Deck 17: Analyzing Starlight

Question 1

When an astronomer measures a color index for a star, what is she measuring?

A) what color the human eye sees when people look at that star
B) the difference between how bright a star looks at two different wavelength regions
C) the total luminosity of the star in all parts of the electromagnetic spectrum
D) the amount of hydrogen in the atmosphere of the star
E) how the color of the star is changed when its light passes through the Earth's atmosphere
Section 17.3: The Spectra of Stars (and Brown Dwarfs)

Answer 1

the difference between how bright a star looks at two different wavelength regions

Question 2

A graduate student has done a careful analysis of the spectrum of a star. While she has found lines from many elements, there was not a trace of the element helium in the spectra she has been analyzing. From this she can now conclude:

A) there is most likely no helium anywhere in the star
B) all the helium must be in the core of the star; there is none of it in the outer regions
C) since helium shows lines only in hot stars, this star must be relatively cool
D) since helium is the kind of element that quickly bonds with others, all the helium in this star must be in the form of molecules
E) the student was not surprised, because NO star ever shows any lines of helium

Answer 2

since helium shows lines only in hot stars, this star must be relatively cool

Question 3

When an astronomer rambles on and on about the luminosity of a star she is studying, she is talking about:

A) what color the star is
B) the total amount of mass in the star
C) the star's apparent size (the size seen from Earth)
D) how much energy the star gives off each second
E) the elements she can see in the star's spectrum

Answer 3

how much energy the star gives off each second

Question 4

Why are astronomers much more interested in the luminosity of a star than its apparent brightness?

A) because luminosity can be measured exactly, but apparent brightness can only be roughly estimated
B) because the luminosity tells us how bright a star really is, while apparent brightness only tells us how bright it happens to look from Earth
C) because the luminosity also tells us what elements the star is made of, while apparent brightness cannot tell us a star's chemical make-up
D) because luminosity can tell us how bright it is inside the star's core, while apparent brightness only tells us about its outside layers
E) you can't fool me, there is no difference between luminosity and apparent brightness; they are merely different terms for the same property of a star
Section 17.2: Colors of Stars

Question 5

Two stars have the exact same luminosity, but star Y is four times dimmer looking that star X. This means that

A) star Y is four times as far away as star X
B) star Y is 16 times as far away as star X
C) star Y is half as far away as star X
D) star Y is twice as far away as star X
E) we can't figure out the relative distance of the two stars from the information given

Question 6

Starting in 2009, astronomers have been discovering really cool objects out there (cool here meaning low-temperature, as well as really interesting), which they have called Y dwarfs. What distinguishes these brown dwarfs from others that astronomers have discovered?

A) they show absorption lines from the ammonia molecule in their spectra
B) they are hotter than other brown dwarfs
C) they orbit hundreds of nearby stars and are much more common than other brown dwarfs
D) they show strong evidence of helium in their spectra
E) they have much larger mass than other brown dwarfs

Question 7

A team of astronomers takes spectra of thousands of different stars in different parts of the sky. The spectra show significant differences. The main reason the spectra of the stars do not all look alike is that the stars

A) are located in many different regions of the Milky Way
B) have different temperatures
C) are made of significantly different elements
D) sometimes have atmospheres and sometimes do not
E) change their spectra as they age, and so young stars have very different spectra from older ones

Question 8

After a lot of work, a group of graduate students has finally measured the wavelengths of many dozens of lines in the spectrum of a distant star. If a number of the lines come from molecules such as titanium oxide, the star is likely to be which spectral type:

A) O
B) B
C) A
D) M
E) we need more information; lines from molecules can be found in stars of every spectral type

Question 9

Which of the following types of star is the coolest (has the lowest surface temperature)?

A) O
B) A
C) M
D) F
E) G

Question 10

Astronomers arrange the stars into groups called spectral classes (or types) according to the kinds of lines they find in their spectra. These spectral classes are arranged in order of:

A) decreasing surface temperature
B) increasing mass
C) increasing amount of hydrogen
D) decreasing distance from us
E) you can't fool me, there is no order to the spectral types (that's why the letters are not in alphabetical order)

Question 11

The first astronomer who did photometry in a systematic way (even though he did not have a telescope) was

A) Hipparchus
B) Ptolemy
C) Kepler
D) Galileo
E) Hubble

Question 12

Which of the following looks the brightest in the sky?

A) a star with magnitude 10
B) a star with magnitude 1
C) a star with magnitude 6
D) a star with magnitude -1
E) you can't fool me, all of the above look equally bright from Earth

Question 13

The astronomer who, at the turn of the century, measured the spectra of hundreds of thousands of stars, leaving a catalog that astronomers used for the rest of the century, was:

A) Edwin Hubble
B) Annie Cannon
C) Cecilia Payne
D) Joseph Fraunhofer
E) James Lick

Question 14

An exhausted-looking astronomer comes off the mountain where her observatory is located and tells you she has been doing photometry all night. What has she been up to?

A) taking photos through bedroom windows in the valley below
B) measuring the positions of stars on photographic plates taken over many years
C) putting the light of stars through a spectrograph to measure what elements are present
D) measuring the brightness of different stars
E) counting the number of stars in different star clusters (groups)

Question 15

If hydrogen is the most common element in the universe, why do we not see the lines of hydrogen in the spectra of the hottest stars?

A) in the hottest stars, hydrogen nuclei are forced to break apart into smaller nuclei
B) in the hottest stars, all hydrogen in the star has quickly fused into helium
C) in the hottest stars, hydrogen can quickly combine with oxygen to make H₂O, whose spectrum consists of completely different lines
D) in the hottest stars, the hydrogen atoms experience a huge Doppler shift, which moves the lines in the spectrum to a completely unrecognizable place
E) in the hottest stars, hydrogen atoms are ionized, and so there are no electrons to produce lines in the spectrum

Question 16

Using a good pair of binoculars, you observe a section of the sky where there are stars of many different apparent brightnesses. You find one star that appears especially dim. This star looks dim because it is:

A) very far away
B) very low luminosity
C) radiating most of its energy in the infrared region of the spectrum
D) partly obscured by a cloud
E) it could be more than one of the above; there is no way to tell which answer is right by just looking at the star

Question 17

Some objects in space just don't have what it takes to be a star (just like many hopefuls in Hollywood don't.) Which of the following is a "failed star", an object with too little mass to qualify as a star?

A) a brown dwarf
B) an M type dwarf
C) an O-type star
D) the Sun
E) any star with high proper motion

Question 18

In recent decades, astronomers discovered stars even cooler than the traditional spectral type M stars recently. Astronomers gave these cool stars a new spectral type, L. If you wanted to go out and find more such type L stars, what kind of instrument would it be smart to use?

A) an x-ray telescope, in orbit above the Earth's atmosphere
B) a sensitive infra-red telescope
C) a small visible-light telescope (something even an amateur astronomer or small college might have)
D) a CCD attached to an ultra-violet telescope
E) a swimming-pool size vat of commercial cleaning fluid, deep in an abandoned mine

Question 19

One similarity in the spectra of T dwarf stars and giant planets in our solar system is that their spectra show:

A) lots of free oxygen
B) lines of ionized magnesium and iron
C) indications of methane
D) nothing that shows that hydrogen was ever present in them
E) temperatures at which human beings could survive unprotected

Question 20

Which color star is likely to be the hottest?

A) red
B) green
C) blue-violet
D) yellow
E) orange

Question 21

At an astronomical conference, an astronomer gives a report on a star that interests astronomers because of hints that it may have a planet around it. In his report the astronomer gives the average speed with which this star is moving away from the Sun. How did the astronomer measure this speed?

A) by seeing how the luminosity of the star has been decreasing as it moves farther and farther away
B) by seeing the whole star become much redder than it used to be
C) by measuring the diameter of the star (which is easy to do) and noticing that it is getting smaller and smaller
D) by looking at the Doppler shift in the lines of the star's spectrum
E) the astronomer must be making up stories to impress his colleagues; there is no way to measure the speed with which stars move away or toward us.

Question 22

A star moving toward the Sun will show:

A) a shift in the spectral lines toward the blue end (as compared to the laboratory positions of these lines)
B) a significant increase in its apparent brightness (magnitude)
C) more and more helium lines as it approaches us
D) a shift in the spectral lines toward the red end (as compared to the laboratory positions of these lines)
E) no change that can be measured with our present-day instruments

Question 23

Studies of the spectra of stars have revealed that the element that makes up the majority of the stars (75% by mass) is

A) stellarium
B) hydrogen
C) helium
D) carbon
E) Einsteinium

Question 24

Astronomers call the motion of a star across the sky (perpendicular to our line of sight) its

A) radial velocity
B) Doppler shift
C) light travel time
D) proper motion
E) spectral type

Question 25

An astronomer whose secret hobby is riding merry-go-rounds has dedicated his career to finding the stars that rotate the most rapidly. But the stars are all very far away, so none of them can be seen to spin even when he looks through the largest telescopes. How then can he identify the stars that rotate rapidly?

A) all stars that rotate show a huge Doppler shift toward the blue end of the spectrum
B) stars that rotate have a significantly lower luminosity than stars that do not rotate
C) stars that rotate have much wider lines in their spectra than stars that do not
D) stars that rotate bring the light atoms (like hydrogen) spinning up to their surfaces; so they can be identified by the elements they contain
E) this astronomer better spend some more time enjoying his hobby, because he is not doing well at his job; there is no way we know about today to identify stars that rotate

Question 26

Imagine that a brilliant but quirky scientist in the biology department manages to put you in a deep freeze and you wake up in a million years. Which of the following statements about the sky you would see in that future time is correct?

A) all the stars and constellations would look exactly the same as they do now
B) all the stars we can see in the sky today will have died in a million years
C) if you could see them up close, almost all the stars in the sky today will have changed their color significantly in a million years
D) because of proper motion, a number of the familiar constellations will look somewhat different in a million years
E) at the present time, astronomers do not know enough about the universe to say what the sky might be like in a million years

Question 27

One key difference that astronomers use to distinguish between brown dwarfs and high-mass planets is that:

A) brown dwarfs are all much larger in diameter than any planets
B) brown dwarfs shine quite brightly in visible light, while planets are only visible from the light they reflect
C) brown dwarfs are able to do deuterium fusion in their cores, while planets can't
D) brown dwarfs are much lower in mass than planets like Jupiter
E) brown dwarfs have a much lower luminosity than any planets
Section 17.4: Using Spectra to Measure Stellar Radius, Composition, and Motion