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Padawan
2004-Apr-05, 06:40 PM
Hiya, i'm new to this forum!

i was wondering if there is an upper mass limit for red supergiants. If the red supergiant becomes massive enough, would it become a blue supergiant?

Does anyone know?

cyswxman
2004-Apr-05, 07:10 PM
Welcome to the board, Padawan! :)

I'm not well versed in this area of astronomy, but my guess is that the colors have no real relation to the size of stars, but rather the fuel they are converting (fusing) in their cores.

Ut
2004-Apr-05, 07:15 PM
Here's some information on blue giants. As you can see, generally speaking, they're just supermassive stars.

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

Though I've heard that red supergiants can contract after the initial shell burning of helium stops, making them blue for a short period. As soon as core and/or shell burning resumes, theough, they'd baloon back up into red supergiants. I'm not sure if stellar evolution theory has tossed this idea aside or not, though.

Ut
2004-Apr-05, 07:19 PM
Welcome to the board, Padawan! :)

I'm not well versed in this area of astronomy, but my guess is that the colors have no real relation to the size of stars, but rather the fuel they are converting (fusing) in their cores.

Colours have to do with the temperature of the star's chromosphere. A red dwarf star is fusing hydrogen, just as our yellow sun is, just as a main sequence blue star is. A red giant star is burning hydrogen in a shell surrounding its core. When massive red giants begin fusing helium, they get even bigger in volume, but remain red.

Padawan
2004-Apr-05, 07:20 PM
Thanks for the input guys! :)

Any other thoughts?

Brady Yoon
2004-Apr-06, 12:49 AM
Is there a definite correlation between size and mass in red supergiants? Are all larger supergiants of the same spectral class more massive?

Normandy6644
2004-Apr-06, 01:32 AM
Is there a definite correlation between size and mass in red supergiants? Are all larger supergiants of the same spectral class more massive?

A believe it's mass and luminosity that correlate, although luminosity is based on fusion power, so I woudl imagine that radiation pressure would be greater and thus produce a more massive star.

AGN Fuel
2004-Apr-06, 02:39 AM
A believe it's mass and luminosity that correlate, although luminosity is based on fusion power, so I woudl imagine that radiation pressure would be greater and thus produce a more massive star.

From memory, the Mass-Luminosity relationship applies only to stars on the Main Sequence. IIRC, implications of the Eddington limit place an upper restriction on the mass a star can be at formation, which I think is somewhere in the 120 solar mass range.

Normandy6644
2004-Apr-06, 02:58 AM
A believe it's mass and luminosity that correlate, although luminosity is based on fusion power, so I woudl imagine that radiation pressure would be greater and thus produce a more massive star.

From memory, the Mass-Luminosity relationship applies only to stars on the Main Sequence. IIRC, implications of the Eddington limit place an upper restriction on the mass a star can be at formation, which I think is somewhere in the 120 solar mass range.

I'm not sure about that main sequence bit, since no giants (red or blue) are on the MS, and yet I'm pretty sure they still obey the mass-luminosity law. Time to get out the old notes... :lol:

Tobin Dax
2004-Apr-06, 04:20 AM
Is there a definite correlation between size and mass in red supergiants? Are all larger supergiants of the same spectral class more massive?

There is a corellation between temperature/luminosity and radius, as radius can be plotted on the H-R diagram. As for radius and mass, you could get a relation from teh L-R relation and the M-L relation. Keep in mind that the M-L relation is sketchy and approximate and the radius curve stretches across the temperature range, so this relation could be tricky to figure out.

Ricimer
2004-Apr-06, 08:19 PM
Normandy: There is no hard and fast mass-luminosity relationship once one leaves the main sequence. The reason for this is because you can have vast differences in luminosity with little difference in mass.

The red giants and supergiants are formed when a star swells, vastly increasing it's luminosity, but maintaining its mass.

Most of the hard and fast relationships, like mass-luminosity are very vague off of the mainsequence.

There brighter does not mean more massive, larger radius does not mean more massive (though it does tend to go with luminosity, but not always).

Normandy6644
2004-Apr-06, 11:17 PM
Normandy: There is no hard and fast mass-luminosity relationship once one leaves the main sequence. The reason for this is because you can have vast differences in luminosity with little difference in mass.

Very true. I suppose the important relationship at that point becomes the Stefan-Boltzmann law, since it relates luminosity to temperature and surface ara.


The red giants and supergiants are formed when a star swells, vastly increasing it's luminosity, but maintaining its mass.

Right, another point for Stefan-Boltzmann.


Most of the hard and fast relationships, like mass-luminosity are very vague off of the mainsequence. There brighter does not mean more massive, larger radius does not mean more massive (though it does tend to go with luminosity, but not always).

Yeah, I'm not sure what I was confusing (if anything). Welcome to the board!! [/quote]

Ricimer
2004-Apr-07, 03:59 PM
Thanks for the welcome.

One small problem with the H-R diagram is that your mind wants to apply a lot of the relationships to everything on it. So people tend to do just that, even if they know better.

A lot of students I work with have the very same problem, and they're much harder to point in the right direction.