I recently read that our humongous Galaxy was probably small in the beginning, but in time it accumulated stars by colliding with other galaxies. This conclusion was drawn from the fact that the outskirts of our Galaxy are composed of very old stars, which have the same composition as stars in dwarf galaxies. Our Galaxy is surrounded by lots of dwarf galaxies, we’ve just started discovering them. There’s at least 30 bright dwarf galaxies in our neighborhood which we already know and many dim ones are probably lurking in our vicinity. These dwarf galaxies contain very old stars, judging by their composition – these stars are very poor in chemical elements heavier than lithium, so they must originate from times when there was very little heavier elements in the Universe, or none at all. It turns out that the stars in the outskirts of our galaxy carry similar spectral signature.
From this I drew two conclusions.
Where to search for life
First of all there is very little sense in looking for life in dwarf galaxies or in the outskirts of our Galaxy, or any other behemoth. The areas which are poor in heavier elements are likely to have low probability in harboring life, because of the deficiency of the needed chemical elements. So the best place to start the search for other life forms is in the richer areas of big galaxies such as ours, or the Andromeda Galaxy which we are going to collide with in a few billion years.
This reasoning may be flawed. I am yet undereducated in the topic of how stars are forming. My guess is that the light gas contracts easier than heavier elements, which instead form clumps around the forming star and turn into planets after a few million years after they sweep all the dust in their orbits.
On the other hand I suspect that the more adventurous places of space are better for evolution. Our spiral Galaxy is thought to be moving its “top” side forward and our star is oscillating between the “bottom” and the “top” in cycles of around 70-90 million years. Whenever our solar system is near the “top”, it is exposed to the influence of the intergalactic medium. This probably induces recurring cataclysmic events, such as the big extinction event 65 million years ago. After every such event the conditions on our planet change significantly and wake up the evolution. Let’s put it this way: the dinosaurs dominated on Earth for about 135 million years and didn’t accomplish much. We, mammals, accomplished much more in only 65 million years after we’ve been given a chance after some rock leveled the evolution’s playing field. Similar cataclysmic events may have triggered the plants and animals moving out of the water to conquer the land and who knows what else.
If you look at the dwarf galaxies, they are dull. They have survived probably 10 or more billion years and remember the times when the Universe was poor in elements needed for life. If they were off to an adventure, they would have already merged with some big galaxy or became one on their own. No, they are small and full of old stars.
I am not claiming that there can’t be any life in these areas where old stars live, but the odds for life are much better in the rich parts of big galaxies.
The other conclusion I drew from the article is that all we have in our Galaxy and the surrounding galaxies has always been here. In the beginning when our area of the Universe formed it was filled with gas. The gas contracted into lots of small galaxies. These galaxies started merging. Two of them started consuming others and became behemoths – our Galaxy and the Andromeda Galaxy. The rest remained small and is probably slowly rotating around the center of our local cluster, which lies in the middle between the two behemoths. I suspect that all the material in our local cluster is the same as it was in the beginning. There was no or little exchange of material with other clusters. We could consider Andromeda as well as all our local dwarfs our siblings.
Not that it matters, the composition of stars in other clusters probably follows very similar patterns. We will eventually start exploring these too and know more about them. But I suspect the only way for galaxy clusters to exchange material is through civilizations capable of intergalactic travel.