The image above shows a representation of our Milky Way galaxy as we think it appears. The blue region indicates the range of stars visible with the naked eye, and the the white dots represent the location of the 118,200 stars precisely measured by the Hipparcos spacecraft. The points represent the best data we have on stars in our galaxy. Since Hipparcos there has been a less precise Tycho catalog of about 2.5 million stars, and the recently launched Gaia spacecraft will precisely map about a billion stars. But even Gaia will map a small fraction of the estimated 100 – 400 billion stars in the Milky Way.  Most of the stars are simply too dim to be seen at galactic distances, or are hidden by the cloud-obscured zone of avoidance toward galactic center. So how is it that we are able to know what our galaxy looks like?

A real map of the Milky Way based largely on H II regions. Credit: Emily Freeland

Although we can only observe a small fraction of the stars in our galaxy, there are other objects such as nebulae that we can observe. In particular, there are star-forming nebulae known as H II regions, which contain large amounts of ionized hydrogen. In the visible spectrum these regions are often seen as red nebulae, but the hydrogen also gives off radio signals at a particular wavelength of 21cm, which is why its often called the 21cm line. Since radio wavelengths aren’t absorbed significantly by the gas and dust in the Milky Way, much of this 21cm radiation is visible to us across the galaxy. As a result, we can map the locations of these H II regions. From this we find that our galaxy has a clear spiral structure to it. In particular, it is a type of spiral galaxy known as a barred spiral.

While hydrogen is by far the most abundant element in our galaxy, the 21cm line hydrogen emits is not particularly strong, so it can be difficult to observe at times. Fortunately, there are other wavelengths we can observe, such as carbon monoxide emission lines, which are much brighter. From all of our observations, we have a pretty good idea of the size and structure of our galaxy.

The artistic renderings of the Milky Way you often see are inspired by images we have of other similar galaxies we can observe such as Andromeda. We know what barred spiral galaxies look like, and our own galaxy likely looks pretty similar. It’s actually amazing how far we’ve come in only a century, given that it wasn’t until 1918 that Harlow Shapley was able to determine the basic structure of our galaxy.

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Last time I talked about how hydrogen gas in our galaxy can be observed by measuring the 21 cm radio spectrum, and how we can use the results to determine how fast our galaxy rotates at different distances from its center.  Usually such a measurement is used as evidence of dark matter (which it is), but it also tells us other things as well.  In particular, it shows us that our galaxy is not just a spiral galaxy, but a particular kind of spiral galaxy known as a barred spiral.

Measuring the 21 cm line tells us how fast hydrogen gas clouds are moving around the center of our galaxy.  Since our galaxy is shaped roughly like a circular disk, it is reasonable to assume that the matter in our galaxy is moving around the galactic center basically in a circle.  If you know a bit of basic physics, you know that if something is moving in a circle there must be a force keeping it in a circle, otherwise things would just fly apart.  In this case the force holding the galaxy together is its own mass.

Now, the thing about gravity is that for a gas cloud moving in a circle at a particular radius, only the mass within that radius works to keep the cloud moving in a circle.  Anything farther out doesn’t really affect things.  (This is true in general.  If you could dig a hole to the center of the Earth, you’d be weightless there.)  This means you can use the speed of the gas clouds to determine how much mass there is within that distance.

In the first image above I’ve plotted total mass as a function of distance from galactic center.  One thing you might notice is that at about 3 kiloparsecs there is a jump in mass.  This type of jump is exactly what you would expect if our galaxy is a barred spiral.  The second image is a NASA drawing of what we think our galaxy looks like (a barred spiral).  You can see that there’s less matter in the center because of the central bar.

So by observing the motion of hydrogen in the galaxy we know that our home galaxy has a bar, just like about 2/3 of the spiral galaxies in the universe.

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Hydrogen is the most common element in the universe, so when you see gas and dust around our galaxy, chances are there is a lot of hydrogen there as well.  We can use that fact to map our galaxy.

It turns out that neutral hydrogen can give off radio signals at a very specific wavelength, known as the 21 cm line.  The 21 cm line has two very nice properties.  First, there isn’t really anything else that produces that radio frequency, so if you observe 21 cm radio waves you know it is from neutral atomic hydrogen.  Second, this particular wavelength isn’t absorbed much by other clouds of gas, so it is less likely to be blocked (unlike visible light).  So if you point a radio telescope in a particular direction, the more intense the 21 cm signal you detect, the more neutral hydrogen there is in that direction.

Stellar velocity vs Distance

Since the 21 cm line is so precise, you can also see the effect of any Doppler motion, which would shift the wavelength by a bit.  So when you measure the signal in a particular direction, you can measure the speed at which the hydrogen gas is moving.  With a little math you can then determine the speed of gas in the Milky Way as a function of its distance from the center of the galaxy.  In the figure below I’ve plotted some speed vs. distance data which was collected by undergraduate students with a small radio telescope.

The distance scale goes from galactic center out to about 8.5 kiloparsecs, which is the distance of our Sun.  You’ll notice the speed keeps going up as you move outward, which is a bit strange.  Given the distribution of gas an dust in the galaxy, we would expect it to level off and start slowing down.  Since it doesn’t, that means something strange is going on.

Of course we now know that strangeness is due to dark matter, but that’s another story.

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