The brightest galaxy in our night sky is the Andromeda Galaxy, our nearest intergalactic neighbor.

Hey…notice we’re talking about intergalactic neighbors now? 😀

In most of the posts on this blog, we’ve talked about interstellar neighbors, interstellar space, etc. But now, we’ve graduated to the intergalactic frontier, and we’re not turning back–at least not for a little while!

Anyway. The Andromeda Galaxy is the brightest galaxy in our night sky. But as you may remember from my eons-old article on apparent visual magnitude…that doesn’t tell us much about how bright it actually is.

It is the closest galaxy to us, the denizens of the Milky Way, so that tells us one thing: if it appears brighter than other galaxies, it probably is.

But…let’s say we put all galaxies in the universe on an even playing field, magically all exactly the same distance from Earth.

How big and bright would they appear then?

First of all…why does distance matter?

Imagine you see two cars, but one is just across the street from you, and the other is all the way down the block. Both cars are identical…but the closer one will appear to have brighter headlights.

On the other hand, let’s say there are two cars right across the street from you, but one is a little sedan and the other is a commercial truck. The truck’s lights will be intrinsically brighter–that is, they are physically more luminous–and they will also appear brighter!

But…let’s say that same truck moves on down the street. Its lights will appear to get fainter as it moves farther from you. But they haven’t actually gotten fainter. If it moves far enough away, that truck’s lights may appear fainter than those of the sedan. But intrinsically, they’re still brighter.

That means there are two factors affecting the brightness of a car’s headlights: the type of car (which will determine the intrinsic brightness) and the distance of the car from the observer (which will determine the apparent brightness).

The same goes for galaxies…with one slight tweak.

A galaxy’s actual brightness–taking into account all the radiation it produces–depends on its surface area. That is, its diameter. So, rather than determining the “type” of galaxy, we need to determine its diameter.

There’s just one problem, and it goes back to our “car headlights” scenario. The distance to an object affects the size it appears to be.

Here’s an example, taken from our very own Earth…

Somehow I doubt there is actually a gigantic shoe about to crunch Stonehenge underfoot!

The person’s foot appears huge because it is very close to the camera (the observer). Stonehenge, on the other hand, is set farther in the background.

The same works for galaxies. Which is bigger: the first galaxy below or the second?

Yes, these are both galaxies–the first one is an elliptical galaxy, which I covered in my “recent” post on the shapes of galaxies. The second is a quite spectacular spiral galaxy.

But the question remains…which is bigger?

Unless we know the distance to each, it’s impossible to tell.

Clearly, we need to get to work on finding the distances to these galaxies.

Good thing we spent the last two posts covering how to do that!

In my post on the distances to galaxies, we covered distance indicators, often known as standard candles.

Standard candles are objects of known luminosities, such as Cepheid variable stars and type 1a supernovae. If we spot an object with a known brightness inside a distant galaxy, we can then calculate how far away the object must be to appear as bright from Earth as it does.

Not all distance indicators are standard candles. The estimated brightness or size of globular clusters can also help us determine the distance to a galaxy. But this method is a bit shakier.

Then, of course, there is the Hubble Law…

According to the Hubble Law, the redshift of a galaxy is directly related to its distance.

Edwin Hubble figured this out by observing galaxies whose distances had already been determined using distance indicators. He found that each galaxy’s spectrum experienced a redshift–indicating that it was receding from Earth. And that redshift was directly proportional to its distance.

The Hubble Law provides a very convenient way to estimate the distance to a galaxy. All you need to know is the redshift!

Okay, now we can find the distances to galaxies! Time for the next step–finding their diameters.

This part is surprisingly simple.

All we need is the distance–which we just found–and the angular diameter, which is the size that the object actually appears in the sky, as pictured above.

Now that we have the size and the distance, all we have left to find is the luminosity! And that is also simple. Since we have the distance, we can use that and the apparent magnitude (how bright it appears in Earth’s sky) to find the luminosity.

Okay, now we’ve got the diameters and luminosities of the galaxies. Let’s see what the observations tell us…

As it turns out…galaxies are a very diverse bunch.

Irregular galaxies probably make up about 25% of all galaxies in the universe. They tend to be small–about 5%-25% the size of our own Milky Way. Due to their tiny surface area, they are also faint and easy to overlook. They’re underrepresented in sky surveys.

Elliptical galaxies are also underrepresented in sky surveys. They are probably the most common type of galaxy.

The largest ellipticals are so big they get their own name–giant ellipticals. (Yes, I know, so creative.)

Giant ellipticals are around 10 times the diameter of our own Milky Way! But then we’ve got the broad smattering of dwarf ellipticals, which are only about 1% the diameter of our galaxy. There’s probably a ton of these small little guys, but they’re very good at hiding from sky surveys.

And then, of course, there are the spectacular spiral galaxies, like our own. (The galaxy pictured below is M77.)

These are by far the most photogenic and they dominate Google search results for galaxies (as well as sky surveys themselves). But, in fact, they’re probably the least common type of galaxy.

Most spiral galaxies are actually smaller and fainter than our Milky Way. But there are also several large ones like ours–the largest known spiral is nearly 5 times the size and 10 times the luminosity of the Milky Way.

Apparently, the size and luminosity of a galaxy doesn’t have much to do with its shape!

But that begs the question…why? What makes a galaxy spiral or elliptical or irregular?

We’ll dive into that soon. But first, there’s one more characteristic of galaxies we need to explore: mass.