From ABC Science, 26 August 2015:
Apart from being home to the only known forms of life in the universe, our Milky Way galaxy isn’t all that unique.
It’s a typical large spiral galaxy, between 100,000 and 180,000 light years wide and containing between 200 billion and 400 billion stars.
The Milky Way has four spiral arms. Two of these arms flow from each end of a ‘bar’ across the middle of the galaxy, like streamers from the ends of a cheerleader’s twirled baton.
Viewed side-on, from far-far away, it’s a disc with a central bulge — or a little like two fried eggs back to back.
How do we know what the galaxy looks like?
Astronomers have painstakingly built up this picture of our galaxy from hundreds of years of studying the night sky in a range of different wavelengths of light.
“The problem is always that we are embedded in the galaxy,” says Professor Fred Watson, Astronomer In Charge at the Australian Astronomical Observatory.
“It’s like trying to draw a sketch of the whole of Sydney from one end of Pitt St, because you’ve got all this obscuration that prevents the view being seen.”
In the 18th century astronomer William Herschel was the first to suggest we might live in a galaxy, after making careful counts of the number of stars he could see in the sky — at least in the northern hemisphere.
“He came to the conclusion that it was more or less equally bright all the way around and therefore we must be at the middle of this disk of stars,” Watson says.
What Herschel didn’t know was that he was only seeing relatively nearby stars — only the first thousand or so light years of our galaxy. It wasn’t until 1919 that American astronomer Harlow Shapley worked out our planet actually sits a long way from the centre of the galaxy.
Then, in the 1950s, Dutch radio-astronomers mapped hydrogen in the galaxy using radio signals that could penetrate through the clouds of dust that had hitherto obscured much of the galaxy to reveal the Milky Way’s spiral structure.
The spiral pattern is thought to be triggered by shockwaves that periodically pass through the galaxy as it rotates.
“The reason why we see the spiral arms is that they’re delineated by very energetic young stars,” says Watson.
These massive young stars glow more brightly so they stand out more in the visual spectrum than the less energetic celestial bodies in the rest of the galaxy.
At the core of our Milky Way galaxy, in the direction of the constellation Sagittarius, lies a supermassive black hole called Sagittarius A*, over 3.6 million times the mass of our Sun.
This black hole contributes a small amount to the central bulge (the yolks of the fried eggs!) but mostly the bulge at the centre of the galaxy is made up of ancient stars that date back to the formation of the galaxy. Read more.