SAO: Making Massive Stars

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SAO: Making Massive Stars

Post by bystander » Fri Sep 10, 2010 9:14 pm

Making Massive Stars
Smithsonian Astrophysical Observatory
Weekly Science Update | 10 Sept 2010
Massive stars -- those with more than about eight times the mass of the sun -- are arguably the most important actors in the universe. Much hotter and more luminous than the sun, they live only hundreds of millions of years before exploding in supernovae, but during their lives their nuclear furnaces produce a wide range of chemical elements (the universe was created with primarily hydrogen and helium). Meanwhile they heat up their galactic neighborhoods and dominate the properties of their environments and their gas and dust. In their dramatic deaths they seed the universe with these elements (and others produced in the cataclysm), disrupt their neighborhoods, and leave behind neutron stars or sometimes black holes.

Massive stars are much less common than sun-sized stars, comprising only a few tenths of a percent of all stars, and astronomers wonder why. It is not clear if standard ideas about star formation apply to massive stars. For example, do they grow like smaller stars by accreting material from a large envelope while also surrounded by a rotating disk of material? Massive stars mature very quickly, however, in less than a few hundred thousand years compared with millions of years for stars like the sun, and as a result there are not many young ones around at any given time in which to study the processes associated with their birth.

Two SAO astronomers, Eric Keto and Qizhou Zhang, argue in a new paper that massive stars do form like smaller ones, at least with respect to their envelopes and disks. Combining observations of molecular gas in one young, massive star with computer models of star formation that were scaled up to fit this more massive case, they find very good agreement, at least with stars of masses up to ten solar masses (stars more massive than this may yet have differences). The results not only suggest that theorists are on the right track, they imply that future observations of massive young stars with new instruments can expect to see the signatures of these disks in their data.
The standard model of star formation applied to massive stars:
accretion discs and envelopes in molecular lines
- Eric Keto, Qizhou Zhang

swainy (tc)

Re: SAO: Making Massive Stars

Post by swainy (tc) » Mon Sep 13, 2010 4:47 pm

bystander wrote:Massive stars are much less common than sun-sized stars, comprising only a few tenths of a percent of all stars, and astronomers wonder why. It is not clear if standard ideas about star formation apply to massive stars. For example, do they grow like smaller stars by accreting material from a large envelope while also surrounded by a rotating disk of material? Massive stars mature very quickly, however, in less than a few hundred thousand years compared with millions of years for stars like the sun, and as a result there are not many young ones around at any given time in which to study the processes associated with their birth.
Is there a whole new process at work? For instance: When the new star got to the size of our sun, What stopped it igniting? The nuclear reactor Must be working. But it carried on getting Bigger, When the New star got to the size of Betelgeuse, Why even then did it carry on Growing? But A Big star only lives so long. So how did one get to be 1 billion times, the size of our Sun? With out Dieing of young age?


http://www.bing.com/videos/watch/video/ ... &gt1=42007

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Re: SAO: Making Massive Stars

Post by Chris Peterson » Mon Sep 13, 2010 5:06 pm

swainy (tc) wrote:Is there a whole new process at work?
As the paper reports, for typical massive stars up to 10 solar masses, the existing models for star formation appear to operate. That is, no new process is required to explain these stars.
But A Big star only lives so long. So how did one get to be 1 billion times, the size of our Sun? With out Dieing of young age?
There are no stars a billion times the size of the Sun. The largest stars that most all astronomers agree exist are about 100 times more massive than the Sun. There is a recent report claiming to have found a star over 300 times as massive as the Sun, but it remains disputed, and many don't believe this finding will hold up. Even so, we are talking much less than a billion!

Note that the paper referenced in this discussion does not suggest that the standard mechanism of stellar formation is necessarily useful for describing the formation of very massive stars.
Chris

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Re: SAO: Making Massive Stars

Post by swainy (tc) » Mon Sep 13, 2010 7:51 pm

Chris Peterson wrote:There are no stars a billion times the size of the Sun.



Yeah, I thought that while watching the very misleading tv program, That I posted above.

http://en.wikipedia.org/wiki/VY_Canis_Majoris#Size

http://en.wikipedia.org/wiki/R136a1

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Re: SAO: Making Massive Stars

Post by bystander » Mon Sep 13, 2010 8:54 pm

swainy (tc) wrote:
Chris Peterson wrote:There are no stars a billion times the size of the Sun.
Yeah, I thought that while watching the very misleading tv program, That I posted above.

http://en.wikipedia.org/wiki/VY_Canis_Majoris#Size

http://en.wikipedia.org/wiki/R136a1
I guess it all depends on what you are referring to as size. According to wikipedia, R136a1 is the most massive known star at 265 time the mass of the sun. It has a radius of 35.4 solar radii which gives it a volume of 45,000 times that of the sun. VY Canis Majoris, the largest in volume, only has a mass of 30 to 40 times that of the sun but a radius 2,000 times as large. That gives it a volume of some 8 billion times that of the sun. You need to specify what measure you are using when talking about size. Mass, radius, or volume could all be used as references to size.

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Re: SAO: Making Massive Stars

Post by Chris Peterson » Mon Sep 13, 2010 10:17 pm

bystander wrote:I guess it all depends on what you are referring to as size. According to wikipedia, R136a1 is the most massive known star at 265 time the mass of the sun. It has a radius of 35.4 solar radii which gives it a volume of 45,000 times that of the sun. VY Canis Majoris, the largest in volume, only has a mass of 30 to 40 times that of the sun but a radius 2,000 times as large. That gives it a volume of some 8 billion times that of the sun. You need to specify what measure you are using when talking about size. Mass, radius, or volume could all be used as references to size.
If unspecified, I would always assume mass. This is a key parameter used to specify the sort of properties a star has; diameter is not. Comparing stars based on volume might be interesting in terms of simply giving the scale of things, but makes no sense astronomically. There is no question how a star can form, from somewhat well understood mechanisms, and end up with a volume a billion times greater than the Sun. The question is how do stars more massive than one or two hundred solar masses form?

VY Canis Major is a fairly typical massive star at less than 100 solar masses. R136a1 has been suggested to have a mass of 265 solar masses, but that report remains controversial. Stellar masses are normally determined by looking at the behavior of orbiting companions. This is a very robust method that produces uncontroversial results. The mass of R136a1 was determined indirectly, looking at other properties. That makes a lot of people uncomfortable, given that the high mass is difficult to explain using existing stellar development models. Whether the report of the high mass of this star (and a couple others in the same region, determined the same way) holds up remains to be seen.
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Re: SAO: Making Massive Stars

Post by bystander » Mon Sep 13, 2010 10:45 pm

Chris Peterson wrote:If unspecified, I would always assume mass. This is a key parameter used to specify the sort of properties a star has; diameter is not. Comparing stars based on volume might be interesting in terms of simply giving the scale of things, but makes no sense astronomically. There is no question how a star can form, from somewhat well understood mechanisms, and end up with a volume a billion times greater than the Sun. The question is how do stars more massive than one or two hundred solar masses form?
Oh, I agree, mass is the key parameter, although the clip Mark referenced was highlighting volume and the inhabitants of Earth at some distant future time (if any) might start considering volume to be very important. :wink:
VY Canis Major is a fairly typical massive star at less than 100 solar masses. R136a1 has been suggested to have a mass of 265 solar masses, but that report remains controversial. Stellar masses are normally determined by looking at the behavior of orbiting companions. This is a very robust method that produces uncontroversial results. The mass of R136a1 was determined indirectly, looking at other properties. That makes a lot of people uncomfortable, given that the high mass is difficult to explain using existing stellar development models. Whether the report of the high mass of this star (and a couple others in the same region, determined the same way) holds up remains to be seen.
Well, I did say according to wikipedia. I was just using Mark's references and explaining that size can mean different things.

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