Dear Urban Astronomer
I recently read that the andromeda system is our closest neighbour. Apparently, our galaxy is on a crash course with the andromeda system. They make it sound like its going to happen in the next few decades. I was just curious how much bigger is andromeda? How close is it to us? And how close are we to impact. Thanks in advance,
First thing first, we are indeed on a collision course with the Andromeda Galaxy, but we’ve got rather more than just a few decades to get our affairs in order, so I wouldn’t worry too much just yet.
The galaxy in question is known by a great many names and catalogue numbers. It’s a prominent naked-eye object (astronomer speak for “It’s a faint smudgy thing that you can see without a telescope if the sky is dark enough”) in the Northern sky, in the constellation of Andromeda. It is a classic spiral form galaxy, about twice the size of our own galaxy. It seems strange now that we didn’t know what it was until the 20th century, but until then it had been assumed to just be a largish spiral nebula, within our own galaxy. The generally accepted view of the universe had generally been confined to what we now call the Milky Way galaxy – our own local grouping of hundred billion or so stars, seen as an island of light in an ocean of black nothing. But by the turn of the 20th century, some astronomers had begun to suspect that there might be more out there, and that perhaps some of those tiny spiral shaped nebulae might in fact be other “island universes”.
It was a revolutionary idea, which ran into a great deal of resistance, until eventually a famous debate was held in which the most prominent astronomers from each camp (island universe vs. spiral nebula) presented the evidence for their position, in the hope of settling things once and for all. One of the strongest arguments produced against the island universe hypothesis was the fact that for these nebulae to contain billions of their own stars and still look like mere smoky smudges, they would have to be impossibly far away – millions of light years. The thought of such a great distance was simply preposterous, the universe simply wasn’t that big, therefore there was no such thing as other galaxies.
Of course, today we know that there are not only other galaxies, but that they outnumber all the stars within our own galaxy, and that the parts of the universe that we can see extend many thousands of times further that the mere millions of light years discussed in those debates. The universe is so much vaster than those old astronomers ever imagined! But even those who had glimpsed the truth did not realise that, despite the immense distances involved, galaxies can and do collide quite regularly. Of course, a galaxy is mostly just empty space, with the stars so widely separated that they simply never collide. But the immense gravitational fields of galaxies can reach out to enormous distances, and as galaxies approach each other they draw long tendrils of stars and dust from each other. Their “collisions” are graceful, beautiful events, which stretch them out into beautiful ethereal forms before eventually merging into a new, larger, single galaxy. This is exactly what will happen to the Milky Way and its larger sister, the Andromeda galaxy.
But it is a slow process. Galaxies are so large that even if they were travelling at the speed of light, it would still take hundreds of thousands of years for them to pass each other by. Since they move a great deal more slowly than light, the entire process of colliding, deforming, swinging around each other and eventually coalescing into a larger elliptical galaxy takes many hundreds of millions of years. By the time it starts, we humans will likely have either gone extinct, or evolved into some new species that we wouldn’t recognise if we could meet it today. And the process itself happens so slowly that whoever is living on Earth at that time would not notice any fireworks or excitement at all. The distorted shapes of the two dancing galaxies would appear as a static backdrop to the night sky, not changing for millennia, just as the Milky Way has done for the full span of human history.
So nothing to worry about then. We’ll be long dead before it happens, and even our ancestors will not see much of a show. Sorry about that!