29:50 Modern Astronomy
Fall 2002
Lecture 26 ...November 11, 2002
The Structure of the Milky Way Galaxy
Exam Wednesday.
Leonid meteors will be prominent one week from today. Monday-Tuesday night,
they will be most conspicuous from midnight to dawn.
The Structure of the Milky Way galaxy
When you look out at the night sky, you see many stars. Clearly these stars are fundamental units in some larger system. What is the size and shape of this system? What processes govern its evolution? How old is it?
I will talk about these subjects today. If you are a person who needs instantaneous
gratification and can't wait for the answer, you can look at the poster at the front of
the room. Or you can look at the picture opposite the
Acknowledgements near the beginning of your textbook.
Image of the Milky Way
It turns out we can learn an awful lot about the structure of the Milky Way from very simple
observations. When you go out in a dark sky, you can see the Milky Way as a band of light across
the sky.
Drawing on blackboard of simulacrum of Milky Way.
It is reasonable to conclude (correctly, it turns out) that this faint light is the
aggregate light of many stars to faint to be individually made out.
The interpretation of this basic observation is simple. We are living in a disk-shaped system
of
stars. When we are looking in the plane of the Milky Way, we are looking into the plane of
this disk. When we are looking far from the Milky Way, in the direction of the contellation Virgo
for example, we are looking through the ``short side'' of the disk, and therefore don't
see many stars.
Drawing on blackboard.
You can even do a little better than this with just naked eye observations without
any numbers or mathematics. The Milky Way is not uniform in its brightness. It is much brighter
in the direction of the constellation Sagittarius than it is towards Orion. The winter
Milky Way is quite faint, while the summer Milky Way is much brighter.
Point to poster.
The interpretation of this is also simple; the center of the galaxy is in the direction of Sagittarius, and we are not near the center of things. An analogy would be looking at the lights at night in a great city. If you are out in the suburbs, you see a lot of light in the direction of downtown, and relatively faint light as you look out towards the countryside.
To progress further, we need to make some measurements and use some numbers. First,
what is the thickness of this disk? You cannot define the thickness of the galactic disk in
the same way that you would define the thickness of a book. Rather, you define it like you
would the diameter of a forest. In the middle of the forest there is a high density of trees
(large number of the number of trees per acre) . As you move out from the center of the forest
the density of trees declines. When the density is zero, you say that you are completely out
of the woods.
We can do something similar with stars. We have talked about how we can measure the distances
to stars. We can look at their spectra, determine their spectral type and thus absolute magnitude,
and then from measurement of their apparent magnitude determine their distance. We can then
(at least in our minds) construct a 3D model of the stellar distribution, and see how the
density of stars (# stars per cubic parsec) depends on height above or below the plane of this
disk.
Drawing on blackboard of N(z) relation.
It is found that this thickness (technically referred to as the scale height of the
stellar density) depends on the type of star. For stars like O stars, it is only about 50
parsecs. For stars like the Sun, the scale height is 350 parsecs. These numbers are extremely
important right off the bat. When we look in the plane of the Milky Way, we can see objects
at much greater distances.
Image of chi and h Persei, distance 2300 parsecs
Image of the Rosette Nebula, distance about 1200 parsecs
This shows that the
galactic disk is very narrow (thin) compared to its diameter, because we can see objects a
couple of kiloparsecs away in the galactic plane, and we know there is more beyond.
Because of the presence of interstellar dust, we cannot see objects much more distant than a couple of kiloparsecs in the galactic plane. We certainly cannot see all the way to the galactic center in Sagittarius.
The final question has to do with the diameter of this disk. How far is it to the center, somewhere off in the direction of Sagittarius? The answer to this was a long time coming. The first value was obtained in the 1930's. I will describe the ``classic'' method of determining this distance.
There are objects in the sky called globular clusters. I have talked about
them some before. They are spectacular objects to see in small telescopes because they appear
as big balls of stars.
Image of the globular star cluster Omega Centauri
Image of the globular cluster M10
This was known for a long time, but people didn't know how far away they are because in the
1920's people weren't sure what kind of stars they were looking at.
There is an interesting thing about globular clusters, which an amateur astronomer quickly comes to realize. They are great summertime objects. Almost all of them are in the early to mid evening sky in the summer when it is pleasant to be outdoors with your small telescope. There almost none to be seen in the winter night sky when you wish you had taken up model trains as a hobby.
The physical reason for this is that they are lying in the direction of the constellation of
Sagittarius. This means most of them must be closer to the galactic center than we are.
Transparency showing globular cluster distribution, real sky and imaginary
sky in which we are at the center of the galaxy.
It was then realized that if one could measure the centroid of the globular cluster distribution, you would presumably have the location of the Milky Way galaxy. The missing piece was a way of determining the distances to these globular clusters.
This ``missing link'' came about in the 1920's with the discovery of an important property of
a kind of star called a Cepheid variable. These stars are named after the prototype of
their class, the star 
Cephei. These stars change in brightness in a periodic fashion.
They have a rhythm of their own. From fairly simple observations, in fact ones you could make
visually through a telescope, one can determine the period of variability, which ranges from
a couple of days on the short end, to perhaps one hundred days on the long end. One of the two brightest
in the star 
Aquilae, which is on your SC1 chart. It has a period of about one week, and its
magnitude range is from about 3.4 to 4.4. You can actually see the variations by observations with
the naked eye.
Diagrams about Cepheids are given on pp445 and 446 of your textbook. This much was known for quite a while. The breakthrough came in the 1920's when two Harvard astronomers, Henrietta Leavitt and Harlow Shapley, found that there was a Period- Luminosity relation for Cepheids. In words, the Period-Luminosity relation says the longer the period of variations, the brighter is the star. This makes Cepheids ideal distance indicators. From measurements of the period of the variations, you can infer the absolute magnitude. If you know the absolute magnitude of a star, you know its distance.
With this knowledge, astronomers were able to measure the distances to globular star clusters.
They are a long ways away. The brightest of the bunch in the northern hemisphere, M13 is
6400 parsecs away. The most prominent one in the whole sky is 4900 parsecs distant. The most
distant ones known are about 15000 parsecs away.
Point to poster showing globulars.
with this information, astronomers were able to determine the centroid of the
globular cluster distribution, and thus the center of the Milky Way galaxy. This distance
is about 8500 parsecs. The present day uncertainy on this number, i.e. the extent to which
the different methods disagree, is about 1000 parsecs.
This is a huge distance. To give an idea, let's go back to our scale model of the sky. Put the universe in a giant shrinking machine so 1 astronomical unit is 1cm. The nearest star is then 2.7 km away, or 1.6 miles. On this scale, the distance to the galactic center is 18500 km, or 3 Earth radii!