Lecture 16: Galaxies Flashcards
1
Q
Interstellar Medium
A
- 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
2
Q
Dust
A
- 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
3
Q
Structures in the Interstellar Medium
A
- 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
4
Q
Cosmic Cycle of the Elements
A
- 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
5
Q
Distance Scale in the Milky Way
A
Trigonometric Parallax
- good to 100 parsecs from the ground
- good to 1000 parsecs from satellites
- Need to use standard candles for beyond 1000 parsecs
6
Q
Standard Candles in the Milky Way
A
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
7
Q
Main Sequence Fitting for HR diagrams
A
- 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
8
Q
Scale of the Milky Way
A
- 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
9
Q
Orbits in the Galaxy
A
- 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
10
Q
Rotation Curves
A
- 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
11
Q
Mass of the Milky Way from its rotation curve
A
- 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
12
Q
Dark Matter
A
- 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
13
Q
Winding Dilemma
A
- spiral arms in a differentially rotating disk will wind up in a few rotation periods, making them tighter
14
Q
Spiral Density Wave Model
A
- 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
15
Q
Structure of the Milky Way
A
- 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
