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
M15
M13
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?
1. Halo
2. Disk
3. Central 1 kpc
4. Bulge
5. 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
Flux A Luminosit yA

Flux B Luminosit yB
 Dist anceB

 Dist anceA



2
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:
1.
2.
3.
4.
5.
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
3
a
M1  M 2  2
P
Where M in solar masses
a in AU
P in Earth years
Mass of the Milky Way within the
Sun’s orbit




9 3
a
1.8 10
M1  2 
8
P
2.3 10
3
2
 110
11
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
4
3
2
a P
G(m1  m2 )
2
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
4
3
2
r P
GM (r )
2
Where M(r) is mass inside r (kg)
Change from P to velocity = v
2r
P
v
4
 4r 
3
r 

GM (r )
 v 
2
GM (r )
v
r
2
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?
1.
2.
3.
4.
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?
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.
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
M74
Which of the following objects are not found
primarily in the spiral arms of the Galaxy?
1.
2.
3.
4.
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:
1.
2.
3.
4.
5.
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
1.
2.
3.
4.
5.
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.
Movie.
The best estimate of the mass of the black
hole at the galactic center was made using:
1. Observations of nearby star orbits in IR
2. Radio observations of accretion disk
3. Orbit of stars in spiral arms [Kepler’s 3rd
law]
4. X-ray observations of coronal gas
5. 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?