Transcript A105 Stars and Galaxies

A105
Stars and Galaxies
Today’s APOD
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This week’s units: 74, 75, 76, 78, 79
News Quiz Today
Galaxies homework due Thursday
Projects due Nov. 30
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
Spirals
Main Types of
Galaxies
Irregulars
Ellipticals
Hubble Deep Field
• Our deepest images
of the universe show
a great variety of
galaxies, many of
them billions of lightyears away
EllipticalGalaxy
Galaxy
Elliptical
Irregular Galaxies
Hubble
Ultra
Deep
Field
Spiral Galaxy
Spiral Galaxy
disk
bulge
halo
Disk Component: stars of all ages, many gas
clouds
Spheroidal Component: bulge & halo, old stars,
few gas clouds
Disk
Component:
stars of all
ages,
many gas
clouds
Spheroidal
Component:
bulge &
halo, old
stars,
few gas
clouds
Blue-white color
indicates ongoing
star formation
Red-yellow color
indicates older
star population
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
Classifying Galaxies
2
3
1
A. Elliptical
B. Spiral
C. Irregular
14
Classifying Galaxies
28
22
24
A. Elliptical
B. Spiral
C. Irregular
30
Thought Question
Why does ongoing star formation lead to a bluewhite appearance?
A. There aren’t any red or yellow stars
B. Short-lived blue stars outshine others
C. 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
Messier 33
 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
The Whirlpool
• Messier 51
• 23 million light
years away
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
Messier 82
• About 12 Mly
• very disturbed galaxy
The
Mice
~ 300 Mly
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
Cepheid variable stars with longer periods have
greater luminosities
Distances
The Nearest Stars
Determine distances of stars out to a few
hundred light-years using parallax
Distances of Galaxies
Galaxies are too far away for parallax
technique
Use “standard candles” (INVERSE
SQUARE LAW)
Cepheid variable
supergiant stars
planetary nebulae
supernovae
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
Identifying
the Main
Sequence
Apparent
brightness of
star cluster’s
main
sequence
tells us its
distance
Establishing
the brightness
of stars
Knowing a star cluster’s distance, we can
determine the luminosity of each type of star
within it, including the distances to Cepheids
Other
Distance
Methods
What clues
give you
information
about
distance?
The Distance Scale
Combination 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
Most distant galaxies
–
Virgo Cluster – about
50,000,000 LY
distant
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 (Mpc)
1000
800
600
400
Distance - Velocity Relation
200
0
0
10000
20000
30000
40000
Velocity (km/sec)
50000
60000
70000
Hubble’s Law
• The correlation
discovered by Hubble was
o
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
Hubble’s
Law …
• The Hubble Space Telescope was launched to the
distance-redshift relation
o
• Why is Hubble’s law so important?
Hubble’s constant tells us age of universe because
it relates velocities and distances of all galaxies
Distance
Age = ___________
Velocity
~ 1 / H0
 Read Units 74, 75, 76, 78, 79
 Galaxies homework due Thurs