So last week, two weeks ago, we were talking about the distribution of optical light in

spiral galaxies.

We looked at the isophodes of the galaxies and realized that they consist of two parts

of spiral galaxies.

One is the bulge, which is more circular, and then the disk, which is a more extended

elliptical thing that you see as ellipses because it's a disk that we see in projection.

We then looked at the distribution of light and we found that if we look in radial direction,

that there is an exponential decrease away from the core in the disk.

That's given by this equation down here with the center brightness of about 22 magnitudes

per square arc second, which you can see nicely here in these figures.

If you ignore the bulge from here to the outside, the galaxy gets dimmer, and that's what's

described by this exponential function.

And then we looked at the radial height distribution, and that height distribution can effectively

also be described by another exponential function that is just given here by this equation down

there and effectively a consequence of the fact that if you look at a thin plate distribution

of mass, that you have a constant gravitational acceleration, which then leads to something

that looks like an atmospheric density distribution here.

And finally, the bulge we found was described by de Beaucaulure's law, which is the R to

the one-fourth power law.

So if you look at the logarithm of the intensity, it essentially goes with distance to the point

25, where the effective radius here is the radius that contains half of the total luminosity.

So bulges here, as you can see, can be very large.

This bulge here is as large as the whole spiral galaxy.

This is why this galaxy here is also called the Sombrero galaxy, but typically bulges

are much smaller, as we've seen in the previous pictures, and are only the inner 10 or 20

percent or so of the overall galaxy.

So this is the distribution of light, but the question now is, are there systematics

in the behavior of galaxies with Hubble type, for example?

And this is summarized here.

So you see that if you plot the brightness of galaxies or the blue or redness or the

color of galaxies as a function of Hubble sequence, starting from the intermediate type

S0s, the lenticular, which are almost elliptical galaxies, and then you go along the Hubble

sequence here.

And by the way, if you're confused here, there are different Hubble sequences that are used.

The one that I showed you so far always was a Hubble sequence that went from SA to SC.

There is an additional classification that was done, I think, by Alan Sandage and by

Duvucouleur, but I might be wrong with the names there, that also introduces SCs and

an SM type, which I wouldn't be able to describe to you how that differs from the SD.

So it's essentially the same overall sequence.

There's only a finer ingrained formal description in the classification.

So the important thing here is if you go from the lenticulars away along the tuning fork,

is there is systematic.

And what you see here is that in general, first of all, along the tuning fork, galaxies

tend to become fainter as you go away from the lenticulars, and galaxies tend to be bluer.

So they're fainter, but they have more younger stars.

They have more OB type stars, and that's why these galaxies are bluer.

Ah, yeah, down here, I wrote this down because I forgot.

So SDs have no bulge, and SM are called Magellanic systems, so they're irregular galaxies, which

kind of look as if there is a disk in there.

I think you yearned that I remembered to put this down.