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The Milky Way Galaxy

If you are standing in your bedroom, you cannot take a picture of your whole house. That is the same problem we, on Earth, have had trying to see what part of the universe we are in. For many years it was assumed that the universe was everything we could see when we looked at the night sky. Some objects up there moved across the sky -- comets, meteors, even planets. Some objects only moved very slowly. Was that because the universe was circling around us, or were we moving? Through the years, both due to mathematics and then to much more sophisticated technology, we have been able to piece together, bit by bit, a lot more about what is up there in the night sky, and the place we have in it.

It was not until the 1920's with Edwin Hubble's work, that astronomers got the shock of finding out that we were not seeing what we thought we were seeing. First of all, we thought the Milky Way "system" was the whole of the universe. We were somewhere in it. Everything we saw, then, was considered part of the Milky Way. Hubble's work showed, to the astonishment of astronomers, that the spiral nebulae were at immense distances, and not part of the Milky Way at all. As they examined Hubble's work and continued it, they realized these nebulae were their own star systems.

That introduced the idea of separate "island universes," as other galaxies were first called. Since then, our knowledge has expanded exponentially regarding what is out there in the night sky. Our Milky Way Galaxy is only one of maybe billions of galaxies in the universe. But it's our home galaxy, so that is where we will start.

Let's look at what we see, and what our ancestors saw as well:

Milky Way northern hemisphere 1 Milky way northern hemisphere 2
Milky Way southern hemisphere Southern cross

Upper left: how we normally see the Milky Way in the northern hemisphere. Upper right: how it looks in the northern hemisphere on a very clear night. Lower left: the Milky Way in the southern hemisphere. Lower right: the southern cross with the Milky Way around it. In all of the pictures, the dark areas show where is a lot of dust and gas which hide the stars behind.

We have learned that what we can see when we look up at the night sky is only the stars in our local arm. We can't even see a good part of our galaxy, let alone the whole thing. The special telescopes make the below illustrations possible and, even then, we are supplying a few guesses along with our knowledge. We recently discovered an extension to the Outer Arm which was hidden before due to the fact that the center of our galaxy hides what is behind it from us.

Clouds of hydrogen gas and star clusters are localized to the spiral arms of the galaxy. They do not occur between the spiral arms. So what we can do is look for the signals from those hydrogen clouds which give signals in the radio region of the spectrum. When these are charted, we get this sort of diagram:

MW radio structure

As you can see above, we cannot discern through the center of our galaxy using this method, and so there is a 'cone' of darkness. Another method of finding out more about our own galaxy is through X-rays and infrared rays. Both allow us to see through the center into the 'dark cone.' When we plot star clusters using this method, we find this sort of thing:

Milky Way star clusters

When it is all put together, here is the conclusion about what we look like:

Milky Way structure

The red you see in the above illustration of our galaxy (this is not a photograph) is indicating where the hydrogen clouds are in red. The red marks are NOT stars.

By using X-ray and infrared telescopes as well as radio telescopes, we have been able to see a lot more of what our home galaxy looks like. The infrared and radio telescopes allow us to penetrate the dust and gas that obscures what is behind them, so we can see what is there. X-ray telescopes add additional information because some of the x-ray length waves can penetrate when others can't.

Milky Way 2

As you can see, there is a lot more to the Milky Way Galaxy than just a bunch of stars rotating around a center. Look at the diagrams below and you will see even more:

Milky Way expanded

note: a "kpc" is a kilo parsec (about 3260 light years)

There are four identifiable areas in a galaxy: the spiral arms, the galactic disk, the galactic halo, and the galactic center. The spiral arms extend out from the galactic center, or nucleus. The brightness of the disk tends to obscure them a bit.

As you can see from several of the illustrations above, there are 'arms' in our galaxy. Each of the arms has a name.

Milky Way arms

We are in the Orion Spur of the Sagittarius Arm. When we look at our night sky, on clear nights, we can see part of the Perseus Arm, part of the Sagittarius Arm, the Orion Spur, and the part of the Crux-Scutum Arm which circles close to us.

What we see in and near the cores of galaxies, including our own, are red stars. They are not the only stars there, but they are the giants which are the brightest and so they are more easily seen. The stars in the spiral arms which are the most easily seen are the blue giants. Again, they are not the only stars there, but because they are so bright and easily seen, spiral galaxies photograph like this:

cstar populations

This photograph is of the great galaxy in Andromeda (M31), The color is true.

The red giants are called Population II stars and the blue giants are the Population I stars. They are given these names because of their apparent ages: the Population II stars generally have less metal content than the Population I stars. We can tell the difference due to spectroscope analyses. Different elements have different 'bar codes' which identify what they are. We can also tell the difference in age by the ratios of parent and daughter elements which indicate radio decay. There are varying reasons for this, as the Explanation page will deal with, but all explanations agree that the older stars are the red giants.

The photograph below, of M101, in Ursa Major (the Big Bear), shows how the red giants extend part way into the spiral arms but then the blue giants take over. The two yellow lines converge on a point showing an exploding star, or supernova (which we saw explode in 2011).


There is a color range, called a "main sequence" of stars in the galaxy spiral arms. Our sun, for example, is a small yellow star -- one of millions. This main sequence looks like this:

main sequence

You will note in the above illustration the brightness of our sun is given the value of "1." This is standard. The luminosity of all other stars are measured according to their comparison to the sun. The bigger the star is, the hotter it is. So it is the blue giants which are the hottest and brightest of the stars in the spiral arms. There are many more of the small red and yellow stars than the blue giants, but they are much fainter and so we don't see them well. These are the younger, or Population I stars. Interestingly, this sequence of colors in the stars is the same sequence of colors you get as you heat an iron bar. The coolest is a dull red and by the time you get to blue, you are immensely hotter.

The Population II stars, the old stars, form their own sequence.


In the above diagram, the temperature decreases as you go to the right (the same as the main sequence graph of spiral arm stars), so the coolest stars are the red stars in the upper right hand corner. But those red giants are also the giant stars and the brightest (unlike in the spiral arms). The brightness, or luminosity increases as you go up the graph.

Altogether there are about 150 billion stars in our galaxy (give or take a few).

We have found planets of various sizes and types circling some of these stars. Both the stars and the exoplanets have their own pages.

In addition, there are the nebulas: reflection nebulas, emission nebulas, absorption nebulas, planetary nebulas, supernova remnants, and molecular clouds.

What is a nebula? It is a cloud of gas and dust. The way the light is handled by these clouds determines their identification.

Reflection nebula -- They reflect the light of the stars that are in and around them. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own.


The Pleiades above show reflection nebula. Right is the central part of the Orion nebula

reflection nebula

Emission Nebula -- They absorb the light from the stars around them and then re-emit it at a variety of other wavelengths. M8 (the Lagoon Nebula in Sagittarius), below, is a good example of this.


Absorption nebula -- These absorb the light from around and behind them and appear dark. The famous Horsehead Nebula in Orion is a good example. The base of the 'head' is part of the absorption nebula, showing up dark.

horsehead nebula

Planetary nebula -- Some nebula appear spherical, and in a small telescope, look like a distant planet. They are actually the gas and dust in a plasma filament around a star.

Lyra nebula Lyra nebula 2
dumbbell nebula spirograph nebula

The upper right nebula is the same as shown in the upper left diagram. It is the Ring Nebula in Lyra (M57). The lower left nebula is called the Dumbbell Nebula (M27) because in the middle of the circle it has the appearance of the two lobes of a dumbbell. It is in the constellation Vulpecula, not far from the constellation of Cygnus as we look at the night sky. The lower right is the lovely Spirograph Nebula in the constellation Lepus, near Orion.

Supernova remnants -- A supernova is a star that has exploded. The debris sent out from the explosion is made up of dust and gas. They both emit and reflect light.

Crab nebula Veil nebula

Above are the Crab nebula (M1) in Taurus on the left, and on the right is the Veil nebula in Cygnus. The Crab nebula was created in 1054 AD when the star was seen to explode (on July 4!). It was visible in daylight for two years. The Veil nebula is the result of an explosion much further back in the past; you can see the debris has spread out into wispy filaments.

Molecular Clouds -- these nebula are composed of molecules of hydrogen, helium and other gases, with a high percentage of dust grains. The other nebula are all plasmas; molecular clouds are not.

Rho_Ophiuchus Eagle nebula

On the left, in true color, is the Rho Ophiuchi Complex near the star Antares in Scorpio. On the right is part of the well-known Eagle Nebula, M16, showing the pillars in false color.

Galactic Disk -- As you move towards the center of the galaxy, the spiral arms are still evident, but there is a flat disk of old Population II stars. The area where the red giants are prominent define the galactic disk. The spiral arms, with the blue giants and other stars are still there, but they are obscured by the brightness of the red giants.

star populations

Galactic Halo -- The halo is made up of globular clusters above and below the disk itself:

Milky Way expanded

A globular cluster is a collection of up to ten million stars in a spherical group from about 100-500 light years across. Some of these clusters can be quite spectacular. The most prominent one in the northern hemisphere is M13:


There are about a million stars in the above cluster. The cluster itself is about 200 light years across.

The most spectacular globular cluster in the southern hemisphere is Omega Centauri:

Omega Centauri

Unlike most globular clusters, Omega Centauri is plainly visible to the naked eye. It contains about ten million stars and is about 500 light years across.

There are about 200 globular clusters in the halo of the Milky Way Galaxy.

The Galactic Nucleus -- Regardless of what it is called, or what we think it is, here is what we see:

Looking from Earth, if we look in the direction of the constellation Sagittarius, we are looking toward the center of the Milky Way. We see a mass of stars and hydrogen clouds and dust. We can see through the dust and clouds using infrared photography and radio waves. What we become aware of at this point is that the very center of everything in the nucleus is an object called Sagittarius A* -- often referred to as Sag A*. We can see stars orbiting this object at a very fast rate. In the combined infrared and radio photograph below, we can see the center of our galaxy. Sag A* is somewhere in the yellow middle in this one.

galactic center

The radio wave photograph below includes about 300 light years from side to side. Sag A* is in the white section (it is not the white section itself.)


X-ray and gamma rays we have measured have given us this view of emissions from the core of our galaxy. The first photograph is of the areas of the x-rays and gamma rays. However the second photograph shows what are referred to as gamma ray bursts. The gamma rays are not continuously emitted, but come in bursts, leaving their signatures in the 'purple' globes, or bubbles.


x-ray lobes

gamma ray blasts

This is all we actually see of our galaxy and its center. What various scientists think about these things can be found in the Explanations and Interpretations.

This photograph of the Milky Way was taken in Australia by Greg Gibbs. It's really quite an incredible shot:

Milky Way


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