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Dark matter shapes the Milky Way

Dark matter was essential to create galaxies like the Milky Way. In part two of this series, Dr Karl explores the galaxy we call home.

Last time, I started talking about this mysterious stuff, dark matter, and then I got diverted into our Milky Way. The detour was relevant because dark matter was essential to create galaxies like our Milky Way.

We used to think that our Milky Way was just a boring middle-aged galaxy, with nothing new or exciting happening, and that had settled into a comfortable and humdrum middle age. But, in the words of Ann Finkbeiner, who wrote about the Milky Way in the journal, Nature: "the ... Milky Way ... galaxy was born in chaos and shaped by violence, (that) it lives in a state of turbulent complexity, and (that) its future holds certain catastrophe."

For example, we now know that in the centre of our galaxy is a mighty black hole, about four million times the mass of our Sun. Now a black hole is a big eater (it can gobble lots, which is why it gets to be so big), but it's not a fast eater. If too much stuff falls into a black hole, it can't all get inside at the same time, and some of it is bounced out before it falls in.

Something big and interesting happened to our black hole about 10 million years ago, and as a result, it squirted out huge jets of energy and created huge shockwaves. The net result was two enormous back-to-back bubbles of hot gas shooting out on each side of our Milky Way. They are very powerful — outlined by x-rays, and with accompanying gamma-ray jets. And they are huge — reaching out about 25,000 light-years away from the black hole.

Just for reference, our visible Milky Way is about 100,000 light-years across, and we are out in the suburb — also about 25,000 light-years from the black hole.

The shape of our Milky Way was dominated by the invisible dark matter, which makes up about 80 or 90 per cent of the mass of our galaxy. But Segue 1, a dwarf galaxy that hangs out with our Milky Way galaxy, has about 1000 times as much dark matter as regular matter. Other structures, such as globular clusters of stars, have virtually no dark matter. We are not sure what's going on here.

Speaking of dwarf galaxies, the Large and Small Magellanic Clouds are also dwarf galaxies. There seem to be a few dozen of these dwarf galaxies associated with our Milky Way. But there may have been many more in the past. In the space just outside our Milky Way, there seems to be remnants of possibly hundreds of dwarf galaxies. What we see are many separate faint lines of stars that rise up out into space away from our Milky Way, and then loop back down into the body of our galaxy.

And there seems to be thin halos of stars extending way past the traditional boundaries of the Milky Way, reaching out some 300,000 light-years from the central black hole. The inner halo is made of quite old stars (say, about 11.4 billion years old) and it rotates in the same direction as our Milky Way. But the outer halo is made of even older stars and rotates in the opposite direction.

And of course, at various locations in our Milky Way, there are stellar nurseries, where gas is being made into stars at the rate of a few times the mass of our Sun every year. These tumultuous areas eject enormous amounts of ultraviolet radiation as well as huge amounts of regular matter in the form of solar winds. Some of the newborn stars are gigantic and die quickly in supernova explosions, while the slightly smaller stars expand into red giants, roughly the size of the orbit of the Earth around the Sun.

But in the fullness of time, our Milky Way galaxy will begin to run out of gas and so the stellar nurseries will shut down. The bigger stars burn brightly for a short time, while the littlies (say, one tenth of the mass of our Sun) can run along for up to a trillion years. But even they will eventually wink out, leaving only darkness, and perhaps lots of dark matter.

But what made the astronomers suspect that this dark matter existed? I'll talk more about that, next time...

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