ASTR 104 – Winter 2007 Intersession – Test 2 answers

Questions on material that we have not covered are greyed out. Blue questions refer to test 3 material.


(1) How can we find the distance to the nearest stars?
– By looking at their size in a photographic image.
– By measuring the amount of starlight that reaches us.
– By looking at their color.
d. By measuring their parallax.

(2) Approximately how far from us is the nearest star (other than the Sun)?
– 4 million miles.
– 4 astronomical units.
c. 4 light years.
– 4 million light years.

(3) Approximately how fast do stars move around us?
– All stars move at a few miles per hour.
– Most stars move at 50-60 miles per hour.
c. Most stars move at tens of thousands of miles per hour.
– All stars move at the speed of light.

(4) Can we tell how large a star is from its size on a photograph?
– Yes, we just need to magnify the photograph and use a ruler.
– Yes, but we have to take into account how far the star is.
– No, because all stars produce images of the same size.
d. No, almost always a large image only means that the star is very bright.

(5) How can we find out the temperature of a star?
a. From the wavelength at which it emits most of its radiation.
– From the amount by which its spectral lines are redshifted.
– By comparing its apparent brightness and luminosity.
– By measuring the rate at which its light flickers.

(6) If we say that a star is among the "cooler" ones, what is its surface temperature, approximately?
– 10 K.
b. 3000 K.
– 30,000 K.
– 10 million K.

(7) What is the main sequence?
a. A band across the HR diagram where most stars lie.
– The sequence of stages any star goes through in its lifetime.
– The sequence of stages a mid-size star like the Sun goes through.
– The set of letters O, B, A, F, G, K, M.

(8) Which stars live longer?
– All stars last about the same amount of time.
– More massive stars, because they have more fuel available.
c. Less massive stars, because they burn more slowly.
– Stars live forever, once they are able to reach the main sequence.

(9) Which of the following have we not mentioned as an example of a binary star?
– Sirius.
– Mizar.
– Algol.
d. Rigel.

(10) Which of the following things do you see when looking at an eclipsing binary star system?
– A two-star system in which both stars can be seen.
b. A star from which the amount of light we receive changes in time.
– One for which the frequency of the spectral lines changes in time.
– One whose visible spectrum shows both emission and absorption lines.

(11) What are the Pleiades?
– Two stars in a very close, tight binary system.
– The four stars at the center of the Orion nebula.
c. An open cluster of stars in Taurus.
– A globular cluster of stars in Taurus.

(12) Which of these contains more stars?
– The Alpha Centauri system.
– The Solar System.
– An open cluster
d. A globular cluster.

(13) Why would we want to plot all stars of one particular cluster together in an HR diagram?
– Because it allows us to find out how far the cluster is.
b. Because it allows us to find out the age of the cluster.
– Because it allows us to find out how hot each star in the cluster is.
– Because it allows us to find out how bright each star in the cluster is.

(14) Which star clusters are usually older, in our galaxy?
– Open clusters, which have opened up and lost stars over time.
b. Globular clusters, most of which are more than 10 billion years old.
– They are equally old, but open clusters just happen to be smaller.
– There is no general pattern, both types can be either young or old.

(15) Why do we see so many stars in close pairs or in clusters in the sky?
– Because when viewed from Earth they appear to be close to each other.
b. Because they form together in large clouds that break into fragments.
– Because stars are so massive that they attract each other over large distances.
– Because many stars break up into smaller stars when they explode.

(16) How large are the clouds that collapse to form stars, initially?
– Tens of millions of km.
– Several astronomical units.
c. Tens of light years.
– Several millions of parsecs.

(17) Which one of the following best describes a protostar?
– The first star that is formed in a given star cluster.
– A totally ionized star, made only of protons without electrons.
– An unstable star, that has already left the main sequence.
d. A hot contracting cloud fragment in the process of becoming a star.

(18) Do stars shine forever?
– Yes.
– Yes, unless other stars hit them and knock them off the main sequence.
– Yes, unless thick clouds of gas and dust surround and obscure them.
d. No, over periods of millions or billions of years they run out of fuel.

(19) What is the first element produced from hydrogen fusion inside stars?
a. Helium.
– Carbon.
– Iron.
– Plutonium.

(20) Why do stars swell up to much larger sizes after the main sequence stage in their lives?
– Because the core grows in size and pushes the other layers outwards.
– Because the temperature and rate of energy production in the core increase.
c. Because the core shrinks, the layer around it contracts, and it becomes hotter.
– Because what makes them leave the main sequence is matter from another star.

(21) What would you call an object that is not quite massive enough to become a star?
– A white dwarf.
b. A brown dwarf.
– A white hole.
– A black hole.

(22) What is a planetary nebula?
– A ring of gas and dust surrounding a newly formed planet.
– The cloud-like debris of a planet shattered by a collision.
c. An expanding, round shell of gas ejected by an old low-mass star.
– A nebula which is too small to form stars, but may form planets instead.

(23) What is a nova?
– A new star that has just formed out of a collapsing cloud.
– A star that is being completely blown away by an explosion.
c. A white dwarf in a binary system whose surface flares up.
– A galaxy whose core has just become a supermassive black hole.

(24) How often do we see novas in our galaxy?
a. Several every year.
– Once every century on average.
– We have only seen one so far.
– We have only seen them in other galaxies.

(25) What is the difference between type I and type II supernovas?
– Type I supernovas are 100 times as bright as type II supernovas.
b. Type I supernovas are binary stars, type II single massive stars.
– Type I supernovas emit visible light, type II only radio waves.
– Type I supernovas produce interstellar gas, type II produce dust.

(26) How often do supernovae normally occur inside our galaxy?
– We see several every year.
b. We expect to see one every 100 years or so on average.
– Only one every 30-40 million years.
– Never; supernovas only occur in young, distant galaxies.

(27) What is a neutron star?
a. The remnant of a massive star after a supernova explosion.
– A star that is too small to start burning hydrogen in its core.
– The hottest and brightest kind of star on the main sequence.
– A star in which the atoms are not ionized and remain neutral.

(28) What is a pulsar?
a. A rotating neutron star emitting a beam of radio waves.
– A neutron star that pulsates by expanding and contracting.
– A neutron star orbiting around a white dwarf and eclipsing it.
– A Cepheid variable that is collapsing to form a white dwarf.

(29) Which of the following is an example of a pulsar?
– The center of the Milky Way galaxy.
b. The center of the M1 (Crab) nebula.
– The center of the M87 globular cluster.
– The center of the Orion Nebula.

(30) According to Einstein's theory of relativity, what produces gravitational forces?
a. What we call gravity is caused by the curvature of spacetime.
– What we call gravity is actually due to the magnetic field of a star.
– Gravity is caused by intense radiation coming from hot objects.
– Gravity is the force between two massive objects a certain distance apart.

(31) What is an event horizon?
a. The surface of a black hole, from inside which not even light can escape.
– The farthest distance you would see if you were standing on a black hole.
– The edge of a galaxy, beyond which there is only dark matter.
– The farthest distance you can see without interstellar matter blocking the view.

(32) What would happen to you if you stood on the surface of a black hole?
a. The surface is not an actual place to stand on, and you would just fall inward.
– Because black holes spin very fast you would be thrown outward at great speed.
– You would be permanently stuck on the surface, unable to see or to lift your feet.
– The strong gravity of the black hole would flatten you down on its surface.

(33) Which one of the following is a way for black holes to form?
– A star is surrounded by so much dust that no light can shine through.
– A star is formed from a cloud of dark matter instead of regular matter.
c. A star collapses to such a small size that its gravity prevents light from escaping.
– A star cools down past the white dwarf stage and in the end emits no light at all.

(34) Are there any black holes in our galaxy?
– No, we don't have good evidence for any black holes anywhere yet.
– No, they have been seen only in some of the most distant galaxies.
– Only one, a supermassive black hole in the core of the galaxy.
d. Yes, there is a massive one at the core plus many smaller ones.

(35) What are the two most common elements in interstellar space?
a. Hydrogen and helium.
– Helium and oxygen.
– Hydrogen and carbon.
– Carbon and oxygen.

(36) On average, how much matter is there in interstellar space?
a. 1 atom per cubic centimeter.
– 10,000 atoms per cubic centimeter.
– 10 million atoms per cubic centimeter.
– 100 billion atoms per cubic centimeter.

(37) What effect does gas in interstellar clouds have on starlight?
– It dims and reddens the light.
– It bends and focuses the light.
– It causes light to be blueshifted.
d. It absorbs light at some specific wavelengths.

(38) What is the name of the galaxy we are in?
– The Solar System.
– The Local Group.
c. The Milky Way galaxy.
– The Andromeda galaxy.

(39) How many stars are in our galaxy?
– One.
– Several hundreds.
– Hundreds of thousands.
d. About 100 billion.

(40) How do we know the size of the halo of our galaxy?
– We look at nearby galaxies, and assume they are roughly the same.
– We observe the 21-cm wavelength radio waves from it.
– We send out radio waves and wait for them to bounce back to us.
d. We map the positions of the globular clusters in our galaxy.