Class 20: The Milky Way

Our own galaxy, the Milky Way. The debate about other galaxies.

The Milky Way

Credit & Copyright Barney Magrath

Credit: Dave Palmer

I: The Milky Way

Milky Way – bright band of light across sky.

Galileo First to use telescope to study MW. Found it was made of millions of faint stars.

What is the Milky Way? Thomas Wright (1750) suggested that solar system

was embedded in enormous shell of stars… Emmanuel Kant (1755) suggested instead that the

MW is a giant disk of stars. Kant also hypothesized that space was full of other,

similar disks of stars (island universes).

LMC

SMC

Inventory of the Milky Way

About 100 billion stars… Disk

50 kiloparsecs (~150,000 ly) in diameter. Generally contains younger stars. Dusty and gassy.

Bulge Contains older stars. Less dust and gas.

Other stuff Galactic nucleus in the bulge. Globular star clusters in the halo.

If our solar system were 1 mm dot… Nearest star would be 100 m away. MW would be size of continental U.S. Halo would be size of Earth.

Other cool facts: Disk is about 0.6 kpc thick. Central bulge is 2 kpc across. Sun is located ~8 kpc from center. Andromeda Galaxy is 750 kpc away.

Spiral Structure

MW is a spiral galaxy – looks like a pinwheel (we’ll see other examples).

Structure difficult to detect because we live inside galaxy – literally can’t see the forest for the trees…

Careful measurement reveals spiral arms, and shows that the disk is rotating differentially. Material closer to center orbits faster than

material farther away.

Evolution of Spiral Structure

Because of differential rotation, any radial feature gets wound up very tightly (see figure). Spiral arms should be tightly wound!

But spiral arms are “open”, “loose”… Modern theory: spiral structure due to

waves in density of stars (& gas). Traffic jam analogy (see figure).

Spiral arms are bright because high density promotes formation of big stars.

• Fast cars slow down to pass paint truck

II : Other galaxies?

What of Kant’s suggestion of other disks? Need to look at Nebulae.

Nebulae Fuzzy blobs in the sky. First systematically catalogued by Messier. Messier was mainly interested in comets…

Credit : A. Dimai

What were Messier’s objects?

Possibilities… Glowing clouds of gas in between the stars. Large collections of stars in our own Galaxy. Whole other galaxies.

Turns out that all of these possibilities are represented…

The Orion Nebula (M42)

Glowing gascloud

The Hercules Cluster (M13)

Credit: Yuugi Kitahara

Ball ofstars in ourown galaxy

Andromeda “Nebula” (M31)

Another galaxy…

But, idea of external galaxies was vigorously debated in early 1900s…

The Great Debate (1920) National Academy of Sciences – downtown DC. Debate between Harlow Shapley and Heber Curtis. Shapley argued for nebulae all being local (i.e. within

the Milky Way, like the Orion Nebula). Curtis argued for “island universe” hypothesis (i.e.,

there are many islands of stars like the Milky Way in the universe).

Really need a good distance indicator to solve the problem… (next class!).