Lectures 19 & 20: Milky Way Galaxy

Star

Large, glowing gas ball generating heat and light via nuclear fusion.

Nebula

Interstellar cloud of gas and dust for star formation.

Galaxy

Island of stars held by gravity, and orbits a center.

Cluster of Galaxies

Group of ~10 to ~1000 galaxies.

Interstellar Medium

Gas and dust between stars, forming new stars.

Disk

Flat component of the Milky Way (stars, gas, dust). Region with active star formation and blue stars.

Bulge

Central spherical region of the Milky Way (mostly stars)

Halo

Outer spherical region (globular clusters, dark matter). Region surrounding the galactic disk with hot gas. Region with no star formation or blue stars.

Gravity ~ Centrifugal force

Galactic Disk is supported by the balance of these two forces.

Galactic Rotation Support Disk

Primarily. Stars in the disk orbit in the same direction.

Thickness of Disk

Secondarily. Stars in disk orbit in same direction with up-and-down motion. Vertical extent of the disk due to star motion.

Random Motions Support Bulge

Orbits of stars in bulge and halo have random orientations.

Star-gas-star cycle

Recycles gas from old stars into new star systems. Gas recycling occurs mostly in disk.

NGC 3603

Region in Milky Way known for star formation.

Sloan Digital Sky Survey

Survey mapping galaxy distribution in the universe.

Filaments

Structures in galaxy distribution indicating matter presence.

Voids

Empty spaces in galaxy distribution across the universe.

Thermal Pressure

Force balancing gravity vertically in the galactic disk.

Random Motions

Unpredictable orbits of stars in bulge and halo.

Orion Constellation

Region in the sky containing notable nebulae.

Planetary Nebulae from low-mass stars

Return gas to interstellar space through stellar winds and planetary nebulae. Core remains as white dwarf.

Stellar Winds from high-mass stars

Strong stellar winds that blow bubbles of hot gas.

Supernova Explosions of high-mass stars

Cooling supernova remnants show newly-made heavy elements. New elements are mixed into interstellar medium.

Emission Line Spectrum

Light spectrum indicating elements in supernova remnants.

Galactic Fountain

Multiple supernovae create huge hot bubbles that can blow out of disk. Gas clouds cool in halo and rain back down on disk (due to gravity). Gas ejected into halo —> expands in halo —> falls down to disk

Gas returned to Interstellar Space

Atomic hydrogen gas forms as hot gas cools (electrons join protons), molecular clouds form after cooling (atoms combine into molecules).

Molecular Clouds in Orion

Mostly H2, 28% He, 1% CO, and other molecules.

Galactic Recycling Summary

Stars make new elements by fusion —> dying stars expel gas and new elements, producing hot bubbles (~10^6K) —> hot gas cools and atomic hydrogen clouds form (~100-10,000K) —> further cooling allows molecules to form, making molecular clouds (~30K) —> gravity forms new stars (and planets) in molecular clouds

21-cm radio waves

Emitted by atomic hydrogen, shows where gas has cooled and settled into disk.

Millimeter-wave emission

From carbon monoxide, shows locations of molecular clouds.

Long-wave infrared emission

Shows where young stars are heating dust grains (thermal radiation form dust)

Short-wave infrared light

Reveals hidden stars whose visible light is blocked by gas clouds.

X-rays

Observed from very hot gas above and below disk.

Ionization nebulae

Found around short-lived high-mass stars, signifying active star formation.

Reflection Nebulae

Scatter light from nearby stars.

Scattering of Light

Process causing blue light to scatter more than red.

Where do we see nebulae?

Halo: no ionization nebulae, no blue stars = no star formation.

Disk: ionization nebulae, blue stars = star formation

Spiral Arms

Regions of high density (traffic jam) where star formation occurs.

Density Wave Theory

Gas clouds get squeezed as they move into spiral arms. —> Squeezing of clouds trigger star formation. —> Young stars flow out of spiral arms.

Gas Compression

Increased density of gas in spiral arms triggers star formation.

Halo Stars

Old stars with low heavy element content, form first, travel high above and far below on orbits with random orientations. Formed first then stopped.

Disk Stars

Stars of all ages, 2% heavy elements, orbit in circles in same orientation with up-and-down motion. Formed later and keep forming.

Galaxy Formation

Galaxy formed from giant gas cloud. —> Halo stars formed first as gravity caused cloud to contract. —> Remaining gas settles into spinning disk (angular momentum must be conserved). —> Stars continuously form in disk as galaxy ages.

Whirlpool Galaxy

Example of a galaxy with star formation in spiral arms.

Milky Way Galaxy

Our galaxy, containing various star formation regions.