.  So, it just quietly shuts down and slowly cools to a black dwarf, which would probably look very much like a brown dwarf or large jovian planet.  The main difference would be that it would have little or no hydrogen, so no methane, water, ammonia, or anything else.  It would be a slowly cooling ball of helium with the few heavier elements it started with.  They'd probably slowly settle to the core, leaving a pure helium outer layer with maybe a thin hydrogen atmosphere.  A red dwarf can't fuse helium, so it can't make anything heavier than that.

As you said, they last for tens of billions to trillions of years, so there are no "black dwarfs", and won't be for a long time to come.  The last habitable planets in the universe will probably orbit little red dwarfs.  Fortunately, there are a LOT of them, so there should be room for everyone.

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  karl.garnham1

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Posted: 2012-November-05 at 3:41am | IP Logged Quote karl.garnham1

I agree with you I think it is very unlikely although have
you heard of the red dwarf that gave off such a powerful
flare it turned blue? it was on and fantastic documentary
called new how the universe works. I personally think as
you do a Red dwarf star is too small and feeble to swell
its not an awful lot bigger considering its a star than
Jupiter its mass is about 80% I think but the star itself is
really quite small. I also believe there may be green stars
out there because if you look at sunlight through a prism
it splits to many different colours and Green is one of
them. Trouble is no one has ever found one so I am
probably wrong.

Thanks for the info I always enjoy our conversations.

Karl


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  nkalanaga

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Posted: 2012-November-05 at 11:45am | IP Logged Quote nkalanaga

Red dwarfs are often flare stars.  These flares can be as large or larger than the Sun's, on a star much smaller, and can temporarily change the color, as well as increasing the dwarf's brightness many times.  They seem to be more common on young dwarfs, but are one reason many researchers doubt that planets orbiting red dwarfs would be good places to look for life.

The main reason there are no green stars is that stars put out the entire spectrum.  Our eyes are sensitive to green, and the Sun produces a lot of green light, but our eyes tend to concentrate on the red and blue.  Also, any continuous-spectrum light above a certain brightness saturates our eyes and looks white.  A star that is bright enough to look green is also bright enough to saturate the red and blue, so it still looks white.

Incidentally, the Sun is white, but looks yellow in the sky because the blue light is scattered, making  the sky blue.  Green light is also scattered, more than yellow,, but not as much as blue, so it's not as noticeable in sunlight.

Going back to hypergiants, here's an article on why there are no yellow hypergiants:
http://phys.org/news/2012-11-hypergiant-star-link-years.html

It seems that their atmospheres are unstable at yellow-star temperatures, so they cross that range very quickly.

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