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 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 earth This is how the universe looks

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

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 observations

Steps on the Distance Ladder Parallax:

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 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

Shift decreases with distance Shift is only measurable out to 1000

pc maximum 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 Relationship

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/4d2

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)

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 wavelengths 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 line 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 other

The Hubble Constant The Hubble constant is found by plotting

velocity versus distance and finding the slope Need accurate distance over a range of

distances 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 finitec = 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

Supernova 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 ellipticals

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 together 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 cluster

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 universe

Things are very far away and so are faint We can see powerful things like quasars 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 end?

List 3 due Friday Quiz 3 on Monday