09 January 2010

Galaxies and Life; why not all galaxies are created equal

The Milky Way Galaxy, our home (in a very broad sense), is what's called a barred spiral galaxy, with something like 300 billion stars. When I took Astronomy 101 in college more than 35 years ago, that sentence would have left out the word ‘barred’ and would have said ‘100’ billion stars, but knowledge is always changing, and in the field of Astronomy, understanding has progressed in the last hundred or so years literally by leaps and bounds.

300 billion is a, well, astronomical, number, so large that we can hardly imagine it, even though we use the same kind of numbers for dollars… which we don’t really grasp either. The mind fogs at the prospect of such numbers. But despite this huge number of stars, a galaxy is almost entirely empty space. Typically there are only one or two atoms per cubic meter on average. All those stars are set amidst an incredibly tenuous gas, which in some places is a bit denser, less so in others, but nowhere, other than in actual objects like stars, comets, and planets, anywhere near as dense as air, just as an example. And the stars are almost unimaginably far apart. Around here (out in the disklike plane of the Galaxy, about 30,000 light years (~250-300 quadrillion kilometers) from the center), stars are typically roughly 3 to 5 light years apart (and this is a slightly denser than average region). A typical star is smaller than the Sun’s roughly 1.4 million kilometer diameter, and the typical distance separating typical stars translates to something like 50 trillion kilometers.That's still an almost unimaginable distance. At the escape velocity of a rocket leaving the Earth, it would take over 100,000 years to get to the nearest star.

M31, the "Andromeda Galaxy;" about 2 million light years distant

That’s hard to imagine. So picture this. If the Sun were a basketball, the next nearest star, which happens to be roughly the same size, would be about 10,000 kilometers away (Alpha Centauri A, ignoring its slightly smaller companion and much smaller distant companion, which just happens to be a shade closer, Proxima Centauri). That's Los Angeles to Eastern Europe. The Earth, in contrast, would only be about 50 meters away. So, you realize that on the interstellar level, the universe is very, very empty, and stars are very very far apart. Two galaxies can pass right through each other without a single stellar collision; in fact it happens all the time, in the grand scheme of things. The Milky Way is in the process of swallowing up a medium small dwarf galaxy right now, and will eventually swallow up its companions the Clouds of Magellan, and in turn be swallowed up after colliding with the Giant Spiral M31 in Andromeda (commonly referred to as the "Andromeda Galaxy"). But don’t worry, that’s billions of years in the future.

Galaxies are gravitationally bound systems of stars, with lots of gas and dust typically, although there are significant differences in their forms and histories that are very important for questions such as the origins of stars like the Sun and planets like the Earth, and, by extension, the origin of life as a sort of scum clinging to the surface of a few particularly situated such planets. Of course, there are all kinds of complications to all that. Galaxies all formed at about the same time, when the universe was cooling and expanding, and knots of density coalesced into, well, galaxies. Almost all galaxies have massive black holes in their central regions, which swallow huge amounts of matter and make the central regions not particularly hospitable. Galaxies, like everything in the universe, are actually mostly dark matter, the specific nature of which no one really knows, but for a basic understanding of what a galaxy is, and why some galaxies are likely abodes of living beings, including possibly, complex biospheres and intelligent creatures somewhat comparable to ourselves; and some are not, this will do as a basic description. They are all but unimaginably vast systems of stars, which can be categorized by their form or shape.

Galaxies are typically found in clusters, which form clusters of clusters. In the center of clusters, collisions are common, and most of the galaxies have already experienced collision and being swallowed up or swallowing up their neighbors. This has an unfortunate consequence for the eventual origination of life. Colliding galaxies, it turns out, typically expel a lot of their interstellar gas back into intergalactic space, where nothing much happens. This gas, however, is the stuff that stars are made out of. And so, galaxies near the centers of clusters typically stopped forming stars a long time ago. This is typical of what are called Elliptical galaxies (whether found in cluster centers or not). Ellipticals range from roughly spherical to elongated ovoids, and range from rather small galaxies to the very largest, in some cases hundreds of times as massive as the Milky Way (these kind have generally absorbed many other galaxies in the process of growing to this size over billions of years). This latter kind of galaxy is common near the center of clusters. Typically, Ellipticals (large and small alike) consist almost entirely of what are called Population II stars… older, very poor in metals (because they formed from what the universe was made of at the time, almost pure hydrogen and helium), and really kind of boring. Such stars can’t have rocky planets like Earth. They probably could never support complex biospheres because all the interesting elements; carbon, nitrogen, magnesium, potassium, phosphorus, oxygen, etc., etc.; necessary for life are pretty much absent. And since whole galaxies consist of these kinds of stars, with little or no gas and dust, and no new star formation, it’s a safe bet that these types of galaxies are essentially devoid of living worlds and life altogether. Wow, boring.

Then there are zillions and zillions of tiny dwarf galaxies, which are common in huge numbers in the general vicinity of other galaxies. These little dwarf galaxies, too, mostly consist of older, boring stars lacking complex chemistry, and almost certainly not capable of being the nursery of living worlds or life. (It turns out the universe is kind of like suds… the galaxies form the soap, and there are big voids where there’s pretty much nothing, but that isn’t really important. We’re talking about the volumes of space where there are galaxies in appreciable numbers. Our galaxy is located on a sort of tail end of a moderately large cluster, called the Virgo Cluster).

So what’s left? The spirals (and some of the irregular small but not tiny galaxies like the Large Cloud of Magellan). These include the barred spirals, more or less shaped like “S”, with a bar in the middle, like ours (this fact about the Milky Way has only been known for a few years). These galaxies are mostly quite flat and disklike, except for their spheroidal central regions and great clouds (haloes) of older stars and clusters that surround the entire galaxies in rough spheres. Picture a pair of CD's glued together, with a marble in the hole surrounded by a tenuous sphere of haze. That's the approximate proportion. Much of the mass is in the central regions, which are a lot like elliptical galaxies themselves, with energetic black holes in the centers of very dense regions, a lot of radiation, almost all Population II stars, and no star formation going on. The haloes of old star clusters and some individual stars are also old, Population II, and therefore lacking complex chemistry. So what’s interesting is the disk regions, which are where the spiral arms are, but also where there is a sort of general “disk population” of stars. The Sun is a disk star, located smack in the Milky Way's galactic disk, about 30,000 light years out from the center. (The disk is a bit over 100,000 light years in diameter).

Here, in these galactic disk populations, is we find active new star formation, and the enrichment of the interstellar medium from recycling of old exploded stars. This is where all those interesting elements beyond helium on the periodic table, which make life possible, come from. Stars which have formed later than the original formation of the galaxy in which they find themselves, and which incorporate this residue (these are referred to as Population I stars). These stars, of which the sun is one, are the kinds of stars that can have rocky planets, and, which, at least in the case of the sun, are capable of providing a suitable location for the evolution of life.

Spiral galaxies (including barred spirals, which are roughly half of them) together only make up about 20% of all galaxies, so right off the bat 80% of the galaxies seem to be unsuitable for the formation of living beings like ourselves anywhere in their vast reaches. There are probably some exceptions to this, since the morphology of galaxies is actually extremely complex, and many galaxies may have no new star formation but still have some Population I type stars, with metals (meaning everything heavier than helium) in their makeup. But however you look at it, not all galaxies are equally likely to be homes of living creatures. The beautiful flat spirals like our galaxy are the best candidates.

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