Lecture 19 - Galaxies Flashcards
what is happening to galaxies as the universe is expanding?
they are moving further away from each other
how can we determine whether a galaxy is moving away/towards us?
measure its rotation and look for red/blue shift
are nearby galaxies similar or different to milky way?
nearby are very similar so formed around 10 billion years ago
how do we study young galaxies?
look very far away!! i.e. look into the past
what is cosmology?
the study of the structure and evolution of the universe
what 2 other variables are a galaxy’s age related to?
- its distance
- age of universe
3 categories of galaxies
- spiral
- elliptical
- irregular
what does a spiral galaxy look like?
what is in the disk?
flat white disk with yellow bulge
cool gas + dust, hot ionized gas, young+hot stars
what are bars in spiral bar galaxies?
where do they come from? (2)
how many spiral galaxies have bars?
temporary structures that can occur due to:
1. a density wave radiating outward from the core
2. tidal interaction with another galaxy
2/3 are spiral bar
what do elliptical galaxies look like?
what are they made of?
all spheroidal –> no disk
redder, rounder, elongated
have very little cool gas and dust, but have hot ionized gas
what does the colour of elliptical galaxies indicate?
red-yellow colour indicates older, low mass star population
why do elliptical galaxies not have many young stars?
very little star-forming gas (little cool gas) and little interstellar medium
every elliptical galaxy has _______
every elliptical galaxy has a black hole
what is an example of an elliptical galaxy?
M87
What is the shape of irregular galaxies?
colour? what does this indicate?
neither disklike or rounded
blue/white indicates ongoing star formation
compare lenticular galaxy to spiral and elliptical galaxies?
has disk like spiral galaxy but much less dusty gas
like intermediate btwn spiral elliptical
what did cosmologists use to determine the types of galaxies?
the hubble tuning fork looked at physical appearance of galaxies
why don’t all galaxies have similar disks?
diff properties of protogalactic clouds:
1. spin
2. density
in general, why do disks form? what happens if stars form too early?
INTERACTIONS!!! if stars form too early there will be very few interactions
how does spin of protogalactic clouds affect galaxy disks?
angular momentum can determine size of disk
how does density of protogalactic clouds affect galaxy disks? what happens if it is more dense?
determines whether it is spiral or elliptical
more dense = cools and form stars before becoming a disk, therefore elliptical
how are spiral galaxies found? elliptical?
spiral galaxies are found in groups (a few dozen)
elliptical galaxies are found in huge clusters (100s to 1000s)
average mass of normal galaxies, what do they contain?
10^10 to 10^12 solar masses
contain about 10^9 stars and a lot of dark matter!
average mass galaxies in galaxy clusters?
why do galaxies form clusters?
10^14 solar masses
clusters form bc of gases interacting and becoming gravitationally bound in a big ball
what are the 3 evidence of dark matter in clusters
- velocities of galaxies
- gravitational lensing
- collisions
how is galaxy velocity evidence of dark matter in galaxy clusters?
measure velocities using Doppler shifts
using velocity we see that the mass of a cluster is 40x larger than the mass of stars
how is gravitational lensing evidence of dark matter in galaxy clusters?
how much galaxies bend spacetime and therefore the light rays of further galaxies will directly indicate the cluster’s mass
how are collisions of clusters evidence of dark matter in galaxy clusters?
1 small and 1 large clusters collide and separate the x-ray emitting hot gas from the galaxies
x-ray emissions shows that mass is much greater than the mass of the stars
what are the 2 groups of galaxy colours?
what does these indicate?
- blue cloud –> large blue-white galaxies with active star formation and young stars producing most of the light
- red sequence –> largest, elliptical galaxies with less star formation and mainly older stars
how do we determine distances within the solar system?
RADAR RANGING –> radio waves transmitted from earth bounce off a planet or other object and return to earth, see how long it takes it to come back
how do we determine distances within the milky way?
PARALLAX (a few hundred Lyr) –> not for distances beyond milky way
how do we determine distances outside the milky way? (2)
use cepheid variable stars as a standard candle –> period directly corresponds to luminosity then we calculate distance (with inverse square law for light)
use white dwarf supernovae (Type 1a) as a standard candle for distances up to 10 billion Lyr
what is a standard candle?
can measure luminosity without measuring distance
why can supernovae type Ia be used as a standard candle?
when it gains enough mass to surpass the Chandresekar limit, it will explode –> it explodes the same way every time!
then measure its apparent brightness to find distance
is there 1 method for determining distances?
which method measures in SI units?
no, use many methods!
RADAR is only method that lets us make measurements in SI units
how do measure the distance of Andromeda galaxy?
using cepheid variables as standard candles
what are the 2 types of redshift ?
- doppler –> things moving away/towards us
- expansion of space itself
if we see galaxies moving away from us does that mean we are in the center of the universe?
no
how do we know that galaxies are moving away?
if a galaxy is 2x as far away as originally, how does its speed change?
if a galaxy is 3x as far away as originally, how does its speed change?
what is causing this?
all galaxies are redshifted on their spectra
2x as far away = moving away 2x as fast
3x as far away = moving away 3x as fast
GRAVITY
what is hubbles law?
velocity = Ho x distance
shows relationship btwn redshift and distance
what are the 2 practical issues with hubble’s law
- Hubble’s law assumes speed is only determined by the expansion of the universe but galaxies experience gravitational tugs from other galaxies which alter speeds
- calculations only as good as our measurement of Hubble’s constant
Hubble’s conclusion:
the more distant a galaxy, the greater its redshift so it is moving away from us faster
how does the expansion rate change thru space?
expansion rate doesn’t change
what is the cosmological principle?
is it proven?
the universe looks the same no matter where you are within it
not proven but consistent with our observation
what does the inverse of Hubble’s constant tell us? why?
tells us the age of the universe i.e. tells us how long it would have taken the universe to reach its present size if the expansion rate never change
bc it relates velocities and distances of all galaxies
age = distance/velocity = 1/Ho
what happens to distances between galaxies when light travels?
distances btwn galaxies change when light travels
what do astronomers use instead of distance?
lookback time
why is it difficult to specify the distance of a faraway galaxy?
distance is always changing
what is the cosmological horizon?
can we see anything beyond?
the limits of the observable universe –> in terms of TIME not space bc when we look far away we are looking back in time so the beginning of the universe is the limit –> i.e. age of universe limits size of observable universe
represents 14 billion year age of the universe
we cannot see anything beyond horizon
in summary, measuring a galaxy’s distance and speed lets us figure out:
measuring a galaxy’s distance and speed lets us figure out how long it took the galaxy to reach its current distance
