astro ch 16 Flashcards
Star A is identical to Star B, but 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
All stars are born with the same basic composition, yet stars can differ greatly in appearance. Which two factors are most important in determining the current appearance of a star?
mass and stage of life
Why do some of the bright stars (such as the one indicated by the arrow) in this photo have cross-shaped spikes over them?
The spikes are an artifact of photography through a telescope.
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?
spectroscopic binary
If the star Alpha Centauri were moved to a distance 10 times farther than it is now, its parallax angle would
get smaller
How did astronomers discover the relationship between spectral type and mass for main sequence stars?
By measuring the masses and spectral types of main-sequence stars in binary systems.
The total amount of power (in watts, for example) that a star radiates into space is called its
luminosity
According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?
Its apparent brightness will decrease by a factor of 9.
Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?
Earth would be inside the supergiant.
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.
What is Rigel’s surface temperature?
10,000 K
Rigel’s luminosity is about _____ times the Sun’s luminosity
100,000
Rigel’s radius is about _____ times the Sun’s radius.
80
Based on its location on the HR diagram, what can we say about Rigel’s mass and lifetime?
Nothing, because it is not on the main sequence.
Which is brighter in our sky, a star with apparent magnitude 2 or a star with apparent magnitude 7?
A star with apparent magnitude 2.
Which units are appropriate for measurement of apparent brightness?
watts per square meter
Compared to a main-sequence star with a short lifetime, a main-sequence star with a long lifetime is
less luminous, cooler, smaller, and less massive
Compared to a high-luminosity main-sequence star, stars in the upper right of the H-R diagram are
cooler and larger in radius
Compared to a low-luminosity main-sequence star, stars in the lower left of the H-R diagram are
hotter and smaller in radius
If star A is closer to us than star B, then Star A’s parallax angle is
larger than that of Star B
What can we infer, at least roughly, from a star’s luminosity class?
its size (radius)
A star with a parallax angle of 1/20 arcsecond is
20 parsecs (~ 65.2 light-years) away
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
mass
Which of these stars has the coolest surface temperature?
a K star
What is the common trait of all main-sequence stars?
They generate energy through hydrogen fusion in their core
Our Sun is a star of spectral type
G
Star A has an apparent magnitude = 3 and star B has an apparent magnitude = 5. Which star is brighter in our sky?
Star A
Which of the following must be true if we are to infer (calculate) a star’s luminosity directly from the inverse square law for light?
No interstellar gas or dust absorbs or scatters light between us and the star.
We have measured the star’s apparent brightness.
We have measured the star’s distance.
What do we mean by a star’s spectral type, and how is spectral type related to surface temperature and color.
Spectral types are a way of classifying stars according to their color or what spectral lines we see in their light. The spectral types run OBAFGKM, where O stars are the hottest and M are the coolest. Hotter stars look bluer to us, and cooler stars look redder.
Before we can use parallax to measure the distance to a nearby star, we first need to know
the Earth-Sun distance
Which of the following is a valid way of demonstrating parallax for yourself?
Hold up your hand in front of your face, and alternately close your left and right eyes.
What is the cause of stellar parallax?
Earth’s orbit around the Sun.
The more distant a star, the
smaller its parallax angle
Approximately what is the parallax angle of a star that is 20 light-years away?
0.16 arcsecond
Suppose that a star had a parallax angle of exactly 1 arcsecond. Approximately how far away would it be, in light-years?
3.3 light-years
Describe what would happen to the surface temperature of a star if its radius increased by 4 times, but there was no change in luminosity.
Luminosity increases both with increasing radius and increasing temperature. Therefore, a lower temperature would be required to maintain the star’s constant luminosity.
