- #1
Baggio
- 211
- 1
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
Thanks
Baggio said: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?
franznietzsche said: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.
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?Baggio said: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?
Nereid said: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)?
SpaceTiger said: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.
A main sequence star is a type of star that is in the longest stage of its life cycle. It is characterized by a stable fusion of hydrogen atoms in its core, which produces energy and makes the star shine brightly.
A star's mass is directly related to its position on the main sequence. The more massive a star is, the higher its luminosity and temperature will be, causing it to appear higher up on the main sequence diagram.
As a main sequence star burns through its hydrogen fuel, it begins to fuse heavier elements in its core. This causes the star to expand and become a red giant, eventually leading to its death as a white dwarf, neutron star, or black hole.
No, a main sequence star cannot have a lifespan longer than the age of the universe. This is because the universe is currently about 13.8 billion years old, and the most massive main sequence stars have a lifespan of only a few million years.
Scientists use a variety of methods to determine the mass of a main sequence star, including measuring its luminosity, radius, and orbital motion in a binary star system. They may also use theoretical models and observations of the star's spectral lines to estimate its mass.