Structure of the Universe Astronomy 315 Professor Lee Carkner Lecture 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/4  d 2

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 )  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 finite c = 3 X 10 8 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