Chapter 15 Astro Surveying the stars
A star with a parallax angle of 1/20 arcsecond is
20 parsecs away.
Ten parsecs is about
32.6 light-years.
Which has a greater luminosity, a star with absolute magnitude +4 or a star with absolute magnitude -6? By how much is it brighter
A star with absolute magnitude -6 is intrinsically more luminous than a star with magnitude +4. The difference is 10 magnitudes, so the difference in luminosity is a factor of 100 for the first 5 magnitudes and a factor of 100 for the second 5 magnitudes, making an overall difference of a factor of 1002 = 10,000.
Which is brighter in our sky, a star with apparent magnitude 2 or a star with apparent magnitude 7? By how much is the star brighter?
A star with apparent magnitude 2 is 100 times brighter than a star with apparent magnitude 7. (Five magnitudes indicates a factor-of-100 difference; larger apparent magnitude stars are always fainter.)
17) Which of the following persons reorganized the spectral classification scheme into the one we use today and personally classified over 400,000 stars?
A) Annie Jump Cannon
36) Which of the following is true about low-mass stars compared to high-mass stars?
A) Low-mass stars are cooler and less luminous than high-mass stars.
12) Which of the following statements about apparent and absolute magnitudes is true?
A) The magnitude system that we use now is based on a system used by the ancient Greeks over 2,000 years ago that classified stars by how bright they appeared. B) A star with apparent magnitude 1 is brighter than one with apparent magnitude 2. C) The absolute magnitude of a star is another measure of its luminosity. D) A star's absolute magnitude is the apparent magnitude it would have if it were at a distance of 10 parsecs from Earth. E) All of the above are true. Answer: E
16) Which of the following statements about spectral types of stars is true?
A) The spectral type of a star can be used to determine its surface temperature. B) The spectral type of a star can be used to determine its color. C) A star with spectral type A is cooler than a star with spectral type B. D) A star with spectral type F2 is hotter than a star with spectral type F3. E) All of the above are true. Answer: E
Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?
A) The star is a member of a binary star system.
On an H-R diagram, stellar masses
A) can be determined for main-sequence stars but not for other types of stars
You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star's
A) mass.
22) Which of the following best describes the axes of a Hertzsprung-Russell (H-R) diagram?
A) surface temperature on the horizontal axis and luminosity on the vertical axis
What is the approximate chemical composition (by mass) with which all stars are born?
A) three quarters hydrogen, one quarter helium, no more than 2% heavier elements
24) On a Hertzsprung-Russell diagram, where would we find stars that are cool and luminous?
A) upper right
25) On a Hertzsprung-Russell diagram, where would we find stars that have the largest radii?
A) upper right
27) On a Hertzsprung-Russell diagram, where would we find red giant stars?
A) upper right
20) Which of the following terms is given to a pair of stars that appear to change positions in the sky, indicating that they are orbiting one another?
A) visual binary
3) What are the standard units for luminosity?
A) watts
9) Suppose you measure the parallax angle for a particular star to be 0.1 arcsecond. The distance to this star is
B) 10 parsecs.
The approximate main-sequence lifetime of a star of spectral type O is
B) 3 million years
11) The most distant stars we can measure stellar parallax for are approximately
B) 500 parsecs away.
31) The faintest star visible to the naked eye has an apparent visual magnitude of about
B) 6.
40) Which of the following statements about an open cluster is true?
B) All stars in the cluster are approximately the same age.
Why do astronomers often measure the visible-light apparent brightness instead of the total apparent brightness of a star?
B) In order to measure the total apparent brightness of a star, you must measure its brightness in all wavelengths, and this is difficult to do. The only wavelengths you can measure from the surface of Earth are visible and radio wavelengths.
According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?
B) Its apparent brightness will decrease by a factor of 9.
15) Why is the spectral sequence of stars not alphabetical?
B) The original alphabetical labeling did not correspond to surface temperature and thus had to be reordered.
7) Which of the following correctly states the luminosity-distance formula?
B) apparent brightness = luminosity/4pie(distance)^2
If Star A is closer to us than Star B, then Star A's parallax angle is
B) larger than that of Star B.
23) On a Hertzsprung-Russell diagram, where would we find stars that are cool and dim?
B) lower right
The axes on a Hertzsprung-Russell (H-R) diagram represent
B) luminosity and surface temperature.
42) Cluster ages can be determined from
B) main sequence turnoff.
Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?
B) spectroscopic binary
13) The spectral sequence sorts stars according to
B) surface temperature.
1) Approximately, what basic composition are all stars born with?
B) three-quarters hydrogen, one-quarter helium, no more than 2 percent heavier elements
Two stars, Fred and Barney, are of the same size. Fred has spectral type F, while Barney has spectral type B. Which one is more luminous?
Barney has a hotter surface temperature than Fred. Since they have the same surface area, Barney is also more luminous
34) A star of spectral type O lives approximately how long on the main sequence?
C) 10 million years
Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
C) Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.
18) Which of the following persons used the ideas of quantum mechanics to describe why the spectral classification scheme is in order of decreasing temperature?
C) Cecilia Payne-Gaposchkin
41) Which of the following statements about a globular cluster is true?
C) Most stars in the cluster are yellow or reddish in color.
14) The spectral sequence in order of decreasing temperature is
C) OBAFGKM
39) Why are Cepheid variables so important for measuring distances in astronomy?
C) Their luminosity can be inferred from their period.
30) On the main sequence, stars obtain their energy
C) by converting hydrogen to helium
43) In order to understand star clusters, we need to be able to estimate their ages. What technique do scientists use for this?
C) finding the main-sequence turnoff point of the stars
38) In a pulsating variable star, which characteristic of the star changes dramatically with time?
C) luminosity
The total amount of power (in watts, for example) that a star radiates into space is called its
C) luminosity
21) Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts
C) spectroscopic binary
26) On a Hertzsprung-Russell diagram, where on the main sequence would we find stars that have the greatest mass?
C) upper left
What is the importance of Cepheid variables in astronomy?
Cepheid variables have a period-luminosity relationship that allows us to determine their luminosity from measuring their periodic variations in brightness. Comparing the luminosity with the apparent brightness then gives us the distance to the star. In this way, we can determine distances to not just Cepheid stars but the clusters and even galaxies that they are associated with.
32) Which of the following is the most common type of main-sequence star?
D) an M star
28) On a Hertzsprung-Russell diagram, where would we find white dwarfs?
D) lower left
2) Since all stars begin their lives with the same basic composition, what characteristic most determines how they will differ?
D) mass they are formed with
6) If the distance between us and a star is doubled, with everything else remaining the same, the luminosity
D) remains the same, but the apparent brightness is decreased by a factor of four.
29) You observe a star in the disk of the Milky Way, and you want to plot the star on an H-R diagram. You will need to determine all of the following, except the
D) rotation rate of the star.
Each choice below lists a spectral type and luminosity class for a star. Which one is a red supergiant?
D) spectral type M2, luminosity class I
35) A star of spectral type G lives approximately how long on the main sequence?
E) 10 billion years
Suppose that you measure the parallax angle for a particular star to be 0.5 arcsecond. The distance to this star is
E) 2 parsecs.
19) Suppose you see two main-sequence stars of the same spectral type. Star 1 is dimmer in apparent brightness than Star 2 by a factor of 100. What can you conclude? (Neglect any effects that might be caused by interstellar dust and gas.)
E) Star 1 is 10 times more distant than Star 2.
37) Which of the following luminosity classes refers to stars on the main sequence?
E) V
33) Which of the following characteristics of stars has the greatest range in values?
E) luminosity
4) A star's luminosity is the
E) total amount of light that the star radiates each second.
5) What are the standard units for apparent brightness?
E) watts per square meter
Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?
Earth would be inside the supergiant.
1) The apparent brightness of a star depends only on its luminosity.
FALSE
A 10-solar-mass star is about ten times more luminous than a 1-solar-mass star.
FALSE
All stars spend approximately the same amount of time on the main sequence.
FALSE
Most stars on the main sequence fuse hydrogen into helium in their cores, but some do not.
FALSE
Two stars have the same luminosity. Star X is spectral type F, while Star Y is spectral type K. Therefore, Star X is larger in radius than Star Y.
FALSE
We can measure stellar parallax for most stars in our galaxy
FALSE
Our Sun is a star of spectral type
G.
What is a white dwarf?
It is the remains of a star that ran out of fuel for nuclear fusion.
What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?
It is the spectral type of the hottest main-sequence star in a star cluster, and it tells us the cluster's age.
How is the lifetime of a star related to its mass?
More massive stars live much shorter lives than less massive stars.
From hottest to coolest, the order of the spectral types of stars is
OBAFGKM.
Which of the following statements comparing open and globular star clusters is not true?
Open and globular clusters each typically contain a few hundred stars.
Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?
Rigel has a higher surface temperature than Sirius.
Two stars, Betty and Wilma, are both on the main sequence. Betty is more luminous than Wilma. Which one has a hotter surface temperature? Which one is more massive? Which one is bigger? If they both formed at the same time, which one will evolve off the main sequence first?
Since they are both on the main sequence but Betty is more luminous than Wilma, Betty must be located higher up on the main sequence. Therefore, Betty has a hotter surface temperature, is more massive, and has a larger radius. Betty will also evolve faster than Wilma, and if they were formed at the same time Betty will turn off the main sequence first.
Star A has an apparent magnitude of 3 and Star B has an apparent magnitude of 5. Which star is brighter in our sky?
Star A
Why is the classification of stellar spectral types not in alphabetical order?
Stars were first classified empirically, based on observed features (location and strength of absorption lines) in their spectra, without understanding what this actually corresponded to in terms of their physical properties. When astronomers understood what caused the spectral lines and that the classification was actually one of temperature, they reordered the classification (and eliminated some letters) to result in the OBAFGKM sequence that we use today.
2) If the distance between us and a star is doubled, the apparent brightness is decreased by a factor of four.
TRUE
Some stars are cool enough to have molecules in their atmosphere.
TRUE
Spectral type, surface temperature, and color all describe the same basic characteristic of a star.
TRUE
The more distant a star, the smaller its parallax.
TRUE
Two stars both lie on the main sequence. Star X is spectral type A, while Star Y is spectral type G. Therefore, Star X is more massive than Star Y.
TRUE
Two stars have the same spectral type. Star X is in luminosity class III, while Star Y is in luminosity class V. Therefore, Star X is larger in radius than Star Y.
TRUE
We can measure the radii of stars in an eclipsing binary system, in addition to the masses
TRUE
Earth is about 150 million km from the Sun, and the apparent brightness of the Sun in our sky is about 1,300 watts per square meter. Determine the apparent brightness we would measure for the Sun if we were located five times Earth's distance from the Sun.
The Sun would appear 1/25 times as bright. So the apparent brightness would be 1,300 ÷ 25 = 52 watts per square meter.
Earth is about 150 million km from the Sun, and the apparent brightness of the Sun in our sky is about 1,300 watts per square meter. Determine the apparent brightness we would measure for the Sun if we were located half Earth's distance from the Sun.
The Sun would appear four times brighter. So the apparent brightness would be 4 × 1,300 watts per square meter = 5,200 watts per square meter.
Earth is about 150 million km from the Sun, and the apparent brightness of the Sun in our sky is about 1,300 watts per square meter. Determine the apparent brightness we would measure for the Sun if we were located one-third of Earth's distance from the Sun.
The Sun would appear nine times brighter. So the apparent brightness would be 9 × 1,300 watts per square meter = 11,700 watts per square meter.
Which of the following statements about spectral types of stars is not generally true?
The spectral type of a star can be used to determine its distance.
What is the common trait of all main-sequence stars?
They generate energy through hydrogen fusion in their core.
How do scientists estimate stellar luminosities?
They must find both the brightness of the star from Earth and its distance, and then use the inverse-square law.
Two stars, Tom and Jerry, have the same spectral type. Tom is luminosity class V and Jerry is luminosity class I. Which star is bigger? Which star is more luminous? Which star has a hotter surface temperature? Explain your answers.
Tom is on the main sequence, while Jerry is a supergiant. They both have the same surface temperature since they have the same spectral type, but since Jerry is much more luminous, Jerry must be much bigger in size.
The star Vega has an absolute magnitude of about 4 and an apparent magnitude of about 0. Based on the definitions of absolute and apparent magnitude, we can conclude that
Vega is nearer than 10 parsecs from Earth.
What is a basic assumption that is made when we measure stellar masses by observing their orbits?
We assume that the stars move around each other under the force of gravity in exactly the same way as planets move around the Sun in our Solar System. That is, Newton's laws are equally valid in these distant stellar systems.
Star A is identical to Star B, except that Star A is twice as far from us as Star B. Therefore
both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A.
On an H-R diagram, stellar radii
increase diagonally from the lower left to the upper right.
All stars are born with the same basic composition, yet stars can look quite different from one another. Which two factors primarily determine the characteristics of a star?
its mass and its stage of life
To calculate the masses of stars in a binary system, we must measure their
orbital period and average orbital distance.
Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes dimmer for about 2 hours. The most likely explanation is that
the star is a member of an eclipsing binary star system.
Based on the definition of apparent brightness, which units are appropriate for its measurement?
watts per square meter
