PHYS 1403 Quiz 4 Ch. 19 and 21

The Orion Nebula is

a large cloud of gas and dust illuminated by the light of newly formed stars within it

If an astronomer wants to find and identify as many stars as possible in a star cluster that has recently formed near the surface of a giant molecular cloud (such as the Trapezium cluster in the Orion Nebula), what instrument would be best for her to use?

an infra-red telescope (and camera)

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

measure the star's Doppler shift from its spectrum

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

Why do Cepheid variables have that strange name?

the first such variable was discovered in a constellation called Cepheus

A star whose temperature is increasing but whose luminosity is roughly constant moves in what direction on the H-R diagram?

to the left

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?

32.6 years

The first ordinary star (other than our own Sun) around which planets were definitely discovered was:

51 Pegasi

The period-luminosity relationship for Cepheid variables was discovered by

Henrietta Leavitt

Why is it so difficult for astronomers to see new stars in the process of birth?

INCORRECT: birth happens very quickly, so it is hard to "catch" stars "in the act"

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

INCORRECT: bouncing radar beams off them

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:

INCORRECT: the same luminosity

Astronomers studying regions like the Orion Giant Molecular Cloud have observed that a wave of star formation can move through them over many millions of years. What sustains such a wave of star formation in a giant molecular cloud?

INCORRECT: when massive stars form, their ultraviolet radiation and later their final explosions compress the gas in the cloud and cause a new group of stars to form

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

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

If you want to find stars that are just being born, where are the best places to search?

in giant molecular clouds

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

When a star settles down to a stable existence as a main-sequence star, what characteristics determines where on the main sequence in an H-R diagram the star will fall?

its mass

In figuring out the evolutionary tracks on the H-R diagram, astronomers

make model stars on a computer and then follow how their characteristics will change with time

What technique did astronomers use to make the first confirmed discovery of a planet around another star like the Sun?

measure the Doppler shift of the lines in the star's spectrum and look for periodic changes in this shift due to the pull of the planet as it orbits the star

Kepler's Laws can give us the relative distance of objects in the solar system. To convert these relative distances into actual distances, we need to:

measure the distance directly to any object orbiting the Sun

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 big surprise about the first planet discovered around another regular star was that it

orbited so close to its star it took only 4 days to go around

If a star appears to move back and forth relative to other stars over a six-month period, this motion is due to the star's

parallax shift

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

A type of star that has turned out to be extremely useful for measuring distances is

the Cepheid variables

The telescope that allowed astronomers to discover most of the planets found with the transit method was called

the Kepler mission

Which of the following are the small regions that are the embryos of stars (where individual stars are most likely to be born)?

the cores within the clumps of molecular clouds

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

the longer the period of its variations

To measure how dense a planet is (to know whether it is made of rock or gas and liquid) they must be able to measure the planet's mass from the Doppler shift and

the planet's radius using the transit method

How do astronomers know that pulsating variable stars are actually expanding and contracting in diameter?

they can measure a regularly varying Doppler shift in the spectral lines

A light curve for a star measures how its brightness changes with

time

Astronomers identify the "birth" of a real star (as opposed to the activities of a protostar) with what activity in the star?

when nuclear fusion reactions begin inside its core

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