The Sun and the End of the World?

OK, I’ve got a question about this article over on the ABC New site. Basically it’s a report on these two scientists who are claiming that all life on Earth will be extinct in 500 million years. (So, don’t make any long term plans.)

The reason for this, they claim, is that the sun is getting brighter. Specifically, they say:

This is where I have my question. The Sun is (I thought) a main sequence start and (I thought) main sequence stars maintained pretty much the same output throughout their lives (except for the beginning and ends). So, how is the Sun “constantly growing brighter”? Am I missing something here?

Sigh… ABC New = ABC News and main sequence start = main sequence star. Believe it or not, I previewed too…

I heard the same thing, tanstaafl that the sun burns hotter the older it gets. I don’t know where I heard it or if it’s true.

This site says the sun has increased about 40% in output. IIRC, it’s generally accepted to be quite a bit longer before Earth becomes uninhabitable.

Isn’t it weird how to threads near the same topic come up at near the same time, or was this thread sparked by my recent one, tanstaafl?

Shoot, and I just bought a timeshare that I was hoping to make my money back on…

Well, there’s your problem right there.

Animal planet caused a stir with the same stuff not too long ago.

tanstaafl - you are right that the sun is an MS (main sequence) star. You are also right that MS stars maintain “pretty much the same” luminosity throughout their stay on the MS. The thing is, a 40% increase (factor of 1.4) is within the bounds of “pretty much the same”. The MS is thick, in that stars move around in it a little over the course of their lives. They begin at one edge, in the ZAMS (zero-age main sequence) and increase in luminosity as they deplete their Hydrogen. Higher-mass stars also become somewhat cooler. When they run out (for the most part) of Hydrogen is when they shoot off the MS. A star like the sun will increase in luminosity by a factor of maybe 1.8 while on the MS, and a factor of maybe 200 off it.

Achernar, thanks. I didn’t realize that MS stars fluctuated that much. Of course, given that I just got out of a meeting where a user was upset because my report was off by 1 passenger out of 3.5 million flights, maybe I was using too strict a definition of “pretty much the same”. :rolleyes:

So, instead of a few billion we’ve only got a few hundred million years left. Guess I better get started on cleaning up the garage…

Clarissa_xx - nope, no connection to your thread. I actually saw yours for the first time when I came by to post this one. I had just seen the report I referenced for the second time and that question just kept coming back to me. Interesting coincidence though…

As I understand it (and it’s been a while since my stellar interiors class…) the hydrogen in the Sun’s core is fused into helium. This helium builds up in the core. It does not fuse (initially) because it takes a lot of pressure and heat to fuse helium into carbon.

As the helium builds up, the core contracts a bit. This increase in pressure in the core also raises the temperature. Fusion rates depend very strongly on temperature (something like T^15, yes, 15). So the fusion rate goes up, and the star increases a bit in size and luminosity.

So as the star ages normally, it heats up and gets brighter. This takes time, millions or billions of years to make a difference.

Things will get worse; the helium will build up in the core, and the hydrogen will fuse merrily around around it. The Sun will expand into a red giant and possibly consume Mercury and maybe Venus. If the Sun has enough mass (and it might), there will be some core helium fusion into carbon, and the Sun will eventually become what’s called an asymptotic red giant (named for its position on a plot of luminosity versus temperature). Then it gets really big, and may very well consume the Earth.

After that, things get really interesting… but I may be writing an article about this soon, and don’t want to let too much information go for free. :slight_smile:

Anyway, see here for more details.

And if we can’t get off this planet in 500 million years, well, then we had our chance.

Isn’t it more like 5.5 billion years from now.

:smiley: [sup]Like it matters, to us. But it might change SwimmingwithChickens’ plans for a condo.[/sup]

I thought that the ancient Mayan Prophecy was the reason. They believe that on December 10, 2012, at 10:45 PM the world is going to end. hmm…it also spoke about the sun’s number of degrees it’d be it was very strange indeed.

Well, the sun may survive for that long, but that doesn’t mean that we will.

;j <-- And what the hell is this? Happy orthodox jewish man? Is that smiley really so necessary?

You’d be amazed how often it comes up. :slight_smile:

Bad Astronomer - Thanks, that cleared everything up. Ignorance has been fought once again.

Terraforming mars wold give humanity a few million extra years when the sun starts to go ker-poof.

While it may be about 4 billion years before the sun swells and possibly eats the earth, the increased solar output while create a run-away greenhouse effect long before that. Probably in about a billion years or so the average earth temperature would be higher than the boiling point of water, and it would quickly rise to venus-like conditions since water is a potent greenhouse gas on its own.

If we started relatively soon however, it would be quite easy for a slightly more advanced civilization to move the earth away from the sun at a rate that would extend it’s habitability by billions of years. (If you have a billion years to work with, even moving the earth is relatively easy.)