Structure of the Universe

Astronomy 315Professor Lee

CarknerLecture 21

“The Universe --

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

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 universe

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 Msun)

All type Ia supernova have the same absolute magnitude are are very bright

Most 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 73

km/s/Mpc

The Hubble Law

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

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

Read Chapter 27 Do homework (last one!) List 3 and Quiz #3 Monday