Discovering the Truth About Antares: From Young Red Supergiant to Blue Giant

[Thecla]

The most recent issue of Sky and Telescope had an article that 100 years ago most asrronomers thought that red supergiants like Antares were young stars and that as they aged they joined the main sequence and became more like the sun. Today we consider blue giants like Spica and Rigel as young stars. Stars like Rigel and Spica didn't start off hot with surface temperatures of 30000 degrees. They had to star off cool (red)and heat up to that temperature. So can we say that a red star is vey old or very young(on its way to becoming a blue giant).

 

[bdrobin519]

Wouldn't it make sense that there's the possibility that heavy elements within star clusters or surrounding areas ( including ejected trajectories) could slowly amass enough gasses to fuse? Could result in a young red star maybe.

 

[Drakkith]

So can we say that a red star is vey old or very young(on its way to becoming a blue giant).

No, as the vast majority of stars are main sequence red stars (red dwarfs). Color by itself is insufficient to determine the age of the star.

Wouldn't it make sense that there's the possibility that heavy elements within star clusters or surrounding areas ( including ejected trajectories) could slowly amass enough gasses to fuse?

Not in the current era of the universe. Not enough time has passed for hydrogen to have been depleted enough to stop making up the vast majority of stellar material.

 

[Vanadium 50]

As a young star, Anatres was out all night partying with his friends. Got some DUIs and a few stints in rehab. Like many young stars today.

As stated, most stars are red dwarfs. By far. They are fully convective so shine for many, many billions of years - perhaps close to a trillion. So 13 billion years doesn't make a whole lot of distance old vs. young. All red dwarfs are young.

You can see how a 10 solar mass (slightly lighter) star evolver here:

 

[Vanadium 50]

As a PS - Antares has a companion, a blue star about two-thirds its mass. Presumably it formed at the same time, but because it is less massive, it has yet to evolve off the main sequence. It sort of shows what Antares was like earlier in its history.

To give you an idea of how young Antares is, despite being in the late stages of stellar evolution, when it formed the dinosaurs had all died. Fifty million years before.

 

[Bandersnatch]

Stars like Rigel and Spica didn't start off hot with surface temperatures of 30000 degrees. They had to star off cool (red)and heat up to that temperature.

This is not true of many stars in general, and of these two (and Antares too) in particular.

As stars coalesce from their molecular cloud, they are at first completely obscured from view. All outgoing radiation - at this time fuelled wholly by gravitational potential energy - leaving the cloud is in infrared. The progenitor gas is optically thick enough to not allow any visible light out. When the protostar finally manages to blow away the remnants of the cloud from which it was born and emerge as a visible object, it may follow on of the three general types of path depending on mass:

- If the star has managed to accrete little mass (<~0.5 solar masses) it stays at the same temperature as it finalises its contraction (therefore dimming), before initiating fusion and entering the main sequence. Constant temperature implies constant colour. For stars of such low mass this means the colour is in some shade of orange or red from the get-go. On the H-R diagram, these stars follow the vertical lines of the Hayashi track.

- Stars of intermediate masses (in terms of solar mass: ~0.5<M<~8) do change colour as they contract, moving at least in part horizontally along the Henyey track. The more massive a star, the larger the horizontal colour change. However, in rough terms, those on the more massive end of the scale, which end up white-blue in colour, do not emerge from the cloud red - they are already somewhere in the yellow region. Similarly, those that start out red don't ever increase in temperature sufficiently to end up blue.

- Stars more massive than roughly 8-10 solar masses - the main components of Rigel, Spica, and Antares all fall into this category - emerge from the protostellar cloud already on the main sequence. That is to say, when they become visible, they're already at the temperature (colour) they'll remain at - almost unchanged - for the reminder of their regular, short life (dying throes excluding).

(note: the transitions between these three are not sharp - e.g. low-mass end intermediate stars mainly follow the Hayashi track before doing a quick, short stint on the Henyey; those on the high-mass end emerge almost-but-not-quite ready for the main sequence)

To throw another wrench into the proposition stated in the OP, some massive stars end up shedding their outer envelope as they near the end of their lives, exposing the extremely hot inner regions. These are the Wolf-Rayet stars, and they used to blue before turning red late in life, and then blue again.

Combine this with the point made by the previous comments, and you should see that the red colour just by itself doesn't tell you much about the age of the star. Where blue-white tells you that the star hasn't lived for long, and won't live for long - because those always burn fast - red could be all sorts of things:
A massive or intermediate-mass star at the end of its life? Check.
A very massive star late in life but before turning into a blue Wolf-Rayet? Check.
A just-born yellow-white (intermediate mass) star? Check.
A red dwarf nearly as old as the universe with still billions+ of years to go, or at any stage of its life whatsoever? Check.
Heck, even a very old white dwarf that's cooled enough to glow red would pass the bar.

The stipulated red-blue-red metamorphosis (probably) can't happen though.

For reference, Hayashi (vertical, blue) and Henyey (horizontal-ish, blue) tracks on the H-R diagram:

(After Wikipedia; blue numbers = mass in terms of solar, red = isochrones of constant age)