This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The Milky Way Galaxy

by Dr Jamie Love Creative Commons Licence 1997 - 2011

We started these lessons with information about far off individual stars and last month you learned about objects so close to us that some of them (meteorites) actually reach the Earth! Now we are changing our attention to objects that are very distant - except for the one we are in!

Galaxies. The universe is made up of galaxies and we are part of a galaxy - the Milky Way Galaxy.

What are galaxies?

Galaxies (singular, galaxy) are a system of stars but much larger and more distant that a star cluster.

You will recall that star clusters are groups of stars that have condensed from the same nebula so they "hang around" each other due to their mutual gravitational attraction (although open clusters disperse over time). The largest star cluster may have up to a million stars but even these giant clusters don't hold a candle to a galaxy.

Our galaxy is pretty typical and it contains about 200 billion stars!
[Note: throughout this lesson please assume that the numbers given are a rough estimate in order to give you an appreciation of the scales involved. Each textbook gives a different value for the number of stars in our galaxy. One book says there are "only" 100 billion stars in our galaxy. Both the number of stars and the size of our galaxy - coming up in a moment - can differ by a factor of two between different authorities. This is because of the way they do the counting, define the edges, etc. Don't worry if you later read of some other size or number. Just stick with the general idea that our galaxy is big! ]
Some galaxies are much smaller. It is convenient to call groups of less than a million stars a "cluster" and those with over a million stars a "galaxy". That's a fair definition, but there is another, more important, difference between clusters and galaxies. Galaxies are very remote and often have clusters of their own. Indeed most of the star clusters you are likely to hear about or see are actually in our own galaxy.

As you studied the night sky during your lessons you may have noticed, on a particularly clear, moonless night, a dim band of light stretching across the entire sky. The Ancient Greeks called this "galaxias kuklo" which translates to "milky circle". They thought it was a kind of magical glowing fluid that was stuck in the Celestial Sphere. They were wrong but the name has stuck. The word "galaxy" is from the Greek "galaxias" meaning "milky".

Today we call this band the "Milky Way" which is a shame because, as you will see shortly, it is a circle so "Milky Circle" would have been more descriptive. Regardless, people have been enjoying its beauty for as long as there have been people. In 1610 Galileo Galilei used his newly invented telescope to discover that this glow is caused by billions of stars. Each star is so far away that it doesn't appear as a star unless viewed through a telescope. Dust and gases tend to block some of the light but a small amount reaches us.
[In my drawings, below, I use dots to represent the stars of the Milky Way but, in fact, these tiny dots would all be part of a more general glow. I have resorted to this type of "dot representation" in order to keep the file size of these images from getting too large! In reality, galaxies have a more gentle, fluffy appearance.]
Eventually, and after much debate, astronomers figured out that the Milky Way was actually our own galaxy which they called "The Milky Way Galaxy". Because most of our astronomy (especially for amateurs) involves only stars in our own galaxy, we tend to refer to our galaxy as simply "the Galaxy" and reserve the word "Milky Way" for the glow we see. But, strictly speaking, we live in the Milky Way Galaxy.

Where is this glow? Is it easy to find?

Yes, it's very easy to find because it's always in the night sky. Unfortunately, you have to have a good clear moonless night to see the Milky Way and it helps to know where to look for it. I found that the summer and autumn months are particularly good for viewing the Milky Way and that is especially true because you know how to find SAGITTARIUS.

You will recall (from last month's lesson) that SAGITTARIUS follows SCORPIUS. Both of them are easy to find. SAGITTARIUS is supposed to look like an archer (yeah, right! ) but to me it looks like a teapot.
If you look carefully at the teapot you will see "steam" rising out of it!
(Adjust the brightness and contrast on your computer screen if you cannot see the glow in this image.)

The "steam" rises out of the top of the teapot and into AQUILA (brightest star, Altair).
It passes through the Summer Triangle covering much of CYGNUS (brightest star, Deneb) in its glow. But it doesn't stop there.
It continues "upwards" through CASSIOPEIA, through PERSEUS (brightest star, Mirphak) and then dips below the horizon.

If you could see though the Earth, or stay up all night, you would see that the Milky Way extends into AURIGA (brightest star, Capella), clips the top of TAURUS's horns, slides past ORION and then dips down into some southern constellations you have not learned and cannot see from the Northern Hemisphere. However, it eventually works its way back up and comes up around SCORPIUS and back to SAGITTARIUS.

The Milky Way is a band that circles the entire night sky!

I don't get it. Why is it shaped like that? And where are we in it?

I can understand your confusion and the questions you ask have been asked for centuries. Indeed, it is difficult figuring out the shape of our galaxy because we are in it! It's like trying to draw a map of your neighborhood having seen it only from your bedroom window. Your neighborhood and the galaxy have depth and hidden portions that cannot be seen from a single vantage point.

Two centuries ago an astronomer named Herschel used a (48 inch reflecting) telescope to survey the Milky Way and the adjacent sky. He found that there were many more stars along the Milky Way than away from it so he concluded that our galaxy is kind of flat. He also noted that along the Milky Way, no matter which part of it he looked at, there were roughly the same number of stars - so he concluded that we were in the center of the galaxy. Herschel argued that if we were on the edge, or even off-center, one part of the Milky Way should have more stars than the other. Because there appears to be equal densities of stars in all parts of the Milky Way, Herschel argued, we must be seeing the galaxy from its center.

That view of our galaxy and our position in it was accepted for a long time. Then, in 1917 Harlow Shapley noted that the globular clusters around our galaxy seem to be in and around the constellation of SAGITTARIUS. (Perhaps you will recall that I told you there were lots of interesting objects in SAGITTARIUS.) He argued that if we were in the center of the Galaxy we should see the same number of clusters in all directions. Instead, we see that most of the clusters are in the direction of SAGITTARIUS so he figured that the Sun was not at the center of the Galaxy. Indeed, the cluster data indicated that the center of the Galaxy was somewhere around SAGITTARIUS. Shapley's simple observation changed our view of our place in the Galaxy. We aren't in the middle!
Shapely didn't stop there. Using variable stars and a technique I will explain to you in November, Shapely estimated we are 54,000 light-years from the center of our galaxy.

But what about Herschel's data?

Herschel did not take into account all the dust in space.

The interstellar dust, or "dust between the stars", is spread thinly throughout the Galaxy. These dust particles are not really the material you think of as dust. Interstellar dust is actually small clumps of frozen water, ammonia and other simple compounds. These particles are about the size of the wavelength of visible light so they absorb and scatter visible light causing distant stars to appear fainter or even hiding them completely. The farther away a star the more dust will be between it and us.

Herschel's mistake was to ignore the dust. When he looked towards the center of the Galaxy most of the stars were hidden by the dust because the distances to those stars are so great that there is a lot of space for the intervening dust. So he under counted the stars in the direction of the center of the Galaxy. When he looked away from the center of the Galaxy he saw the closer stars and there was little dust to hide them. The overall affect was to mistakenly believe that there were equal numbers of stars in all parts of the Milky Way and to conclude that we are at the center.

Shapely also didn't account for the intervening dust. It turns out that his measurements of the distances to those clusters was off. (The details are a long story, but it has to do with the interstellar dust.) We are not 54,000 light-years from the galactic center - we are about half that far from it.

DUST!!!
It's an astronomer's nightmare.
But it's also important. A particularly dense cloud of dust causes a "rift" in the Milky Way between CYGNUS and AQUILA. Have a look at that area of the Milky Way on a particularly clear night and you will see a blank patch where the glow seems to have been sliced into two sections. Dust distorts our ability to see the Milky Way clearly.

Fortunately, radio waves are not affected by dust, because radio waves have much longer wavelengths, so our galaxy can be "seen" better using radiotelescopes. Radioastronomy (astronomy using radio waves) not only allows us to see through the dust - it allows us to "see" the interstellar hydrogen! In 1944 an astronomer in Holland, by the name of van de Hulst, predicted that interstellar hydrogen should emit radio waves exactly 21.1 centimeters in wavelength. How and why that happens is beyond the scope of this course but the nice thing about 21.1 centimeters wavelengths is that they are about 100,000 times longer than visible light and completely unaffected by dust. Armed with this information radioastronomers mapped the positions, and even the velocities, of hydrogen clouds. Naturally, they could only do this from Earth, but when they were done they were able to produce a three dimensional map of the hydrogen.

Their map showed that the hydrogen accumulated in the center of the Galaxy and in a ring about a third of the galaxy's radius out from the center.
This is followed by four spiral arms and produces a galaxy about 100,000 light-years across or 50,000 light-years in radius.
We are in one of the arms about 30,000 light-years from the center. SAGITTARIUS is slightly "inward" (more towards the center) from our position.

More recently, radioastronomers collected data from other wavelengths representing ionized hydrogen and molecules. Their maps are very similar to the one made using neutral hydrogen. That is not a great surprise. All of these (neutral hydrogen, ionised hydrogen and molecules) are mass and it is mass that makes stars. Indeed, wherever you find accumulations of mass you find stars!

By the way, my drawing is very crude. The Galaxy is much fluffier. And prettier!
Individual stars would be impossible to see at this scale. All the stars, collectively, should appear as a blob of glowing light.
Every star you see in the night sky with your naked eye would be within a sphere only a couple millimeters in diameter in this drawing - centered around the Sun (the "you are here" point). Galactic scales are enormous.

Our Sun and Solar System are orbiting the galactic center, at a speed of about 250 kilometers per second! We complete one orbit of the Galaxy in about 220 million years, so the last time our Solar System was in its current position, dinosaurs ruled the Earth! Our Sun and its planets have gone around the Galaxy about 20 times since they formed from the accretion disk four and a half billion years ago. We don't notice this motion but it is there and it is real. When you take into account this motion we, and all the stars in our neighborhood, are rotating in the direction of 21 hours 12 minutes right ascension and +48o19' declination which is roughly in the direction of CYGNUS. So, the next time you look at the Swan imagine that we (and the Swan and every star you see with your naked eye) are speeding along in that direction at 250 kilometers per second. (With a little imagination you can feel the wind in your face! )
If you scroll up to the image of the galaxy that I showed you before, you can get oriented. SAGITTARIUS is slightly "inward" (up from the Sun in the drawing). In my drawing the Galaxy rotates counter clockwise and CYGNUS is slightly to the right of the Sun.

Of course the Galaxy has a thickness to it that determines it's true three dimensional shape.

The center of the disk is swollen to a thickness of about 15,000 light-years but the thickness tapers away to a few thousand light-years in our neighborhood and even thinner farther out.

Think back to the stars you have learned about in your lessons. Most of them have been within a few hundred light-years of us.

The center of the Galaxy is shaped like a flattened sphere with the rest spread out as a disk. Some folks compare it to the shape of two fried eggs stuck bottom to bottom with the yolk as the center and us somewhere in the white of the egg.

(The individual points of light above and below the disk are star clusters. Notice how, if viewed from our position, most of the star clusters would be in or near SAGITTARIUS.)

When you take into account the direction in which we are rotating, the Galaxy's "North Pole" is at the point on our Celestial Sphere of 21 hours 12 minutes right ascension and + 27o declination. That's roughly between BÖÖTES and LEO.

Why is the Milky Way at such a strange angle to the rest of the sky? It doesn't follow the ecliptic or make any sense.

That's because the plane of the Galaxy is not the same as our orbital plane. You will recall that our orbital plane, like that of all the Sun's planets, is derived from the plane of the accretion disk from which the Solar System was created billions of years ago. That accretion plane had nothing to do with the plane of the Galaxy. As a matter of fact, the Galaxy is rotating in a direction almost opposite the direction of rotation of the planets in our Solar System. Or, more correctly, our Solar System is travelling through the Galaxy upside down! If that worries you, or you have trouble imagining it, don't worry. It's just another of the complications of living in a three dimensional space.

This brings to three the total number of planes an amateur astronomer should understand.

  1. The celestial plane is the imaginary plane you get if you were to project the Earth's Equator into the sky. In fact, this plane is really derived from the Earth's rotational axis (90o from the equator).
    The declination of a star is based upon this system of celestial coordinates, so it is the most important plane to understand.

  2. The Earth's orbital plane (ecliptic) is the plane the Earth makes as it orbits the Sun. We have seasons because the Earth's axis of rotation is tilted 23.5o away from the orbital plane's perpendicular direction.

  3. The galactic plane is the plane of our Galaxy and it is seen as the Milky Way. It's obvious when looking at the Milky Way that our Galactic plane is very tilted from the Earth's axis. The good news is that this plane has nothing to do with star coordinates or seasons. In fact, all it does is explain the position of the Milky Way.

Yeah, OK. So which way to the center of the Galaxy?

The center of our galaxy is at 17 hours 45 minutes right ascension and -28o 56' declination. Or, to put that into more meaningful terms, just look towards the spout of the teapot! The center of our galaxy is just to the edge of the teapot's spout - almost on the edge of the scorpion's tail.

Using radiotelescopes astronomers have found a powerful radiosource in SAGITTARIUS that they call "Sagittarius A *". (You pronounce "*" as "star" even though there isn't a star there! ) The huge amount of energy and the powerful gravitational force calculated to be at Sagittarius A* (about 2 million solar masses) makes it very likely that it's a black hole. We, and every star we see, are (probably) orbiting a black hole at the center of the Galaxy! It "eats" nearby stars and as it eats them, Sagittarius A* produces powerful X-rays and other energy. Calculations based upon its energy output suggest that the black hole at the center of our Galaxy needs one Sun-sized star every 4000 years to keep it fed.

There's a lot of mass around that black hole. Within a few light-years of the center there are millions of stars. That is an amazingly high density of stars. The average distance between stars near the Galactic center is about 1000 AUs (Astronomical Units - the distance from the Earth to the Sun). If we lived near the Galactic center the starlight at "night" would be about as bright as a cloudy day (on Earth)!

Of course, astronomers have used some pretty sophisticated techniques in recent times to get a better picture of our galaxy.

One particularly fruitful project has been the COsmic Background Explorer (COBE) satellite that mapped our Galaxy at near-infrared wavelengths.

When the data were assembled by computer this panoramic view of the Milky Way Galaxy was created - as if seen from a location outside the galaxy. Of course, this is just a "trick". We really couldn't get a satellite out there!

This image is courtesy of NASA & the COsmic Background Explorer (COBE) Project.

You can see from this image that there is more to our galaxy than a flat spiral shape. The entire Galaxy has a spherical "halo" made of very old stars in globular clusters. (Remember, you cannot see individual stars in this image. What you see are clusters of many stars). Stars in these outer globular clusters are very old and often called "Population II" stars to distinguish them from the younger "Population I" stars in the arms of the Galaxy. But I'm getting ahead of myself. We will return to the subject of Population I and II stars next month.

Are there other galaxies?

Oh, yes. Billions of them.

If you like you can continue on to the next lesson to learn about some of our neighboring galaxies.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.