Structure of the Universe

1. Structure of the Universe “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 Astronomy 315 Professor Lee Carkner Lecture 21

2. 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

3. Early Model of the Universe

4. 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

5. Steps on the Distance Ladder • Parallax: • Spectroscopic Parallax: • Cepheid Period/Luminosity Relationship: • Supernova Standard Candle: • Redshift: • out to limits of universe

6. 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

7. 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

8. 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

9. Read off luminosity from main sequence Find spectral type

10. 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)

11. Variation in Cepheid Properties

12. P-L Relation for Cepheids

13. 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

14. Most Distant Supernova

15. 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

16. 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

17. 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:

18. 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

19. 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

20. 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

21. The Hubble Law

22. Look Back Time • Light is the fastest thing in the universe, but its speed is finite c = 3 X 108 m/s • For other galaxies we can see things as they were billions of years ago, when the universe was young

23. Using the Distance Ladder • We can use the distance ladder to map the structure of the universe • Parallax and Spectroscopic Parallax • Cepheid variables • Supernova

24. 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

25. Local Group • The local group extends out over several million light years • Most other galaxies are small companions to these two

26. The Local Group

27. 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

28. Large Scale Structure

29. 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

30. The Virgo Cluster

31. Hubble Deep Field

32. 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

33. Next Time • Read Chapter 27 • Do homework (last one!) • List 3 and Quiz #3 Monday