Final Exam

Question 1

41- a) What is an exit cone for light, and why does it exist? Use diagram(s) and a few sentences. Show what happens to light shining within the exit cone, and outside it.

Answer 1

a) The lowest escape velocity from any object is straight up, otherwise the curvature of spacetime requires greater and greater velocity as the object has to overcome the curvature. For huge objects, sometimes the escape velocity gets so high that nothing can escape it- not even light. An exit cone are the possible paths that light can take to escape (the more direct/less curvature regions.)

Question 2

41b) What happens to the exit cone at an event horizon?

Answer 2

b) At an event horizon, the exit cone for light disappears. This is because the curvature of space at event horizon is so great that not even light can escape it.

Question 3

43- a) What do astronomers look for to detect a stellar black hole, and why?

Answer 3

a) Astronomers look for a binary system with a red giant and a dark companion. They hope that the large volume of the red giant would push it beyond the Lagrangian point of the system, so that material from it enters the black hole’s accretion disk. This material is then heated up and emits tons of xrays.

Question 4

43b) What was the name of the first one discovered, in what decade was it discovered?

Answer 4

b) Cygnus X1- 1964! Notes say 74…

Question 5

45- a) Describe the density wave theory of galactic spiral structure. How do the arms maintain themselves in this theory, and how does it change our thinking about galactic rotation rates?

Answer 5

a

Question 6

45b) How does this theory help explain the birth of stars?

Answer 6

A

Question 7

46- a) Draw a graph of the Hubble law.

Answer 7

A

Question 8

46b) How did astronomers gather the information needed to establish the law?

Answer 8

A

Question 9

46c) What is the definition of the Hubble constant, and what is its value thought to be today?

Answer 9

A

Question 10

47- a) How and when was the first quasar discovered? Include its name and a graph and diagram illustrating the method.

Answer 10

A

Question 11

47b) Why are quasars hard to explain?

Answer 11

A

Question 12

48- a) Describe how our universe could be finite in spatial size but not bounded in any direction.

Answer 12

A

Question 13

48b) Also use a diagram and a few sentences or equations to derive an expression for the approximate age of the universe from the Hubble constant. Assuming gravity has always been the ruling force over long distances, is that estimate an upper or lower limit, and explain why?

Answer 13

A

Question 14

49- a) Draw the Friedmann and Steady State models of the universe on a graph of scale parameter vs. time. Indicate where the present time is on the graph.

Answer 14

A

Question 15

50- Why does the cosmic background radiation (CBR) favor the Friedmann model over the steady state model? Explain how the CBR originated and when, using a graph and a few sentences.

Answer 15

A

Question 16

51-a) What is the evidence for dark matter? Include as much as you can, and use diagrams where appropriate to explain it.

Answer 16

A

Question 17

53- a) Name 4 problems with cosmology as it existed about 25 years ago, and briefly describe each one.

Answer 17

A

Question 18

1- a) List 3 physical properties that are vector quantities, and 3 that are not.

Answer 18

a) vectors: acceleration, pressure, velocity

not vectors: energy, time, mass

Question 19

1b) Write a few words or a sentence that describes conceptually what each one means.

Answer 19

b)
velocity: change in the position of an object over time
acceleration: change in the velocity of an object over time
pressure: force (push or pull) applied to a unit of area
mass: resistance to acceleration.
time: an amount of time, another dimension of space
energy: potential to do work.

Question 20

5- a) Why are radio telescopes sometimes linked together to form an “interferometer”? In other words, state what problem this helps to solve.

Answer 20

a) Interferometry is combining more than one radio telescope to gather data. Combining data from more than one telescope effectively creates one telescope as large as the distance between the telescopes involved. This allows for much greater resolution and smaller beam width when studying the sky.

Question 21

5b) Discuss why “adaptive optics” are used in modern optical telescopes, and basically how that technology works.

Answer 21

b) Adaptive optics is when a reflecting mirror is made of many small individual mirrors that can be adjusted. They are adjusted to counter atmospheric turbulence and thus gain a much sharper image.

Question 22

7- a) Name and briefly describe two effects that prove that the Earth rotates on its axis.

Answer 22

a) 1. coriolis effect: Geometry of the rotation of earth deflects massive objects to appear to turn clockwise in northern hemisphere and counterclockwise in southern hemisphere.
2. foucault pendulum: As the earth spins on its axis, a pendulum not restricted to one plane will precess and change its plane

Question 23

7b) Illustrate one of these effects with a diagram.

Answer 23

b) Draw coriolis, twirl on atmosphere going LOOK UP DIRS

Question 24

9- a) In a few words, name 2 reasons we have the seasons on Earth?

Answer 24

a) Because of the Earth’s 23.5° tilt, the sun’s light hits the northern and southern hemispheres of the Earth at different angles at different times of the year (as Earth revolves around the sun).
1. During the winter, a shaft of sunlight hits a wider area of the Earth’s surface than it does in the summer. This means that the same amount of light is spread across a wider area, thus heating that area less than the smaller area it would hit in summer.
2. The hemisphere in winter spends less time on the day side of the terminator. This means the day is shorter, meaning that it is exposed to sunlight for shorter periods of time. Less time being heated means lower temperature.

Question 25

9b) Include an Earth diagram, showing the Tropics, Arctic and Antarctic circles with their latitudes, plus incoming sunlight and the terminator for late June.

Answer 25

b) Tropic of cancer is 23.5°N, Capricorn is 23.5°S, artic circles are 66.5(CHECK)°, incoming sunlight+terminator for late june has terminator bending L->R.

Question 26

14- a) Use 2 diagrams of sun, earth and moon to show: 1- the three types of lunar eclipses; 2- the three types of solar eclipses. Label each type of eclipse, and describe its appearance.

Answer 26

a) Refer to notes to plan out diagram. Definitions;
Lunar Eclipses
Total LE: When the moon is caught completely within the umbra (shadow) of the Earth, moon is much darker and has a reddish appearance
Partial LE: When the moon is between the umbra and penumbra of the Earth. - part of the moon looks dark and red, the rest is normal.
Penumbral lunar eclipse: When the moon is in the penumbra of the Earth. Moon looks no different, has to be measured to pick up difference.

Solar eclipses:
Total solar eclipse: The moon completely blocks the sun (earth is in the umbra), only the sun’s corona is visible.
Partial solar eclipse: The moon partially blocks the sun, earth is in both the umbra and penumbra of the moon, giving the sun a crescent shape.
Annular solar eclipse: The moon is at or near aphelion and appears too small to completely obscure the sun. Moon looks like a concentric circle within the sun.

Question 27

14b) Draw a diagram showing the forces that cause the tides on Earth.

Answer 27

b) Gravitational force arrows around the planet cause bulging on both sides, direct pull of moon+tidal forces on other side.

Question 28

15- a) Describe what it would be like to stand on the dayside of the moon. Include a description of the ground, approximate temperature compared to earth, gravity, appearance of the sky and how you see the earth move in the sky above you.

Answer 28

a) On the dayside of the moon it would would be extremely hot, around 240° fahrenheit. All around you is the bleak, black and white lunar landscape. Your boots are in 2-6” of lunar dust. The sky is night sky except for the earth and maybe the sun. The earth doesn’t move. The gravity there is 1/6th that of earth so you can jump really high

Question 29

15b) At midday on the moon, what phases are possible for the Earth as you would see it, and draw a diagram to illustrate this answer?

Answer 29

b) Draw diagram but remember ALWAYS DAYSIDE OF MOON.

Question 30

18- a) Describe two substantially different astronomical measurements that are made possible by the Doppler shift of light. Include a diagram for each.

Answer 30

a) 1. One measurement that is made possible by the doppler effect is measuring the radial velocity of something. By calculating the “real” wavelength of the object, and knowing the speed of light and the doppler’d wavelength, we can know if the object is moving to or away from us and at what speed.
2. Another measurement is the rotational speed of a body. Measure the blueshift of the side coming towards you and the redshift of the side moving away to get its rotational velocity.

Question 31

18b) Draw a diagram of a spectrograph, and label as many of the parts as you can.

Answer 31

Look this up.

Question 32

19- a) Write “Bode’s Law”, and indicate the location of each planet in the scheme.

Answer 32

a) Bode’s law states that if you take a sequence of numbers that goes 0,3,6,12,24,48, keep doubling, add four, then divide by 10, you get the distance from the sun in AU (earth’s avg). The planets go in order lowest to greatest.

Question 33

19b) State where the law originally appeared not to be accurate.

Answer 33

b) The law appeared inaccurate between Mars and Jupiter, Jupiter skips a step and there is no planet at the distance 2.8AU. However, the minor planet Ceres was found there, which lead to the discovery of an asteroid belt at this location.

Question 34

21- a) Name 2 major features on the surface of Mars, and compare each in size to a familiar feature on Earth.

Answer 34

a) Two major features on the surface of Mars are Olympus Mons, an extinct mountain volcano that is four times as tall as Mt. Everest and the tallest mountain in the solar system. Another is the Valles Maranes, an ancient canyon carved by water that is as long as the United States is wide.

Question 35

21b) Name 4 of the probes (rovers etc.) that were soft-landed on Mars before 2012.

Answer 35

b) Viking 1 and 2, Sojourner+Pathfinder, Spirit, Opportunity.

Question 36

33- a) Draw an H-R diagram, showing the major domains where stars are found. Label the axes as completely as you can.

Answer 36

a) Draw the curve of stellar heat vs stellar brightness, I think, and the L->R line going down. Draw the white dwarfs down below and the “red giants” up above. Brigher abs mag on Y going up, going right on X from cooler to hotter.

Question 37

34- a) Who discovered the temperature sequence of the Draper spectral types, and at what University?

Answer 37

a) Annie Cannon at Harvard.

Question 38

34b) Who discovered the period-luminosity relationship for Cepheid variable stars, and in what decade?

Answer 38

b) Henrietta Leavitt in 1912 I BELIEVE

Question 39

39- Discuss white dwarf stars.
a) In what decade was the first one discovered, in what decade was it found to be unusual, what is its name, and why was it thought to be unusual?

Answer 39

a) The first white dwarf star to be discovered was Sirius B, discovered in 1862, found to be unusual from spectral diagram in 1915, unusual because it was hugely massive but small: way too dense to be real.

Question 40

39b) What was the stability problem, who explained it and in what decade?

Answer 40

b) The density problem stated that white dwarfs such as Sirius B should not be stable, as they were so dense that they should collapse under their own gravity. It was solved by S. Chandrasekar in 1931.