Question 1

How did Henrietta Leavitt "calibrate" her period-luminosity relationship for Cepheid variable stars? In other words, how did she make the general idea into a numerical rule?
A) by finding cepheids in star clusters whose distance was known in another way
B) because the star closest to us is a Cepheid variable and we know its distance
C) by noting that the period was related to the luminosity in all stars
D) by measuring the Doppler shift in the spectral lines of Cepheids as they pulsated
E) by assuming that the Cepheids that appeared the brightest in the sky were closest to us
Section 19.4: The H-R Diagram and Cosmic Distances

Accepted Answer

Henrietta Leavitt calibrated her period-luminosity relationship for Cepheid variable stars by finding cepheids in star clusters whose distance was known through other methods. This allowed her to establish a numerical relationship between the period of variability and the intrinsic luminosity of Cepheids, which could then be used to determine distances to other galaxies.

Question 2

Astronomers must often know the distance to a star before they can fully understand its characteristics. Which of the following properties of a star typically requires a knowledge of distance before it can be determined?
A) its luminosity
B) its radial velocity
C) its temperature
D) its apparent brightness
E) all of the above

Accepted Answer

Luminosity is a measure of a star's overall energy output, and it is dependent on both the star's temperature and size. However, in order to determine a star's luminosity, astronomers must first know its distance, as apparent brightness decreases with distance.

Question 3

Which type of star has the least amount of pressure in its atmosphere?
A) main sequence stars
B) subgiants
C) giants
D) supergiants
E) you can't fool me; all stars have roughly the same pressure

Accepted Answer

Supergiants have the least amount of pressure in their atmospheres due to their large size and low density compared to other types of stars.

Question 4

An astronomer is interested in a galaxy called M31, the nearest galaxy that resembles our Milky Way. It is about 2 million lightyears away. Which technique would be able to give us a distance to this galaxy?
A) parallax
B) radar reflections
C) period-luminosity relation for Cepheid variables
D) Kepler's laws
E) you can't fool me: there is no way at present to get a distance to an object so far away

Accepted Answer

The period-luminosity relation for Cepheid variables is a reliable technique for measuring distances to galaxies. Cepheid variables are stars whose brightness varies periodically in a well-known way, and by measuring their apparent brightness and period of variation, astronomers can determine their absolute brightness and thus their distance. This technique was used by Edwin Hubble to determine that the galaxy M31 was located 2 million lightyears away from us.

Question 5

If an astronomer wants to find the distance to a star that is not variable and is located too far away for parallax measurements, she can:
A) use the star's light curve
B) find the star's luminosity class from its spectrum and read the luminosity from an H-R diagram
C) use Kepler's laws as modified by Newton
D) search for planets around the star since it is much easier to get the distance to planets
E) only throw up her hands in desperation; there is no way to even estimate the distance to such a star

Accepted Answer

By finding the star's luminosity class from its spectrum and reading the luminosity from an H-R diagram, the astronomer can determine the star's intrinsic brightness. With the apparent brightness, which can be measured, the astronomer can use the inverse square law to calculate the distance to the star.

Question 6

Which of the following stars is a Cepheid variable?
A) Sirius
B) Betelgeuse
C) Rigel
D) Mizar
E) Polaris

Accepted Answer

Polaris is a well-known Cepheid variable star, meaning its brightness varies in a regular cycle over time. This property makes it useful for measuring distances to other objects in the universe.

Question 7

The measurement of cosmic distances was helped tremendously by the discovery, in the early part of the 20th century, that in Cepheid variable stars, the average luminosity was related to:
A) their radial velocity
B) the abundance of hydrogen in their atmospheres
C) their distance from the Sun
D) the length of time they took to vary
E) their parallax

Accepted Answer

The average luminosity of Cepheid variable stars was found to be related to the length of time they took to vary, allowing astronomers to use them as standard candles to measure cosmic distances. This relationship is known as the period-luminosity relationship.

Question 8

To get the distance to a Cepheid variable star, astronomers must take several steps. Which of the following is NOT one of these steps?
A) observe the star getting brighter and dimmer to establish that it varies
B) measure the period of variations by timing when the star is brightest and dimmest
C) estimate the apparent brightness of the star
D) measure the star's Doppler shift from its spectrum
E) apply the period-luminosity relationship

Accepted Answer

Measuring the star's Doppler shift is not necessary for determining the distance to a Cepheid variable star. The other steps listed (observing the star's brightness variations, measuring the period of those variations, estimating the star's apparent brightness, and applying the period-luminosity relationship) are all important steps in determining the distance to a Cepheid variable star.

Question 9

The period-luminosity relationship for Cepheid variables was discovered by
A) John Goodricke
B) Henrietta Leavitt
C) Edward Pickering
D) Henry Norris Russell
E) Annie Cannon

Accepted Answer

The period-luminosity relationship for Cepheid variables was discovered by Henrietta Leavitt while working at the Harvard Observatory in the early 1900s. She observed thousands of variable stars in the Magellanic Clouds and found a pattern between their brightness and period, which allowed astronomers to determine the distance to other galaxies in the universe.

Question 10

The higher the luminosity (intrinsic brightness) a Cepheid variable is,
A) the smaller its mass
B) the lower it is on the main sequence of the H-R diagram
C) the longer the period of its variations
D) the closer it is to us
E) the larger the telescope we need to observe it

Accepted Answer

The period of a Cepheid variable is directly related to its luminosity, with brighter Cepheids having longer periods. This relationship, known as the period-luminosity relation, allows astronomers to use Cepheids as standard candles to determine distances to other galaxies.

Question 11

The luminosity class of a star tells an astronomer
A) whether the star is close to us or far away
B) whether or not the star is surrounded by planets
C) whether the star is a supergiant, a giant, or a main-sequence star
D) how long ago the star formed
E) none of the above

Accepted Answer

The luminosity class of a star tells an astronomer whether the star is a supergiant, a giant, or a main-sequence star based on its size, temperature, and brightness. It does not provide information about the star's distance, planetary system, or age.

Quiz 19: Celestial Distances

How did Henrietta Leavitt "calibrate" her period-luminosity relationship for Cepheid variable stars? In other words, how did she make the general idea into a numerical rule?

Astronomers must often know the distance to a star before they can fully understand its characteristics. Which of the following properties of a star typically requires a knowledge of distance before it can be determined?

Which type of star has the least amount of pressure in its atmosphere?

An astronomer is interested in a galaxy called M31, the nearest galaxy that resembles our Milky Way. It is about 2 million lightyears away. Which technique would be able to give us a distance to this galaxy?

If an astronomer wants to find the distance to a star that is not variable and is located too far away for parallax measurements, she can:

Which of the following stars is a Cepheid variable?

The measurement of cosmic distances was helped tremendously by the discovery, in the early part of the 20th century, that in Cepheid variable stars, the average luminosity was related to:

To get the distance to a Cepheid variable star, astronomers must take several steps. Which of the following is NOT one of these steps?

The period-luminosity relationship for Cepheid variables was discovered by

The higher the luminosity (intrinsic brightness) a Cepheid variable is,

The luminosity class of a star tells an astronomer