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The Milky Way Galaxy. Greeks called the hazy band of light around the sky ‘galaxias kuklos’ – milky circle Romans called it ‘via lactia’ – milky road, or milky way But what is it? By the mid-18 th century, astronomers new that it was made up of an enormous number of distant stars.
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Greeks called the hazy band of light around the sky • ‘galaxias kuklos’ – milky circle • Romans called it ‘via lactia’ – milky road, or milky way • But what is it? • By the mid-18th century, astronomers new that it was made • up of an enormous number of distant stars
1785: William and Caroline Herschel try to map out the distribution • of stars: published the ‘Grindstone model’ – the Sun at the center of • An irregularly shaped disc of stars The Sun
1922: Jacobus Kapteyn’s model The Sun
Harlow Shapley: noticed that although open clusters were • randomly scattered about the sky, globular clusters were • concentrated in the direction of Sagittarius • Therefore center of our • system of stars must be • somewhere towards • Sagittarius
Previously, astronomers had thought that galaxy was much • smaller and that we were near the center because they did not • take into account the dimming of light from stars
The Disk: • - Contains most stars and dust • - Contains most GMCs, so most star • formation takes place in disk • - Contains all open clusters, a few million • to a billion years old • - By proportion, the disk is thinner than • a pizza crust (not deep dish!) • The Halo: • - Contains about 200 globular clusters, • average age of 11 billion years • Spiral Arms: • - Long spiral patterns of bright stars, HII • regions, star clusters, gas and dust • - Sun is located on inner edge of one • Galactic year: • - The galaxy is rotating: our solar system takes • 225 – 250 million years to orbit the galactic • center
Differences between disk stars and halo stars • Astronomers define metals to be any elements that are not H or He • Population I stars are metal rich (2 to 3% of their mass is metals) • Population II stars are metal poor (0.1% metals) • Population I stars are located in the disk, population II stars in the halo • Population II stars must be very old – the gas clouds they were formed out of were not enriched with metals by supernovae of previous stars
Mass of the Milky Way • From Kepler’s third law, mass of galaxy is about 400,000,000,000 MSun
We seem to be missing about 90% of the mass of the galaxy! • Most of it cannot be emitting or absorbing light • Astronomers name it Dark Matter
