How Far Away are Galaxies?

Well, I’ll give you a spoiler: they’re ridiculously far away.

Let’s consider for a moment what a light-year actually means. It sounds like a unit of time, but it’s actually the distance that light travels in one Earth year.

Think of it this way: if your name is Bob, and you can travel a certain distance in one year, that distance could be called a Bob-year.

I know it’s strange to think of light traveling at a certain speed. When you flip a light switch, the room immediately brightens. When you shine a flashlight, its beam immediately falls across the nearest surface.

But that just goes to show how insanely fast light travels. If it takes 2 million years for light to get from one object to another…imagine how far apart those objects are?

Well, that’s the case for our home galaxy, the Milky Way, and our nearest galactic neighbor, the Andromeda Galaxy.

But…wait a second. How do we know that?

I don’t know about you, but I’m not sure there’s a tape measure quite long enough for this.

First things first: let’s get our measurement units straight. When measuring the distances between galaxies, even light-years–insanely large as they are–are too small to be convenient.

When was the last time you heard your house described in thousands of millimeters, rather than feet, yards, or meters? Well…it’s not very convenient to talk about millions of light-years between galaxies, either.

When talking about the distances between galaxies, astronomers use the megaparsec, which is 1 million parsecs. A parsec, by the way, is about 3.26 light-years. (And a megaparsec is 3.26 million light-years.)

That makes the Andromeda Galaxy roughly 0.89 megaparsecs from the Milky Way. There…that’s a much easier measurement to work with, huh?

Alright, so…how the heck do we find the distance to a galaxy?

If we were just measuring the distance to stars within our own galaxy, it would be a bit easier–we’d just use parallax.

As the Earth orbits the sun, nearby stars appear to shift position a little against more distant, “background” stars. I described this method of measuring distances to stars in detail here.

Problem is…parallax isn’t much good for measuring distances to galaxies. Galaxies are just too far away. There is some parallax, but it’s barely noticeable at all–even for nearer galaxies.

Fortunately for astronomers, though, parallax is still helpful–indirectly. We can still use the distances of stellar objects within galaxies. Parallax helps us learn about those that are within our own galaxy, and we can build from there.

This is where our old friends the Cepheids come in.

You might recognize Cepheid variable stars from recent posts. In the 1920s, they helped us discover the size of our own Milky Way…and, only four years later, Edwin Hubble used them to measure the distance to Andromeda Galaxy.

Cepheids are a type of distance indicator–objects that can be used to find the distance to a galaxy.

But…what makes them so great at that, again?

Cepheids are expanding giant stars. They are stars that have run out of hydrogen nuclei in their cores and have lost stability. And that instability makes them pulsate rhythmically in brightness.

They’re so useful to astronomers because there is a direct relationship between their average luminosity and their pulsation period: fainter Cepheids pulsate much faster than luminous Cepheids.

If you can measure a Cepheid’s pulsation period, you can determine its intrinsic brightness (absolute magnitude). And from there, you can use how bright it appears from Earth (apparent magnitude) to determine its distance.

See where I’m going with this?

This is how Edwin Hubble was first able to measure the distance to Andromeda Galaxy and finally determine that it was a galaxy outside our own and rivaling ours in size, not merely a swirl of gas and stars inside the Milky Way.

And it doesn’t just work on Andromeda.

Meet M100, a galaxy 16 megaparsecs from Earth. And that pixelated little star in each frame above is the Cepheid variable that helped us figure that out.

In this image, you can see the Cepheid fluctuate in brightness over several different shots of its home galaxy. By measuring its period, astronomers could determine how far away M100 was.

Like Cepheids, most distance indicators have known luminosities–so astronomers often refer to them as standard candles. And standard candles are absolutely invaluable in measuring the distances to galaxies.

But Cepheids aren’t the only standard candles we’ve got to choose from.

And thank goodness for that…because in galaxies beyond around 30 megaparsecs away, they’re just too faint to reliably measure.

Then, astronomers can rely upon globular clusters.

Ah, yes…back to globular clusters. We’ve sure been talking about them a lot lately, haven’t we?

Well…they’re useful objects! Not only do they contain Cepheids (which was how Harlow Shapley first measured the size of the Milky Way), they also work as distance indicators.

But how?

Because globular clusters contain Cepheids, it’s relatively easy to find a nearby galaxy that has both Cepheids and globular clusters. Cepheids in nearby galaxies are bright enough to work as distance indicators.

When we find the galaxy’s distance, we also know the distance of the globular clusters in that galaxy–and we can find those globulars’ absolute visual magnitude.

Globular cluster G1 here is in fact a resident of our neighbor Andromeda, not the Milky Way.

It turns out that the brightest globular clusters have absolute visual magnitudes of about -10. We can then assume that globular clusters in more distant galaxies also have absolute visual magnitudes of -10.

That means they can work as a standard candle, even for galaxies too distant to be measured using Cepheids!

But that’s not all.

Globular clusters also tend to be around 25 parsecs in diameter. Assuming that all globular clusters are 25 parsecs across, astronomers can measure their angular diameter and calculate the distance to a faraway galaxy that way.

However…even globular clusters can only take us so far. So, for even more distant galaxies, we need to find a standard candle that’s even brighter.

Enter type 1a supernovae–supernovae produced by the collapse of a white dwarf, rather than the death throes of a massive star.

Above is Messier 82, a galaxy 3.7 megaparsecs away. (That’s 12 million light-years!) And that fuzzy little dot of light indicated by the arrow is a type 1a supernova.

But…wait. What’s a type 1a supernova again?

It’s worth reviewing–because it explains why the heck we can use these as a standard candle.

We begin with a white dwarf, orbiting a giant companion.

This diagram describes the white dwarf as a “white dwarf star”–but it’s not really a star. At least not if we define a “star” as an object that counteracts the weight of its own mass by nuclear fusion in its core. A white dwarf would be more accurately described as a stellar remnant.

More specifically, it is a compact object–an object so dense it defies the laws of physics.

Well…sort of. It’s more that different laws of physics–those from the insane world of quantum mechanics–begin to kick in.

The key is, a white dwarf cannot be more massive than 1.4 solar masses (1.4 times the mass of the sun). This is called the Chandrasekhar limit, named for the astronomer who first calculated it.

If mass from a companion star falls onto a white dwarf until the white dwarf’s mass exceeds 1.4 solar masses…


Now, here’s the crux. A type 1a supernova always occurs when the white dwarf surpasses the Chandrasekhar limit. That means that these supernovae will always be similar–and they will always reach roughly the same peak luminosity, no matter where they are in the universe.

Type 1a supernovae can be identified by their light curve, the graph of their luminosity over time. And it’s pretty easy to tell them apart from other types of supernovae…

A type 1a supernova in the Andromeda Galaxy will reach roughly the same peak luminosity as a type 1a supernova in a galaxy at the limits of the observable universe.

The only difference between them will be their apparent luminosity. But that’s exactly how we’ll find their distances from Earth–by comparing those apparent luminosities with the peak intrinsic luminosity we know they both have.

That’s how they work as standard candles.

The magnificence of type 1a supernovae as standard candles is that we don’t even need to calibrate the measurement using other distance indicators–all type 1a supernovae look roughly the same, so we already know their intrinsic luminosity.

That means we don’t need to find nearby objects that contain both globular clusters or Cepheids and type 1a supernovae. And a good thing too, because type 1a supernovae are quite rare.

…and that’s the catch.

They would be perfect standard candles–except that they’re just not that common. And they’re short-lived events. The chances of spotting one are slim.

And so, there’s one last distance indicator we can fall back on…

The luminosities of galaxies themselves.

This one is a bit shakier than the others. After all, every galaxy is unique, and just like stars, some are larger than others. The intrinsic brightness of galaxies varies–that’s why we needed to use standard candles in the first place.

But in the farthest reaches of the observable universe, there just aren’t any standard candles visible. Everything’s too far away. It’s impossible to resolve individual stars, at least with current telescopes. Even if type 1a supernovae did occur while we were watching, chances are we wouldn’t notice them.

What we have to do is take some really rough estimates.

This is not one of the “deep field” images of galaxies that I’ve been showcasing for you–this is a galaxy cluster, something we’ll be exploring shortly. Just as a star cluster is a cluster of stars, a galaxy cluster is a cluster of galaxies.

By studying nearby galaxies, we’ve found that a normal, large spiral galaxy like the Milky Way is generally about 16 billion times as luminous as the sun. Assuming that’s the case for all similarly sized spiral galaxies…we can apply that to galaxy clusters.

What astronomers can do is estimate the luminosity of several of the brightest spiral galaxies in any one cluster. Then, after estimating their distances from Earth, they can average those distances to find a likely middle-ground estimate.

That still leaves a lot of uncertainty. It’s based on a lot of assumptions, such as that the brightest spiral galaxies in any given cluster are actually similar to the Milky Way. But when it comes to those most distant galaxies, it’s the best we’ve got.


That’s not quite true.

There is one very surefire way to estimate the distance to just about any galaxy, without even using standard candles.

But that is a story for my next post!

Questions? Or just want to talk?

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