ASTRO WK 6 mastering astronomy chp 19

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

The diameter is about 100 times as great as the thickness

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

How do disk stars orbit the center of the galaxy?

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

What are basic characteristics of stars' orbits in the disk, halo, and bulge of our galaxy?

The disk stars have nearly circular orbits. They have vertical motions out of the plane, making them appear to bob up and down, but they never get "too far" from the disk. Orbits of stars in the bulge and the halo of the galaxy are much less orderly, traveling around the galactic center on elliptical orbits with more or less random orientations.

Where are most of the Milky Way's globular clusters found?

in the halo

Why do disk stars bob up and down as they orbit the galaxy?

because the gravitational pull of other disk stars always pulls them toward the disk

How do we determine the Milky Way's mass outside the Sun's orbit?

from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun

The story of how we came to learn the structure of the Milky Way is an excellent example of how science progresses.

Select the сorrect statements.

The Milky Way is much wider than it's thickness. So it is a narrow band with many stars in it, while the sky outside the band has much fewer stars.

Shapley's 20th-century observations of globular cluster orbits, which center on a point about 30,000 light years from our Sun, showed we weren't in the center of the galaxy.

Listed following are several locations in the Milky Way Galaxy. Rank these locations based on their distance from the center of the Milky Way Galaxy, from farthest to closest.

farthest to closest

-a globular cluster in the outskirts of the halo

-a cloud of gas and dust in the outskirts of the disk

-our solar system

-the edge of the central bulge

Imagine a photon of light traveling the different paths in the Milky Way described in the following list. Rank the paths based on how much time the photon takes to complete each journey, from longest to shortest.

Hints

longest to shortest

-across the diameter of the galactic halo

-across the diameter of the galactic disk

-from the sun to the centre of the galaxy

-across the diameter of the central bulge

-through the disk from top to bottom

Match the words in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each word only once.

CONTINUED

The circular but relatively flat portion of the galaxy is the ______.

disk

The first portion of the galaxy to form was the ______.

halo

A tightly packed group of a few hundred thousand very old stars is a ________.

globular cluster

A ______ stands out in a photo of a galaxy because it shines brightly with light from massive young stars and glowing clouds of gas and dust.

spiral arm

Our Milky Way galaxy is a ______.

spiral galaxy

Stars orbiting in the ______ near the galaxy's center can have orbits highly inclined to the galactic plane.

bulge

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.

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.

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

in the spiral arms

Why do star clusters make superbubbles?

Superbubbles form when the hottest, most massive stars in a cluster explode as supernova within a few hundred thousand years of each other. Their bubbles merge into a giant bubble, called a "superbubble."

What happens to those bubbles when they expand beyond the galactic disk.

There is no longer anything to keep them from expanding in the vertical direction, resulting in blowouts.

Which part of the galaxy has gas with the hottest average temperature?

the halp

Which of these forms of radiation passes most easily through the disk of the Milky Way?

infrared light

Where are large dust clouds predominantly located in the galaxy M51?

within or on the edges of the spiral arms

Where are the ionization nebulae predominantly located in the galaxy M51?

within or on the edges of the spiral arms of the galaxy

Why are the ionization nebulae so bright?

They are regions where gas is ionized by hot, young 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.

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.

Match types of stars with the appropriate characteristics.

CONTINUE

The _____ population of stars contains both young and old stars, all of which are made up of about 2% heavy elements.

disk

The ____ population stars are all old and have low masses.

halo

The stars in the ______ are lower in heavy elements than the _______ population of stars.

halo, disk

Which kind of star is most likely to be found in the halo?

an M star

Each item below belongs either with the population of disk stars or the population of halo stars of the Milky Way Galaxy. Match each item to the appropriate population.

Disk Stars (Yellow): Youngest stars, the sun, stars that all orbit in nearly the same plane, high-mass stars

Halo Stars (Green): Oldest stars, globular clusters, stars whose orbit can be inclined at any angle, stars with the smallest 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

Where would you be most likely to find an ionization nebula?

in a spiral arm

We measure the mass of the black hole at the galactic center from:

the orbits of stars in the galactic center.

To calculate the dashed orbits from the stellar positions, astronomers had to assume that __________.

if they observed for many more years, the dots would trace out ellipses

Notice that some of the stars on the diagram are represented by a series of dots that are very close together, while others have their dots farther apart. Keeping in mind that all the stellar positions were measured at approximately one-year intervals, which stars are moving the fastest in their orbits during the time period indicated by the dots?

The fastest stars are the ones with

the dots farthest apart

To determine the mass of the central object, we must apply Newton's version of Kepler's third law, which requires knowing the orbital period and average orbital distance (semimajor axis) for at least one star. We could consider any of the stars shown in the figure, so let's consider the star with the highlighted orbit (chosen because its dots are relatively easy to distinguish). What is the approximate orbital period of this star?

20 yr

Look again at the orbit of the star with the highlighted orbit. By comparing the orbit to the scale bar shown on the diagram, you can estimate that this orbit has a semimajor axis of about _____.

1150 AU

The following equation, derived from Newton's version of Kepler's third law, allows us to calculate the mass (M) of a central object, in solar masses, from an orbiting object's period (p) in years and semimajor axis (a) in astronomical units:

M=a3p2

Using this formula with the values you found in Parts C and D, what is the approximate mass of the central object?

4 million solar masses

From Part E you know the mass of the central object. Now consider its size. Based on what you can see in the diagram, you can conclude that the diameter of the central mass is __________.

no more than about 70 AU

You've now found that the central object has a mass of about 4 million solar masses but is no more than about 70 AU in diameter—which means it cannot be much larger than the size of our planetary system. Why do these facts lead astronomers to conclude that the central object is a black hole?

There is no known way to pack so much mass into such a small volume without it collapsing into a black hole.