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)

Accepted Answer

A color index is the difference in brightness (magnitude) between a star measured through two different wavelength regions, typically blue and red or blue and green. By comparing the amount of light in each wavelength region, astronomers can determine a star's temperature and composition.

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

Accepted Answer

Helium shows lines only in hot stars, so the absence of helium lines in this star's spectrum indicates that it must be relatively cool. Options A, B, D, and E are incorrect because they make assumptions that are not necessarily supported by the absence of helium lines in the spectrum.

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

Accepted Answer

Luminosity refers to the amount of energy a star gives off each second, so option D is the correct choice. The other options do not accurately describe luminosity.

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

Accepted Answer

Astronomers are more interested in the luminosity of a star because it tells us how bright the star really is, regardless of its distance from Earth, and also gives us information about the star's size and temperature. On the other hand, apparent brightness only tells us how bright a star appears from Earth and can be affected by distance and interstellar dust.

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

Accepted Answer

The brightness of a star depends on both its luminosity and its distance from us. If two stars have the exact same luminosity, but one appears four times dimmer than the other, we can conclude that the dimmer star must be four times farther away than the brighter star. This is because the apparent brightness of a star decreases with distance squared, so if we double the distance, the star's brightness decreases by a factor of four (2^2), and if we quadruple the distance, the brightness decreases by a factor of sixteen (4^2). Therefore, choice D is the best answer: star Y is twice as far away as star X.

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

Accepted Answer

Y dwarfs are distinguished by showing absorption lines from the ammonia molecule in their spectra, which is not seen in other types of 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

Accepted Answer

The spectra of stars are influenced by their temperature, which determines the amount and distribution of energy in their light. Thus, stars of different temperatures produce spectra with different characteristics.

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

Accepted Answer

Titanium oxide lines are common in the spectra of cool stars, particularly those of spectral type M.

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

Accepted Answer

The M-type star is the coolest with surface temperatures ranging from about 2,400 to 3,700 K, while the other types listed have higher surface temperatures ranging from 5,000 K to 50,000 K.

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)

Accepted Answer

Spectral classes are arranged in order of decreasing surface temperature, with the hottest stars being classified as O-type and the coolest stars being classified as M-type. The spectral lines present in a star's spectrum provide information about its temperature, composition, and other physical properties. Therefore, by grouping stars with similar spectra, astronomers can infer their properties and classify them into spectral classes.

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

Accepted Answer

Hipparchus was a Greek astronomer who lived in the second century BCE and was known for his extensive work on star cataloguing and brightness measurements. He used a system of six magnitude classes that is still in use today, with first magnitude stars being the brightest and sixth magnitude stars being the faintest visible with the naked eye. Even though he did not have a telescope, he developed a method of comparing stars of different brightness by eye, which is considered the first instance of photometry.

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

Accepted Answer

The lower the magnitude number, the brighter the star. A star with magnitude -1 is the brightest, followed by a star with magnitude 1, then 6 and 10, respectively.

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

Accepted Answer

Annie Cannon was the astronomer who classified and measured the spectra of hundreds of thousands of stars in the early 1900s, leaving the famous Henry Draper Catalogue.

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)

Accepted Answer

Photometry is the measurement of the brightness of celestial objects, typically stars, using a photometer. In this case, the astronomer has been doing photometry all night, which means she has been measuring the brightness of different stars.

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

Accepted Answer

In the hottest stars, the extreme temperatures cause hydrogen atoms to lose their electrons, resulting in ionization. Without electrons, these atoms cannot absorb or emit light at the specific wavelengths required to produce the characteristic hydrogen spectral lines.

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

Accepted Answer

A star can appear dim for multiple reasons, including being very far away, having low luminosity, radiating most of its energy outside the visible spectrum (such as in the infrared), or being obscured by interstellar matter. Without additional information, it's impossible to determine the exact reason from just observing its apparent brightness.

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

Accepted Answer

Brown dwarfs are sometimes referred to as "failed stars" because they have insufficient mass to sustain nuclear fusion in their cores and thus cannot become true stars. M type dwarfs and O-type stars are still considered stars, albeit with different characteristics and mass ranges. The Sun is a main sequence star, also known as a G-type dwarf. Proper motion is a measure of a star's apparent motion across the sky and is not directly related to whether it is a failed star or a true star.

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

Accepted Answer

Cool stars emit mostly in the infrared part of the spectrum, so a sensitive infrared telescope would be the best choice to find more type L stars.

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

Accepted Answer

The presence of methane is a feature in the spectra of both T dwarf stars and giant planets in our solar system.

Question 20

Which color star is likely to be the hottest?
A) red
B) green
C) blue-violet
D) yellow
E) orange

Accepted Answer

Blue-violet stars are likely to be the hottest because they have the highest surface temperatures, sometimes over 30,000 Kelvin, which causes them to appear blue in color. Red stars, on the other hand, are among the coolest with surface temperatures under 4,000 Kelvin.

Quiz 17: Analyzing Starlight

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

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:

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

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

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

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 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

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:

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

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:

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

Which of the following looks the brightest in the sky?

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:

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?

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?

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:

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?

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?

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

Which color star is likely to be the hottest?