I find pictures of spiral galaxies to be one of the most beautiful cosmic images. These galaxies are those grand disks of stars with 2 or more “arms” that appear to spiral out from a central bright region.
Our own Milky Way galaxy – the one that houses our Sun and solar system – happens to be a spiral galaxy. Since we can’t get outside our galaxy to take a picture (we’ve only just gotten the second spacecraft outside our Solar System, let alone the galaxy), how do we know what kind of galaxy we live in?
First, let’s take a step back. Astronomers have identified three basic types of galaxies – spirals, ellipticals, and irregulars. Spirals are as I described above – flattened disks, usually with identifiable arms encircling the center region. Elliptical galaxies are giant balls of stars without any regular structure other than appearing generally spherical. Irregulars are basically the “miscellaneous” category – galaxies that aren’t strictly elliptical or spirals, but might be somewhat of a mess.
The first clue that the Milky Way is a spiral has been part of the human experience for millennia…
… there’s band of stars that we can see in the sky when it’s very dark. You can’t see this in the city or anywhere with a lot of light pollution, but if you get into the country, you can see a bright band of stars. This band is how we see the disk of our galaxy. Ancient observers didn’t know this, but once we started identifying other galaxies, it became clear that it’s exactly what you’d expect to see as from inside a spiral galaxy.
However, more recently, astronomers can actually map out the individual structures of the Milky Way. One way to do this is by measuring the density of neutral hydrogen in our home galaxy which is done with radio observations and was first attempted in the 1950s. Another way to do this is by mapping out bright nebulas of ionized hydrogen, which are tracers of star formation.
In both cases, these maps show distinct bands of material – the galaxy’s spiral arms.
Now, the spiral arms aren’t the only feature of our (or any spiral) galaxy. There’s the bulge, which is a “ball” of stars, dust and gas the forms the center of the galaxy. There’s also a “halo” – a huge sphere that’s not terribly dense, but surrounds the entire disk. Dotted throughout the halo are globular clusters – ancient groups of stars that are among the oldest in the galaxy. The youngest stars are found in the disk and spiral arms.
So, where do we fit into this? The Sun lies in one of the minor spiral arms about half-way out from the center of the galaxy. And we lie a bit above the central plane of the galaxy.
And, in case you missed my Milky Way Interlopers video, be sure to check it out.
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