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
If the star Alpha Centauri were moved to a distance 10 times farther than it is now, its parallax angle would
get smaller.
What do we need to measure in order to determine a star's luminosity?
apparent brightness and distance
What two pieces of information would you need in order to measure the masses of stars in an eclipsing binary system?
the time between eclipses and the average distance between the stars
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.
If star A is closer to us than star B, then Star A's parallax angle is _________.
larger than that of Star B
found in upper right of H-R diagram &
very cool but very luminous
red giant or supergiant stars
the Sun, for example,
a very hot & very luminous star
main sequence stars
not much larger in radius than Earth &
very hot but very dim
white dwarfs
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
Which of these stars has the longest lifetime? A, G, or M?
M
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?
The star is a member of a binary star system.
observe directly
color, apparent brightness, spectral type, parallax angle
infer indirectly
mass, radius, surface temp, luminosity
What must we measure directly so that we can infer a star's surface temperature?
spectral type
we can measure a star's mass directly if __________.
it is a member of an eclipsing binary system
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.
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
Based on the definition of apparent brightness, which units are appropriate for its measurement?
watts per square meter
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.
A star with a parallax angle of 1/20 arcsecond is _________.
20 parsecs away
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.
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.
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.
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
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
The approximate main-sequence lifetime of a star of spectral type O is _________.
3 million years
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 choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest?
The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.
rank these stages in order:
protostar
white dwarf
red giant
planterary nebula
main sequence g star
contracting cloud of gas and dust
protostar
main-sequence G star
red giant
planetary nebula
white dwarf
A brown dwarf is
an object not quite massive enough to be a star
Stars can form most easily in clouds that are
cold and dense
What kind of gas cloud is most likely to give birth to stars?
a cold, dense gas cloud
What happens to a low-mass star after helium flash?
Its luminosity goes down.
What would you be most likely to find if you returned to the solar system in 10 billion years?
a white dwarf
List the stages of life for a low-mass star in the correct order
protostar, main-sequence star, red giant, planetary nebula, white dwarf
The main source of energy for a star as it grows in size to become a red giant is _________.
hydrogen fusion in a shell surrounding the central core