The Star–Gas–Star Cycle

The matter in our Galaxy emits different kinds of radiation, depending on what stage of the star–gas–star cycle it is in.

Phases of the ISM

Ionization Nebulae •  Found around high-mass

stars (OB associations) •  O & B stars (T > 25,000K)

make enough UV photons to ionize hydrogen in the nebula

•  Gas re-emits Hα line (red)

“Whirlpool” galaxy

19. Our Galaxy

Immanuel Kant (1724 – 1804) German philosopher

The infinitude of creation is great enough to make a world, or a Milky Way of worlds, look in comparison with it what a flower or an insect does in comparison with the Earth.

Regions of the Milky Way Galaxy

diameter of disk = 100,000 l.y. (30,000 pc)

number of stars = 200 billion

thickness of disk = 1,000 l.y. (300 pc)

Sun is in disk, 28,000 l.y. out from center

play brf

Our orbit (with sun) in the Milky Way

8.5 kpc

Galactic disk, bulge & halo...

Globular cluster M80

HRD for Globular Cluster M3

Regions of the Milky Way Galaxy

• Disk •  younger generation of stars •  contains gas and dust •  location of the open clusters

•  Bulge • mixture of both young and old stars

• Halo •  older generation of stars •  contains no gas or dust •  location of the globular clusters

Halo vs. Disk •  Stars in disk relatively young.

•  fraction of heavy elements same as or greater than the Sun •  plenty of high- and low-mass stars, blue and red

•  Stars in halo are old. •  fraction of heavy elements much less than in Sun •  mostly low-mass, red stars

•  Stars in halo must have formed early in Milky Way’s history. •  when few heavy elements existed •  no ISM in the halo •  star formation in halo stopped long ago when all gas flattened into

the disk

Stellar Orbits •  All Stars orbit Galactic

center.

•  Stars in disk orbit: •  in same direction •  in same plane (like planets) •  just “bobble” up and down •  like merry-go-round

•  Stars in bulge & halo orbit: •  in different directions •  with various inclinations •  like swarming bees

Galaxy Song - Composers: Eric Idle & John Du Prez ���Author: Eric Idle Singer: Eric Idle From the 'Meaning of Life' Just remember that you're standing on a planet that's evolving And revolving at nine hundred miles an hour, That's orbiting at nineteen miles a second, so it's reckoned, A sun that is the source of all our power. The sun and you and me and all the stars that we can see Are moving at a million miles a day In an outer spiral arm, at forty thousand miles an hour, Of the galaxy we call the 'Milky Way'.

Our galaxy itself contains a hundred billion stars. It's a hundred thousand light years side to side. It bulges in the middle, sixteen thousand light years thick, But out by us, it's just three thousand light years wide. We're thirty thousand light years from galactic central point We go 'round every two hundred million years, And our galaxy is only one of millions of billions In this amazing and expanding universe.

The universe itself keeps on expanding and expanding In all of the directions it can whiz As fast as it can go, at the speed of light, you know, Twelve million miles a minute, and that's the fastest speed there is. So remember, when you're feeling very small and insecure, How amazingly unlikely is your birth, And pray that there's intelligent life somewhere up in space, 'Cause there's bugger all down here on Earth.