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Structure of the Universe
“The Universe --
Astronomy 315
Professor Lee Carkner
Lecture 21
Size: Bigger than the biggest
thing ever and then some. Much
bigger than that in fact, really
amazingly immense, a totally
stunning size, real "wow, that's big,"
time. ... Gigantic multiplied by
colossal multiplied by staggeringly
huge is the sort of concept we're
trying to get across here.”
--Douglas Adams, The Restaurant at
the End of the Universe
The Universe
One of the earliest models of the universe
had everything outside of the solar system
fixed to a celestial sphere
Everything was the same distance from the
This is how the universe looks
We have no depth perception when viewing the
We have to somehow find the distance to
celestial objects to understand the true
nature of the universe
Early Model of the Universe
The Distance Ladder
There is no single method that can be
used to find the distances to all objects
We use many methods, each building
on the other
Called the cosmic distance ladder
Each method takes us one step further
away, out to the limits of our
Steps on the Distance Ladder
out to ~1000 pc
Spectroscopic Parallax:
out to 100,000 pc
Cepheid Period/Luminosity Relationship:
out to ~5,000,000 pc
Supernova Standard Candle:
out to 4 billion pc
out to limits of universe
As we have seen parallax is the
apparent motion of a star as you look at
it from two different points of view
Shift decreases with distance
Shift is only measurable out to 1000 pc
From space with the Hipparcos satellite
Spectroscopic Parallax
We can use spectroscopy and
photometry to get the spectral type and
the apparent magnitude (m) of a star
We can estimate the absolute
magnitude (M) from the spectral type
With the two magnitudes we can get
the distance:
m-M = 5 log d - 5
Example: We know how bright an A0
should be, so we can find its distance by
how bright it looks
Cepheid Period-Luminosity
Cepheids are bright pulsating variable stars
As the star get larger and smaller the
brightness goes up and down in a very
regular way
There is a direct relationship between period
and luminosity
Long period (slow changes) means brighter star
Again we can get the distance from the
luminosity and flux (flux measured directly):
F = L/4pd2
Variation in Cepheid
P-L Relation for Cepheids
Supernova Standard Candles
Type Ia supernovae are not exploding
massive stars, but rather a white dwarf
that accretes mass from a companion
until it exceeds the Chandrasekhar limit
(1.4 Msun)
When this occurs the WD collapses and
rapidly burns its carbon
All type Ia supernova have the same
absolute magnitude are are very bright
We can use them to find distance to very
distant objects
Most Distant Supernova
Distance Indicator Limitations
All methods have limits where they
can’t be used and problems that can
lead to errors
Parallax -- Motion has to be large
enough to resolve
Even from space can’t resolve parallax
beyond 1000 pc
Spectroscopic Parallax -- Have to be
able to resolve star and it must be
bright enough to get a spectrum
Exact spectral type is uncertain
Standard Candle Problems
Cepheids and supernova have to be
bright enough to see
Can see supernova further than Cepheids
but, supernova are transient events (have to
wait for one to occur)
Largest source of error is extinction
along the line of sight
Makes things appear more distant
Red Shift
The spectral lines from distant galaxies
are greatly shifted towards longer
The galaxies are moving away from us
very quickly
The degree to which the lines are
shifted is represented by z
High z = large red shift = high velocity
We can find the velocity with the
Doppler formula:
z = v/c
The Hubble Flow
Spectra of all distant galaxies are red shifted
This means that everything in the universe is
moving away from everything else
This in turn means that he universe is expanding
Objects can have other motions as well, but
the motion due to expansion is called the
Hubble flow
The Hubble flow velocity is related to the
object’s distance
The Hubble Law
If a plot is made of recession velocity
versus distance, the result is a straight
Larger distance, larger velocity
The two are related by the Hubble
Constant H, through the Hubble law:
V = Hd
We can always get V from the red shift,
so if we know d or H we can find the
The Hubble Constant
The Hubble constant is found by
plotting velocity versus distance and
finding the slope
Need accurate distance over a range of
Use the distance ladder methods
H is given in units of kilometers per
second per megaparsec (km/s/Mpc)
Megaparsec is one million parsecs
Our best determination for H is about
70 km/s/Mpc
The Hubble Law
Look Back Time
Light is the fastest thing in the universe, but
its speed is finite
c = 3 X 108 m/s
When we look at distant objects we are
seeing them the way they were when the light
left them, not the way they are now
For other galaxies we can see things as they
were billions of years ago, when the universe
was young
Distance in light years gives the look back time
Using the Distance Ladder
We can use the distance ladder to map the
structure of the universe
Parallax and Spectroscopic Parallax
Use to find the dimensions of our galaxy
Cepheid variables
Use to find the distance to near-by galaxies
Use to find distances for very distant galaxies
Local Neighborhood
Our galaxy is about 100,000 light years
in diameter
We are surrounded by near-by, smaller
companion galaxies
LMC and SMC are two examples
These companions are a few hundred
thousand light years away
Companions tend to be dwarf
Local Group
The Milky Way is in a cluster called the
Local Group
The local group extends out over
several million light years
Group is dominated by the two largest
spirals: M31 and the Milky Way
Most other galaxies are small
companions to these two
The Local Group
Beyond the Local Group
If we photograph the sky, we clearly
see places where galaxies are grouped
The universe is full of clusters
Clusters tend to be millions of light
years across and 10’s of millions of light
years apart
Clusters gathered into superclusters
Supercluster size ~ 100 million light years
Large Scale Structure
The Virgo Cluster
One of the nearest clusters is the Virgo
More than 2000 galaxies and covers 100
square degrees in the sky
15 Mpc or 50 million light years away
Centered on giant ellipticals larger than
the entire local group
Local group is a poor cluster, Virgo is a
rich one
The Virgo Cluster
Hubble Deep Field
The Distant Universe
It is hard to see into the distant
Things are very far away and so are faint
We can see powerful things like
Can see other objects in the 10 day long
exposure of the Hubble Deep Field
Can see back to when the universe was
only 1 billion years old
See things that may be protogalaxies
Next Time
Read the rest of Chapter 19
Question of the Day:
How did the universe form and how will it
List 3 due Friday
Quiz 3 on Monday