The Milky Way

1
The Milky Way

• Center, Shape
• Globular cluster system
• Galactic coordinates

2
Milky Way from Iowa
3
Milky Way from Australia
4
Milky Way from Australia
5
How do we locate the center of the Milky Way?

• Cant see center directly with visible light
  because of obscuring clouds in the plane of the
  Galaxy

• Look above the plane of the galaxy

6
(No Transcript)
7
(No Transcript)
8
M15
9
M13
10
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

11
Globular cluster system
12
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).

13
Globular clusters in Sagittarius
14
Galactic coordinates
15
Globular clusters are found primarily in what
part of the Milky Way?

• Halo
• Disk
• Central 1 kpc
• Bulge
• Disk and Bulge

16
Size of the Milky Way

• Pulsating stars
• Size of the Milky Way

17
But how do we find the distance to the Galactic
center?

• We need the distances to the globular clusters
• Use pulsating stars in the clusters
• Animation

18
If a star is neither expanding nor contracting,
we may assume that throughout the star there is a
balance between pressure and

• temperature
• density
• luminosity
• gravity

Do mass on spring demo
19
Pulsating stars
20
Pulsation cycle
Rate of fusion in the core stays
constant. Transport of energy through outer
layers of star oscillates.
21
Pulsating stars
22
Pulsating stars
23
Why is this useful?

• 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.
24
A Cepheid has the same pulsation period, but is
1/16 the brightness of another Cepheid known to
be at a distance of 2 kpc. How far away is the
dimmer star?

• 2 kpc
• 4 kpc
• 8 kpc
• 16 kpc
• 32 kpc

25
Size of Milky Way
26
What causes Cepheid variables to vary in
brightness?

• they pulsate
• eclipses
• changes in nuclear energy generation rate
• obscuration by clouds of dust

27
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?

28
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

29
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.

30
(No Transcript)
31
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 stars orbit and
  its distance from the center, we can figure out
  the total mass lying inside the orbit of the star

32
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 Barnards Star

33
(No Transcript)
34
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

35
Mass of the Galaxy

• We can use Keplers Third Law to estimate the
  mass of the Milky Way inside the Suns orbit
• Suns distance from center of Milky Way
  8,500 pc 1.8 x 109 AU
• Period of Suns orbit around the center of the
  Milky Way 230 million years (2.3 x 108 yr)

36
Simplified form of Keplers 3rd law using
convenient units
Where M in solar masses a in AU P in Earth years
Example a 0.05 AU, P 1 day 1/365 yr, M1
M2 16.6 Msun
37
Mass of the Milky Way within the Suns orbit

• Where M in solar masses, a in AU, P in Earth
  years
• Mass within Suns orbit is 1011 M?
• Total mass of MW Galaxy is 1012 M?
• Total number of stars in MW Galaxy ? 2 x 1011

38
Keplers 3rd Law applied to Binary Stars

• Where
• G is gravitational constant
• G 6.6710-11 m3/kg-s2 in SI units
• m1, m2 are masses (kg)
• P is binary period (sec)
• A is semi-major axis (m)

39
Keplers 3rd Law applied to Galaxy
Where M(r) is mass inside r (kg)
Change from P to velocity v
40
Rotation curves
41
Rotation curve of the Milky Way
42
Rotation curve of Milky Way
43
Mass of the Milky Way
44
Dark Matter

• Dark it doesnt produce light (any kind)
• Does have mass, produces gravity
• Nature is unknown
• Might be normal matter in a form that doesnt
  emit much light very small and dim star, little
  black holes
• More likely it is elementary particles other than
  normal matter

45
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

46
The Milky Way in various wavelengths

• Scattering of light, or why is the sky blue?
• Milky Way in infrared, radio
• The 21 cm line of Hydrogen

47
Milky Way from Australia
48
Scattering of light

• Light is completely absorbed by very dense clouds
  of dust
• For less dense clouds, some light is transmitted
• Does the transmitted light have the same color as
  the scattered light?
• Do demo 6F40.10

49
Scattering light

• Blue light is scattered more
• Red light is transmitted more
• This is why the sky is blue
• 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?

50
Electromagnetic spectrum
---------- radio ------------
51
Milky Way in optical light
52
Milky Way in infrared light
53
Milky Way in radio waves
54
Hydrogen emits 21 cm radio waves
55
Same effect in other atoms is used to do magnetic
resonance imaging (MRI)
56
Spiral arms

• Tracers of spiral structure
• Objects found in spiral arms
• How are the spiral arms formed?

57
Spiral arms
58
Tracing spiral arms
59
Spiral arms can be traced from the positions of
clouds of atomic hydrogen
60
21 map of spiral arms
61
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

62
Spiral arms
63
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.

64
So what causes spiral arms?
65
(No Transcript)
66
(No Transcript)
67
Density waves
68
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

69
Density waves
70
Density waves
71
M74
72
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

73
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.

74
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

75
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

76
The Galactic Center

• Center of the Milky Way
• Stars orbiting the black hole
• X-ray flares from the black hole

77
Fish eye MW
78
Radio image, central 3 ly

• Center is the red ellipse at the center
• Called Sgr A

79
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.

80
Stellar Orbits in the Galactic Center
81
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

82
X-ray image, central 3 ly

• Sgr A is the bright object in the center of the
  image.
• Makes flares in X-rays.
• Movie.

83
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 Keplers 3rd law
• X-ray observations of coronal gas
• Dark matter studies using rotation curves

84
Review Questions

• What properties of a stars 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?