Astronomy: The Milky Way

Bulge

Center of the galaxy

Spiral arms are not permanent structures

they spread out over the whole disk

How did our galaxy form?

• Our galaxy formed from a cloud of intergalactic gas. • Halo stars formed first as gravity caused gas to contract. • Remaining gas settled into a spinning disk. • Stars continuously form in disk as galaxy grows older.

How do stars orbit in our galaxy?

-Orbits of stars in the bulge and halo have random orientations. -Stars in the disk all orbit in the same direction with a little up-and-down motion.

how is gas recycled in our galaxy?

-low mass stars return gas to interstellar space through stellar winds and planetary nebulae. -High-mass stars have strong stellar winds that blow bubbles of hot gas. -High mass stars explode in supernovae and spread newly made heavy elements into the interstellar medium. -As expanding gases cool, they form new molecular clouds. -gravity forms stars out of the gas in molecular clouds, completing the star-gas- star cycle.

Halo Stars

0.02-0.2% heavy elements (O, Fe, ...), only old stars

Globular clusters

1000s of stars that were all born from the same cloud at the same time -are distributed uniformly around the Milky Way. • The center of this distribution is located at the galactic center

Disk Stars

2% heavy elements, stars of all ages

Halo

Spherical distribution of stars surrounding the galaxy

If spiral arms are not permanent structures, then what is making these locations become active star forming regions?

Spiral arms are caused by spiral density waves - spiral shaped disturbances traveling through the disk of the galaxy that compresses gas and triggers star formation.

What type of galaxy is the milky way?

The Milky Way is a spiral galaxy

Disk

Thin disk around bulge

Where are we in the milky way?

We are located in the disk about 28,000 ly out from the center; a little more than halfway from the center to the edge.

What is the milky way made out of?

gas, dust and stars

Stellar Lifetimes

the stars in the spiral arms are mostly bright, hot, and young stars - Since these stars have such a short lifetime, it must be that the arms of spiral galaxies are where star formation happens

Summary of Galactic Recycling

• Starsmakenewelementsbyfusion. • Dyingstarsexpelgasandnewelements,producing hot bubbles (~106 K). • Hotgascools,allowingatomichydrogencloudsto form (~100-10,000 K). • Furthercoolingpermitsmoleculestoform,making molecular clouds (~30 K). • Gravityformsnewstars(andplanets)inmolecular clouds.