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Lecture 25

The Milky Way Galaxy

November 28, 2018

Size of the Universe• The Milky Way galaxy is very much larger

than the solar system

• Powers of Ten interactive applet

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Galaxies

• Large collections of stars, dust and gas

• Held together by their gravity.

• Contain millions to billions of stars.

• Stars rotate around the center of the galaxy.

• Our galaxy is the Milky Way

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Structure of the Milky Way

• Hard to observe our Galaxy -- we are inside.

• William Herschel -- tried to find where Sun was

in the galaxy by counting stars

– Found same density of stars on all sides

– Concluded we are in the center.

Dust Blocks Our View of the

Center of the Milky Way

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Shapley Uses Globular Clusters

to Find the Center

• Globular Clusters

often orbit outside

disk of MW.

• Shapley observed

them mainly on one

side of the sky.

• We are not at the

center of the galaxy.Figure 23.9,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

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Structure of the Milky Way

Figure 23.10,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall Animation

Harlow Shapley first located the center of our Galaxy in 1917 by

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A. observing supernova explosions throughout

the Galaxy.

B. observing the distribution of globular clusters.

C. making redshift measurements of stars in the

galactic disk.

D. observing the distribution of hydrogen gas in

the Galaxy

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Components of the Milky Way

• Disk

– Young and

older stars.

– Much gas and

dust.

– Extends ~30

kpc in diameter

– Sun is about

halfway out

from center of

the disk.

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Motions of gas and stars• Disk

– Takes 230 million yrs for Sun to go about the center.

– Stars rotate differentially

• Takes different amounts of time for stars to get around the

center

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• Bulge

– Young and

older stars

– Gas and dust.

– At center of

disk.

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• Halo

– Mostly old

stars.

– Many stars in

globular

clusters

orbiting center.

– Little gas so no

new star

formation.

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• Bulge

– Some stars move

in elongated orbits

coplanar with

disk, others have

random

orientations.

• Halo

– Stars move in all

directions around

the center

Motions of Stars

Figure 23.13,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

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Edge-on Spiral Galaxy

Galaxy NGC 4565 Figure 23.3b,

Chaisson and McMillan, 6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

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Milky Way

Milky Way galaxy from our perspective and at visible wavelength of light

Astronomers deduce that the Milky Way is a

disk because they

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A. see stars arranged in a circular pattern around the

north celestial pole.

B. see far more stars along the band of the Milky

Way than in other directions.

C. see a large, dark circle silhouetted against the

Milky Way in the Southern Hemisphere.

D. see the same number of stars in all directions in

the sky.

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Mapping the Structure of the

Milky Way

• Dust obscures our view of the galaxy

• Need to use radio and IR wavelengths of light

• 21-cm emission line

– Comes from neutral atomic hydrogen (not ionized)

– The radio wavelength is not obscured by dust

– H very common in the universe

• Hydrogen emits 21-cm-wavelength radio

waves because of a “spin-flip” transition.

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Milky Way at different wavelengths (Interactive Figure)19

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Distribution of Gas

• Much of the gas is

concentrated in arms

in the disk spiraling

around the center

Figure 15.18,

Arny and Schneider,

5th ed. Explorations, © 2008 The McGraw-Hill

Companies

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Spiral Structure in the Milky Way

A "God's view" map of Milky

Way as seen from far Galactic

North (in Coma Berenices).

The star-like lines center in a

yellow dot representing the

position of Sun. The spokes of

that "star" are marked with

constellation abbreviations,

"Cas" for "Cassiopeia", etc.

The spiral arms are colored

differently in order to highlight

what structure belongs to

which arm. HII regions are

marked as dots colored in the

same color as their spiral arm.

From Krisciunas and Yenne,

The Pictorial Atlas of the

Universe, p.145 (ISBN 1-

85422-025-X)

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Spiral Structure in the Milky WayRecent (2008) data from the

Spitzer Space Telescope

indicates our galaxy has a

large bar and perhaps only two

major arms with several minor

arms. Note in this diagram our

Sun is at the bottom. (NASA)

Our galaxy may therefore

resemble NGC 1365 (below),

click here for image info.

Radio waves of 21-cm wavelength originate from

which component of the interstellar medium?

A. Cool, neutral atomic hydrogen

B. Cool, carbon monoxide, CO

C. Cold, molecular hydrogen, H2

D. Hot, ionized atomic hydrogen

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Destruction of Spiral Structure

If the spiral arms consisted of matter moving together

with the stars, they would “wind up” and be destroyed

within a few rotations. See animation applet. (Applet from M. Seeds, Foundations of Astronomy 12th ed. ITP Nelson; Dr. Brian Martin, The King’s University College)

Figure 23.17, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall

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Density Waves Form Spiral Arms

• Spiral structure may be caused by waves of

higher density.

• Same material is not always in the arm, just

temporarily compacted. (See Wikipedia

animations and Interactive Figure)

– Slinky

– Traffic on the highway

Slow moving carDensity waveLower density

Area

Lower density

Area

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Spiral Arm Formation

Figure 23.18,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

Density Wave Theory holds that the spiral

arms are waves of gas compression that

form stars as they go. In the painting at

right, gas enters an arm from behind, is

compressed, and forms stars. The inset

shows spiral galaxy NGC 1566.

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Spiral Arm Formation

• Stars form in arms

since they have higher

density.

• Recent star formation

indicated by some O

and B stars.

– Arms are bright, blue

regions

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Spiral Arm Formation

• Between arms, star

formation less recent,

less bright.

• Arms have only 5%

more stars than the

other areas, but they

are mostly very

luminous (O and B

spectral types)

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Self-Propagating Star Formation

• Stars form in higher density area.

• Supernovae later on compress nearby areas, start

new star formationFigure 23.19,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

How can you tell the difference between a young

collection and an old collection of stars?

A. Collections of young stars are redder.

B. Collections of young stars are darker.

C. Collections of young stars move faster.

D. Collections of young stars are bluer.

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Center of the Galaxy

• Center suspected to contain billions of stars

• Cannot observe center in visible light due to dust,

so we use IR and radio observations

• Bright source near center in radio - Sagittarius A*

• Probably a black hole at the center…

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Galactic CenterRadio image of Galactic center.

~60 parsecs across

Filaments may be associated with

magnetic field.

Radio image of Galactic center

~7 parsecs across

Sagittarius A* at center.

Black Hole at the Center

• Stars are orbiting center (Sagittarius A*) very quickly.

• Estimated 4.3 million solar masses contained in an area 0.30 AU across

• Likely black hole.

IR image of Galactic Center movie

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The best measurements of the mass of the

black hole at the galactic center come from

A. the orbits of gas clouds near the center.

B. analysis of the X-ray emission from the accretion disk.

C. the orbits of stars near center.

D. the rate at which the black hole orbits its companion black hole.

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