AST 102 Test 2

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

How far away would a star with a parallax of 0.2 arcsec be from us?

5 parsecs

The comet-hunting astronomer who made a list of over 100 nebulae and galaxies that could be mistaken for comets was

Charles Messier

The period-luminosity relationship for Cepheid variables was discovered by

Henrietta Leavitt

What was the first evidence that gravity outside our solar system worked the same way as it does inside?

Herschel measured the two stars that make up the Castor system moved around each other

Who was the astronomer who is the "H" in H-R diagram?

Hertzsprung

Which law do astronomers use to determine the masses of the stars in a spectroscopic binary system?

Kepler's Third Law

Which of the following stars is a Cepheid variable?

Polaris

What is the closest star to the Sun?

Proxima Centauri

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

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

The Messier Catalog is

a list of nebulae, star clusters, and galaxies that might be mistaken for a comet far from the Sun

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

The Local Bubble is

a region of low density interstellar gas extending several hundred LY around the Sun

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 infra-red telescope in orbit around the Earth

An HII region is

a zone around a hot star where hydrogen atoms are ionized

Where is the outer rim of the Local Bubble located today?

about 200 light-years from the Sun, in the rough direction of the constellations of Orion, Perseus, and Auriga

In an H-R diagram, where can you see the spectral type of a star (whether it is an O type star or a G type star, for example)?

along the bottom (the horizontal axis)

One of your good friends who is on a diet asks you to point out the stars with the smallest mass on an H-R diagram that you are studying. Where are you sure to find the stars with the lowest mass on any H-R diagram?

among the stars at the bottom right of the main sequence

Which of the following statements about spectroscopic binary stars is FALSE?

an analysis of the ways the lines in the spectrum change allows us to calculate the star's distance directly

Supposing we launched a very fast dart from the Space Shuttle, pointed in some direction away from any planet, so that it could travel beyond the solar system. What would it be most likely to hit first after traveling outward for a while?

an atom of interstellar gas

What happens in the process of fluorescence?

atoms absorb ultraviolet rays and convert them to visible light as their electrons cascade to lower energy levels

The light element lithium (which, on Earth, is part of medications that improve the lives of people with mental health illnesses) is more common in cosmic rays than it is in the Sun and the stars. What do astronomers think is the reason for this?

because cosmic rays move so fast, they can collide with atoms of carbon, nitrogen, and oxygen and break them apart, producing more light elements like lithium

Some "superstars" give off more than 50,000 times the energy of the Sun. Why are there no such stars among the stars that are close to the Sun?

because such very luminous stars are extremely rare, and thus any small neighborhood in the Galaxy is unlikely to contain one of them

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

Two stars that are physically associated (move together through space) are called

binary star

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

bouncing radar beams off them

Stars that do not have what it takes to succeed as a star (i.e. do not have enough mass to fuse hydrogen into helium at their centers) are called:

brown dwarf

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

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

Why can astronomers not tell from what direction a particular cosmic ray started toward the Earth?

cosmic rays are charged particles and are thus spiral around the Earth and the Galaxy's magnetic field

Which of the following is a method for measuring the diameter of a star?

e. more than one of the above

For what type of star can astronomers measure the diameter with relative ease?

eclipsing binary stars

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 identify the main sequence on the H-R diagram with what activity in the course of a star's life?

fusing hydrogen into helium in their cores

Astronomers use the term interstellar matter to refer to:

gas and dust that lies between stars

In order for a cold atom of hydrogen 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

Cosmic rays are

high-speed atomic nuclei, electrons, and positrons

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

hydrogen

Which of the following statements about interstellar matter is FALSE?

if the matter were spread out evenly, it would be about as dense as the Earth's atmosphere

Which of the following is not a place where astronomers believe interstellar dust grains can form?

in HII regions

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

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

in giant molecular clouds

Imagine that powerful telescopes in the future give us a truly representative sampling of all the stars in the Sun's cosmic neighborhood. Where on the H-R diagram would most of the stars in our immediate vicinity lie?

in the lower right, among the least luminous main sequence stars

Which of the following descriptions of interstellar dust is FALSE?

interstellar dust must be at a temperature equal to the outer layers of a star

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

Measurements show a certain star has a very high luminosity (100,000 x the Sun's) while its temperature is quite cool (3500o K). How can this be?

it must be quite large in size

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

Which of the following characteristics of a single star (one that moves through space alone) is it difficult to measure directly?

its mass

Given the presence of dust throughout the disk of the Milky Way Galaxy, what is the best technique for learning about more distant regions of our galaxy's disk.

look for radiation at long wavelengths, for example in the infrared region of the spectrum

The most common kinds of stars in the Galaxy have

low luminosity compared to the Sun

Stars on the main sequence obey a mass-luminosity relation. According to this relation

luminosity is proportional to mass to the fourth power (luminosity increases strongly with mass)

Stars that lie in different places on the main sequence of the H-R diagram differ from each other mainly by having different:

masses

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

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

The largest types of clouds found in interstellar space are

molecular clouds with supplies of dust and molecules

Most of the stars we can see with the unaided eye from Earth are

more luminous (intrinsically brighter) than the Sun

Cosmic rays differ from other forms of interstellar matter by Correct Response

moving much faster, at typical speeds of 90% the speed of light

A team of astronomers discovers one of the most massive stars ever found. If this star is just settling down in that stage of its life where it will be peacefully converting hydrogen to helium in its core, where will we find it on the H-R diagram?

near the very top of the main sequence, in the upper left

The luminosity class of a star tells an astronomer

none of the above

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

Where on the H-R Diagram would we find stars that look red when seen through a telescope?

only on the right side of the diagram and never on the left

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

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

A white dwarf, compared to a main sequence star with the same mass, would always be:

smaller in diameter

The amount of interstellar matter present in our Galaxy is always changing. Which of the following processes is NOT a major contributor to that change?

some atoms of gas combine in dusty clouds to make more complex molecules

Why can astronomers not measure the diameters of stars directly?

stars are so far away, we cannot resolve (distinguish) their diameters

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 ultra-violet 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

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

supergiants

An H-R Diagram plots the luminosity of stars against their:

surface temperature

I am measuring the spectrum of the stars in a spectroscopic binary system. When one of the stars is moving toward the Earth in its orbit, we observe

that the lines in its spectrum show a blue-shift

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

the Cepheid variables

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

To establish the scale of the solar system, we need to measure the distance to one object orbiting the Sun. Venus was first used for this purpose, but in the 1930's astronomers organized an international campaign to measure the distance to:

the asteroid Eros

An astronomical unit is:

the average distance between the Earth and the Sun

A friend of yours who has not taken an astronomy class looks at your textbook and really likes the picture of the Pleiades, a cluster of stars surrounded by a bluish reflection nebula. She wants to know what causes that beautiful blue glow. To explain it to her, you want to compare the process that causes the blue glow to something that is in your friend's everyday experience. Which of the following terrestrial phenomena is the result of the same type of process that makes a reflection nebula in space?

the blue color of the Earth's sky

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

Why do Cepheid variables have that strange name?

the first such variable was discovered in a constellation called Cepheus

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 length of time they took to vary

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

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

the longer the period of its variations

Among interstellar clouds, the hotter the cloud, the

the lower the density of particles in it

Ninety percent of all stars (if plotted on an H-R diagram) would fall into a region astronomers call:

the main sequence

The term "baryon cycle," as astronomers use it, refers to:

the movement of interstellar material into the Galaxy and into stars and then the movement out when stars end their lives

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

Astronomers use the term interstellar extinction to refer to:

the scattering and absorption of starlight by dust grains in space

Which of the following has the smallest mass?

the smallest mass star that can still have fusion of hydrogen to helium in its core

Which of the following will show the smallest parallax shift?

the star 51 Pegasi, about 50 lightyears away

Most of the really bright stars in our sky are NOT among the stars that are very close to us. Why then do they look so bright to us?

these stars are intrinsically so luminous, that they can easily be seen even across great distances

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

The dust in the dust clouds in interstellar space consists of

tiny solid grains

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

An astronomer is observing a star which puzzles her. The lines in the star's spectrum indicates that the star is very hot and should therefore be blue. But the star looks reddish in photographs and in measurements of the continuous spectrum. What is one possible explanation of this puzzle?

we are seeing the light of the star through layers of interstellar dust

A star that is quite hot and has a very small radius compared to most stars is called

white dwarf

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