Physical Universe Chapter 19

Primary Galactic Features (2)

• we see the Milky Way from edge-on

• Primary features: disk, bulge, halo, globular clusters

Our galaxy: Interstellar Medium (2)

• Our view is obscured by dusty gas clouds, as they absorb visible light

• these clouds make new star systems

Galileo

• didn’t know shape of our galaxy, but used his telescope to discover many new stars

Herschel

• counted how many stars lies in each direction, suggested that the width was larger than thickness

Kapteyn

• confirmed Herschel result, suggested that
  our Sun is at the center of the Milky Way (wrong)

Shapley

• found that globular clusters appeared to be
  centered far from the Sun, at the true center of our
  galaxy

Stellar Orbit: Disk

• all stars in the disk orbit in the same direction with a
  little up-and-down motion

Stellar Orbit: Bulge and halo

• stars here orbit with random orientations

Getting stellar orbits (3)

• found by measuring stellar motion relative to the sun

• doppler effect can only tell up radial velocity

• tangential velocity is harder to measure, changes apparent position

Star-gas-star cycle: Recycling old star’s gas (9)

• low mass stars return gas to space through stellar winds and planetary nebulae

• high mass stars have strong winds that blow bubbles of hot gas

• gas is then ionized around exploding stars, new heavy elements in supernova remnants 

• remnant cools and expands, emitting visible light

• new elements made by supernova mix into the interstellar medium 

• radio emission in remnants from particles accelerated to near light speed (could also be source of cosmic rays 

• Atomic Hydrogen Gas Atomic forms as hot gas cools, allowing electrons to join with protons. This gas emits a spectral line with wavelength
  at 21 cm (radio portion of the electromagnetic
  spectrum)

• molecular clouds forms after atomic hydrogen gas, after the gas cools enough for molecule formation

• Gravity forms stars out of the gas in molecular clouds, completing the star–gas–star cycle

Galactic Recycling Summary (5)

• Stars make new elements by fusion

• Dying stars expel gas and new elements, producing hot bubbles (~106 K).

• Hot gas cools, allowing atomic hydrogen clouds to form (~100–10,000 K).

• Further cooling permits molecules to form, making
  molecular clouds (~30 K).

• Gravity forms new stars (and planets) in molecular
  clouds.

Observing star-gas-cycle (5)

• observed using light wavelengths

• Radio waves: atomic hydrogen shows where gas has cooled and formed a disk, carbon monoxide shows the locations of molecular clouds

• Infrared: reveals where young stars are heating dust grains and stars who’s light is blocked by gas

• x-rays: produced by the hot gas found above and below the disk

• gamma rays: reveals where cosmic rays collide with atomic nuclei

Ionization nebulae (3)

• located around short-lived, high-mass stars, aka regions of active star formation

• none are found in the halo

• mostly in disk where star formation occurs (spiral arms)
  formation

Spiral Arm star formation (3)

1. Gas clouds get squeezed as they move into spiral arms.
2. Squeezing of clouds triggers star formation.
3. Young stars flow out of spiral arms.

Halo stars

old stars, roughly 0.2% heavy elements

Disk stars (2)

• ages, 2% heavy elements

• continually form as galaxy grows older

Galactic center (2)

• stars appear to orbit a massive (4Mx mass of sun), invisible black hole

• shown by x-ray flares from tidal forces tearing apart matter