PHYSICS 406 - Introduction to Modern Astronomy

PRACTICE FINAL EXAM

Interactive Exam

Chose the best answer in the Table Below each Question

Correct answers will go to the next question

Incorrect answers will send you to the course review

1. Stars form from the collapse of cool clouds of gases ("stellar nurseries"). The collapse is due to the force of gravity, which tries to pull the cloud together. Why must a cloud be cool in order to collapse?

1. High temperatures would tend to resist gravity, and cause a cloud to expand instead of collapse.
2. The force of gravity depends on a particle's speed, and acts only on slow-moving particles in the gas cloud.
3. Newborn stars would be evaporated if the gas cloud were hot
4. Newborn stars are clumsy and can only catch slow atoms

2. The nuclear fusion in the sun's core consumes hydrogen fuel. With time there is less and less hydrogen available, the fusion reactions become less likely, and the energy production will go down. What is the reaction of the sun's core?

1. Because the energy production goes down the core will cool and the energy production will die out very rapidly.
2. The energizer bunny will bring a new long life battery.
3. Because of the reduced energy production the core will shrink under the pressure of the overlying material and be heated to a higher temperature, which increases the energy production again.

3. No stars exist with masses higher than 60 times the mass of the sun. What is the reason?

1. It is impossible to gather more mass out of the thin interstellar gas in one place to form a star.
2. The energy flux is so high that the radiation pressure of this flux would literally blow the star apart, if it was formed.
3. A larger mass than 60 solar masses would break apart due to centrifugal forces.

4. Jupiter has about 1% of the mass of the sun and the same composition as the sun. Why is it not shining like the sun?

1. A few photon torpedos from the Enterprise would ignite the fusion reaction; we haven't got there yet.
2. The little amount of fuel in the relatively small body of Jupiter was burnt up already.
3. The total mass of Jupiter is too small to produce a high enough temperature and density in the center to start the fusion reaction.
4. The fusion reaction inside Jupiter has not yet started, but it will within a few million years.

5. What is the early sun paradox?

1. The sun produced more energy in the past so that the Earth must have been burnt and could not produce life.
2. The sun did not have enough mass in the beginning to keep all the planets in orbit.
3. The sun produced less energy in the past so that it was much dimmer when life begun on Earth.

6. All planets in the solar system orbit the sun approximately in the same plane and move in the same direction. What is the most likely explanation for these observations?

1. The planets were captured when the sun passed regions with small planetary bodies in the galaxy.
2. The planets were formed out of the flat rotating disc of the material during the formation of the solar system.
3. The planets were ejected out of the rotating sun after its formation.

7. The early solar nebula rotated much too fast so that the centrifugal force would have torn the sun apart before it was formed. What happened with most of the excess angular momentum of this rotation?

1. The sun was slowed down by giant explosions at the beginning of its life.
2. Excess angular momentum ended up in the orbital motion of the planets.
3. An alien spaceship used tractor beams to slow the sun down.
4. The crash of many comets into the sun caused it to slow down the rotation.

8. During a Type II supernova the interior of the star collapses into a neutron star. Which of the following statements on the neutron star are not true? More than one correct answers

1. The neutron star spins much more rapidly than the original star.
2. The neutron star is heavier than the original star.
3. The neutron star has a much stronger magnetic field than the original star.
4. The neutron star is much brighter than the original star.

9. Material from a neighboring star is falling towards a black hole. While it is closing in on the black hole the material is gaining

1. angular momentum
2. gravitational energy
3. kinetic energy
4. magnetic energy

10. Boston Globe science page headline: "Another candidate for a black hole found". Which of the following remarks in the article contain the crucial information that makes astronomers believe they found a black hole and not a neutron star?

1. An X-ray emitting star was found in a gas cloud.
2. It turned out that the star system is an X-ray emitting binary with a visible and an invisible component.
3. From the motion of the binary stars the mass of the invisible component was determined to be at least 8 times the mass of the sun.
4. The star seems to be in the center of an ancient supernova remnant.

11. A star is fighting a battle against gravity. But gravity helps the star during its lifetime. How does gravity help a star?

1. Gravity makes the star slowly shrink and get dimmer, so the star conserves its nuclear fuel.
2. Gravity is responsible for the high pressure in the star's core, which makes it hot and dense enough to keep nuclear fusion going.
3. Shrinkage due to gravity is the only energy source of a star.
4. Gravity attracts photons that are reflected on the star's surface and thus makes it shine.

12. The color of a star is determined by its

1. Its distance
2. Its temperature
3. Its size
4. Its Age

13. In the Hertzsprung -Russell diagram below, where are the youngest stars?

14. If the Hertzsprung -Russell diagram above were that of a star cluster at the age of 2 billion years, there would be

1. no stars in the lower right portion of the main sequence.
2. no stars in the upper right corner.
3. stars along the entire main sequence.
4. no stars in the upper left corner of the main sequence.

15. Stars on the main sequence that have a small mass are found

1. in the upper left.
2. in the center of the main sequence.
3. in the lower right.
4. throughout the entire main sequence, because the position on the main sequence only tells us the star's age.

16. A star has a temperature of 40,000K. (The sun's surface temperature is 5,800K. ) Most of the star's energy is emitted at

1. ultraviolet wavelengths.
2. infrared wavelengths.
3. radio wavelengths.

17. The detection of a White Dwarf with a mass of 2 times the mass of the sun has been announced in a newspaper. What is your response to the article?

1. Oh this is the most massive White Dwarf that has been found.
2. Not interesting, there are millions of White Dwarfs with similar masses.
3. They got it wrong. White Dwarfs cannot hold more than 1. 4 solar masses.
4. This is the smallest White Dwarf that has ever been found.

18. What will happen to our Earth when the sun turns into a red giant?

1. It will become cold on Earth since the sun runs out of fuel and turns red, i.e. cooler on its surface.
2. There will be many earthquakes and volcano eruptions because the rapid growth of the sun shakes the Earth violently.
3. The Earth will become a hot desert because of the increased luminosity of the sun and finally may be swallowed by the giant sun.
4. The Earth will be pushed farther away from the sun when it expands into a red giant.

19. What are planetary nebulae?

1. Clouds of planetary material around stars.
2. Interplanetary gas clouds orbiting a star at a great distance.
3. Dust storms observed on planets.
4. Gas clouds, which are ejected from stars in the late stage of their life.

20. The supernova is the end of a heavy star, which has exhausted all its available nuclear fuel. What is the energy source of the tremendous event?

1. Nuclear fission energy as in a uranium atomic bomb.
2. The energy of the gravitational field of the star when it collapses to a tiny body.
3. Energy from annihilation of matter and antimatter.
4. Supernovae get their energy from gigantic space battles with phasers and photon torpedos.

21. The composition of heavy elements found in planets, meteorites, the stars and the interplanetary gas can be explained by

1. how many times protons collided with other protons during the beginning of the universe.
2. chemical reactions in the interstellar gas.
3. the sequence of nuclear reactions during the life of the stars and recycling of some of the stars' material.
4. the horrendous nuclear reactions inside massive black holes in the centers of galaxies.

22. The gravity of a white dwarf and therefore pressure of its material in the center is enormous. What keeps it from collapsing?

1. Centrifugal force, due to its rapid rotation.
2. Intense nuclear fusion throughout the star.
3. The huge degeneracy pressure exerted by electrons.
4. A White Dwarf is all iron, and iron is stable enough to keep the star from collapsing.

23. Channel 11 broadcasts "How to squeeze Mt. Washington into a sugar cube?". What will the broadcasting probably be about?

1. A very efficient food packing technique.
2. The formation of the sun out of the interstellar gas.
3. The formation of a neutron star.
4. The formation of a white dwarf

24. A pulsar, although a dead star, radiates an enormous energy in its pulses. What is its energy source?

1. The decay of its strong magnetic field is the energy source.
2. All pulsars will shrink further under their gravity and send pulses until they turn into black holes.
3. The slowing down of the fast rotation of the pulsars through magnetic braking feeds the radiation of pulsars.
4. Pulsars are strong radio transmitters of other civilizations in the galaxy that use energy techniques yet unknown to us.

25. The space ship in the following figure soars through space at 50% of the speed of light.

All four scientists can directly measure the speed of the light that comes from the light source inside the space ship. Who will find the slowest speed?

e. Nonsense, all four will measure the same speed.

26. Why can't we ever get an object to move faster than the speed of light?

1. When an object is moving almost at the speed of light, any added energy goes into increasing the object's mass, rather than its speed.
2. When an object is moving almost at the speed of light we can't see it, so we don't know where to push.
3. When an object is moving almost at the speed of light, all the light, which it reflects, exerts such high radiation pressure that we can't get it faster.
4. When an object is moving almost at the speed of light, it generates strong gravitational waves that it looses all the additional energy immediately. It therefore is not accelerated any further.

27. In the age of widespread interstellar travel a biologist experiments with turtles which normally live for about 300 years. To extend his life in order to see the results of his experiments at the end of the turtles' lives the scientist decides to make use of the relativistic time effects at high speeds. What does he do?

1. He takes his turtles along on an extended space trip, since then he will live as long as the turtles.
2. He sends the turtles on an extended space trip, since the time and the life of the turtles will be accelerated at high speeds.
3. He goes on an extended space trip, since his time and thus his life span will be stretched at high speeds.

28. The mass provides an object with inertia, i.e. its ability to resist a force that tries to accelerate it, and with gravitation, i.e. its ability to attract other objects or to be attracted. Einstein's theory of general relativity is based upon the principle that

1. the gravitational mass is greater than the inertial mass of an object
2. the gravitational mass is less than the inertial mass of an object
3. the gravitational and the inertial mass of an object are the same
4. the gravitational mass is positive and the inertial mass is negative

29. A test of Einstein's theory of general relativity was first performed

1. on the extended space flights of the Voyagers to the outer solar system.
2. by observing the deflection of starlight in the gravitational field of the sun during a solar eclipse.
3. by observing the rotation of the sun.
4. by experiments with rockets in the earth's gravitational field.

30. An object in space from which even light cannot escape is called a

1. Planetary Nebulae
2. Black Hole
3. Neutron Star

31. How can we ever hope to detect the presence of a black hole?

1. Gamma rays are the most energetic radiation. We will still find gamma rays coming out of the black hole.
2. The strong gravity of the black hole affects objects in its vicinity. That is what we will see.
3. Neutrinos will escape from the black hole, since they do not interact with matter.
4. We have to search for absolutely black spots in the sky.

32. Consider that photons are bunches of energy and that energy is equivalent to mass. Note that

high energy of photon <-> short wavelength

low energy of photon <-> long wavelength

If you take a spectrum of a white dwarf (very strong gravity), how would you see the spectral lines?

1. The spectral lines are in the same location in the spectrum as in the laboratory.
2. The lines are blue-shifted, since the photons gain energy in the gravity field.
3. The lines are red-shifted, since the photons loose energy in the gravity field.
4. No light escapes from a white dwarf.

33. What kind of galaxy is our Milky Way galaxy?

1. Elliptical.
2. Spiral.
3. Irregular.
4. Dwarf.
5. Unknown, since we can't see it from outside.

34. How do we know that we live in a disc shaped galaxy?

1. We see that all the other galaxies are disc shaped.
2. All globular clusters of the Milky Way galaxy are arranged in a band across the sky.
3. We see the stars of the Milky Way arranged in a band across the sky.
4. Astronomers have observed our Milky Way galaxy from all sides during a full orbit of the solar system about the center of the galaxy.

35. Where is the center of our Milky Way galaxy in the following figure? (Click on the appropriate point in the image)

36. What was the reason that Herschel and Kapteyn came to the wrong conclusion that we are in the center of the Milky Way?

1. They saw that all stars in the galaxy were orbiting around the sun.
2. They had the religious belief that our solar system was the center of the universe.
3. Stars beyond a certain distance are obscured by interstellar dust and cannot be seen.
4. Their distance measurements of the stars were wrong.

37. Cepheid variables are important to us in calculating

1. the ages of globular clusters.
2. the distances to galaxies.
3. the sizes of stars.
4. the composition of the interstellar medium.
5. the masses of stars.

38. The Hubble team in the press announces the observation of two supernovae of Type I in two different galaxies. You read that the supernova in galaxy A reached 4 times the brightness than the supernova in galaxy B. You conclude:

1. The supernova in galaxy A is 4 times more luminous than the one in galaxy B.
2. Galaxy B is 4 times farther away than galaxy A.
3. Galaxy B is 2 times farther away than galaxy A.

39. A new galaxy is observed with the Hubble space telescope. It has no gas and dust, and it contains no young stars. Which type is this galaxy?

1. spiral galaxy
2. barred spiral galaxy
3. elliptical galaxy
4. irregular galaxy

40. During a collision of 2 galaxies a lot of things may happen in this turmoil. What will happen to the stars in this region?

1. Because the density of stars is increased during the galaxy collision, many stars will collide.
2. Because the stars of one galaxy feel the gravity of the many stars of the other galaxy, they will be drawn into the other galaxy.
3. Because the distances between stars in a galaxy are so large, the stars will travel through the other galaxy unharmed and stay with their own galaxy.

41. Here is a view of a spiral galaxy seen edge-on. Photons from one side of the galaxy are red-shifted, while photons from the other side of the galaxy are blue-shifted, as shown. What do you conclude?

1. The galaxy is moving away from us.
2. The galaxy is expanding.
3. There is more dust on the right side than the left side.
4. The galaxy is rotating.

42. Astronomers have concluded that there is a missing mass problem in galaxy clusters. What is the reason for this conclusion?

1. The galaxy clusters do not have enough mass to produce the energy they emit.
2. All galaxy clusters are moving away from us, and this indicates that there is not enough mass to keep them from flying away from us.
3. The velocities of the individual galaxies in the clusters are so high that the sum of the masses of all individual galaxies would not be enough to keep the galaxy clusters together.

43. Clusters with only elliptical galaxies show a bright X-ray emission, homogeneously distributed over the entire cluster region. This is evidence for

1. black holes in each of the galaxy centers.
2. the young age of the galaxies which then were extremely hot.
3. collisions between galaxies, which have driven all gas and dust out of the galaxies and left behind a huge cloud of extremely hot gas.
4. galactic star wars which must have wiped out all the civilizations in the member galaxies of these clusters.

44. In their survey of the structure of the universe Margret Geller and John Huchra found that distant galaxies are

1. evenly distributed throughout the universe with no obvious structuring.
2. concentrated around the Milky Way galaxy and that much less are found farther away.
3. concentrated along the surfaces of giant bubbles that seem to be empty in their interior.
4. only found in one direction, namely the center of the universe.

45. Why do we believe there are very massive objects (possibly super-massive black holes) at the centers of galaxies?

1. The centers of galaxies are very dark.
2. Stars have been observed to disappear from view.
3. Doppler measurements show that there are very rapid motions around the centers of galaxies, and a very massive object is needed to keep things from flying away.
4. Recently astronomers have been able to detect the black hole in the center of our galaxy on an infrared picture.

46. With the Big Bang model scientists describe

1. the explosion of a supernova.
2. the cataclysmic end of the universe.
3. the supersonic bang of the Earth's bow shock in the solar wind.
4. the formation and the evolution of the entire universe.

47. You want to compute the distance of a celestial object by the use of a "Standard Ruler" method. Which parameters of the object do you have to determine?

1. the size and the mass of the object
2. the actual size of the object and the angular size in the sky
3. the mass and the velocity of the object
4. the velocity of the object and the angular size in the sky
5. the luminosity and the apparent magnitude of the object

48. Quasars were found as the objects with the largest red shift in the universe. From this we can conclude

1. that they are objects of the early universe because large red shift means a large distance and thus their light traveled for a long time.
2. that they must be much more massive than galaxies because they move away from us so fast.
3. nothing, because this is not enough information.

49. Quasars produce the energy of several 100 galaxies, i.e. greater than 10¹³ suns in a region the size of the solar system. What is the most likely energy source?

1. A star with the mass of 10000 suns can produce this luminosity with nuclear fusion according to the mass-luminosity relationship of stars.
2. The reconnection of strong magnetic fields provides the energy.
3. The accretion of enormous amounts of mass into a giant black hole with the speed of light provides the most efficient energy source for this purpose.
4. The quasars are giant wormholes out of which the energy from other universes flows into our universe.

50. Which observation taken from quasars tells us that the size of the energy-emitting region must be smaller than the size of our solar system?

1. Hubble space telescope has measured the size of the quasar centers directly.
2. The luminosity of some quasars varies strongly within one day. Thus their size can only be as large as the distance light can travel in one day.
3. Quasars are so bright that we can measure their angular sizes with a good telescope on Earth.
4. Radiotelescopes have been used to measure the size of quasars with a radar technique by bouncing strong radio pulses off a quasar.

51. Which age problem of the universe has emerged through recent observations?

1. The oldest rocks are 4. 5 billion years old, but bible interpreters say the Earth can only be 4500 years old
2. Stars cannot have enough fuel to be still seen for a universe several billion years old.
3. Derived from Hubble space telescope observations the expansion of the universe seems to be younger than the oldest stars.

52. The early universe was

1. dark and cold, since there were no stars yet.
2. very hot, because everything must have been very compressed, and compressed material is hot.
3. empty, since nothing had been created yet.
4. looked the same as today, since the universe can never change.

53. The age of the universe can be derived from Hubble's Law. In which way?

1. From the velocity and distance of galaxies in Hubble's Law we can compute the time it took for the galaxies to reach their present distance. This is close to the age of the universe.
2. The galaxies with the largest red shift must be the oldest things in the universe. Knowing their distance in light years give the age of the universe in years.
3. From the distance, mass and velocity of the galaxies we can calculate how much time was needed to accelerate them to the present velocity. This is close to the age of the universe.

54. If the average density of the universe is higher than the critical density, the universe

1. will expand forever.
2. will eventually collapse.
3. is a steady state universe.
4. violates the laws of mass conservation.

55. The stuff of the early universe was very compressed. What was the state of the original stuff of the universe?

1. Since it was incredibly compressed, it must have been like a neutron star.
2. Since compressed material had to be very hot, material was completely disintegrated into energy. There was only radiation.
3. Since it was incredibly compressed, it must have been like a white dwarf.

56. Which question is the "Anthropic Principle" related to?

1. The formation of the planets.
2. The question why the universe is exactly the way that we are able to exist in it.
3. The evolution of the life on Earth.
4. The evolution of our Milky Way galaxy.
5. The search for extraterrestrial intelligence in the universe.

57. What do we mean by the "Horizon Problem" of the universe?

1. We cannot see beyond the edge of our universe and cannot get any information from beyond the horizon of the universe.
2. The background radiation is exactly the same in opposite directions, but when the radiation was released no information could have made it from horizon to horizon of the universe yet to make it the same.
3. The universe is infinitely large. Therefore, it cannot have a horizon.

58. The 2. 7K background radiation is extremely smooth, i.e. is the same from all directions. What does this tell us?

1. It shows that the universe looked the same all the time and that the Steady State model of the universe is correct.
2. It was quiet in the early universe and there was no explosion.
3. There were so many stars in the early universe that their light was blurred into one smear.
4. The early universe was very uniform everywhere.

59. The 25% helium in the typical interstellar material stems from

1. nuclear fusion in stars
2. nuclear reactions in the interstellar gas
3. the exhaust of starships
4. nuclear reactions during the first 3 minutes of the universe

60. Olbers' Paradox of the universe refers to the fact that

1. the universe expands, although gravity is pulling the galaxies together
2. the sun is still shining, although it should have already used up its energy sources
3. the night sky is dark, although there is the same density of galaxies everywhere in the universe and no limit to their distances had been found yet

Bonus Question 1:

Why do have all the new planets that were found around other stars, so far, have masses that are comparable to or are higher than that of Jupiter?

1. There are no planets smaller than Jupiter in orbit around other stars.
2. These planets are so large that they could be directly seen through available telescopes.
3. Planets with such large masses are the easiest to detect, because they cause the center star to move most visibly.
4. Planets of the size of Jupiter emit their own light and thus can be seen easily.

Bonus Question 2:

In a spiral arm of a galaxy a stream of material approaches a region where the gravity is stronger, because the density of matter (interstellar gas and stars) is higher here. What will happen to the stream of material while it is passing this region?

1. The density of the material remains constant, since no material can be lost.
2. The density of the material is lowered in the region with strong gravity, since the material is slowed down and therefore the material flux is reduced.
3. The density of the material is increased in the region with strong gravity, since the material is slowed down, but the total flux of the material must remain constant.