Lecture 16: Galaxies

Question 1

Interstellar Medium

Answer 1

• most of the volume of the milky way is the gas between the stars
• Made of atoms, molecules, and dust
• Hydrogen is by far the most common element
• also contains cosmic rays, energetic protons, electrons, nuclei
• Very weak magnetic fields

Question 2

Dust

Answer 2

• Causes reddening and extinction of starlight
• Dust is only 1% by mass of Interstellar Medium
• Causes obscuration of visible light
• reddens visible light by scattering the bluer photons more
• Provides an efficient coolant for clouds
• Polarizes visible light
• Composition of graphite, Silicon carbide, silicates, hydrogen gas, and water

Question 3

Structures in the Interstellar Medium

Answer 3

• ISM divided into clouds
• Diffuse Interstellar Clouds, low extinction, 5-100 parsecs in diameter
• Molecular clouds, high extinction, same diameter
• Very diffuse inter-cloud medium, very low density gas between clouds
  HII regions surround massive, hot, short-lived stars
• These stars emit strongly in the UV, ionizing nearby Hydrogen
• Mark the birth sites of massive stars
  Planetary Nebulae
  Supernova Remnants
• Only 150 known in Galaxy
• Central role in chemical evolution of Galaxy

Question 4

Cosmic Cycle of the Elements

Answer 4

• Stars are formed form interstellar gas, mostly Hydrogen
• Stars convert elements to heavier elements
• When stars die they put enriched gas back into ISM through planetary nebula or supernova
• next generation of stars then formed from more enriched material
• Metallicity is the fraction of atoms larger than helium and is constantly increasing

Question 5

Distance Scale in the Milky Way

Answer 5

Trigonometric Parallax
- good to 100 parsecs from the ground
- good to 1000 parsecs from satellites
- Need to use standard candles for beyond 1000 parsecs

Question 6

Standard Candles in the Milky Way

Answer 6

Variable Stars
- Cepheid and RR Lyrae
- Supernovae 1a
Main Sequence fitting
- Use observed HR diagram of objects at a common distance and compare to calibrated HR diagram

Question 7

Main Sequence Fitting for HR diagrams

Answer 7

• Use observed HR diagram of objects at a common distance and compare to calibrated HR diagram
• Have to compare brightness of a cluster instead of an individual star

Question 8

Scale of the Milky Way

Answer 8

• Observations of RR Lyrae stars in globular clusters in our galaxy led to the discovery that we are not at the centre of the milky way
• Best estimate is that we are 8.5 kPc from the centre

Question 9

Orbits in the Galaxy

Answer 9

• Orbits are set by the mass of the Galaxy
• Objects in the disc of the galaxy have close to circular orbits in the same plane
• objects in the halo have more random and more eccentric orbits

Question 10

Rotation Curves

Answer 10

• Plot of orbital velocity (rotation) vs distance from centre of mass of the orbit in the disk
• Need to measure both velocity and position at various distances from centre of mass

Question 11

Mass of the Milky Way from its rotation curve

Answer 11

• larger mass means a larger orbital velocity to travel in circular orbit at a given distance
• larger distance requires a smaller velocity for a circular orbit
• rotation curve of milky way is nearly flat out to large distances from the centre
• More mass beyond visible edge of milky way

Question 12

Dark Matter

Answer 12

• Standard methods suggest there is lots of mass in the galaxy that is not visible
• Dark matter is matter we detect gravitationally
• No light is detected from these objects and no light is obstructed by other objects
• Possibly dark dwarfs

Question 13

Winding Dilemma

Answer 13

• spiral arms in a differentially rotating disk will wind up in a few rotation periods, making them tighter

Question 14

Spiral Density Wave Model

Answer 14

• Stars and gas pass through a wave where they slow down
• this area has greater density
• star formation can begin due to non-linear fashion of material

Question 15

Structure of the Milky Way

Answer 15

• Disk, bulge, halo, bar are the largest structures in the galaxy
• disk has spiral arms, bright and patchy
• patchy because of other processes like explosions and super-bubbles
• most ISM material concentrated in spiral arms

Question 16

Centre of the Milky Way

Answer 16

• Evidence of a very massive but unseen object at centre of mily way
• thought to be supermassive black hole of 3 million solar masses

Question 17

Galaxies Classified by their morphology (appearance)

Answer 17

• Spirals, spiral structure
• ellipticals, large round balls of stars with little gas and dust, vary greatly in size
• Ienticulars, cross between spirals and ellipticals
• Barred Spiral, spiral galaxies with a bar in the middle
• irregular, other forms, lacking symmetry
• peculiar, all of the weird forms, some very bright

Question 18

Supernovae (Type 1a)

Answer 18

• all white dwarf/binary supernova explosions are nearly identical

Question 19

The Hubble Law

Answer 19

• Edwin Hubble measured distances and radial velocities of nearby galaxies
• Discovered universe is expanding
• Velocity is proportional to distance
• More distant a galaxy is, the faster it is receding
• slope of line is Hubble constant