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A105 Stars and Galaxies
This week’s units: 74, 75, 76, 78, 79 News Quiz Today Galaxies homework due Thursday Projects due Nov. 30
Today’s APOD
Announcements…• Kirkwood Obs. open Weds night
6:30-8:30 PM• Rooftop Session on Thurs, Nov.
16 @ 8 PM• Leonid Meteor Shower, Friday
AM or PM
Main Types of Galaxies
Spirals
Ellipticals
Irregulars
Hubble Deep Field
• Our deepest images of the universe show a great variety of galaxies, many of them billions of light-years away
Spheroidal Component: bulge & halo, old stars,few gas clouds
Disk Component: stars of all ages, many gas clouds
Disk Component:stars of all ages,many gas clouds
Spheroidal Component:bulge & halo, old stars,few gas clouds
Blue-white color Blue-white color indicates indicates ongoing star ongoing star formationformation
Red-yellow Red-yellow color indicates color indicates older star older star populationpopulation
Properties of Elliptical Galaxies
• Round or elliptical in shape• Contain no visible gas or dust• No young stars or star-forming
regions• The largest galaxies are
ellipticals (and also some of the smallest)
Elliptical Galaxy: All spheroidal component, virtually no disk component
Red-yellow color indicates older star population
Properties of Irregular Galaxies
• Chaotic mix of stars, gas and dust
• No bulge or spiral arms• May contain star forming
regions• Usually small galaxies
Blue-white color indicates ongoing star formation
Thought Question
Why does ongoing star formation lead to a blue-white appearance?
A. There aren’t any red or yellow starsB. Short-lived blue stars outshine othersC. Gas in the disk scatters blue light
Famous Galaxies!
• The Local Group – about 3 dozen galaxies– Milky Way– Large and Small
Magellanic Clouds– Andromeda– M33– Dwarfs
About four times smaller than our Milky Way Galaxy
Near M31 more than twice the angular size of the
full moon visible with a good pair of binoculars
Messier 33
Messier 87
• Giant elliptical galaxy at the center of the Virgo Cluster of galaxies
• ~60 million light years away
• note – globular clusters– jet– other galaxies
The Puzzle of “Spiral Nebulae”
• Before Hubble, some scientists argued that “spiral nebulae” were entire galaxies like our Milky Way, while others maintained they were smaller collections of stars within the Milky Way
• The debate remained unsettled until someone finally measured their distances
How did Hubble prove that galaxies lie far beyond the Milky Way?
Standard Candles! Pulsating Stars
Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles
Cepheid Variable Stars
Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles
Period = 3 days
The Nearest StarsDetermine distances of stars out to a few hundred light-years using parallax
Distances
Distances of GalaxiesGalaxies are too far away for parallax
techniqueUse “standard candles” (INVERSE
SQUARE LAW)Cepheid variablesupergiant starsplanetary nebulaesupernovae
Image “graininess” – The smoother the distribution of stars in a galaxy the farther away it is
Steps to the Distance Scale
How do astronomers measure distances to objects?
Brightness alone does not provide enough information to measure distance
Start with nearby objects, move to greater distances
Identifyingthe MainSequence
Apparent brightness of star cluster’s main sequence tells us its distance
Knowing a star cluster’s distance, we can determine the luminosity of each type of star within it, including the distances to Cepheids
Establishingthe brightnessof stars
The Distance ScaleCombination of methods allows
us to measure distances to nearby galaxies, and then to further and further distant galaxies.
Andromeda – about 2,500,000 LY distant
Virgo Cluster – about 50,000,000
LY distant
Most distant galaxies –
By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way
“Redshift” of Galaxies
Remember the Doppler Shift?
The spectral lines of galaxies are redshifted, i.e. galaxies are moving away from us.
Plot the velocity of recession against the distance to the galaxy: the farther away a galaxy is, the faster it recedes from us!
Hubble Law
Distance - Velocity Relation
0
200
400
600
800
1000
0 10000 20000 30000 40000 50000 60000 70000
Velocity (km/sec)
Dis
tanc
e (M
pc)
Hubble’s Law
• The correlation discovered by Hubble was reported in 1929 as the formula:
Velocity of Recession = Ho x Distance
• Ho is now know as the Hubble constant, and is measured in kilometers per second per megaparsec
o
Hubble’s Law …
o
• The Hubble Space Telescope was launched to the distance-redshift relation
• Why is Hubble’s law so important?
Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies
Age = ___________ ~ 1 / H0
Distance
Velocity