The time of becoming a red giant

[fawk3s]

How long would it take for a star, say approximately the mass of our Sun, to become a red giant? What I mean is the process of actually changing into a red giant. I know the star changes over its lifetime, but what I haven't found much information on is how fast the forming of a red giant happens.

Thanks in advance,
fawk3s

 

[Kracatoan]

The time a star spends in the stable main-sequence phase of its life depends on its mass, in general, lower mass stars will be main-sequence for longer, whereas higher mass stars move on more rapidly.

As for our sun, the current estimate is that is about half way through its 10 billion year long main-sequence phase and hence will become a red giant in about 5 billion years.

Main-sequence stars become red giants when the exhaust their reserves of hydrogen and instead start fusing heavier elements. The reason higher mass stars go red giant sooner is because they burn fuel much, much faster than lighter stars, even though they have more fuel to "burn".

This is not an exact science and is more based on observations than theory, so any times are merely approximate, not that it matters due to the enormous time scales.

 

[fawk3s]

Im not sure you fully understood my question. I asked how long does the process of actually changing into a red giant take, aka the expansion.
From what I understand, the star changes over its lifetime anyway, but is there a line from which on we can tell the star is actually changing/expanding into a red giant? And how long does this process take before the star stops expanding and has fully become a red giant?

Thanks in advance,
fawk3s

 

[Kracatoan]

Oh sorry, um, I don't think we're that sure as to how long the transition of main-sequence to red giant takes. All I can find are vague estimates of around 1 billion years (wikipedia). I think this is because the star doesn't suddenly become a red giant, it just slowly and steadily gets larger and larger of the course of around a billion years or so, becoming more red as the outer layers cool down. The fastest process is the implosion when it reaches the end of the red giant stage, and I believe this takes a matter of minutes.

 

[qraal]

How long would it take for a star, say approximately the mass of our Sun, to become a red giant? What I mean is the process of actually changing into a red giant. I know the star changes over its lifetime, but what I haven't found much information on is how fast the forming of a red giant happens.

Thanks in advance,
fawk3s

Hi fawk3s
The process takes about 2 billion years from when the Sun goes off the Main Sequence. Most of that time the Sun changes a bit, increasing to about 2.7 times it's current luminosity during the Redwards Traverse. Then in the last 100 million years the luminosity increases dramatically, hitting about 2700 times it's current luminosity in a rapid climb during the last 10 million years.

By this stage the Sun's core is ready to burn helium, but the Triple Alpha reaction has a very high temperature sensitivity and is initially unstable, causing the core to explode in the Helium Flash. The Flash takes mere minutes, but may have no visible change at the surface while it occurs, as most of the Flash's energy goes into heaving the core onto the Helium Main Sequence, which lasts a 100 million years at a steady ~50 times current luminosity. Once the helium is all burnt, the Sun then undergoes a series of pulses which rapidly inflate it into a Red Giant like state which lasts just a few thousand years each time. This is the Asymptotic Giant Branch, which ends with almost all fusion fuel gone and the Sun a hot, young white dwarf.

 

[fawk3s]

Hi fawk3s
The process takes about 2 billion years from when the Sun goes off the Main Sequence. Most of that time the Sun changes a bit, increasing to about 2.7 times it's current luminosity during the Redwards Traverse. Then in the last 100 million years the luminosity increases dramatically, hitting about 2700 times it's current luminosity in a rapid climb during the last 10 million years.

By this stage the Sun's core is ready to burn helium, but the Triple Alpha reaction has a very high temperature sensitivity and is initially unstable, causing the core to explode in the Helium Flash. The Flash takes mere minutes, but may have no visible change at the surface while it occurs, as most of the Flash's energy goes into heaving the core onto the Helium Main Sequence, which lasts a 100 million years at a steady ~50 times current luminosity. Once the helium is all burnt, the Sun then undergoes a series of pulses which rapidly inflate it into a Red Giant like state which lasts just a few thousand years each time. This is the Asymptotic Giant Branch, which ends with almost all fusion fuel gone and the Sun a hot, young white dwarf.

This is what interests me the most. How fast does the star inflate?
What are these pulses though and how do they inflate the star?

Thanks in advance,
fawk3s

 

[Vanadium 50]

If you want an exact time, you need to decide what exactly your starting and stopping points are. How do you decide on what day your star is not yet a red giant and on what day it is? If you can do that, you can get a crisp answer. As long as the boundaries are fuzzy, you can only get answers like "several hundred million years".

 

[fawk3s]

Well, let's take the above. Say the star has entirely run out of helium.
I can see that the process is slow, but what I don't grasp yet is how fast does the expansion of the star happen? Is it like with the brightness and the color change, or once ran out of helium, the star expands rapidly?

 

[qraal]

When I say "run out" I mean in the core. There's still some in an outer shell which contracts onto the core and undergoes a mini Helium Flash style fusion runaway. This extra energy puffs up the outer layers via heating - the only exit for heat is via radiation from the star's surface,

 

[fawk3s]

Previously you said the inflation happens quite rapidly. What I would like to know is how fast is this 'rapid'? Havent found an answer to that.

 

[qraal]

Previously you said the inflation happens quite rapidly. What I would like to know is how fast is this 'rapid'? Havent found an answer to that.

Takes about 10,000 years with each pulse. So, for a star, quite rapid.

Here's an older, but still wortwhile reference which details the Sun's evolution...

http://adsabs.harvard.edu/abs/1993ApJ...418..457S"

...from Arnold Boothroyd, Juliana Sackmann & Kathleen Kraemer.

A somewhat more recent paper, with similar conclusions is by Schroeder & Smith...

http://adsabs.harvard.edu/abs/2008MNRAS.386..155S"

...together they should give you sufficient to get a rough idea of how long the different phases take. But a detailed timeline requires actual modelling to describe properly. There are several unknowns, like opacity, mass-loss rates and the Sun's core Z fraction, which are all areas of active research, but the broad features are correct.

 

[qraal]

A somewhat more recent paper, with similar conclusions is by Schroeder & Smith...

http://adsabs.harvard.edu/abs/2008MNRAS.386..155S"

...from that paper there's a diagram of the Sun's radius evolution over the last 4 million years of its climb up the Red Giant Branch to its maximum size as a Red Giant (as opposed to an Asymptotic Giant, a shorter lived phase.) The Sun grows from 0.4 AU in radius to its maximum of 1.19 AU in that last 4 million years.

Then the Helium Flash occurs and turns it into a Helium Star, with a hydrogen burning shell. About 0.3 Solar masses of gas and dust is blown off as a very strong stellar wind in the last ~1.5 million years prior to the Flash and it's during this time that Mercury, Venus and probably Earth would be engulfed by the Red Giant Sun, due to tidal forces and direct gas drag.

The Shell Helium Flashes during the AGB phase are relatively rapid, taking mere thousands of years to inflate the Sun, and the AGB phase blows off about another 0.12 Solar Masses, perhaps enough to make a short-lived Planetary Nebula. The final White Dwarf mass is about 0.54-0.55 Solar Masses.

 

[fawk3s]

Thank you for the info. I think I got what I needed!