“Contact” A105 Movie Special Wednesday, April 18, Swain West 119, 7:30 pm (153 minutes) Counts as an out-of-class activity

M53 (NGC 5024), one of about 250 globular clusters located in the halo of the Milky Way, has in excess of 250,000 stars.

Homework #8 due Thursday, April 12, 11:30 pm. Note the due dates for out-of-class activities. Everyone should do at least one out-of-class activity. No more than 80 points (1/2 of maximum total activity points) can come from out-of-class activities

halo disk bulge Edge-on views of spiral galaxies. Disks in spiral galaxies are very thin.

Spiral arms are embedded in the disks of spiral galaxies, such as the Milky Way

Population I and II Stars in Milky Way Younger stars in disk Enriched in heavy elements (up to 5%) Population II Older stars in bulge and halo Low in heavy elements (less 1%)

Based on Main Sequence turnoffs for globular clusters, the Milky Way is 12 – 14 billion years old.

Populations I and II can be understood from the perspective of the star-gas-star cycle.

Our galaxy probably formed from a giant gas cloud

Halo stars (Pop II) formed first as gravity caused cloud to contract – little remaining gas, so no young stars in halo.

Remaining gas settled into a spinning disk Stars (Population I) continuously form in disk as galaxy grows older – range of ages

This model is oversimplified: mergers and accretion of smaller galaxies continue through today.

Halo Stars: 0.02-0.2% heavy elements (O, Fe, …), only old stars Disk Stars: 2% heavy elements, stars of all ages

Halo stars formed first, then stopped 0.02-0.2% heavy elements (O, Fe, …), only old stars Halo stars formed first, then stopped Disk Stars: 2% heavy elements, stars of all ages

Halo Stars: 0.02-0.2% heavy elements (O, Fe, …), only old stars Halo stars formed first, then stopped Disk Stars: 2% heavy elements, stars of all ages Disk stars formed later, kept forming

Halo stars are all old, with a smaller proportion of heavy elements than disk stars, indicating that the halo formed first Our galaxy formed from a huge cloud of gas, with the halo stars forming first and the disk stars forming later, after the gas settled into a spinning disk (star-gas-star cycle functioning)

We observe star-gas-star cycle operating in Milky Way’s disk using many different wavelengths of light

Radio (atomic hydrogen) Visible 21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk. Cool gas is considered a Pop I constituent.

Molecular gas is considered a Pop I constituent. Radio (CO) Visible Radio waves from carbon monoxide (CO) show locations of molecular clouds. Molecular gas is considered a Pop I constituent.

Young stars are Pop I objects. IR (dust) Visible Long-wavelength infrared emission shows where young stars are heating dust grains. Young stars are Pop I objects.

Infrared Visible Infrared light reveals stars whose visible light is blocked by gas clouds

Visible X-rays X-rays are observed from hot gas above and below the Milky Way’s disk

Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds

Spiral Arms, Star Forming Regions, and Population I objects

Ionization nebulae are found around short-lived high-mass stars, signifying active star formation Orion nebula

Halo: No ionization nebulae, no blue stars  no star formation Disk: Ionization nebulae, blue stars  star formation

Most star formation in the disks of spiral galaxies happens in spiral arms Whirlpool Galaxy

Most star formation in the disks of spiral galaxies happens in spiral arms Ionization Nebulae Blue Stars Gas Clouds Whirlpool Galaxy

Spiral arms are density waves in the interstellar medium. Gas orbits faster than the spiral arms, eventually “catching up” with an arm. As it orbits about the Galaxy, gas slows down when it encounters a spiral arm

Spiral arms are waves of star formation Gas clouds get squeezed as they move into spiral arms Squeezing of clouds triggers star formation Young stars flow out of spiral arms

Model of Milky Way’s spiral structure by Steiman-Cameron, Wolfire & Hollenbach (2010). Four spiral arms.

How is gas recycled in our galaxy? Gas from dying stars mixes new elements into the interstellar medium which slowly cools, making the molecular clouds where stars form Those stars will eventually return much of their matter to interstellar space Where do stars tend to form in our galaxy? Active star-forming regions contain molecular clouds, hot stars, and ionization nebulae Much of the star formation in our galaxy happens in the spiral arms

Where will the gas be in 1 trillion years? (red) Blown out of galaxy (yellow) Still recycling just like now (blue) Locked into white dwarfs and low-mass stars

Where will the gas be in 1 trillion years? (red) Blown out of galaxy (yellow) Still recycling just like now (blue) Locked into white dwarfs and low-mass stars

The Galactic Center & the beast that lurks there x

In visible light, the Galactic center (GC) is heavily obscured by gas and dust Only 1 out of 1012 photons of visible wavelengths makes its way from the GC towards Earth! The rest are scattered into other directions or are absorbed. Galactic center

Emissions near the Galactic Center Radio emission Infrared light Emissions near the Galactic Center Swirling gas Orbiting stars Motions near the Galactic Center Note the changing scales

a super-massive black hole! Stars appear to be orbiting something massive, but invisible, at the Galactic Center a super-massive black hole! Orbits of stars indicate a mass of about 4,000,000 MSun

X-ray flares from galactic center suggest that tidal forces of central black hole occasionally tear apart chunks of matter that are about to fall into the black hole Download a great movie of star motions from: http://www.astro.ucla.edu/~jlu/gc/images/orbits_pause.gif