Astronomy Exam - 3 Mix

In an H-R diagram, where can you see the spectral type of a star?

Along the bottom (the horizontal axis)

The most common kinds of stars in the Galaxy have:

Low luminosity compared to the Sun

A star appears especially dim through binoculars. Why does it look dim?

It could be any of these; there is no way to tell which answer is right by just looking at the star

Why are astronomers much more interested in the luminosity of a star than its apparent brightness?

Because the luminosity tells us how much energy the star emits, while apparent brightness only tells us how bright it happens to look from Earth

Astronomers identify the main sequence on the H-R diagram with what activity?

Fusing hydrogen into helium in their cores

Which color star is likely to be the hottest?

Blue-violet

Two stars have the same luminosity, but star B is three times farther away than star A. Star B will look:

Nine times fainter

A star moving toward the Sun will show:

A shift in the spectral lines toward the blue end

A star has very high luminosity (100,000x the Sun) but cool temperature (3500K). How?

It must be quite large in size

Most bright stars in our sky are NOT close to us. Why do they look so bright?

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

In a binary star spectrum, a few lines don't shift at all while others shift back and forth. How do we explain the non-shifting lines?

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

Why did it take astronomers until 1838 to measure stellar parallax?

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

Among interstellar clouds, the hotter the cloud, the:

The lower the density of particles in it

How does interstellar gas get heated to millions of degrees?

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

What baseline do astronomers use to measure stellar parallax?

Half the diameter of Earth's orbit around the Sun

How far away is a star with a parallax of 0.2 arcseconds?

5 parsecs

Which property of a star requires knowing its distance before it can be determined?

Its luminosity

What does interstellar dust consist of?

Tiny solid grains

Which technique gives us the distance to a galaxy 2 million light-years away like M31?

Period-luminosity relation for Cepheid variables

How do fragile molecules like acetaldehyde survive in interstellar space?

They are found only in dense clouds where dust shields them from high-energy radiation

The Orion Nebula is:

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

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

To the left

What characteristic determines where a star falls on the main sequence of an H-R diagram?

Its mass

What was the big surprise about the first planet discovered around another regular star?

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

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

Measure the Doppler shift of the star's spectral lines and look for periodic changes due to the planet's gravitational pull

How would the habitable zone of Proxima Centauri (spectral type M) differ from our Sun's habitable zone?

It would be significantly closer to Proxima Centauri than ours is to the Sun

Planets in the habitable zone of their stars:

Are at a temperature where water can exist as a liquid

What telescope allowed astronomers to discover most planets found with the transit method?

The Kepler mission

Why couldn't Kepler find planets smaller than Mars?

Such planets make dips in starlight that are too small for Kepler to detect

What observations about dusty disks around young stars suggest planets may be forming?

The disks show lanes that are empty of dust within them

What characteristic of an H-R diagram do astronomers use as a good indicator of a cluster's age?

The point on the main sequence where stars begin to turn off and move toward the red giant region

Which type of star spends the longest time on the main sequence?

K

What type of star cluster contains mostly very old stars?

A globular star cluster

How do really massive stars differ from Sun-like stars?

They can fuse elements beyond carbon and oxygen in their hot central regions

What event in a star's life begins its expansion into a giant?

Almost all the hydrogen in its core that was hot enough for fusion has been turned into helium

How do first generation stars in the Galaxy differ from stars forming today?

First generation stars contain little or no elements heavier than helium

Why can a Sun-like star not fuse elements beyond carbon and oxygen?

They cannot get hot enough for the fusion of heavier nuclei

Where are elements heavier than oxygen produced in the universe?

In the cores of significantly more massive stars than the Sun, which can get hotter in the middle

How are globular clusters distributed in the Milky Way?

Mostly in a large spherical halo surrounding the flat disk of the Galaxy

When a star begins becoming a red giant, why does the hydrogen layer around the core suddenly become hot enough for fusion?

The core is collapsing under its own weight and heating up from compression, which heats the next layer up

Why haven't astronomers seen a supernova in our Galaxy since 1604?

The disk of our Galaxy contains a great deal of dust which blocks the light of supernova explosions from more distant parts of the Galaxy

A young cluster has stars 3x the Sun's mass still on the main sequence but already contains white dwarfs. How is this possible?

Stars lose a lot of mass on their way to becoming white dwarfs, so the white dwarfs could have started out as quite massive stars

What is the most stable (tightly bound) atomic nucleus in the universe?

Iron

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

Smaller in diameter

What band of the spectrum is best for observing a dying Sun-like star surrounded by its own expelled dust and gas?

Infrared

What allows a nova explosion to happen to a star more than once?

The star has a companion that continuously dumps material onto it even after the first nova explosion

How do astronomers know that visible filaments in the Crab Nebula are moving toward us at great speed?

From the Doppler shift in the line radiation from the nebula

A neutron star is as dense as:

The nucleus of an atom

Elements heavier than iron can be created during:

A supernova explosion

When a single star with a mass equal to the Sun dies, it will become a:

White dwarf

Approximately, what basic composition are all stars born with?

three-quarters hydrogen, one-quarter helium, no more than 1 percent heavier elements

Since all stars begin their lives with the same basic composition, what characteristic most determines how they will differ?

Mass they are formed with

What are the standard units for luminosity?

Watts

A star's luminosity is the

total amount of light that the star radiates with each second

What are the standard units for apparent brightness?

Watts per square meter

If the distance between us and a star is doubled, with everything else remaining the same, the luminosity

remains the same, but the apparent brightness is decreased by a factor of four

Which of the following correctly states the luminosity-distance formula?

apparent brightness = luminosity/(4pi*distance^2)

Why do astronomers often measure the visible-light apparent brightness instead of the total apparent brightness of a star?

In order to measure the total apparent brightness of a star, you must measure its brightness in all wavelengths, and this is difficult to do. The only wavelengths you can measure from the surface of Earth are visible and radio wavelengths

Suppose you measure the parallax angle for a particular star to be 0.1 arc second. The distance to this star is

10 parsecs

Suppose that you measure the parallax angle for a particular star to be 0.5 arc second. The distance to this star is

2 parsecs

The most distant stars we can measure stellar parallax for are approximately

500 parsecs away

Which of the following statements about apparent and absolute magnitudes is true?

The magnitude system that we use now is based on a system used by the ancient Greeks over 2,000 years ago that classified stars by how bright they appeared,

A star with apparent magnitude 1 is brighter than one with apparent magnitude 2,

The absolute magnitude of a star is another measure of its luminosity,

and A star's absolute magnitude is the apparent magnitude it would have if it were at a distance of 10 parsecs from Earth

The spectral sequence sorts stars according to

surface temperature

The spectral sequence in order of decreasing temperature is

OBAFGKM

Why is the spectral sequence of stars not alphabetical?

The original alphabetical labeling did not correspond to surface temperature and thus had to be reordered

Which of the following statements about spectral types of stars is true?

The spectral type of a star can be used to determine its surface temperature,

The spectral type of a star can be used to determine its color,

A star with spectral type A is cooler that a star with spectral type B,

and A star with spectral type F2 is hotter than a star with spectral type F3

Which of the following persons reorganized the spectral classification scheme into the one we use today and personally classified over 400,000 stars?

Annie Jump Cannon

Which of the following persons used the ideas of quantum mechanics to describe why the spectral classification scheme is in order of decreasing temperature?

Cecilia Payne-Gaposchkin

Suppose you see two main-sequence stars of the same spectral type. Star 1 is dimmer in apparent brightness than Star 2 by a factor of 100. What can you conclude? (Neglect any effects that might be caused by interstellar dust and gas.)

Star 1 is 10 times more distance than Star 2

Which of the following terms if given to a pair of stars that appear to change positions in the sky, indicating that they are orbiting one another?

Visual binary

Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?

Spectroscopic binary

Which of the following best describes the axes of a Hertzsprung-Russell (H-R) diagaram?

Surface temperature on the horizontal axis and luminosity on the vertical axis

On a Hertzsprung-Russell diagram, where would we find stars that are cool and dim?

Lower right

On a Hertzsprung-Russell diagram, where would we find stars that are cool and luminous?

Upper right

On a Hertzsprung-Russell diagram, where would we find stars that have the largest radii?

Upper right

On a Hertzsprung-Russell diagram, where on the main sequence would we find stars that have the greatest mass?

Upper left

On a Hertzsprung-Russell diagram, where would we find red giant stars?

Upper right

On a Hertzsprung-Russell diagram, where would we find white dwarfs?

Lower left

You observe a star in the disk of the Milky Way, and you want to plot the star on an H-R diagram. You will need to to determine all of the following, except the

rotation rate of the star

On the main sequence, stars obtain their energy

by converting hydrogen to helium

The faintest star visible to the naked eye has an apparent visual magnitude of about

6

Which of the following is the most common type of main-sequence star?

An M star

Which of the following characteristics of stars has the greatest range in values?

Luminosity

A star of spectral type O lives approximately how long on the main sequence?

10 million years

A star of spectral type G lives approximately how long on the main sequence?

10 billion years

Which of the following is true about low-mass stars compared to high-mass stars?

Low-mass stars are cooler and less luminous than high-mass stars

Which of the following luminosity classes refers to stars on the main sequence?

V

In a pulsating variable star, which characteristic of the star changes dramatically with time?

Luminosity

Why are Cepheid variables so important for measuring distances in astronomy?

Their luminosity can be inferred from their period

Which of the following statements about an open cluster is true?

All stars in the cluster are approximately the same age

Which of the following statements about a globular cluster is true?

Most of the stars in the cluster are yellow or reddish in color

Cluster ages can be determined from

Main sequence turnoff

In order to understand star clusters, we need to be able to estimate their ages. What technique do scientists use fo this?

Finding the main-sequence turnoff point of the stars

The apparent brightness of a star depends only on its luminosty

False

If the distance between us and a star is doubled, the apparent brightness is decreased by a factor of four

True

The more distant a star, the smaller its parallax

True

We can measure stellar parallax for most stars in our galaxy

False

Spectral type, surface temperature, and color all describe the same basic characteristic of a star

True

Some stars are cool enough to have molecules in their atmosphere

True

We can measure the radii of stars in an eclipsing binary system, in addition to the masses

True