The Milky Way Galaxy
16-1 Our Galaxy
1. The Milky Way Galaxy is the galaxy of which the Sun is a part. From Earth, it appears as a
band of light around the sky.
2. About 200 billion stars make up the Milky Way Galaxy.
3. Most stars in the Galaxy are arranged in a wheel-shaped disk that circles around a bulging
4. The diameter of the Galaxy is about 50,000 parsecs (160,000 light-years).
5. The Sun and our solar system is about a third of the way out from the Galaxy’s center (8000
pc or 26,000 light-years).
6. With the exception of the Magellanic Clouds and the Andromeda galaxy, every naked-eye
object in our sky is part of the Galaxy.
7. Interstellar dust and gas in our Galaxy prevented Herschel (who measured the number of
stars along different directions in the 1780s) and Kapteyn (who measured the number and
distance of stars along different directions in the early part of the 20 century) from getting
accurate star density counts from visual observations. These inaccurate data led them to the
mistaken conclusion that the Earth is at the center of the Galaxy.
A globular cluster is a spherical group of up to hundreds of thousands of stars, found
primarily in the halo of the Galaxy.
The average separation of stars near the center of a globular cluster is 0.5 light-year.
Stars in the region of the Sun average 4–5 light-years apart.
Shapley attempted to determine the Sun’s location in the Galaxy using globular clusters.
In order to determine the distance to these clusters, he used Leavitt’s discovery of the
Cepheid variables’ period luminosity relationship.
o Shapley found RR Lyrae variables in the globular clusters population II cephied
o Shapley published the distances and direction to the 93 know GCs Shapley showed in 1917 that globular clusters are distributed evenly around the sky,
about a point 50,000 light-years from the Sun. His discovery also showed that the Galaxy
is larger than the Herschel had imagined.
o Shapley assumed that they revovle around the centre of the galaxy
o His idea of the galaxy was way larger then Herschels
o Showed galaxy not centered about the solar system
In the 1920s, Oort and Lindblad studied the motions of great number of stars near the
Sun and found that there is a pattern in these velocities. They concluded that the center
of the Galaxy is thousands of light-years away in the direction of Sagittarius.
o Using Kepler's 3rd law, the stars opposite sagittarius slowerd, in the direction of
o Not all the stars were moving the same way, had to study huge number of stars
to see the pattern
o Think about a highway, the cars can be changing lanes but still moving in the
In 1930, the interstellar dust was discovered, resolving the conflict between the
discoveries of Shapley, Oort, and Lindblad and the star counts of Herschel and Kapteyn.
o Interstellar dust had prevented Herschel and Kapeyn from seeing the egdes of
the galaxy a
o Interstellar dust had also messed up Shapley because he observed clusters that
were above or below the plane of the galaxy and calibrated the period luminosity
relationship using cephieds in the plane which had effected their apparent
magnitude. He was off by a factor of 2 but he had still proven that the gaaxy was
way bigger then previously thought
16-2 Components of the Galaxy
1. The Galaxy contains four components: the disk (which contains the Sun), the nuclear
bulge, the halo, and the galactic corona.
2. The disk is the large, flat part of the Galaxy that rotates in a plane around the nucleus.
The disk contains stars and most of the gas and dust in the Galaxy; it is about 1,000
o Star formation maybe because of all the O and B
o 50 000 parsec diameter
o Thickness is 2% of the diameter, looks like flat plate/plane
o Most crowded in the plan, less as you move away
o Has open clusters
Almost all O-type stars lie within about 100 parsecs of the galactic plane. The disk
appears bluish because of the presence of the hot O and B main sequence stars. o Star formation most likely in high dense areas so most are born near the galactic
o Very massive stars have short lifespans so are found mainly near their birth so O
mainly in first 100 parsecs of the plane and less massive can be found further out
o Difficult to determine the structure of the disk b/c interstellar dust is obscuring
4. The nuclear bulge is the central region of the Galaxy; it is about 2000 parsecs in
diameter. It contains both young and old stars and appears reddish because of the
presence of many red giants and supergiants.
o Most dense area of stars dust and gas
o No star formatin
5. The Galactic halo is the outermost part of the Galaxy; it is fairly spherical in shape and
lies beyond the spiral component. The outer halo is sometimes called the Galactic corona
and may contain large amounts of unseen matter.
o Contains the globular clusters that Shapley studied
o Besides the clusters mainly old population II stars, metal poor
o Small amounts of gas and dust
o Clusters feel gravitational pull of the galactic nucleus and orbit it
They pass through the disk 2 times per orbit
150 have been found to orbit MW
The gravational force exerted on galaxies appears to be much greater then the observed
o More then 6X the invisible mass then visible mass in the galaxy
o Thus the idea of the extended halos comes up
2-3X the radius of disk and halo
Don’t know what it consists of
Cool WD, Black Holes, neutrinos etc.
38 new WD found in 2001
35% of unseen matter is made of the normal stuff (neutrons, protons,
6. Milky Way properties: Radius of disk = 80,000 light-years; Radius of nuclear bulge =
3000 light-years; Total radius of halo = 200,000 light-years; Sun’s distance from center =
26,000 light-years; Sun’s orbital period = 250,000,000 years; Thickness of disk = 3000 light-
years; Number of stars = 200 to 400 billion.
1. If we assume that the average velocity of all globular clusters, relative to the Galactic center,
is zero, then we can use the Doppler effect to measure the velocities of the globular clusters
relative to the Sun and attribute the average motion that is observed to the motion of the Sun. Boat and planes analogy
2. The Sun is traveling in a nearly circular path around the Galactic center at a speed of about
220 km/s. It is now moving toward the constellation Cygnus.
3. With the radius of the Sun’s orbit equal to 8000 parsecs, the Sun takes about 230 million
years to complete one revolution around the center of the Galaxy.
20 orbits since birth
4. The galactic rotation curve is a graph of the orbital speed of objects in the galactic disk as a
function of their distance from the center.
If all the mass of the galaxy was in the center it would be following Keplers 3rd Law (like
the solar system) but it doesn’t, the sun has a very weird speed of orbit
Stars that are farther out are not moving slower then the sun
Indications are that there is a lot of mass outside of the suns orbit
A galactic rotation curve for our Galaxy indicates that large amounts of unseen mass
orbit the center far beyond the Sun’s orbit.
Stars have some weird orbits but are generally nearly circular
The Mass of the Galaxy
1. Oort and Lindblad discovered in 1927 that the Galaxy in the Sun’s neighborhood
undergoes differential rotation. This allows the use of Kepler’s third law to find the mass
of the Galaxy inside the Sun’s orbit.
A3/p2 = (m1+ m2)/msun
2. The mass of the inner Galaxy is estimated at 100 billion solar masses. Recent analysis
of the rotation patterns in the outer parts of the Galaxy indicates that the total mass of
the Galaxy is about 10 solar masses (10 times more mass than calculated for the inner
Historical Note: The Shapley-Curtis Debate
1. In the late 1910s, there was considerable controversy about the size of the universe and the nature of the spiral nebulae. In
1920 the National Academy of Sciences held a pubic debate concerning the size of the universe and the nature of the spiral
2. Harlow Shapley estimated a large size of our Galaxy, and concluded that the Magellanic Clouds, the Andromeda nebula, and
other spiral nebulae were also part of it.
3. Heber Curtis estimated the Galaxy to be smaller and that the Andromeda nebula and other spiral nebulae are outside and are
other “island universes”.
4. We now know the nature of our Galaxy and other galaxies is closer to Curtis’ explanation. Shapley had made use of some
incorrect data and misinterpreted observations of Cepheid variables because it was not known at the time that there were
different types. Shapley was more correct in his ideas about the overall size of the universe however.
16-3 The Spiral Arms 1. A spiral galaxy is a disk-shaped galaxy with arms in a spiral pattern.
2. In 1951, the spiral nature of our Galaxy was first hinted at by the distribution of O- and B-
type stars. Confirmation came from radio telescope observations of the 21-cm radiation.
3. The 21-cm radiation is radiation from atomic hydrogen, with a wavelength of 21.1
centimeters. It results from a transition that a hydrogen atom makes from a higher energy level
to a lower one.
4. Hydrogen gas clouds detected by 21-cm radiation are located at the same place as newly
5. Applying the Doppler effect, astronomers use the 21-cm radiation to provide further
evidence for the spiral nature of the Galaxy.
Doppler allows determination of radial motion of the hydrogen
If hydrogen where distributed evenly there would be uniform blueshifting while there
o Proof of spiral nature of galaxy
6. Recent evidence suggests that the Milky Way Galaxy is actually a barred spiral galaxy.
16-4 Spiral Arm Theories
1. It may seem that the differential rotation of a spiral galaxy can explain the presence of spiral
arms. However, such a rotation would result in a fairly uniform disk.
If the spiral nature was just because of uniform velocity and shorter orbital distances