In a galaxy far far away . . .

Peter Shakes, of Wellington, asked:

If light takes 13 billion years to get here from a recently spotted distant galaxy, did the galaxy form that long before ours, and consequently would any life there have a 13billionyear headstart on us?

Duncan Steel, of the Centre for Space Science Technology in Alexandra, responded:

Let me start with the answers to the two questions posed: no, and no. The interest lies in the reasons for those answers.

So far as cosmologists understand it, the universe began about 13.7 billion years ago in the socalled Big Bang. It is generally thought that over the following halfabillion years, vast clouds of hydrogen and helium accumulated, such clouds being held together by gravity, and they collapsed so as to form individual stars in the massive structures we term galaxies. A typical galaxy contains over 100 billion stars.

The first galaxies coalesced around 13.2 billion years ago. It is thought that the majority of galaxies formed at that time, with only a few latecomers.

The above is the cosmological viewpoint, but it is confirmed by astrophysicists who study how stars evolve over time. The oldest stars in the Milky Way (our own galaxy) are a bit more than 13 billion years old.

The reason for the newsworthiness of a galaxy being charted more than 13 billion light years away (i.e. the light has taken that long to reach us), is that we are seeing it quite soon after it was formed, and that can tell us much about how the universe has altered over the aeons.

That explains my first answer ‘‘no’’ above: that galaxy formed around the same time as our own Milky Way, and we are seeing it far back in time because it is a vast distance away from us.

Would any life there have a 13billionyear headstart on us? Again I answered ‘‘no’’. The reason is that life needs heavier elements than hydrogen and helium, and these atoms, such as carbon, oxygen and nitrogen, take time to form in the cores of stars. All elements lighter than iron can be generated in nuclear fusion reactions in stellar interiors, with stars much more massive than the sun being required to produce many of those atoms (such as sulphur and phosphorus) essential for life. Elements heavier than iron (such as lead and uranium, but also some elements used in living things, such as zinc and iodine) can only form in the extreme circumstances of a supernova explosion.

Similarly, the formation of rocky planets like Earth requires elements such as silicon, sodium and magnesium to be available. This means that planetary systems like our own could not form until after many stars had gone through their lifetimes, spewing out the building blocks from which rocky bodies could agglomerate.

Although the Milky Way is around 13 billion years old, the sun and the solar system are only about onethird of that age.

It is likely, however, that there are planetary systems a few billion years older than Earth, in the Milky Way and other galaxies. Any life originating there could indeed have a ‘‘headstart’’ on terrestrial life. The fact that we see no evidence for extraterrestrial life (yet) can be used as an argument for life being a rare phenomenon, perhaps unique to our own planet.

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