View Full Version : The Sun's future

Hat Monster
2006-Mar-29, 03:05 PM
There's a conflict here. It relates to what will happen to the sun in ~3Ga. Commonly held is that it'll expand to be a red giant and engulf Mercury, Venus and maybe Earth.

But that's lacking common sense.

No, bear with me, this isn't a pseudoscience ramble. When was the last time we observed a one solar mass, 1AU diameter star? Five solar masses, ten solar masses, perhaps. But not something as lightweight as Sol. Want some examples?

Name Mass (Solar) Radius (Sol) Spectral Type Perception
Sol 1 1 G2Vp Hydrogen fusing main sequence dwarf
Arcturus 1.5 26 K1III Helium fusing giant
Aldebaran 1.0 40 K5III Aged helium fusing giant
Sigma Gem. 1.5 9.2 K1III Helium fusing giant
Anser 1.5 45 M0III Transitional giant
Eta Sag. 1.5 62 M3.5III Transitional giant or new helium fusing giant
Gamma Sco. 2 110 M3III Second stage shell giant
Pollux 1.7 9 K0III Helium fusing giant

I hope that lines up right. The biggest of the lot there is Gamma Scorpii, a two solar mass star at 110 solar radii. As one solar radius is 0.0047AU, that equates to 0.51AU, barely enough to roast Venus. We don't seem to get a full AU in radius until stars are kicking around four or five solar masses.

So my question would then be, where are people getting the idea that Sol will get much larger than any other star of its type?

2006-Mar-29, 04:04 PM
I have a feeling that the story is coming from parents who need to tell their kids something when the question comes up.

2006-Mar-29, 04:19 PM
I have a feeling that the story is coming from parents who need to tell their kids something when the question comes up.

No, it has a stronger basis in observation than that. We may not have an example of a 1 solar mas star getting that big, but the measurements of star masses and sizes have improved since the old text books. The Sun will expand. We don't know wht the maximum size will be, and there's a fairly wide range of possibilities.

Ken G
2006-Mar-29, 04:24 PM
I've generally heard that the Sun will not engulf the Earth. Note that it will get even bigger the second time it expands to a giant (asymptotic giant), but by then it will have lost mass and Earth will be farther from the Sun.

2006-Mar-29, 04:39 PM
I don't think there is a conflict; the astrophysical equations governing stellar evolution are known quite well. I'm guessing that we don't see many stars with 1 solar mass with a diameter of 1 AU is for the following reasons:

1.) They don't stay in this phase--which I'm guessing is the Asymptotic Giant Branch--long at all, only a tiny fraction of the time they spent on the main sequence. That favors none being close by. For our own Sun this stage will take only 21 million years:


2.) It is difficult to estimate the mass of any non-main sequence star (such as an AGB giant) in a non-binary system, so there may be plenty out there that are indeed ~1 solar mass.

3.) Stars of 1 solar mass are not particularly common anyway; the G-class as a whole only consists of maybe ~5-7% of stars, and of those an even smaller number are 1 solar mass, and a smaller number yet are in the AGB phase.