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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?.

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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?

  • 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



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

Globular cluster system

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).

Globular clusters in Sagittarius

Galactic coordinates

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

  • Halo

  • Disk

  • Central 1 kpc

  • Bulge

  • Disk and Bulge

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.

Pulsating stars

Use known luminosity to determine distance

Flux versus luminosity relation

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.

Size of Milky Way

Luminosities of components

  • With distances, can do star counts, correct for dust absorption and estimate luminosities

    • Disk: 19109 L

    • Bulge: 2109 L

    • Halo: 2109 L

    • Total: 23109 L

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

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?

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

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.

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

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

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

  • 800 pc

  • 8 kpc

  • 8,000 kpc

  • 8 Mpc

  • Variable, between 1 and 10 kpc

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)

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

Where M in solar masses

a in AU

P in Earth years

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

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)

Kepler’s 3rd Law applied to Galaxy

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

Change from P to velocity = v

Rotation curves

Rotation curve of the Milky Way

Rotation curve of Milky Way

Mass of the Milky Way

density  R-2

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

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

  • mass and orbital speed

  • mass and distance from the center

  • mass and age

  • orbital speed and distance from the center

The Milky Way in various wavelengths

  • Milky Way in infrared, radio

  • The 21 cm line of Hydrogen

Milky Way from Australia

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?

Electromagnetic spectrum

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

Milky Way in optical light

Milky Way in infrared light

Milky Way in radio waves

Hydrogen emits 21 cm radio waves

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

Spiral arms

  • Tracers of spiral structure

  • Objects found in spiral arms

  • How are the spiral arms formed?

Spiral arms

Tracing spiral arms

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

To do this yourself, read 19.5-19.6

21 map of spiral arms

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

Spiral arms

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

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

  • Interstellar dust blocks our view.

  • Too many stars are in the way.

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

So what causes spiral arms?

Density waves

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

Density waves

Density waves


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

  • white dwarf stars

  • HII regions

  • supernovas

  • O and B stars

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.

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

  • Definitely in the form of brown dwarfs

  • Probably cold, dark hydrogen molecules

  • Likely super-massive black holes

  • Definitely cold gas, unknown composition

  • Not known

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

  • A concentration of dark matter inside 2 kpc

  • A spherical distribution of dark matter

  • Dark matter only outside 2 kpc

  • Uniformly distributed dark matter in the disk

  • No dark matter is needed to explain this plot

The Galactic Center

  • Center of the Milky Way

  • Stars orbiting the black hole

  • X-ray flares from the black hole

Fish eye MW

Radio image, central 3 ly

  • Center is the red ellipse at the center

  • Called Sgr A*

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.

Stellar Orbits in the Galactic Center

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

X-ray image, central 3 ly

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

Makes flares in X-rays.


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

  • Observations of nearby star orbits in IR

  • Radio observations of accretion disk

  • Orbit of stars in spiral arms [Kepler’s 3rd law]

  • X-ray observations of coronal gas

  • Dark matter studies using rotation curves

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?

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