Monique Keiran: A collision of galaxies is coming - in about four billion years

A century ago, our idea of what the night sky was showing us was pretty simple.

The perceived wisdom of the time was that the universe consisted entirely of the Milky Way galaxy — a single, expansive field of stars, glowing clouds of dust and gas and luminous, sometimes spiral-shaped nebulae (fancy talk for “clouds”).

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Although some pointy-headed people speculated that, perhaps, some of those ­nebulous formations may be “island ­universes of their own,” that talk was ­controversial.

Even by 1917, the latest research ­suggested that the Milky Way — that is, the universe — was 300,000 light years across and 30,000 light years thick at its bulging centre. These dimensions exceeded any ­previous estimates by 10 times.

That was just the start. Throughout the 1920s and ’30s, the universe underwent several major changes. A series of discoveries made on the biggest, most advanced telescopes of the time, including the one here on Little Saanich Mountain, revealed that our place in the universe is not just not central, it was downright peripheral and insignificant.

Of course, it was our puny, imperfect ­perception of the universe that shifted. The universe itself continued on just as it had done for the previous 14 billion years.

The revolution in cosmic understanding accelerated in late-December 1924 when American astronomer Edwin Hubble revealed that, actually, the Milky Way wasn’t it. Some of those mysterious luminous blobs that had, to that point, been called “nebulae” were in fact systems of stars as vast and numerous as the Milky. There was, Hubble announced, more to the universe than the Milky Way, and the Milky Way galaxy was just one of a to-be-determined number of galaxies.

Scant years later, our own John Stanley Plaskett, of the Dominion Astrophysical Observatory, completed a 10-year, detailed inventory of the speeds and direction of travel of some 500 of the oldest, most distant (to us) stars in the Milky Way. He used this information to confirm a discovery made by Dutch astronomer Jan Oort the year before, in 1927.

Using data observed from 1,000 stars about 1,000 light years from our sun, Oort had demonstrated that the Milky Way galaxy disc doesn’t just float passively through space, it spins like a giant, spoked wheel.

Hubble topped that in 1929. He said the universe — which, by then, was understood to consist of many, many galaxies — is expanding, and the stars most distant from us are receding the fastest. He arrived at this conclusion by using speed and distance data for 24 nearby galaxies. One of these was M31, the Andromeda galaxy, our neighbour, located just 2.5 million light years away.

Discovery after discovery followed. Plaskett made another appearance on the world science stage in 1936. He provided the first detailed analysis of the structure of the Milky Way galaxy. He demonstrated that our solar system is located in a backwater region on a spiral arm two-thirds out from the galaxy’s centre and that the galaxy rotates once every 220 million years — measures still considered accurate today.

We’re still learning about our galaxy and the greater universe today. Astronomers revealed a few years ago that the Milky Way and Andromeda are heading straight for each other. About four billion years from now, a galactic pile-up will start changing the night sky.

Some researchers say the collision will mean the two galaxies will merge completely over a period of two billion years, but others say the galaxies will merely ricochet off each other.

More recently, new information suggests the first steps of the galactic mash-up have already begun. As spiral galaxies, both the Milky Way and Andromeda are shaped and spin like discs, but they are each also enveloped in their own diffuse, spherical cloud of stray stars, star clusters, dust and gas.

These “halos” stretch far beyond the visible boundaries of their galaxies. Scientists determined last year that the Andromeda halo extends 1.3 million lightyears from the galaxy — about halfway to our Milky Way — and as far as two-million lightyears in some directions.

Because the Milky Way has its own halo, the two galactic halos may already be ­bumping into each other and stepping on each other’s toes.

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