astro ch 19

If a star is 20 parsecs away, its parallax must be:

1/20th of an arcsecond

If a star is 10 parsecs away, how long ago did the light we see from it tonight begins its journey toward us? 0.1 years 32.6 years 10,000 years 6100 years 10 years

32.6 years

Which of the following stars is a Cepheid variable?

Polaris

Why did it take astronomers until 1838 to measure the parallax of the stars?

because the stars are so far away that their annual shift of position in the sky is too small to see without a good telescope

Today, astronomers can measure distances directly to worlds like Venus, Mars, the Moon, or the satellites of Jupiter by

bouncing radar beams off them

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?

by finding cepheids in star clusters whose distance was known in another way

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:

find the star's luminosity class from its spectrum and read the luminosity from an H-R diagram

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?

its luminosity

An astronomer is observing a single star (and one which does not vary) which she knows is located about 30 light-years away. What was the most likely method she or her colleagues used to obtain that distance?

measuring the star's parallax

As astronomers use the term, the parallax of a star is

one half the angle that a star shifts when seen from opposite sides of the Earth's orbit

The original definition of a meter was

one ten-millionth of the distance from the Earth's equator to its pole

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?

period-luminosity relation for Cepheid variables

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

supergiants

The instrument astronomers are now using to make the most precise measurements of stellar parallax we have ever had is

the Gaia satellite in space

An astronomical unit is:

the average distance between the Earth and the Sun

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

the length of time they took to vary

The apparent brightness of stars in general tells us nothing about their distances; we cannot assume that the dimmer stars are farther away. In order for the apparent brightness of a star to be a good indicator of its distance, all the stars would have to be:

the same luminosity

Which of the following will show the smallest parallax shift?

the star 51 Pegasi, about 50 lightyears away

The luminosity class of a star tells an astronomer

whether the star is a supergiant, a giant, or a main-sequence star

What is the baseline that astronomers use to measure the parallax (the distance) of the nearest stars?

½ the diameter of the Earth's orbit around the Sun