Day: August 26, 2007

Milky Way Census

Stars and life-homes estimated for you

I am rather puzzled by just how many stars there are in the Milky Way too. Different sources give different figures, but ask an astronomer and they usually say 100 billion, roughly. That figure comes from actually measuring the light put out by the Milky Way and doing the sums.

If you look at the mass of the Milky Way – for example by taking the orbital radius and velocity of the stars at the galactic periphery, then working backwards – you get hundreds of billions of solar masses. However a BIG fraction is dark-matter and dark gas etc. and we really don’t know how much there is of either. If you look at the Milky Way from M31 and measure its mass via its satellite galaxy orbits you get about 1.2 trillion solar masses.

The total luminosity gives a less theory laden measure. That works out at about 55 billion solar luminosities and a baryon mass of about 60 billion solar masses. For a recent study check out this link. Divide that mass-figure by the average stellar mass and multiply by the fraction that is stars, and you only get about 100 billion stars. About 20 billion of those are roughly Sun-like. Assuming a Galactic disk age of 10 Gyr, a random spread of ages, and an oxygenic biosphere life-time of 1 billion years, thus there’s about 2 billion stars that could have planets with oxygen.

I’ll put my head out and say that 50% have terrestrial planets (Geoff Marcy’s estimate) and 50% of those systems have a planet in the habitable zone (Kasting’s estimate.) Thus there’s 500 million planets as old as Earth and in the right place for life-as-we-know-it. Not a lot different from Stephen Dole’s estimate from 1964 of 640 million. What’s different is that we now KNOW there are planets out there. Dole only know of a few possible planets – none of which are correct, though 61 Cygni is still a maybe.

But how many actually have life? A new study by researchers from my Alma Mater has demonstrated actual microbial remains from 3.5 billion years ago which is a boost to prospects for figuring out just when Life got started here. But does it tell us about Out There? Many popularists for SETI – the Search for ExtraTerrestrial Intelligence – argue that because Life arose very soon after Earth became stable (any time after 3.9 billion years ago) then it must be ‘inevitable’ and arise wherever it can. However the mystery of Life’s origin is still a matter of debate and very few facts. We do know that DNA-RNA style life is incredibly complex compared to basic organic chemistry, but we also know that cells make themselves using relatively small amounts of information. Their constituent molecules assembly themselves into an ordered whole very easily. How?

Until we know that the numbers of planets with Life might just be one.

Engineer the Sun!

Trying to imagine Life billions of years from now seems kind of futile to me. But if there’s any trace of us left then I can imagine they’d be familiar and yet very strange too. Certainly no end of SF writers have tried – Stephen Baxter, Poul Anderson and Charles Sheffield have had the balls to take readers to the end of Time, and beyond. Let’s be a bit more prosaic and stick to our little Solar System. So what would billions of years of technological advancement let us do?

We all know the Sun will evolve into a bloated, overluminous giant about 5 billion years from now – a timeframe that depends on the amount of heavy elements in the Core. According to a fairly standard model, the Sun’s future is as follows (in gigayears of the Sun’s age. Subtract 4.6 Gyr to get the date from now)…

(A) Core burning ends, t = 9.4 Gyr
(B) Redwards Traverse, end of Main Sequence, t = 10.9 Gyr (Sun pretty stable, Mars’ temperature rather nice)
(C) First RedGiant ascent, t = 11.6 Gyr (Sun goes from about 3 times present luminosity to about 2,400)
(D) Sun’s Core explodes, Helium burning begins, t = 12.1 Gyr (Sun pretty stable, Jupiter’s rather nice)
(E) Asymptotic Giant Branch, t = 12.2 Gyr (Sun goes from about 45 to 6,000 times present)
(F) Planetary Nebulae shed off, Sun dies as White Dwarf, t = 12.25 Gyr

Of course Earth, left to Nature’s course, dies long before the Sun even finishes Core burning. In about a billion years photosynthesis will crash and/or the oceans will be ploughed into the mantle by plate tectonics. Bacteria might survive for another billion, but eventually it’s all desert and a slow warming towards a Venus-like greenhouse. For Earth’s biosphere – and our “descendents” – the usual options are:

(1) extinction
(2) Move the Earth
(3) Live in mobile space colonies
(4) Put up sunshades
(5) Move to another planet

Instead of migrating, moving the Earth, or moving into space permanently – and they’re all options that might be taken – I would suggest a more radical option: engineer the Sun.

A few facts suggest this might be worthwhile.

First, the Sun will go red giant after using a tiny fraction of its total energy potential. This seems rather wasteful to me.

Second, magnetic fields can potentially reach all the way down into the Sun’s core. Thus we might be able to control the Sun’s energy output and its chemical evolution by inducing convection.

So just how much energy is available? If all the Sun’s mass converted to energy at current output it would last 14.5 trillion years. But it’s a giant fusion reactor instead. Proton-proton fusion, and associated reactions, convert 0.7% of the mass into energy. As the Sun is currently 74% hydrogen, proton-proton fusion would last 75 billion years using all the hydrogen. If we ignited helium fusion after that we might get another 30 billion years.

That sounds pretty good, but could we go further?

Some of the energy involved in the Sun’s evolution is from gravitational collapse. About half the Sun’s mass will collapse into a white dwarf liberating a few billion years worth. If the Sun could be collapsed further then even more would be liberated. The absolute limit is, of course, when the Schwarzschild radius is reached and we’ve made a black hole. If we collapsed the Sun into a quark-star just 6 km in radius we might extra a few trillion years of energy out of it.

Via reverse baryogenesis we might then extract all the mass-energy out of the remaining quark mass, thus getting the full 14.5 trillion years. All up we might extract 20 trillion years out of the Sun. But what happens then?

Instead of burning the Sun’s mass up perhaps we could change power sources. There’s a lot of dark matter around and the evidence is good that it self-annihilates with a release of real energy. Perhaps the Sun could be converted into a dark matter reactor? This is believed to happen naturally in white dwarf stars – but the power level is low. We might clever enough to develop a means of funnelling dark matter in to improve the output.

After all the options of this universe are tried perhaps we’ll have to look into higher dimensions to extend the Sun’s life even longer. We have a long, long time to figure out what to do.