Read more: 鈥�Mysteries of the Milky Way鈥�
The Milky Way and Andromeda are siblings: two great spirals that dominate our local group of galaxies. They have about the same total mass, and we used to think they were near-twins.
Not any more. 鈥淎s we look in more detail, we see that they are quite different,鈥� says of the Herzberg Institute of Astrophysics in Victoria, Canada. Andromeda is the favourite child. It is brighter, with a wider disc of stars. The black hole at its heart is more than a hundred times as massive as ours. And while our galaxy is strewn with about 150 of the bright galactic baubles known as globular clusters, Andromeda boasts more than 400.
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One might ask, a little plaintively, whether Andromeda is an exceptional specimen of galaxyhood. It seems not. In 2007, Fran莽ois Hammer and his colleagues at the Paris Observatory in France compared Andromeda and our galaxy with a sample of more distant galaxies. They found that whereas Andromeda is a pretty well-adjusted spiral, the Milky Way is an oddball 鈥� .
That is probably because typical spirals such as Andromeda are transformed by collisions with other galaxies over their lifetimes. These violent events shake up the galaxy鈥檚 gas to form new stars and globular clusters, stir up the disc so it spreads farther out, and perhaps send some gas and stars plunging into the galactic heart to feed a more monstrous central black hole.
If this is to explain our sibling inequality, then the Milky Way must have lived relatively undisturbed. Except for encounters with a few little galaxies such as the Sagittarius dwarf, which the Milky Way is slowly devouring, we wouldn鈥檛 have seen much action for 10 billion years.
Perhaps that is why we are here to note the difference. More disturbed spirals would have suffered more supernova explosions (see 鈥淢ilky Way mysteries: Where are all the supernovae?鈥�) and other upheavals, possibly making the Milky Way鈥檚 rare serenity especially hospitable for complex life. 鈥淲e are still a long way from being able to answer that,鈥� says McConnachie, 鈥渂ut it鈥檚 not a crazy suggestion.鈥�