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

Galaxies Structure, Evolution and Dynamics

by Dr Jamie Love Creative Commons Licence 1997 - 2011

How did the Milky Way Galaxy form?

Good question.
No one was around to see how our galaxy came about so we have to find clues to its formation. Those clues come from details about the structure of the Milky Way Galaxy including its distribution of elements, comparisons with other galaxies, and computer simulations based upon well-understood laws of gravity and energy distribution.

It's best to think of our galaxy, and any disk galaxy, as being made of three parts -

  1. a flat, rotating disk
  2. a spherical, very slowly rotating central bulge
  3. a distant halo of star clusters.
Analysis of the elements present in these parts of the Galaxy causes us to conclude that the disk is made of Population I stars while the central bulge and halo are made of Population II stars.

Population I and II? What are you talking about?

Our Sun is a Population I star. (If you have been paying attention, that should make sense. We are in the disk of the Galaxy and I just said that Population I stars inhabit the disk.) All Population I stars are rich in heavy elements - those elements with more than two protons, so the heavy elements are all elements except hydrogen and helium. You've heard me go on (and on) about how we use spectroscopy to determine a star's composition so I won't go into it again. As you know, these heavy elements are produced by nuclear fusion so we would not expect those elements in the photosphere or chromosphere of a star unless it had collected those elements from elsewhere.
Of course, you know how those heavy elements got into our Sun because I told you how our Solar System formed. Our Solar System, including our Sun, is the product of the planetary ring nebulas produced by dying giants and the remnants of novas and supernovas. These materials were drawn together by their mutual gravitational attraction and formed nebulas which eventually condensed into our Solar System. Our Sun and all other Population I stars are "second generation stars" spiked with the heavy elements of previous stars. There are Population I stars forming right now and not too far away from here. Remember that the Orion Nebula and the Pleiades (to name two) are "star nurseries" where new stars are being born. This new star formation goes on a lot out here in the arms of our galaxy. Remember, the disk of spiral (and barred spiral) galaxies have varying amounts of gas and dust. Well, that gas and dust were produced by planetary nebula over the billions of years.
This means Population I stars can be as young as 0 years (the ones being made today) to over 10 billion years (the ones that were made after planetary nebulas had been produced). The youngest Population I stars are the ones richest in heavy elements. Indeed, when you think about it you realize that we can use the abundance of heavy elements in a star's spectrum to infer its age!

Population II stars are very old stars mostly red in color but not giants. Indeed, they are probably red dwarfs and they haven't changed much since they formed billions of years ago. (Remember, red dwarfs age very slowly.) Some Population II stars may be nearly as old as the universe itself! Among a group of Populations II stars you will not find any young stars or any signs of star formation - not even dust or gas nebulas. More importantly - Population II stars have absorption lines strong in hydrogen and helium (the two light elements) but very weak absorption lines for the heavy elements.
That means Population II stars formed very early in the history of the galaxy before the formation of planetary nebulas. Therefore Population II stars are the oldest stars in a galaxy! They could be as old as 15 billion years (the age of the universe) but no younger than 10 billion years (which is assumed to be a good age for a galaxy before it starts to get cluttered with old and then dead star pieces that make planetary nebulas rich in heavy elements).
Of course, even these stars may have very small amounts of heavy elements from recent nebulas that swept through the neighborhood AFTER the Population II stars had formed. It turns out that Population II stars in the bulge of our galaxy have slightly more heavy element "contamination" than Population II stars in the halo.

This diagram may help you to review what you have learned.

Population I stars are found in the disk of our Galaxy and are formed, at least in part, from the debris of old planetary nebula. They are second generation stars. (Maybe third generation or more, but we don't know about that.) Astronomers have decided to define Population I stars as those with heavy elements making up more than 2% of their chromospheres (atmospheres).

Population II stars are found in the central bulge and halo of our Galaxy and have been around for over 10 billion years. They are first generation stars. Heavy elements make up less than 0.8% of the chromosphere of Population II stars, with stars in the bulge being richer in the heavy elements than stars in the halo.

Before you ask - the range between 0.8% and 2.0% heavy element abundances is where we find arguments among the astronomers. Stars with heavy elements in this range cannot be clearly assigned as belonging to one population or the other, although their location could resolve the conflict.

We don't have any Population II stars in our neighborhood?

Yes, that's right. Out here in the disk are nothing but the (young) Population I stars.
The Population II stars in the center of our Galaxy (which, you will recall is in the direction of SAGITTARIUS) are pretty far away - 30,000 light-year. The distance isn't really the problem - it's all that intervening dust in the way. You cannot see the Population II stars in the Galaxy's central bulge because of the dust.

Globular clusters are your best chance of seeing Population II stars. These are the clusters that make up the halo of our galaxy. You may remember I introduced you to star clusters several months ago. The open star clusters, like the Pleiades, are Population I stars in the disk and I told you all about them because they are the home to the really young stars - all Population I stars, and now you know why! However, I said little about the globular clusters other than to explain that they were pretty far away. Now you know why. They are in the halo. Over a hundred globular clusters have been found in (around) our Galaxy. Messier found 28 of them.

I told you how to find M3 as an example of a globular cluster. This beautiful cluster can be found easily with binoculars but it will take a telescope with an aperture larger than 8 centimeters to actually make out some of its stars. All of them are Population II stars.

M3 is 48,000 light-years away, farther away then the center of our Galaxy, but there is very little dust in the way because it is away from (above or below) the disk of the Milky Way.

Our closest globular cluster is a mere 17,000 light-years away but it's in the constellation of CENTAURI so you have to be very far south (further south than the USA or Europe) to see it.

The brightest globular cluster in the Northern Celestial Hemisphere is M13, in HERCULES. During the May lessons I gave you directions on how to find M13. It's about 22,500 light-years away with a diameter between 100 and 160 light-years, depending on how you define its edge. Binoculars give a good view but a small telescope will help you to resolve the individual Population II stars.

M3 is circumpolar for most people in the Northern Hemisphere so you should be able to find it year round. M13 will be visible in the western sky this month. Make a point of going out and finding both of them (again) and look at them in a new light. They are made of very old Population II stars located in the halo of our Galaxy!

I'll look for them.
How did the Galaxy get this distribution of Population I and II stars?

Astronomers have pieced together a plausible story of our Milky Way based upon the evidence you've just learned. It explains the current properties of our Galaxy. "Once upon a time ....

Many billions of years ago our "protogalaxy" was nothing more than a slowly rotating cloud of hydrogen and helium (produced in the "Big Bang" which I will tell you about in December). This idea of a slowly rotating cloud may sound familiar because that's how all stars start out, but the protogalaxy cloud was about 300,000 light-years across and devoid of heavy elements. Gravitational attraction caused the cloud to slowly collapse or condense in localized areas.
The first stars to form were in isolated clusters in what has become the halo. Of course, these stars were (are) without any heavy elements because there were no planetary nebulas around during their formation. At this time the young Galaxy was still pretty spherical and disorganized. It didn't have a disk yet. It was also rotating pretty slowly. So, these star clusters took up disordered orbits around the central mass but with no relationship to the position that the yet-to-be-formed disk would have. (Like the Oort Cloud, but made up only of hydrogen and helium.) Those stars that were of low mass are still out there today in the halo as Population II stars. The high mass stars that formed at the same time have since exploded as nova and supernova and enriched the collapsing cloud with heavy elements. Many of the stars in the most disorganized orbits ended up getting captured in new orbits much closer to the center of mass - the central bulge of the Galaxy.
As time went by the cloud continued its collapse into a structure similar to the accretion disk that formed our Solar System. This disk of material contained the heavy elements produced by large mass stars in the halo that had exploded. These materials in the disk went on to form the Population I stars so these stars are rich in heavy elements (made by the halo stars as well as any disk stars that have gone nova). Also, by this time, the disk had collapsed into a dense flat structure so the orbits of these new stars were orderly and nearly circular.

But that means Population II stars came BEFORE Population I stars!

That's right. (Confusing ain't it. )

If our Sun were a Population II star we would not be here because Population II stars would only have light elements as part of their environment and you can't build worlds out of them! Dust and gas in the disk make this part of the galaxy the place where Population I stars are constantly being formed and the most likely place to look for planets.
So a spiral galaxy (like our own) or a barred spiral galaxy has a halo and a core of old, metal-poor Population II stars while its disk is made of dust, gas and recycled Population I stars full of metals.

What keeps those old clusters from collapsing into a single, massive star?

No one knows for sure. Some astronomers believe that globular clusters have complex gravitational interactions among their own stars that keeps them from collapsing into the center of the cluster while other astronomers insist that the collapses occur but that it takes a great deal of time to happen.

By the way, there is a slightly different theory to explain the lack of dust and debris in the halo populations. Remember, stars are not stationary and even those out in the halo are attracted by gravity to the core of the galaxy, where most of the mass is located. However, these stars and clusters do not fall into the center for the same reason that stars in the disk do not fall to the center - they are in orbit. The difference is that stars in the disk all orbit near a similar plane.
Stars in the halo must also orbit the center of the galaxy but they do that at extreme inclinations to the disk. At some time those stars must pass through the disk - twice per orbital period. Some astronomers have suggested that as a cluster of halo stars passes through the disk they are "cleansed" of any loose debris by the disk stars. That is, the mass of the disk stars simply tug the debris away from the halo stars as they pass through the disk. This would explain why Population II stars are poor in the heavy elements produced by old, dead stars. The disk scavenges up the heavy elements.

Ah, again, please.

OK. Imagine these halo clusters started out just like any group of stars except in orbits inclined from the plane of the galaxy's disk. Some were small and aged slowly while others were large and aged fast. The large, fast aging stars would go nova and produce clouds of debris containing heavy elements. Normally, those materials would eventually condense again, attracted by their mutual gravity, and form new stars - Population I stars like our Sun. However, the halo stars are "cleansed" of any loose debris by passing though the disk. That means these halo clusters cannot recycle their materials into new, young stars. Only stars in the disk can have generation after generation of stars because only they are able to retain their old materials after going nova. After (say) 10 billion years the only stars left in the halo will be those that were so small and aged so slowly that they never gave up their original materials. What we are left with, after all this stellar evolution, is a population of stars in the disk (Population I stars) representing a variety of ages because they recycle their "dead" and a population of stars in the halo (Population II stars) which have never "died" in the first place. (The debris from those halo stars that that did die ended up in the disk and was recycled by the Population I stars.)
It makes sense (to me ).

Why isn't the Galaxy a complete disk? Why does it have spiral arms?

Like many things the short answer is "we aren't completely sure", but here is what we think is going on.

Stars far away from the center of the disk should slowly orbit the galaxy's center (like a stream of water, but in a circle). Stars closer to the center of the galaxy should orbit faster. This for the same reason planets closer to the Sun orbit it more quickly than more distant planets - recall Kepler's third law.
However, careful observations and measurements prove that most of the stars in spiral galaxies (including our own) move at a speed independent of their distance from the center! How can that be? Well, astronomers have good evidence that there is hidden mass in our galaxy and many other galaxies. They call this material "dark matter" because it does not shine. Dark matter is (speculated to be) a mix of brown dwarfs and gas giants (too small to for nuclear "ignition"), dead stars (white dwarfs, neutron stars and black holes) and clumps of hydrogen. This material makes the galaxy a lot bigger and more massive than it looks, spreading out the gravitational forces such that the expected slowness at the edges does not occur.
It takes a lot of matter to do this. The dark matter I mentioned, along with the stars we see, have been calculated as accounting for only 10% of the mass! Most astronomers consider the missing 90% to be an exotic "non-baryonic" matter but this is not a course in advanced (and theoretical) particle physics so I will not discuss it further.

Most of the stars in our galaxy move at a speed of about 200 kilometers per second. Of course, the ones with shorter orbits, those closer to the center, complete their orbits more quickly than those on the edge because they have less distance to travel before they return to their position. Our Sun and our neighboring stars take about 200 million years to orbit the Milky Way. The gravitational effects of this complicated race around the center might influence the shape of the galaxy.

Many astronomers believe that the spiral arms are shaped by density waves. These waves do not actually move the stars or dust in the arms (at least not very far). Instead they briefly compress the materials. If you understand how sound waves travel, you can understand this phenomenon but don't take the analogy too far. Sound waves travel by passing their energy along in collisions with their neighboring molecules but density waves (in space) travel by a series of gravitational pushing and pulling that compresses and expands the local materials.

As the gas is compressed it heats up. These materials become ionized by the compression and, if there is a large density of it in a tight ball of gravity, the density waves may provide the trigger that "ignites" nuclear fusion and thus gives birth to a star!
These density waves move at different speeds depending upon their distance from the galactic center. Waves near the center orbit more quickly than those further away and the effect is to produce a spiral zone of compression. Keep in mind that these density waves are NOT in the same position as the current arm pattern. That pattern is created by many successive waves of compression passing by. It has to do with resonance (the frequency of an event occurring at a point) and reiteration (the repeating of a pattern). It's too complicated to get into and too complicated for me to explain well. Try to think of it as similar to the complicated process that creates ripples of sand on a beach. Our Galaxy's spiral arms are those ripples arranged in a circle!

Of course, this story of how the Galaxy was formed is open to interpretation and ridicule but the scenario above is a likely explanation for the origin of our Galaxy and other disk galaxies. However it does not explain how other types of galaxies formed or the properties of other types of galaxies.
Population I stars are not found in elliptical galaxies (or they are so rare there that they haven't been found) but they are the dominant type of stars in irregular galaxies. Indeed, irregular galaxies have many of the opposite features of elliptical galaxies. Irregular galaxies have more gas and dust than any other kind of galaxy and they are made mostly of Population I stars (although a few Population II stars are found there too).

It seems that irregular galaxies are full of Population I stars because they are full of planetary nebula and that means they are retaining their "corpses" for recycling. We see a great deal of star formation occurring in irregular galaxies, probably because they have so much material for making stars. But why haven't they settled down into a disk? Maybe it's because they have been disturbed by interacting with other galaxies.
"Galaxy interactions" is a hot topic in astronomy. It's complicated and still not well understood but this topic might provide us with an answer to the origins of irregular and elliptical galaxies as well as the fate of our own.

The universe is a big place but it's full of big galaxies. Galaxies are usually found in clusters like our Local Group of galaxies. Most galaxies are bound to other galaxies by their mutual gravity. These groups are called clusters. Our Local Group is considered a particularly poor cluster because it has only a few dozen galaxies. The Virgo Cluster is an exceptionally rich cluster containing over 2500 galaxies!
Galaxies in clusters are often separated by distances of about 20 times their diameters so you might imagine they would be attracted by each other's gravity. You would be right. Powerful telescopes have found galaxies undergoing gravitational interactions, collisions and mergers!

You must understand that most of the volume (space) a galaxy takes up is just empty space. So when two galaxies collide it isn't like two objects striking each other. Instead, galaxies can pass through each other with very few collisions among their stars. Remember how I asked you to visualize the distance between two stars as like a pair of ping-pong balls separated by a thousand kilometers?

Most of these galactic "collisions" are like a ghost passing through a mist. They collide but keep on going although they might exchange some dust, gas and maybe even a few stars. These interactions can leave "trails" of dust or stars connecting the two galaxies for millions of years until their separation is complete. Because these bridges are caused by a gravitational connection, rather than an actual bridge, we refer to these as a tidal bridge of stars.


[Image credit: 1.1 Meter Hall Telescope, Lowell Observatory, Bill Keel (U. Alabama)]
The Whirlpool Galaxy (M51), shown on the left, is connected by a tidal bridge to its small neighbor below it but you cannot see the bridge very well in this image. (Astrophotography is always a compromise. Here I want you too see the blue colors for reasons that will be explained shortly.) This is the result of a high-speed encounter in which the galaxies just grazed each other.

A dead-center collision, where one galaxy passes through the center of another, is a rarer event but it happens.
The Cartwheel Galaxy (on the right) is an example of such a collision. (Pretty, isn't it! )


[Image credit: Kirk Borne (STScI) & NASA]

When collisions occur the excessive amounts of gas and dust, along with the increased density and the heat of collision, cause a burst of new star formation to occur. The large number of new, BIG stars creates areas of blue produced by the O and B type stars (big, hot stars) newly formed at that position. Remember, these bright, blue stars won't last long because they burn hot and fast. That means astronomers can use these blue patches as evidence that the collision occurred recently.

Sometimes two galaxies collide at lower speeds and that allows them to become trapped by each other's gravity. They will fall back into each other and eventually merge to form a new, larger galaxy. Large galaxies often cobble up their neighbors and this behavior is called galactic cannibalism. (I'm not making this up! )
(Recall, I told you that the Milky Way and Andromeda galaxies could merge into a giant galaxy.)

Here on the right you see the Antennae galaxies. They have been caught in the act of merging or cannibalizing each other. You can see how the tidal forces have torn the spirals into curved shapes. Also, notice all the telltale signs of new star formation - blue areas.

When large galaxies merge, their large mass and different angles of merger lead them to eventually settle down in an ellipsoid object. This is how (we suspect) elliptical galaxies form, but where all the Population I stars have gone is still unclear.


[Image credit: Ohio State Galaxy Survey Project]

Rich galaxy clusters often have many elliptical galaxies but few spiral disk galaxies because most of them have been merged into elliptical galaxies during low-speed collisions. Careful analysis of the area BETWEEN these elliptical galaxies show it has plenty of intergalactic gas, so we assume this has been stripped from the galaxies during the collisions(s) leaving clean Population II stars to form. (But this doesn't completely explain the lack of Population I stars in elliptical galaxies.) Near the center of a rich galactic cluster there is often a giant elliptical galaxy, and you can just image how it got that way! [One fat cannibal! ]

There are many galaxies and many kinds of galaxies. Hubble's system for classifying them is a convenient and useful way for astronomers to understand the different types. We are still learning about galactic evolution and dynamics so some of this information may change as we learn more - but most of it is probably correct. There is a lot to learn!
You need a pretty good telescope to take part in this kind of galactic astronomy but every amateur astronomer understands this information that I've presented here in this lesson. Also, an amateur astronomer with an amateur telescope can enjoy trying to find all the Messier objects.
Next month I'll teach you about some very unusual galaxies (as well as some other things) so be sure you understand this lesson before moving on to next month's materials.



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