|Posted on Wednesday, January 30, 2008 – 06:49 am:|
Found this really interesting update on what the Sun is going to end up like in a few billion years – and whether Earth will end up in a decaying orbit ploughing through red-hot solar material.
Distant future of the Sun and Earth revisited (pdf format file)
Table of important phases of the Sun…
First thing to note is that the official Main Sequence ends in about 5.42 billion years – but contrary to its TV depiction the Sun won't suddenly balloon into a humungous Red Giant. For about a billion years it will be bigger, brighter, but cooler than today, about bright enough to warm Mars to nice temperatures. This remains relatively stable. Then it will slowly swell as the core contracts until it peaks at ~ 12.17 Gyr of age, some 7.59 billion years from now.
A good time to move to the outer planet moons will be during the Helium Main Sequence – lasts about 100 million years, and the Sun will be ~ 54 times brighter. While a Red giant the Sun will lose ~ 1/3 its mass, and the planets will orbit ~ 1.5 times their present orbits to conserve angular momentum. Except Mercury and Venus, which get swallowed. The Galilean Moons will receive as much sunlight as Earth currently does, while Titan will get as much as Mars, during the Helium Main Sequence – though how well they will survive the Sun's climb to Red Gianthood is debatable.
The acronyms need some explaining:
ZAMS – Zero Age Main Sequence, the Sun at year zero
MS – Main Sequence, the Sun burning hydrogen in its core. About 10 billion years.
RGB – Red Giant Branch, the tip of which is its peak luminosity
ZA-He – Zero Age Helium Main Sequence, the Sun burns hydrogen in a shell around a helium burning core.
AGB – Asymptotic Giant Branch, the death-throes of the Sun,as it really runs out of fuel, throws off mass, and becomes a white dwarf.
And what of Earth? If its orbit isn't modified then it will be engulfed when the Sun hits its RGB maximum. The main cause will be due to tidal interactions with the bloated Sun's atmosphere, not merely drag with its outer layers.
At the Sun's RGB-tip the Habitable Zone will be beyond 52 AU. Which is roughly where Neptune will be, thanks to RGB mass-loss. What's left of the Kuiper Belt will form a vast band of comets across the sky as their volatiles boil away.
Jupiter's moons will experience about x45 Earth's present heat levels, Saturn's moons about x13, so none of them will remain as they are before hitting more clement weather on the He MS. But even at those energy levels their mass-loss won't be fast – Callisto & Ganymede will probably end up much like Europa, while Europa might keep much of its ocean.
Once the Sun's Core explodes and it heaves itself onto the He MS, then Jupiter might have some decent real estate orbitting it. In the table the symbol for the Sun (a circle with a dot in the middle) didn't transliterate from Unicode to WordPress.
So L/LiN is the luminosity in present Solar luminosities.
Similarly R/RiN, MSun/MiN, are the radius and mass in present day solar values.
Teff/K is the effective temperature of the Sun in Kelvin.
As you can see the Sun only gets a bit hotter, but then gets much cooler. Inside the Sun the Core rises in temperature as it shrinks – it needs +100 million degrees to turn helium into carbon/oxygen. That super-hot core, some 54% of the Sun's mass, is much smaller than the present Sun and so hot that it causes the outer layers to swell into a bloated red-giant. At the end of the AGB the last outer layer is blown away, exposing a white dwarf, hotter than any normal star. But it's no longer producing energy from fusion and rapidly cools to more star-like levels.
Ultimately all that carbon will crystallise – from the Core outwards gradually the Sun's corpse will become a gigantic diamond, solidifying totally perhaps a trillion years from now.