Transcript Structure of the Universe

Structure of the Universe
Astronomy 315
Professor Lee Carkner
Lecture 23
Quiz #3
On Friday
Same format as Quiz 1 and 2
Covers lectures 16-23
Bring pencil and calculator
The Universe

Everything was the same distance from the earth

We have no depth perception when viewing the universe
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

We use many methods, each building
on the other

Each method takes us one step further
away, out to the limits of our
observations
Steps on the Distance Ladder
Parallax:

Spectroscopic Parallax:

Cepheid Period/Luminosity Relationship:

Supernova Standard Candle:

Redshift:
out to limits of observation
Parallax
As we have seen parallax is the
apparent motion of a star as you look at
it from two different points of view


From space with the Hipparcos satellite
Standard Candle
A common way to find distance is to use a
standard candle

We can get a value for the intrinsic brightness or
luminosity (L) in joules/second

We can then find the distance from:
i.e., the closer the object, the greater flux we will will
measure for a given luminosity
Spectroscopic Parallax
We can use spectroscopy to get the spectral
type of the star

We can then estimate its luminosity from the
spectral type


We know how bright a star should be and
then we compare to see how bright the star is
Read off
luminosity
from main
sequence
Find spectral type
Cepheid Period-Luminosity
Relationship
Cepheids are bright pulsating variable stars

There is a direct relationship between period
and luminosity

Again, we can get the distance from the
luminosity and flux (flux measured directly)
Variation in Cepheid
Properties
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)

All type Ia supernova have the same absolute
magnitude are are very bright

Distant Supernova
Distance Indicator Limitations

Parallax -- Motion has to be large enough to
resolve

Spectroscopic Parallax -- Have to be able to
resolve star and it must be bright enough to
get a spectrum

Standard Candle Problems
Cepheids and supernova have to be
bright enough to see
Can see supernova further than Cepheids

Largest source of error is extinction
along the line of sight

Red Shift
The spectral lines from distant galaxies are
greatly shifted towards longer wavelengths

The degree to which the lines are shifted is
represented by z

We can find the velocity with the Doppler
formula:
The Hubble Flow
Spectra of all distant galaxies are red shifted
This means that everything in the universe is moving
away from everything else


The Hubble flow velocity is related to the object’s
distance
The Hubble Law

Larger distance, larger velocity
The two are related by the Hubble Constant
H, through the Hubble law:
We can always get V from the red shift, so if
we know d or H we can find the other
The Hubble Constant
The Hubble constant is found by plotting
velocity versus distance and finding the slope

Use the distance ladder methods

Megaparsec is one million parsecs
Our best determination for H is about 71
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

For other galaxies we can see things as they were
billions of years ago, when the universe was young

Using the Distance Ladder
We can use the distance ladder to map the
structure of the universe
Parallax and Spectroscopic Parallax

Cepheid variables

Supernova

Local Neighborhood

We are surrounded by near-by, smaller
companion galaxies

These companions are a few hundred
thousand light years away
Companions tend to be dwarf
ellipticals
Local Group

The local group extends out over
several million light years

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 together

Clusters tend to be millions of light years
across and 10’s of millions of light years apart

Supercluster size ~ 100 million light years
Large Scale Structure
The Virgo Cluster
One of the nearest clusters is the Virgo cluster

15 Mpc or 50 million light years away

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 universe

We can see powerful things like quasars

Can see back to when the universe was only 1
billion years old
See things that may be protogalaxies
Next Time
Quiz #3
Read Chapter 27.1-27.5 for Monday