Chapter 19-20

When astronomers discuss a nebula, what are they talking about?

A giant cloud of gas and dust between or among the stars

An astronomer wants to observe a cloud of cold neutral (not ionized) hydrogen, far away from any stars. What would be an instrument that could help in this task?

A radio telescope, tuned to a wavelength of about 21 centimeters

An astronomer wants to observe a cloud of dust in a relatively close part of the Galaxy. Unfortunately, this dust cloud is not located in the direction of a crowded region of stars. What instrument would be the most help in finding this cloud.

A sensitive infrared telescope in orbit around the Earth

An HII region is:

A zone around a hot star where hydrogen atoms are ionized

Which of the following is NOT a way astronomers discover clouds of interstellar matter that have a large amount of dust in them?

By giving off x-rays from hot gas surrounding the dust cloud

Astronomers now understand that the dark regions or rifts visible in parts of our Galaxy that are otherwise crowded with stars are caused by:

Clouds with a considerable amount of dust which blocks the light of the stars behind them

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 use the term interstellar matter to refer to:

Gas and dust that lies between stars

In order for a cold atom of hydrogen in an interstellar cloud to emit a 21-cm wave, it must first be in a slightly higher energy state. What event usually "kicks" the hydrogen atom up to this higher state?

Gas atoms within the cloud collide

The red color we see on a lot of photographs of nebulae comes from which element?

Hydrogen

You are an astronomer whose supervisor has just forbidden drinking alcoholic beverages near the observatory. In frustration, you undertake a search for alcohol molecules in space. Where would your search be most likely to succeed?

In an interstellar cloud of gas and dust

Astronomers have found large quantities of cold, neutral hydrogen gas in our galaxy. How is this gas distributed?

It is found mostly in a flat layer extending throughout the disk of our galaxy

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

If an astronomer wanted to find some relatively complex molecules in space, what technique should she use?

Point a radio telescope into regions in the galaxy where there is a lot of dust

How do fragile structures like acetaldehyde (CH3CHO) molecules survive in the harsh environment of interstellar space? Why are they not destroyed by high-energy radiation from stars?

Such molecules are found only in dense clouds that have a lot of dust; the dust keeps the radiation from hot stars from reaching the molecules

Astronomers were at first surprised to find complicated molecules in the interstellar medium. They thought ultraviolet light from stars would break apart such molecules. What protects the molecules we observe from being broken apart?

Such molecules form in clouds that contain significant amounts of dust, and the dust in the cloud protects them from ultra-violet rays

You are observing a binary star system and obtain a series of spectra of the light from the two stars. In this spectrum, most of the absorption lines shift back and forth as expected from the Doppler Effect. A few lines, however, do not shift at all, but remain at the same wavelength. How can we explain the behavior of the non-shifting lines?

The lines come from interstellar matter between us and the star, not from the stars themselves

Astronomers use the term interstellar extinction to refer to:

The scattering and absorption of starlight by dust grains in space

Some of the interstellar gas in our Galaxy has been heated to millions of degrees, a temperature that surprised astronomers when it was first discovered. How do we now think that gas between stars gets that hot?

Very powerful shock waves from exploding stars heat the gas they come into contact with

The luminosity class of a star tells an astronomer:

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