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The Milky Way • Center, Shape • Globular cluster system • Galactic coordinates • Size of the Milky Way

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The Milky Way. Center, Shape Globular cluster system Galactic coordinates Size of the Milky Way. Milky Way from Iowa. Milky Way from Australia. Milky Way from Australia. How do we locate the center of the Milky Way?. - PowerPoint PPT Presentation

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Page 1: The Milky Way

The Milky Way

• Center, Shape

• Globular cluster system

• Galactic coordinates

• Size of the Milky Way

Page 2: The Milky Way

Milky Way from Iowa

Page 3: The Milky Way

Milky Way from Australia

Page 4: The Milky Way

Milky Way from Australia

Page 5: The Milky Way

How do we locate the center of the Milky Way?

• Can’t see center directly with visible light because of obscuring clouds in the plane of the Galaxy

• Look above the plane of the galaxy

Page 6: The Milky Way
Page 7: The Milky Way
Page 8: The Milky Way

M15

Page 9: The Milky Way

M13

Page 10: The Milky Way

Globular clusters

• Compact, spherical group of stars

• Up to several 100,000 stars

• All stars formed together, same age

• Form a halo around the Milky Way

Page 11: The Milky Way

Globular cluster system

Page 12: The Milky Way

Globular cluster system

• Centered on the center of the Milky Way

• Extends far above and below the plane

• By observing globular clusters, we can determine the direction to the center of the Milky Way (and, later, our distance from the center).

Page 13: The Milky Way

Globular clusters in Sagittarius

Page 14: The Milky Way

Galactic coordinates

Page 15: The Milky Way

Globular clusters are found primarily in what part of the Milky Way?

1. Halo

2. Disk

3. Central 1 kpc

4. Bulge

5. Disk and Bulge

Page 16: The Milky Way

Size of the Milky Way

• How do we find the size of the Galactic halo or the distance to the Galactic center?

• Star counts, as described in the text book, are a bad way.

Page 17: The Milky Way

Pulsating stars

Page 18: The Milky Way

Use known luminosity to determine distance

Flux versus luminosity relation

2

A

B

B

A

B

A

Distance

Distance

Luminosity

Luminosity

Flux

Flux

We can figure out the luminosity of a pulsating star by timing the pulsations. Since, we can measure its flux, we can then find the distance to the star.

Page 19: The Milky Way

Size of Milky Way

Page 20: The Milky Way

Luminosities of components

• With distances, can do star counts, correct for dust absorption and estimate luminosities– Disk: 19109 L – Bulge: 2109 L – Halo: 2109 L – Total: 23109 L

• Average star is dimmer than the Sun, total number stars is ~200 billion.

Page 21: The Milky Way

Review Questions

• What are globular clusters? How are they distributed in the Galaxy?

• What are Galactic coordinates?

• Why do some stars pulsate? Why are pulsating stars useful in measuring distances?

• What is the size and shape of the Milky Way?

Page 22: The Milky Way

Orbits of stars in the Milky Way

• Stellar orbits in disk and halo

• Finding the mass from the orbit

• Mass of the Milky Way

• Rotation curves

• Dark matter

Page 23: The Milky Way

What keeps the planets in orbit around the Sun?

• The force of gravity from the Sun

• To orbit, a planet at a particular distance from the Sun must have a particular orbital speed.

Page 24: The Milky Way
Page 25: The Milky Way

Orbits of stars in the Milky Way

• The orbit of a star is determined by the total mass lying inside the orbit

• By measuring the speed of the star’s orbit and its distance from the center, we can figure out the total mass lying inside the orbit of the star

Page 26: The Milky Way

Stellar Orbits in the Galaxy• Stars in the disk all orbit the

Galactic center:• in the same direction

• in the same plane (like planets do)

• they “bobble” up and down• this is due to gravitational pull

from the disk

• this gives the disk its thickness

• Stars in the bulge and halo all orbit the Galactic center:• in different directions

• at various inclinations to the disk

• they have higher velocities• they are not slowed by disk as

they plunge through it

• nearby example: Barnard’s Star

Page 27: The Milky Way
Page 28: The Milky Way

1. 800 pc2. 8 kpc3. 8,000 kpc4. 8 Mpc5. Variable, between 1 and 10 kpc

The distance between the Sun and the Galactic center is closest to:

Page 29: The Milky Way

Mass of the Galaxy

We can use Kepler’s Third Law to estimate the mass of the Milky Way inside the Sun’s orbit• Sun’s distance from center of Milky Way:

8,500 pc = 1.8 x 109 AU• Period of Sun’s orbit around the center of the

Milky Way: 230 million years (2.3 x 108 yr)

Page 30: The Milky Way

Simplified form of Kepler’s 3rd law using convenient units

Where M in solar masses

a in AU

P in Earth years

2

3

21 P

aMM

Page 31: The Milky Way

Mass of the Milky Way within the Sun’s orbit

Where M in solar masses, a in AU, P in Earth years

• Mass within Sun’s orbit is 1011 M

• Total mass of MW Galaxy is ~ 1012 M

• Total number of stars in MW Galaxy 2 x 1011

1128

39

2

3

1 101103.2

108.1

P

aM

Page 32: The Milky Way

Kepler’s 3rd Law applied to Binary Stars

Where:

• G is gravitational constant

• G = 6.67·10-11 m3/kg-s2 in SI units

• m1, m2 are masses (kg)

• P is binary period (sec)

• A is semi-major axis (m)

23

21

2

)(

4Pa

mmG

Page 33: The Milky Way

Kepler’s 3rd Law applied to Galaxy

Where M(r) is mass inside r (kg)23

2

)(

4Pr

rGM

v

rP

2

23

2 4

)(

4

v

rr

rGM

Change from P to velocity = v

r

rGMv

)(

Page 34: The Milky Way

Rotation curves

Page 35: The Milky Way

Rotation curve of the Milky Way

Page 36: The Milky Way

Rotation curve of Milky Way

Page 37: The Milky Way

Mass of the Milky Way

density R-2

Page 38: The Milky Way

Dark Matter

• Dark – it doesn’t produce light (any kind)

• Does have mass, produces gravity

• Nature is unknown

• Might be normal matter in a form that doesn’t emit much light – very small and dim star, little black holes

• More likely it is elementary particles other than normal matter

Page 39: The Milky Way

What properties of the sun could be used to measure the total mass enclosed within the sun's orbit?

1. mass and orbital speed

2. mass and distance from the center

3. mass and age

4. orbital speed and distance from the center

Page 40: The Milky Way

The Milky Way in various wavelengths

• Milky Way in infrared, radio

• The 21 cm line of Hydrogen

Page 41: The Milky Way

Milky Way from Australia

Page 42: The Milky Way

Scattering of light

• Blue light is scattered more

• Red light is transmitted more

• Stars seen through dust appear redder than they really are

• If we want to try to see through dust, what kind of light should we use?

Page 43: The Milky Way

Electromagnetic spectrum

---------- radio ------------

Page 44: The Milky Way

Milky Way in optical light

Page 45: The Milky Way

Milky Way in infrared light

Page 46: The Milky Way

Milky Way in radio waves

Page 47: The Milky Way

Hydrogen emits 21 cm radio waves

Page 48: The Milky Way

Same effect in other atoms is used to do magnetic resonance imaging (MRI)

Page 49: The Milky Way

Spiral arms

• Tracers of spiral structure

• Objects found in spiral arms

• How are the spiral arms formed?

Page 50: The Milky Way

Spiral arms

Page 51: The Milky Way

Tracing spiral arms

Page 52: The Milky Way

Spiral arms can be traced from the positions of clouds of atomic hydrogen

To do this yourself, read 19.5-19.6

Page 53: The Milky Way

21 map of spiral arms

Page 54: The Milky Way

Tracers of spiral arms

• Young stars and related objects also trace spiral arms

• Emission nebulae = H II regions

• Molecular clouds

• Clusters of young (O and B) stars

Page 55: The Milky Way

Spiral arms

Page 56: The Milky Way

Why can't we see visible radiation from the central region of the galaxy?

1. No visible light is emitted by the central region of the galaxy.

2. Interstellar dust blocks our view.

3. Too many stars are in the way.

4. Gravity curves the light away from the earth and Sun.

Page 57: The Milky Way

So what causes spiral arms?

Page 58: The Milky Way
Page 59: The Milky Way
Page 60: The Milky Way

Density waves

Page 61: The Milky Way

Spiral arms are patterns

• According to the density-wave theory, spiral arms are created by density waves that sweep around the Galaxy

• The gravitational field of this spiral pattern causes stars and gas to slow down near the arm

• This compresses the interstellar clouds, triggering the formation of stars

• The entire arm pattern rotates around the Milky Way once every 500 million years

Page 62: The Milky Way

Density waves

Page 63: The Milky Way

Density waves

Page 64: The Milky Way

M74

Page 65: The Milky Way

Which of the following objects are not found primarily in the spiral arms of the Galaxy?

1. white dwarf stars

2. HII regions

3. supernovas

4. O and B stars

Page 66: The Milky Way

Review Questions

• How can we measure the mass of the Milky Way?• Why do stars behind dust clouds appear red?• Why is the sky blue?• Why are wavelengths of light outside the visible useful

in studying the Milky Way?• How is the 21 cm line of Hydrogen produced?• Describe the spiral arms of the Milky Way and what

causes them.

Page 67: The Milky Way

1. Definitely in the form of brown dwarfs2. Probably cold, dark hydrogen molecules3. Likely super-massive black holes4. Definitely cold gas, unknown

composition5. Not known

The nature of the dark matter in the Milky Way is:

Page 68: The Milky Way

1. A concentration of dark matter inside 2 kpc

2. A spherical distribution of dark matter

3. Dark matter only outside 2 kpc

4. Uniformly distributed dark matter in the disk

5. No dark matter is needed to explain this plot

From the rotation curve of a hypothetical galaxy as shown in the figure, one could

infer

Page 69: The Milky Way

The Galactic Center

• Center of the Milky Way

• Stars orbiting the black hole

• X-ray flares from the black hole

Page 70: The Milky Way

Fish eye MW

Page 71: The Milky Way

Radio image, central 3 ly

• Center is the red ellipse at the center

• Called Sgr A*

Page 72: The Milky Way

Infrared image, central 3 ly

• Sgr A* does not appear.

• There are about 1,000,000 stars in the area covered by this image.

• Stars are only 1000 AU apart.

Page 73: The Milky Way

Stellar Orbits in the Galactic Center

Page 74: The Milky Way

Mass of Sgr A* can be measured using stellar orbits

• Fastest moving star moves at 2% of the speed of light, 5000 km/s

• Mass is about 3 million solar masses

• Emits radio and X-rays

• Almost certainly a black hole

Page 75: The Milky Way

X-ray image, central 3 ly

Sgr A* is the bright object in the center of the image.

Makes flares in X-rays.Movie.

Page 76: The Milky Way

1. Observations of nearby star orbits in IR2. Radio observations of accretion disk3. Orbit of stars in spiral arms [Kepler’s 3rd

law]4. X-ray observations of coronal gas5. Dark matter studies using rotation

curves

The best estimate of the mass of the black hole at the galactic center was

made using:

Page 77: The Milky Way

Review Questions

• What properties of a star’s orbit around the Galaxy enable one to measure the mass inside its orbit?

• What is the shape of the rotation curve of the Milky Way and why is was it unexpected?

• What lies at the center of the Milky Way?