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How does the diameter of the disk of Milky Way Galaxy compare to its thickness?

The diameter is about 100 times as great as the thickness.

What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy?

the galaxy's bulge

The Sun's location in the Milky Way Galaxy is _________.

in the galactic disk, roughly halfway between the center and the outer edge of the disk

What do we mean by the interstellar medium?

the gas and dust that lies in between the stars in the Milky Way galaxy

What are the Magellanic Clouds?

two small galaxies that orbit the Milky Way Galaxy

How do disk stars orbit the center of the galaxy?

They all orbit in roughly the same plane and in the same direction.

How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?

by applying Newton's version of Kepler's third law to the orbits of the Sun or other nearby stars around the center of the Galaxy

Fill in the blank: Elements heavier than hydrogen and helium constitute about _________ of the mass of the interstellar medium.

2 %

What do we mean by the star-gas-star cycle?

It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium.

What are cosmic rays?

subatomic particles that travel close the speed of light

The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with _________.

radio telescopes observing at a wavelength of 21 centimeters

Which of the following models best explains why our galaxy has spiral arms?

The spiral arms are a wave of star formation caused by a wave of density propagating outward through the disk of the galaxy.

What do we mean by a protogalactic cloud?

a cloud of hydrogen and helium that contracts to become a galaxy

Most stars in the Milky Way's halo are _________.

very old

What is an ionization nebula?

a colorful cloud of gas that glows because it is heated by light from nearby hot stars

What do halo stars do differently from disk stars?

They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.

Where does most star formation occur in the Milky Way Galaxy?

in the spiral arms

Based on observations, which of the following statements about stars in the Milky Way is generally true?

The older the star, the lower its abundance of heavy elements.

What kind of object do we think lies in the center of the Milky Way Galaxy?

a 3 to 4 million solar mass black hole

If we could see our own galaxy from 2 million light-years away, it would appear _________.

as a flattened disk with a central bulge and spiral arms

How does the interstellar medium affect our view of most of the galaxy?

It prevents us from seeing most of the galactic disk with visible and ultraviolet light.

Applying the Newton's version of Kepler's third law (or the orbital velocity law) to the a star orbiting 40,000 light-years from the center of the Milky Way galaxy allows us to determine ______.

the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center

How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?

It should have a higher fraction of elements heavier than hydrogen and helium.

Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?

The 10% figure is too high because there are not enough heavy elements to make that much dust.

The most common form of gas in the disk of the Milky Way galaxy is _________.

atomic hydrogen gas

How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?

The total amount of gas will be much less than it is today.

Over time, the star-gas-star cycle leads the gas in the Milky Way to _________.

have a greater abundance of heavy elements

Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?

the Chandra X-ray Observatory

If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?

Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles.

What observational evidence supports the galactic fountain model (which describes how gas cycles between the disk of the galaxy and regions high above the disk)?

We see hot gas high above the region of the disk near our solar system, along with cool gas that appears to be raining down from the halo.

All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except _________.

globular clusters

Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found _________.

only in or near star-forming clouds

Which of the following statements comparing halo stars to our Sun is not true?

Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime.

Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?

It is probably a halo star that is currently passing through the disk.

What evidence suggests that most of the mass of the Milky Way is in the form of dark matter?

The orbital speeds of stars far from the galactic center are surprisingly high.

Spiral arms appear bright because ________.

they contain more hot young stars than other parts of the disk

How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?

The stars that formed first could orbit the center of the galaxy in any direction at any inclination.

If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?

Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.

What is the best evidence for an extremely massive black hole in the center of the Milky Way?

The orbits of stars in the center of the galaxy indicate that the presence of 3 to 4 million solar mass object in a region no larger than our Solar System.

Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?

It is by far the brightest source of visible light lying in the direction of the galactic center.

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