Lecture 26 Flashcards
Approximately how big is a Schwarzschild radius of a black hole?
About 3km per solar mass
How did William & Caroline Herschel map the Milky Way and what assumption did they make?
They counted stars while assuming that they all have roughly the same luminosity.
What was the problem with William & Caroline Herschel’s conclusion that the Sun is at the centre?
- they only observed stars in the local neighbourhood which are distributed isotopically.
- they were assigning the wrong distances to the few intrinsically bright (and distant) stars.
How did Harlow Shapley map the Milky Way?
By observing globular clusters. He measured distances using Cepheid Variables and used globular clusters as a “standard ruler”.
What conclusion about the Sun did Shapely arrive at?
He concluded that the Sun must be about 2/3 of the way of of the galaxy based on the known luminosity and apparent brightness of star clusters
What are the 3 types of galaxies?
- spirals
- elliptical
- irregular
What is the anatomy of a spiral galaxy?
- disk: flat distribution, contains spiral arms where new stars are formed and contains gas and dust
- bulge & halo: spherical distribution and globular clusters
What are some key features of the Milky Way?
- approximately 100 billion stars
- approximately 100,000 ly diameter
- disk galaxy
- mostly empty space between stars
- the Sun is located ~ 28,000 light years from the centre
How do stars move in the galaxy?
- disk stars move in near-circular orbits and oscillate up and down
- bulge and halo stars have random orientations
- disk stars all orbit in the same direction
What does the motion of bulge and halo stars suggest about their age?
Their motion suggests that they formed before the disk existed as their orbits are less affected by gravity
What happens if disk stars move too far above or below the disk?
The gravity of everything in the disk pulls them back in but they go too far and end up on the other side
How does the composition of halo and disk stars differ?
- halo stars contain between 0.02-0.2% heavy elements- they are only old stars
- disk stars are ~1-2% heavy elements- they have varying ages
What do multi-wavelength observations tell us?
Different processes have signatures in different types of light so by viewing an object at different wavelengths we can get a fuller picture
What do the different wavelengths show in the Milky Way?
- optical- light from stars, blocked by dust
- infrared- some penetrates dust that obscures optical
- longer-wavelength IR- from dust reradiating absorbed light
- radio- emission from hydrogen and molecules. Reveals molecular clouds where stars form
- x-ray- high temperature gas (and x-ray binaries)
How is gas recycled from low mass stars?
Through stellar winds and planetary nebulae
