I Why are there so many different kinds of stars?

[sjp9220]

There are red giants, blue giants. white dwarfs, red dwarfs, brown dwarfs, etc., etc., etc,... Why? do they have different chemistries? Are they each at different stages in their lives? Is there a combination of reasons?

 

[mfb]

Mainly different initial mass and age. All stars are about 75% hydrogen and 25% helium when they form. While the amount of heavier elements varies a lot they are always rare.

 

[Drakkith]

It's a combination of age and mass mostly. Smaller mass stars follow different life-cycles than higher mass stars. For example, a star of 0.2 solar masses will live a very, very long time but will never be able to do anything besides turn hydrogen into helium in its core. As it ages and runs out of fuel, turns into a white dwarf, and then starts to cool off until, many billions of years later, it finally cools off to ambient temperature in space (less than a few kelvin). However, a star of 10 solar masses ages extremely rapidly, staying on the main sequence for only a few million years or so before igniting helium fusion in its core. It then undergoes a series of events that ping-pong it back and forth between expansion and contraction phases, moves up from burning helium to burning heavier and heavier elements, until finally it explodes as a type-II supernova.

The only difference between these two stars is their initial mass, yet this single difference causes a drastic change in their overall life-cycle. While different compositions can also create differences in their life-cycles, these are less drastic by comparison.

 

[snorkack]

It's a combination of age and mass mostly. Smaller mass stars follow different life-cycles than higher mass stars. For example, a star of 0.2 solar masses will live a very, very long time but will never be able to do anything besides turn hydrogen into helium in its core. As it ages and runs out of fuel, turns into a white dwarf, and then starts to cool off

Actually, it does many other interesting things. Like, starts off being fully convective and turning protium into helium all over the star - then, as the molar mass increases, heats up and forms a core.
But that does not contribute to diversity of stars, because it takes more time than world has existed. Red dwarfs are only represented by main sequence and protostars.

 

[Chronos]

Since we don't know what any star [aside from the sun] actually looks like, our models are necessarily theoretical. Lacking a solid basis to doubt them is insufficient to justify faith in their accuracy. We have been surprised in the past when actual data caught up with modeling and we can rest assured this will happen again. Whether this happens with stars remains to be seen. An example of some of the surprises still awaiting us are discussed here; https://www.space.com/35297-green-glow-from-ancient-galaxies-surprise-scientists.html.

 

[lomidrevo]

I found this article as a quite nice overview of different stages of the stellar evolution. As already mentioned, the initial mass of the star is the most important parameter determining the destiny of the star.

 

[lomidrevo]

Since we don't know what any star [aside from the sun] actually looks like

Of course, most detailed information we gathered about the Sun, but plenty of information can be retrieved from observational data of other stars as well. So in this sense we know how they look like. For that, theoretical models are required, but this is the case for Sun as well.

 

[snorkack]

igniting helium fusion in its core. It then undergoes a series of events that ping-pong it back and forth between expansion and contraction phases, moves up from burning helium to burning heavier and heavier elements, until finally it explodes as a type-II supernova.

The only difference between these two stars is their initial mass, yet this single difference causes a drastic change in their overall life-cycle. While different compositions can also create differences in their life-cycles, these are less drastic by comparison.

Qualitatively, the effect of heavy elements is to cause the star to bloat with radiation blocked by heavy elements, and to lose gas by stellar wind. A star which is poor in heavy elements compared to hydrogen and helium will be smaller and hotter on main sequence, and lose less gas through evolution.

 

[mfb]

@Chronos: We have reliable spectrum and size measurements of millions of stars. We have mass measurements of many of them as well. We know this zoo exists and we can group it by mass.
We also have much more beyond the scope of this thread. Chemical composition of the surface, rotation data, variability, limb darkening, a handful of spatially resolved star images, just to name some examples.
Sure, there is still much to learn.

What do you mean by “actually looks like”?

 

[Chronos]

'Looks like' in the sense of any observable property. That leaves open most features imaginable - like the color of their spots

 

[mfb]

Well, obviously images of the Sun have a much higher resolution, but here is Antares:

 

[Vanadium 50]

Antares seems to have gone missing.

 

[mfb]

My phone could display it, but it looks like the forum and other platforms have problems with it. Changed to a JPG version.

 

[Vanadium 50]

It's back!

 

[Chronos]

Are those dark/light blotches Antares-spots?

 

[mfb]

Sort of, convection cells. As you can see it has far fewer convection cells than the Sun, despite being larger.

 

[stefan r]

Are those dark/light blotches Antares-spots?

I thought they were more like granules. Supergiant red stars have extra-super-duper giant granules.

Sunspots are linked to concentrations of magnetic flux. I believe slow rotation period makes the tangling of flux lines less likely. But I am not sure.

 

[unusually_wrong]

There are red giants, blue giants. white dwarfs, red dwarfs, brown dwarfs, etc., etc., etc,... Why? do they have different chemistries? Are they each at different stages in their lives? Is there a combination of reasons?

To help answer this question you have to understand the life cycle of stars.

Ex 1: Red giants have left main sequence and are on their deathbed. Stars spend about 90% of their life in main sequence and 10% dying. So imagine being a baby until you're 90 years old and then age rapidly catches up to you and when you reach 100 years old you die (in one hell of an explosion).

Ex 2: A white dwarf isn't really a star, but the dead body of a star. It is the iron core left over from a medium mass star. When the medium mass star (like our sun) dies, it creates a planetary nebula.

Ex 3: A red dwarf star can potentially live trillions of years. We do not know what will happen when one dies because the age of our universe is nowhere near that time.

Ex 4: A brown dwarf is basically a failed star. They do not have enough mass to fuse hydrogen. That raises the question of if Jupiter is a brown dwarf (but you'll have to look that up on your own).

 

[sjp9220]

It's a combination of age and mass mostly. Smaller mass stars follow different life-cycles than higher mass stars. For example, a star of 0.2 solar masses will live a very, very long time but will never be able to do anything besides turn hydrogen into helium in its core. As it ages and runs out of fuel, turns into a white dwarf, and then starts to cool off until, many billions of years later, it finally cools off to ambient temperature in space (less than a few kelvin). However, a star of 10 solar masses ages extremely rapidly, staying on the main sequence for only a few million years or so before igniting helium fusion in its core. It then undergoes a series of events that ping-pong it back and forth between expansion and contraction phases, moves up from burning helium to burning heavier and heavier elements, until finally it explodes as a type-II supernova.

The only difference between these two stars is their initial mass, yet this single difference causes a drastic change in their overall life-cycle. While different compositions can also create differences in their life-cycles, these are less drastic by comparison.

OH o.k., so you're saying that the reason their are blue stars, red, white, etc is just a matter of mass in their initial ignition? Thank you.

 

[Nik_2213]

IMHO, A 'local' example would be the Sirius system. Initially, a fairly close binary, so probably a very similar mix of ingredients. The slightly more massive sib developed a much hotter core, ran through its fuel significantly faster. It became a Red Giant, collapsed to a white dwarf and is now Sirius B on a wider orbit.

IIRC, the Sirius system is but 300~400 million years old. Sirius B blew through its life cycle in half of that. By comparison, our Sol is 4+ billion years old, and some old red dwarf stars are close to thrice that...

 

[Vanadium 50]

OH o.k., so you're saying that the reason their are blue stars, red, white, etc is just a matter of mass in their initial ignition?

That's not what he said. The very first sentence was:

It's a combination of age and mass mostly.

 

[Drakkith]

OH o.k., so you're saying that the reason their are blue stars, red, white, etc is just a matter of mass in their initial ignition? Thank you.

Initial mass, current mass, age, and a few other factors. Like I said, the main factors are initial mass and age. The Sun will swell into a red giant in a few billion years, during which it will be much more reddish than it is now, despite having nearly the same mass as it initially had.

 

[stefan r]

Initial mass, current mass, age, and a few other factors. Like I said, the main factors are initial mass and age. The Sun will swell into a red giant in a few billion years, during which it will be much more reddish than it is now, despite having nearly the same mass as it initially had.

Metalicity is a major factor. All of the universe that we see came out of one big bang so we see similar stars everywhere. If, for example, there were a stellar mass of plastic (1 part carbon, 2 part hydrogen) it would collapse into a star that looks different from the Sun.
How homogeneous the initial cloud was is also a main factor. Grains of dust and meteors will evaporate in the protostar stage so stars are usually homogeneous. If you had the Sun's composition of elements and it was sorted the star would be a red giant.

The stars we see usually came from well mixed clouds that were in our galaxy.

 

[Drakkith]

Metalicity is a major factor.

Like you said, stars come from clouds of material that is mixed, so the metallicity of real-world stars doesn't appear to matter a great deal except perhaps at the extreme ends (population I stars for example).