Space Physics: Exploring the Universe's Mysteries

1. Units to cover: 76, 78, 79, 80

2. Tasks for those who want to get extra points (not obligatory): • 1. If you could go shopping on the Moon to buy a pound of chocolate, would you get more chocolate than if you bought a pound of chocolate on Earth? 2. Two kids of the same height are throwing balls. One kid throws the ball horizontally with an initial speed of 5 mph. At the exact same instant, the other kid lets the ball fall to Earth by dropping it — with no horizontal velocity. Which ball strikes the ground first and why?

3. Additional tasks 3. a. Is there gravity between Earth and the Space Shuttle as it orbits Earth? b. Is the satellite accelerated as it goes around the Earth? If yes, what is causing the acceleration? c. Does the satellite need to fire its engines constantly in order to maintain its speed? 4. In the constellation Orion, the star Betelgeuse is orange-red in color, and another star in the constellation, Bellatrix, is blue. What does that tell you about the temperature of each of these stars? 5. Why does the 2.4 m Hubble Space Telescope give images with better resolution than a 2.4 m telescope on the ground?

4. Tasks (continuation) 6. Absorption lines of two different transitions are shown above on the left. Energy level diagrams of these elements are shown on the right. Which energy level diagram corresponds to absorption line 1 located at 1? 7. The Sun has been fusing hydrogen into helium for about 4.6 billion years now. How much has the amount of helium on the surface (i.e., in the photosphere) increased over this time?

5. continued 8. The Sun is a low mass star and so will never have the core fusion reactions of massive stars. Yet, the Sun has iron in its atmosphere, as well as gold, silver and other elements much heavier than iron. Why? 9. If iron is the heaviest element that a massive star can make in its core, how and where are metals like gold, silver, tin and uranium made

6. 10. If the universe were contracting instead of expanding, how would we know (what would the observations be)? • 11. The Andromeda galaxy and the Milky Way are rushing toward each other at a velocity of 130 km/s (or, 300,000 mph!). We will collide in about 60 billion years. Andromeda is about one and a half times larger in diameter and in mass than our Milky Way. Describe what might happen during and after the collision. 12. The Hubble Deep Field image shows (in true color) that some galaxies are blue. Why is this?

7. Additional tasks • 13. The volume of the nucleus of an atom, which contains 99.98% of the mass, is only one hundred millionth the size of the whole atom. Explain, using the electron cloud model of the atom, why we cannot walk through walls if the wall and we are mostly empty space. • 14. As we look around the sky at night, we can distinguish about 6,000 individual stars with our eyes. Many of these stars are close together in the sky and cultures all over the world have connected the dots to draw figures from them, much like seeing figures in passing clouds. We call these traditional groupings of stars "constellations." Are these stars physically related?

8. Additional tasks • 15. The extremely elliptical orbital path of an imaginary planet is shown in the picture above. According to Kepler's Laws, where would the star have to be located?

9. A neutron star can be detected from Earth as a pulsar by its regular radio pulses only if • A. Earth lies within the path of the narrow beam of radiation generated by the neutron star • B. Earth lies in the neutron star’s “equator”, the plane perpendicular to its spin axis • C. Earth lies directly above the rotation axis of the rotating neutron star • D. none of the above

10. The main reason for the observed slow down of pulsars is • A. the slowing of rotation caused by slow expansion of the neutron star • B. friction between the stellar surface and interstellar medium • C. loss of rotational energy to provide energy for the emission of radiation • D. a build-up of the magnetic field with rotational energy being transferred to magnetic energy

11. Black holes are named because • A. all of their electromagnetic radiation is in the X-ray range • B. they emit a perfect blackbody spectrum • C. no light or any other electromagnetic radiation can escape from inside them • D. their most prominent spectral lines are in the radio

12. Energy Source for Active Galactic Nuclei • Active galactic nuclei emit a tremendous amount of radiation over a broad range of wavelengths • A black hole can be both very small, and have an accretion disk that can emit enough radiation • Likely that at the centers of these galactic nuclei, there are supermassive black holes • Intense magnetic fields in the accretion disk pump superheated gas out into jets that leave the nucleus • There are still many questions to be answered…

13. Model of AGN

14. Quasars are • A. the remnant cores of exploding stars • B. the focused image of a distant galaxy by the gravitational lens effect of a closer galaxy • C. the central nuclei of very distant, very active galaxies

15. Figure 78.06

16. The Redshift and Expansion of the Universe • Early 20th century astronomers noted that the spectra from most galaxies was shifted towards red wavelengths • Edwin Hubble (and others) discovered that galaxies that were farther away (dimmer) had even more pronounced redshifts! • This redshift was interpreted as a measure of radial velocity, and it became clear that the more distant a galaxy is, the faster it is receding!

17. The Hubble Law • In 1920, Edwin Hubble developed a simple expression relating the distance of a galaxy to its recessional speed. • V = H  d • V is the recessional velocity • D is the distance to the galaxy • H is the Hubble Constant (70 km/sec per Mpc) • This was our first clue that the universe is expanding!

18. Which two quantities are shown to be related to one another in Hubble Law? • A. distance and brightness • B. distance and recession velocity • C. brightness and recession velocity • D. brightness and dust content

19. Large Scale Structure in the Universe • Using modern technology, astronomers have mapped the location of galaxies and clusters of galaxies in three dimensions • Redshift is used to determine distance to these galaxies • Galaxies tend to form long chains or shells in space, surrounded by voids containing small or dim galaxies • This is as far as we can see!

20. How are galaxies spread through the Universe? • A. They are grouped into clusters that in turn are grouped into clusters of clusters (superclusters) • B. Galaxies are spread more or less evenly throughout the Universe • C. They are grouped around our galaxy • D. none of the above

21. Seyfert Galaxies • Seyfert galaxies are spiral galaxies with extremely luminous central bulges • Light output of the bulge is equal to the light output of the whole Milky Way! • Radiation from Seyfert galaxies fluctuates rapidly in intensity

22. Radio Galaxies emit large amounts of energy in the radio part of the spectrum Energy is generated in two regions Galactic nucleus Radio lobes on either side of the galaxy Energy generated by energetic electrons Synchrotron radiation Electrons are part of the gas shooting out of the core in narrow jets Radio Galaxies

23. What type of object does explain activity at the very center of our galaxy? • A. a rapidly rotating neutron star • B. a supermassive black hole • C. a giant molecular cloud • D. a supernova explosion

24. Why do the spiral arms show up so clearly in spiral galaxies? • A.The brightest stars occur mostly in the spiral arm, making arms stand out • B. Stars occur only in the spiral arms • C. The number of stars in spiral arms is orders of magnitude larger than in the regions between • D. The dust forms a spiral pattern

25. What is an elliptical galaxy? • A. A galaxy with an elliptical outline and a smooth distribution of brightness • B. A spiral galaxy seen from an angle • C. Any galaxy with an extended radio halo • D. A spiral galaxy with an elliptical bulge

26. Missing Mass • In Unit 73, we calculated the mass of the Milky Way by measuring the orbital velocities of dwarf galaxies in orbit around our galaxy • We can also count the number of stars in the galaxy, and estimate the galactic mass. The two numbers do not agree! • Rotation curves do not show the expected decrease in stars’ orbital velocities with distance from the galactic center, so there must be much more mass present in our galaxy • Astronomers cannot find a large majority of this mass! • Astronomers call the missing mass dark matter

27. Many galaxies have flat rotation curves! Dark matter is not unique to the Milky Way!

28. 99 percent of the stars in a galaxy are within 20 kpc of the center Gas extends far out into the disk, but is not very massive! Galaxies are now thought to be embedded in a dark matter halo that surrounds the entire galaxy Unfortunately, dark matter cannot be detected directly. Figure 78.03

29. Dark Matter in Clusters of Galaxies • Missing mass is also a problem in clusters of galaxies! • Not enough visible mass to hold the clusters together by gravitation, and to keep hot gas in their vicinity • Cluster mass must be 100 times greater than the visible mass! • Once again, dark matter seems to be the solution

30. Gravitational Lenses • Dark matter warps space just like ordinary matter does • The path of light rays bends in the presence of mass • A galaxy or other massive object can bend and distort the light from objects located behind it, producing multiple images • This is called gravitational lensing

31. Gravitational lensing of light by clusters of galaxies A. Indicates the existence of dark matter B. Proves that the Universe has large positive curvature C. Proves the expansion of the Universe D. Indicates the existence of dark energy in the Universe

32. The expansion of the Universe is not like the explosion of a bomb sending fragments in all directions Space itself is expanding! We can detect photons that appear to have moved at different speeds through space Rather, the speed of light is constant, and it is space that was moving relative to the photon If each galaxy is like a button attached to a rubber band, an ant walking along the band as it is stretched will appear to have a velocity slower than it really does. The buttons (galaxies) are fixed relative to space, but space itself is moving. An Expanding Universe

33. One More Analogy • The expansion of the universe and the increasing distance between galaxies is similar to the increase in distance between raisins in a rising loaf of raisin bread. • The raisins are fixed relative to the dough, but the dough expands, increasing the space between them. • Problem with these analogies – loaves and rubber bands have edges! • We have seen no ‘edge’ to the Universe; there are an equal number of galaxies in every direction! • Also, galaxies can move relative to space, as sometimes gravity can accelerate one galaxy toward another faster than space expands!

34. As light waves travel through space, they are stretched by expansion This increases the wave’s wavelength, making it appear more red! An objects redshift, z, is Here,  is the change in wavelength, and  is the original wavelength of the photon This is equivalent to: The Meaning of Redshift

35. The Age of the Universe • Thanks to the Hubble Law, we can estimate the age of the universe • At some point in the distant past, matter in the universe must have been densely packed. • From this point, the universe would have expanded at some high speed to become today’s universe • Assuming a constant expansion over time, we find that the age of the universe is around 14 billion years.