I have heard from time to time that when our sun goes off main sequence, in another five or six billion years, it will expand into a red giant and consume the earth. But what does that mean, really? If you take the mass of the sun and fill a volume 93 million miles in radius with it, do you have a noticeable gas, or is it so diffuse that a guy on earth (probably in a space suit) would barely perceive it? What density of gas would it take to put sufficient aero-drag on the earth to pull it into the sun?
Anything past the photosphere (the surface) of the sun is opaque with light/photons.
The sun by then will have shed about 30% of its mass, but it will still be bright and hot as hell. The density of the sun depends a lot on where you are within the sun’s atmosphere and its inner workings. But no, no one will want to be around for this.
We’re still not sure where Earth is going to end up at this stage, whether it’ll be engulfed or move outward as the sun looses mass, but due to the sun increasing in intensity with age, it’ll be way too hot on earth to support life well before we burn out the main sequence.
We’ve got about probably less than a billion years to find a new home.
One might say that Earth is already inside the Sun, if you count the solar wind, which can be considered an extension of the Sun. Not only that, it has a temperature of around 150,000 K around Earth; the extremely low density prevents it from having any heating effect.
You might, but that’s not the definition anybody else uses. I know we get into pointless nitpicking and extremes in these threads, but I think this goes too far to be useful or informative.
Or figure out how to move the earth out of the danger zone.
We can use the moon as a tug boat.
Then again, whatever our descendants have become in 1 BY there’s no telling if we’ve moved on from earth, or if we’ll even still be around, at all.
Quite.
When I was reading Phil Plait’s book “Death from the skies” he seemed quite sure that the death of the sun was a trivial concern compared to the other risks to the earth out there in the universe.
Like the collision with the Andromeda Galaxy and the Milky Way…
First of all, it’s not as likely as the sensationalists like to make out. We’re currently getting closer together, it’s true, but that’s about all we can say. It’s more likely that we’ll swing close by each other and then move back further out than it is that we’ll actually collide. And second, even if we do, it’s not likely to do anything at all to us. Galaxies are composed almost entirely of the vast empty spaces between the stars, not by the stars themselves, so even if galaxies collide, the stars will mostly just pass by each other.
Don’t we base our projections on how big our sun will become in the red giant phase on the estimated size of Betelguese (which would extend to at least the asteroid belt)? Just something I seem to remember reading…
That isn’t the problem; the problem is that the surface of the Sun being so much larger and closer, or Earth being actually inside the Sun means that the Earth is likely to be boiled away.
There’s quite a few red giants out there, so not just Betelgeuse. It’s much more massive than the Sun anyway.
Betelgeuse is surely one significant data point used in calibrating our stellar models, but it’s not in any way a direct model for what the Sun will be like. At the simplest level, you’d find a bunch of red giants of various known mass and radius, fit a curve to them, and then predict the radius of giant-Sun by finding its mass on the curve. You’d be using the data from other stars, but none of them would necessarily be all that close to the Sun.
Golly, we better get moving on this. Reminds me of Sinclair on Babylon 5 justifying exploring space. We have to, since this whole planet is gonna blow eventually.
I did some rough math using a 95 million mile radius for the current mass of the sun and came up with an average density of around 415 milligrams to the cubic meter, similar to the upper edge of the stratosphere. But, being an average, that is a high-ish number.
Gravity would keep a significant fraction of the sun’s mass near the core, but heat would also drive a fair bit outward, I have no idea how the math works out, but it seems like there would probably be a drag-worthy atmosphere out here. Yet, would it pull the earth in or would it push the earth out as it expands (like skipping a stone)?
One thing is for sure: before the sun gets here and fully erodes the earth’s atmosphere, there are going to be some awesome auroras all over the place.
Oh, and I just ran across this cool image of a star like our sun going through the much later white dwarf phase.
Oh, yeah, planetary nebulae make for some of the best astrophotographs.
Is this true? I was under the impression that we had another 4 billion or so years. If so, I need to change my plans.
We have about that long until the Sun goes red giant. However the Sun is warming over time, and it’s estimated that in about a billion years the Sun will have warmed to the point that the Earth undergoes a runaway greenhouse effect and ends up like Venus. Assuming no intervention by whatever civilization exists that far in the future, if any.
So…you see global climate change isn’t humans’ fault! Hah! Gotcha!
Even a billion years is being too generous. Plant life has around half that time left, and once the plants go…
It’s a supergiant with a mass at least ten times that of our sun, so I don’t think its evolution and eventual fate are the same as our sun.