The Milky Way Galaxy

A River of Stars

Warm UpPlease complete the following KWL

chart.

What I know about the Milky Way is…..

What I would like to know about the Milky Way is…..

What I learned about the Milky Way is…..

Warm Up1. Where did the name “Milky Way” most likely originate?

2. What German philosopher proposed that the Milky Way was a flattened swarm of stars?

3. How is our view of our galaxy like an ant’s inside a fruit salad?

4. What conclusions did Sir William Herschel make about the Milky Way?

5. How was Harlow Shapley’s model different from Jacobus Kapteyn’s?

6. What did Shapley use to determine the extent of the Milky Way?

Warm Up

1. What is our solar system’s tilt relative to the galactic disk?

2. Where is our Sun located relative to the Milky Way’s galactic center?

3. What is another name for the galactic center?

4. How quickly is the Sun moving around our galactic center?

5. How far has the Milky Way rotated since the dinosaurs dies out 65 million years ago?

6. How is galactic density measured?

7. What is the galactic density is the region of our Sun?

8. What is the galactic density near the center of our galaxy?

9. What is the approximate mass of the Milky Way Galaxy?

10. If the mass of the Milky Way is greater, what is responsible for the discrepancy?

Warm Up-12/19/121. How is galactic density measured?

2. What is the galactic density is the region of our Sun?

3. What is the galactic density near the center of our galaxy?

4. What is the approximate mass of the Milky Way Galaxy?

5. If the mass of the Milky Way is greater, what is responsible for the discrepancy?

Warm Up-12/18/121. What is our solar system’s tilt relative to the galactic

disk?

2. Where is our Sun located relative to the Milky Way’s galactic center?

3. How quickly is the Sun moving around our galactic center?

4. How long does it take our galaxy to make one complete galactic revolution?

5. How far has the Milky Way rotated since the dinosaurs dies out 65 million years ago?

Warm Up

1. What are population I stars?

2. Where are population I stars found?

3. What are population II stars?

4. Where are population II stars found?

5. What is the center of the galaxy called?

6. What do astronomers believe is at the center of the Milky Way?

WORD Definition

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Used correctly in a sentence

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Word Wall Elements

Words for Word Wall Elements

1) Galaxy

2) Milky Way

3) Globular cluster

4) Galactic disk

5) Galactic halo

6) Galactic bulge

7) Spiral arm

8) Galactic nucleus

9) Population I stars

10) Population II stars

11) Open clusters

12) scattering

13) Dark nebulae

14) Zone of avoidance

15) Emission nebula

16) Reflection nebula

The Milky WayThe origin of the term “Milky Way” has been lost to antiquity. The Greek word for galaxy is “galactos” for milk. The Latin version of Milky Way is “via lactea”, with “via" meaning “way" or “road" and “lactea" meaning “milk".

The Milky WayOur understanding of the Milky Way as a star system dated back to the 18th century when English astronomer Thomas Wright and German philosopher Immanuel Kant both proposed that the Milky Way was a flattened swarm of stars

There’s Ants in the Fruit Salad!!

Here’s a good analogy for understanding the way the Milky Way looks from our vantage. Imagine that you’re an ant that has fallen into a round fruit salad. Sitting there on your berry, you look up and see a few other berries and down and you see a few more berries, but when you look straight you see a sea of berries.

The Milky Way: Sir William Hershel Late in the 1800’s, Sir William Hershel counted the number of stars that were visible in different directions. From this count he concluded that stars laid in a disk and that the disk was approximately 5 times longer than it was thick.

The Milky Way: Sir William Hershel

Herschel further concluded that because the number of stars appeared to be the same in each direction that our Sun lay roughly in the middle of this disk.

HerschelSketch of theMilky Way

Measuring the GalaxyThe Galaxy was first measured in the early 1900’s by Dutch astronomer Jacobus Kapteyn (cap-tine) and American astronomer Harlow Shapley.

Measuring the Galaxy

Shapley’s model differed from Kapteyn’s in that it was larger and the Sun resided about 2/3 of the way from the center. Shapley’s method was different as well. Instead of plotting stars, he plotted globular clusters.

Globular ClustersGlobular clusters are groups of up to millions of stars. Shapley used them because of their great luminosity. They could be seen at greater distances than single stars. He noticed that globular clusters were scattered evenly with the stars.

Measuring the Galaxy

Shapley determined the distances by methods that we are familiar with: variable stars in the globular clusters, true and apparent brightness and the inverse square law. Shapley believed that the clusters filled an elliptical region about 100,000 parsecs (300,000 light-years) across.

Oop!

Unfortunately for science, each scientist was partially right and partially wrong. Shipley correctly placed the Sun within the galaxy, but he grossly overestimated the size. Kapteyn came closer to the actual size of our Galaxy. Neither knew of the attenuating effects of interstellar gases. Neither knew that there were two types of variable stars. Looking back at the controversy, it’s funny that they were wrong about so much.

The Structure and Content of the Milky Way Galaxy

The Milky Way is a disk that measures about 30 parsecs (80,000 light-years) across. It consists of three main parts: the disk, the bulge and the halo.

Where Are We?Our solar system lies about 8.5 parsecs (28,000 light-years) from the galactic center. It is tilted about 600 relative to the galactic disk. That is why you see the Milky Way at such a steep angle in the sky.

What Else is Out There?Along with the stars, there is a lot of gas and dust. In fact, it makes up about 15% of the galaxy’s mass. Most of these regions reveal themselves by blocking the light of stars.

What Else is Out There?

It is because of this dust and gas that we are unable to see to the center of our own galaxy. Optically, we can only see about 3,000 parsecs (10,000 light years) toward the galactic center known as the galaxy’s nucleus. However using x-ray, infrared and radio frequencies, we can “see” that the core contains a vast number of stars and a giant black hole.

Forces of the GalaxyGravity keeps the galaxy together, but it would quickly pull the entire thing together if it weren’t for our galactic spin. The Sun moves around the center of the galaxy at about 220 km/sec. It is this rotational velocity that gives the Milky Way its flat, disk-like structure.

Forces of the Galaxy

Just as a point on the edge on a merry-go-round travels faster than one toward the center, so to do the stars at the outer edge of the galaxy.

Forces of the Galaxy

While traveling 220 km/sec might seem fast, it still takes the Sun about 240 million years to make one complete galactic revolution. The Sun has only made about a quarter of a revolution since the dinosaurs died.

Galactic Density

While the Sun may seem as if it’s lost in a sea of stars, that is not true. Stars are relative scattered. Our closest neighbor is about 4.2 light-years away, comparable to two pinheads fifteen miles apart.

Webquest

• http://olc.spsd.sk.ca/DE/webquests/landofthelivingskieswq/milkyway.htm

Galactic DensityNear the core of the galaxy, stars are packed more densely, some 1500 times closer together. The analogy would be like two pinheads at either end of a football field.

Living in the Burbs

Star density is measured by stars per cubic light-year. Near the core the density is approximately 10 million stars per cubic light-year, while in our region of the galaxy it falls to around 0.003 stars per cubic light-year.

The Mass and Number of Stars in Our Galaxy

Astronomers can calculate the mass of the solar system from the gravitational attraction needed to keep the Sun in orbit. It turns out that the mass of the Milky Way is about 1011 solar masses.

Dark MatterHowever, our gravitational effects on other galaxies implies that our mass could be as high as 2 X 1012 solar masses. This discrepancy implies some missing mass. Scientist believe that our galaxy is embedded in a vast halo of material that we can not see called dark matter.

Stellar CensusBy dividing the total number of stars in our galaxy by its age, astronomers estimate that every year between 3 and 5 stars are born in our galaxy. This is several times less than those that were probably born earlier in our galaxy’s history.

Dark Matter

Observations indicate that this dark matter fills a region with a radius of 100 kilo-parsecs (326,000 light years), dwarfing the observable portion of our galaxy.

How Many Stars Are in Our Galaxy?

Assuming that most stars have masses similar to our Sun, it is estimated that about 100 billion stars reside in the Milky Way.

Age of the Milky Way

Scientists estimate that the age of our galaxy is about 13 billion years old. During that vast expanse of time, the Milky Way has consumed most of its original gas. It is believed that in about 10 billion years that the Milky Way will contain no material to make new stars and will begin to fade.

Stellar CensusThe Milky Way contains all the types of stars that we discussed during stellar evolution. However, despite the great diversity of stars, the typical star in the Milky Way is very much like our own Sun in age and size.

Population I and II Groups

Walter Baade, during the blackouts of WWII, made a series of photographs at the Mt. Wilson observatory in California. In them, he noticed that stars in galaxies were often segregated by color.

Population I and II Groups

The disks of galaxies contained blue stars. He called these population I stars.

Population I and II Groups

The galactic bulges and haloes contained red stars. These he called population II stars.

Population I and II Groups

Population I and II stars differ in almost every respect: age, color, location, motion and composition. Population I stars are younger, averaging from 106 to 109 years old. Most are blue and lie in the galactic disk following approximately circular orbits. Their composition is mainly hydrogen and helium with only about 3% of their mass as heavy elements.

Population I and II Groups

Population II stars are generally older and red whose ages are about 1010 years old. They lie in the bulge or halo and have highly elliptical orbits that may be heavily tilted with respect to the galactic disk. These stars are mostly hydrogen and helium as well with only a few 100th’s of a percent as heavy elements.

Star ClustersSome of the stars in the Milky Way are gravitationally bound together into groups called star clusters.

Star ClustersWithin a star cluster, each star moves along its own orbit around the center of the mass of the cluster. One of the most famous is the constellation Pleiades (daughter of Atlas). This constellation, visible from August to March, is located just above the “V” in Taurus.

Types of ClustersThe Milky Way contains two types of star clusters. The first is the open cluster. Open clusters generally contain up to a few hundred stars and typically span 7 to 20 light-years.

Open ClustersScientists believe that open clusters form from interstellar gas caught between the spiral arms of galaxies. The strong gravitational fields compress the gas creating stars that are gravitationally bound together. These clusters eventually break apart. Approximately 20,000 star clusters are believed to exist in the Milky Way. Our own Sun may have originally formed this was and become disassociated with the rest of its cluster over time.

Open Clusters

Globular Clusters

Globular clusters contain many more stars than open clusters. Their populations can range from a few hundred thousand stars to several million per cluster. Their radii usually range form 40 to 160 light-years. Because they are more massive, they pull their stars into a tighter ball. Astronomers estimate that between 150 to 200 globular clusters exist within the Milky Way.

Globular Clusters

Measuring the Milky Way

We discussed ways that Shapley used to determine the size of our galaxy previously. Using globular clusters remains one of the best ways. Astronomers first locate a large number of bright star clusters. They look for clusters that are symmetrically positioned around a central point and assume that this is the galactic center and measure from that point.

Measuring the Milky Way

From this, astronomers estimate (and it is a rough estimate) that the Milky Way has a diameter of about 40 parsecs with our Sun lying about 8.5 kiloparsecs (28,000 light-years) from the center.

The Galactic Center

Observation of the galactic center is difficult due to dust clouds that block the light. But, using x-ray, infrared, gamma ray and radio wave telescopes, the galaxy’s center can be glimpsed. Astronomers have noticed an intense source of radio waves known as “Sagittarius A”.

The Galactic Center

The Galactic Center

Around Sagittarius A, which measures about 10 AU, can be seen gas and stars swirling around at an extremely rapid rate. Some of these bodies travel at hundred of kilometers per hour.

The Galactic Center

Kepler's Law says that the body around which these stars orbit has a mass of 4 million solar masses, yet nothing is seen there. Therefore, many astronomers almost inescapably conclude that a massive black hole lies at the heart of our galaxy. This evidence is perhaps the best yet that black holes actually exist.

Galactic Center Applet

Galactic center applet.

Galactic Formation applet.

Merging Galaxies Movie.