Ch 16 assignment 1

Which methods did Kapteyn and Shapley use to estimate the size of the Milky Way?

Kapteyn used stellar motions and Shapley used the period-luminosity relation.

Why did interstellar dust affect Kapteyn's estimate of the Sun's location in our Galaxy, which used stars, more than it did Shapley's estimate, which used globular clusters?

Kapteyn's observations of the disk stars were blocked by dust clouds, whereas Shapley observed clusters above and below the disk

Match the mass ranges with the descriptions of how common different types of stars are.

Mass between 0.1 to 0.5 M☉ The most common values Nearly 1 M☉ About average Masses greater than 20 M☉ Fairly rare

When we look at the night sky with our eyes, most of the stars we see are massive stars, even though those are much rarer than smaller ones. Why?

Massive stars are so bright they can be seen even when far away, but less massive stars are so faint that only the closest few can be seen easily.

We live in the ______ ________ Galaxy

Milky Way

Since we are embedded in the disk of our Galaxy, the stars in it stretch across a dark sky like a faint white band. This lead to the Greeks to name it galaxias kyklos, which translates into the name ______.

Milky Way Galaxy

Match the properties to the star types.

Population I found in the disk millions to billions of years old many are main sequence stars Population II very old, perhaps ten billion years many are red giants found in the bulge and halo

Match the features of Galaxy models with the astronomers who proposed them.

Shapley-The Sun is two-thirds out from the center of the Galaxy. Shapley-The Galaxy is extremely large (100,000 parsecs). Kapteyn-The Sun located near the center of the Galaxy. Kapteyn-The Galaxy is relatively small (18,000 parsecs).

If Population I stars are younger the Population II stars, why do they have more heavy elements than stars that have been around much longer?

The gas clouds that Pop I stars were born from had been seeded with heavy elements because many Pop II stars had already died.

Compared to a globular cluster, the stars in an open cluster ____. (Select all that apply.)

are fewer in number are more spread apart are younger

Select all that are mostly found in our Galaxy's disk.

Spiral arms Young blue stars Interstellar gas and dust clouds

Which of the following best describes the density of stars in the Milky Way's disk?

Stars have a very high density near the Galaxy's center but are much less closely packed at the Sun's location.

Our Galaxy (in fact, all galaxies and especially all spirals) exhibits differential rotation. Select the phrases that describe this phenomenon.

Stars pass each other as they orbit. Stars farther from the center take more time to finish an orbit. it is a result of Kepler's laws

Select all that correctly describe Kapteyn's model of the Galaxy.

The Sun is located near the center of the Galaxy. The Galaxy is shaped like a disk.

Select all that correctly describe the Sun's motion around the galactic center.

The Sun takes a long time to orbit the Galaxy, compared to stars close to the galactic center. The Sun takes about 220 million years to make one trip around the galactic center.

How do astronomers estimate the age of the Galaxy?

The ages of the oldest known stars provide an estimate of how the Galaxy's age.

Select all the statements that correctly describe the formation of stars in the Milky Way.

The average rate is about eight stars each year. Stars are continuously being formed. Astronomers believe during certain time periods, significant bursts of star formation might have occurred.

How do astronomers use O and B stars to map the Galaxy's spiral arms?

The bright, blue O and B stars form, live, and die in spiral arms, making it easy to spot the arms.

Put the statements below in order to argue that the Milky Way Galaxy contains a large amount of dark matter.

1. Astronomers observe the Motion of Stars 2. using the motions of the star, astronomers calculate how much mass is needed to keep the stars in our galaxy 3. the mass needed to keep fast moving stars in our galaxy is 2 x 1o ^12 M 4. The mass of visible matter detected in the galaxy is about 10 to 20% of what is needed 5. The difference in the two masses is composed of unobserved material called dark matter

Which item below does the Milky Way Galaxy most closely resemble in shape?

A frisbee

What recent discovery made it possible for Shapley to use globular clusters to study the structure of the Milky Way?

A period-luminosity relation for variable stars

Which of the following best describe the edge of the Milky Way's disk? (Select all that apply.)

About 50,000 light-years from the galactic center, the density of stars thins out to nearly nothing. The density of stars gradually decreases far from the Galaxy's center. The Galaxy disk has no sharp edge.

Select each stellar property that is different between Population I and Population II stars.

Age Location in our Galaxy Orbit Color

Where is our Sun located within the Milky Way Galaxy?

In the disk, in a spiral arm, about two-thirds out from the center

Roughly speaking, there are several hundred ____ stars in our Galaxy.

billion

If you were to plot the positions of globular clusters and found that they were scattered uniformly across the sky, then the Sun must be located in the __________ of the Galaxy.

center

Stars in the inner and outer parts of our Galaxy not completing an orbit in the same amount of time is called ______ rotation.

differential

True or false: The Milky Way Galaxy has more mass in interstellar gas and dust clouds than it does in stars.

false

You observe a cluster that looks like a tightly packed ball of stars. This is most likely a(n)_____ cluster

globular

Considering the ages of the stars in each part of the Milky Way, it is likely that the __________ was the first part of the Galaxy to form.

halo

Compared to open clusters, globular clusters ____. (Select all that apply.)

have more stars are larger are typically farther away from us

Stars in open clusters and associations spread out over time primarily because ____.

there aren't enough of them to hold the cluster together by gravity. Random and orbital motions separate them

Which group of stars is older?

stars in the halo

How do astronomers estimate the age of the Galaxy?

the ages of the oldest known stars provide an estimate of how the Galaxy's age.

Place these parts of our Galaxy in order by size, from the smallest (top) to the largest (bottom).

1. Nucleus 2. Bulge 3. Disk 4.Halo

Approximately how many stars are in the Milky Way Galaxy?

100s of billions

By measuring the ages of the oldest stars in the Galaxy, astronomers have determined that the Milky Way is roughly billion years old. (Round your answer to the nearest billion.)

13

Match the main parts of the Milky Way Galaxy to their descriptions.

Disk- Contains the Sun, spiral arms, and clouds of gas and dust Halo- Spherically shaped, contains many globular clusters Bulge- Flattened, elongated shape containing many stars, located near the middle of the Galaxy

Match the astronomers to their work regarding the shape and size of our Galaxy.

Galileo Galilei-Discovered that the Milky Way is made up of stars William Herschel-The first to estimate the size of the galaxy by counting stars Jacobus Kapteyn-Argued the Galaxy was a 18 kpc disk with the Sun near the center Harlow Shapley-Learned the center of the Galaxy is in Sagittarius by studying star clusters

You try to make an H-R diagram of all the stars you can see in a big region of the night sky using a small telescope and a camera. Select all the reasons why your results might not accurately describe the numbers of stars of different types.

Generally, bright stars are more massive ones. You can see brighter stars at farther distances.

Which astronomer was the first to present convincing evidence that the Sun was located about two-thirds of the way out along the Galactic disk, and not at the center of the Milky Way?

Harlow Shapley

Select all the ways that Sir William Herschel estimated the shape and size of the Milky Way Galaxy.

He estimated the size by measuring the apparent brightnesses of the faintest stars he could see. He estimated the shape by counting the numbers of stars in different directions on the sky.

How did Kapteyn estimate the distances to remote stars and thereby measure the size of the Galaxy?

He realized that the most distant stars will have the smallest apparent motions over time.

Select all that correctly describe Population II stars.

Their orbits are elliptically shaped. Their heavy element content is low. Their color is redder.

Select all the reasons why globular clusters are useful when measuring the distance to the center of the Galaxy.

They can have up to a million stars, thus are very easy to see even far away. Most are not in the Galactic plane, so they are seen without being obscured by stars or gas clouds.

How do astronomers estimate the average number of stars born in the Galaxy each year?

They divide the total number of stars in the Galaxy by the Galaxy's age.

Select all the reasons why O and B stars are useful for mapping spiral arms.

They only last a few tens of millions of years. They are very bright.

True or false: Dim, cool stars are more common than bright, hot stars.

True

If dark matter cannot be observed, how do astronomers know it exists?

Without extra, unseen mass, the Galaxy's gravity would not be enough to hold in fast-moving stars.

Both Shapley's and Kapteyn's models of the Milky Way contain errors as a result of neglecting to consider interstellar dust. If there is dust between you and a star or cluster, select all the possible effects it can have on your measurement.

You might not be able to see a star at all.You might estimate a star is more distant than it is.

You are repeating Herschel's estimation of the size and shape of our Galaxy. If you assume a star has the same luminosity as the Sun, but in reality it is 10 times more luminous, how does that affect your estimate of the star's distance?

You will think it is closer than it really is.

The particular collection of stars shown in the image is known as _______.

a globular cluster

A star cluster forming out of a molecular cloud would mostly like be a(n) ____.

open cluster

At which position in the diagram must the Sun be located, according to Shapley's estimate of the shape of the Galaxy?

position B