Structure of the Universe Astronomy 315 Professor Lee Carkner Lecture 23

Quiz #3  On Friday  Same format as Quiz 1 and 2  Covers lectures  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

Find spectral type Read off luminosity from main sequence

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 M sun )   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 10 8 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 for Monday