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The Milky Way galaxy contains about 400 billion stars...
...give or take a dozen. But who cares ? Stars are rubbish.
They form from hydrogen gas which is way more awesome.
Let's take a look at it.
To do that, we'll need a radio telescope...
...like the massive Arecibo Observatory in Puerto Rico.
It can map the hydrogen in exquisite detail. There's just one problem : Arecibo is...
...too bloody big. It can't turn !
This means it can only observe part of the sky.
To map the rest, we have to use smaller telescopes.
The Lediden-Argentine-Bonn survey has mapped the entire sky.
The resolution is much worse than Arecibo, but it's still enough to show the major...
...features of the galaxy.
Hydrogen is particularly awesome because we can measure both how bright it is...
...and how fast it's moving toward or away from us. The animation shows the hydrogen...
...detected at different velocities.
We can also pretend that velocity is the same as distance...
...and explore a three-dimensional velocity space.
If velocity was really the same as distance, the Galaxy...
... would look like this. This is clearly very stupid. We have to do some mathematics...
properly convert velocity into distance. As we will see in a moment...
...this is not as good as we if could directly measure the distance.
The mathematics is not difficult, but it does have some limitations.
As you can see, it's not perfect.
There are some gaps where the equations break down...
... but it shows the major structures in the outer parts of the galaxy.
To fill in the gaps, we have to use other methods like measuring distances...
...to stars. Maybe they're not so rubbish after all.