Okay, so welcome to today's lecture in galaxies and cosmology.

And the topic of today's lecture is observations and observing methods.

Okay, so how do we observe now the universe and galaxies and everything that's in there?

Well, the obvious thing is that we start just looking and looking with our eyes or using a telescope or whatever

then just means that we are observing just a very, very small fraction of the electromagnetic spectrum.

So, actually only this small area here, visual, this is the range of the electromagnetic spectrum that we can perceive with our eyes.

However, the electromagnetic spectrum is a lot larger than that.

So, while our visual range is here in the order of, in terms of wavelengths,

something like 10 to the minus 6 or 7 or so, the electromagnetic range in wavelengths goes up to like meters or even kilometers of wavelengths,

but can be as short as something like 10 to the minus 12 and even shorter,

if it just already still makes sense to talk about wavelengths at these energies.

So, there is a lot more or a lot more areas which we can use to observe the universe and try to understand the universe.

And we will see in this lecture that different wavelength ranges have different advantages,

but also of course require different observing technologies.

So, for example, if you start here from the visual range,

if you go to the longer ranges, then longer wavelength ranges, then we have first of all the infrared, in popular terms known as heat.

Then we have the microwaves, which is like similar to the microwave oven or radar, these things.

And at the very long wavelength ranges, we have typical radio waves.

On the other side, we have first of all the ultraviolet, which we know from getting tan in the summer.

Then there are X-rays, soft X-rays and harder X-rays, also used at the doctors.

And then we have the really hard stuff like gamma rays and anything that's beyond that.

To give you an impression, what that means typically, there are a number of visual things here.

So, for the radio wave, for a single wave, you have, for example, the size of a soccer field.

In the microwaves, it's the typical size of a ball.

Then in the visual range, we are already in the size of a bacterium or virus.

And then we go to even smaller stuff like molecules or single atoms in this area.

So, as you can imagine, to observe the universe in these different energy ranges,

we will really need a lot of different technology, a different understanding of what's going on.

But also, there will be lots of quite different phenomena that will be radiating

or will be visible at these different energy ranges. So, it's going to be interesting.

However, before we start just starting to observe the universe in all these kinds of different wavelength ranges,

we'll find a problem.

And the most pressing problem is that a big part of the electromagnetic spectrum is actually blocked by our atmosphere.

So, luckily, the visual range is not, so we can actually see something.

If the atmosphere would be opaque in the visual, we would just live in a very dark world.

Luckily, it's transparent in the optical.

Then there are a few more bands in the ultraviolet and so on, and in the infrared, where something is a little bit visible.

And then there is the big fraction here in the radio, which is also transparent.

Everything else is more or less opaque or is absorbed by the atmosphere.

So, for example, it's completely impossible to observe X-rays or gamma rays or even infrared from the ground.

If you are lucky, the atmosphere is not totally opaque to these energy ranges,

then for example, it might be sufficient to use a high-altitude plane or a high-altitude balloon to go to them.

It seems like 20 kilometers in height, and then we can already observe some stuff.

But for most of the time, we will have to go to outer space to really do some science to observe the universe in these energy ranges.

Okay.

Light or the electromagnetic spectrum can be characterized or is often referred to either in wavelengths, as I've just been doing.

These are then typically measured either in meters, centimeters, millimeters, nanometers, angstrom.

Actually, this is just all the same.

It's more convenient in the typical area you are working in, for example, if you always talk about very small objects or very short wavelengths,

then it's typical to use nanometers or angstrom, while if you're something very long, you start talking about centimeters, meters, kilometers, and so on.