Main Sequence Stars: Mass & Beyond

[Baggio]

Straight forward questions that's been bugging me a little. Why do most stars lie on the main sequence whilst others don't? Is it just purely characterised by the mass?

Thanks

 

[SpaceTiger]

Straight forward questions that's been bugging me a little. Why do most stars lie on the main sequence whilst others don't? Is it just purely characterised by the mass?

The main sequence is basically the location on the Hertzsprung-Russel diagram where hydrogen-burning stars sit. Every star more massive than a brown dwarf (~0.1 solar masses) will be on it at some point in its life. In fact, stars spend the majority of their lives on the main sequence, so it shouldn't be surprising that most of what we see is on it. They leave the main sequence when they run out of hydrogen to burn in the core. Examples of non-main sequence stars are white dwarfs, asymptotic giant branch stars, and protostars.

 

[franznietzsche]

Space tiger summed it up pretty well. A Main sequence star is simply one that is predominantly burning hydrogen as its source of energy. One thing to note is that stars do not really progress down the Main sequence from Upper left to lower right on an HR diagram. They do move along it, but at a certain point they basically 'jump' off, when the hydrogen runs out. Where they go depends on their mass and composition.

 

[SpaceTiger]

One thing to note is that stars do not really progress down the Main sequence from Upper left to lower right on an HR diagram. They do move along it, but at a certain point they basically 'jump' off, when the hydrogen runs out. Where they go depends on their mass and composition.

That's right, the motion along the main sequence during their lifetime is very small. To zeroth order, a given star will only sit at one position on the main sequence for most of its life. Moving up the main sequence is basically equivalent to moving to higher-mass stars.

 

[Nereid]

Straight forward questions that's been bugging me a little. Why do most stars lie on the main sequence whilst others don't? Is it just purely characterised by the mass?

Just to make your life a little more exciting Baggio (and extend the excellent answers from ST, so keeping this thread alive a bit longer) ... why do H-burning stars end up with this particular relationship between (surface) luminosity and mass (or surface temperature or ...)? I mean, the 'H-burning' occurs deep, deep down, in the core, yet what we see is the photosphere - even if 'H-burning' cores are all the same (varying only by mass?), why should the photospheres all end up the same too?

You also know that there is a considerable range of 'metalicity' in main sequence stars (astronomers are funny folk, they say 'metal' for any element heavier than He ... so to them even O and N and C are 'metals'!) - does that make a difference?

It may be intuitively OK that protostars and white dwarfs are different, but why should 'shell-burning' make red giants (etc) so much different from their twins, stars of the same mass but burning in the core (not a shell)?

 

[neurocomp2003]

math wise-prolly stability and fundamental burning that was stated above.

 

[SpaceTiger]

why do H-burning stars end up with this particular relationship between (surface) luminosity and mass (or surface temperature or ...)? I mean, the 'H-burning' occurs deep, deep down, in the core, yet what we see is the photosphere - even if 'H-burning' cores are all the same (varying only by mass?), why should the photospheres all end up the same too?

Actually, believe it or not, the physical conditions on the surface of the star are not determined by nuclear burning. In fact, we had a fairly good idea of the structure of the sun before we even knew about nuclear burning. The majority of a star's structure is determined by battle between pressure and gravity. The thing determined by fusion is how long it can maintain this equilibrium. The star is cooling via the light it emits, so the fusion is needed to keep the temperature up and maintain the pressure. In other words, the energy source determines the lifetime, not the structure or appearance.

You also know that there is a considerable range of 'metalicity' in main sequence stars (astronomers are funny folk, they say 'metal' for any element heavier than He ... so to them even O and N and C are 'metals'!) - does that make a difference?

Metals have two effects. First, their absence or presence will alter the radiative opacity of the atmosphere. This will, in turn, alter the luminosity. Specifically, metals tend to make the atmospheres more opaque, decreasing the luminosity. Thus, metal-poor stars (subdwarfs) are dimmer. The other effect of metals is to alter the spectrum and, therefore, the color. Metals contribute a lot of absorption lines/bands/edges, so a star of equivalent luminosity and temperature will have a different color if it has fewer metals.

It may be intuitively OK that protostars and white dwarfs are different, but why should 'shell-burning' make red giants (etc) so much different from their twins, stars of the same mass but burning in the core (not a shell)?

The envelopes of stars expand into giants expand because their cores contract (conservation of energy). Their cores contract because they can no longer support themselves with nuclear reactions, so they collapse until degeneracy pressure kicks in. If the star is massive enough, it will eventually start burning helium and the core will expand again, allowing the star to shrink back to a more reasonable size.

 

[Spin_Network]

Actually, believe it or not, the physical conditions on the surface of the star are not determined by nuclear burning. In fact, we had a fairly good idea of the structure of the sun before we even knew about nuclear burning. The majority of a star's structure is determined by battle between pressure and gravity. The thing determined by fusion is how long it can maintain this equilibrium. The star is cooling via the light it emits, so the fusion is needed to keep the temperature up and maintain the pressure. In other words, the energy source determines the lifetime, not the structure or appearance.




Metals have two effects. First, their absence or presence will alter the radiative opacity of the atmosphere. This will, in turn, alter the luminosity. Specifically, metals tend to make the atmospheres more opaque, decreasing the luminosity. Thus, metal-poor stars (subdwarfs) are dimmer. The other effect of metals is to alter the spectrum and, therefore, the color. Metals contribute a lot of absorption lines/bands/edges, so a star of equivalent luminosity and temperature will have a different color if it has fewer metals.




The envelopes of stars expand into giants expand because their cores contract (conservation of energy). Their cores contract because they can no longer support themselves with nuclear reactions, so they collapse until degeneracy pressure kicks in. If the star is massive enough, it will eventually start burning helium and the core will expand again, allowing the star to shrink back to a more reasonable size.

I came across(I was looking into luminosity function), an interesting lecture/seminar by Anthony Aguirre entitled Enigmas In Galaxy Formation:http://streamer.perimeterinstitute....aspx?cid=a9b1d20a-efa7-485f-8d5d-3b62fb7d3e4c

the seminar is on page 7 of 11, and you will need WMP 9 or later,
half way through he gives a specific overview on the reasons for the existence of how:Metals exist outside of Galaxies, and the process of Heavy metal diffusion implimented by the Galactic Wind(which Iam delving into its correlation with Luminosity), you may find some interesting recent thoughts on Star formation, Dark Matter, and 'Surface Critical Density' which may have a baring on your post?