structure of the universe astronomy 315 professor lee carkner lecture 23
Post on 20-Dec-2015
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TRANSCRIPT
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
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
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)
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
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
Look Back Time Light is the fastest thing in the universe, but its
speed is finitec = 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
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
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 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