Download Lecture 19 The Milky Way Galaxy

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-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
Finding Spiral Arms: 21cm Radiation
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
Center of the
Milky Way