Video Transcript: Distance to Galaxies

So one approach to solving that problem is to take a spectrum of those fuzzy glowing things, and when you do that, you see that some of them have very sharp lines, very clear spectrum that say this thing consists of just a few key elements, and it's a cool-ish gas; it's just glowing at a particular wavelength, which tells you it's a nebula of some kind.  But other ones of those fuzzy, cloudy things were giving spectrum that looks a lot like stars, just like stars, in fact, and so it suggested that this cloud consisted of lots and lots of stars.

So the challenge started to become, well, are these groups of stars?  Are they inside our galaxy, or are they outside of our galaxy?  Because there must be so many stars there that we can't even see the stars?  So is that really far away or what's going on?  

So it became necessary to measure the distance to these clouds.  And that's really, that was really essential for the whole idea of a galaxy.  We didn't have these pictures where you could see the whole spiral; it just didn't happen yet.  But if we could somehow measure the distance to a galaxy, or to one of these clouds, we could know how far away it is, whether it's inside our Milky Way or outside our Milky Way.  Okay, okay.  So how do we do this?  How do you measure the distance to a galaxy?  That's what we're going to focus on in this video and you know, amazingly, there are several ways that you can do that.  I mean, it's astounding that that's even possible, but you can.  

One of the first ways that you can measure the distance to a galaxy is using supernovas.  Now, remember that a supernova is a single star at the end of its life that explodes with this tremendous explosion.  An explosion that’s so big that the star becomes so bright that it can outshine the entire galaxy that it's in.  It can shine brighter than these millions and millions of stars. 

So here's an example.  This first picture in the gallery shows a real picture of a supernova that's happening in a galaxy.  This is a single star, you can see it down here in the bottom left, that has exploded.  And it becomes so bright that you can see that star even though it's just one in a billion of the stars in this particular galaxy.  Now, why is that important? Well, because it shines so bright.  In these distant galaxies, we can see them; you can look at a galaxy and you'll see this bright flash of the supernova.  

Now here's the thing about supernovas is they happen relatively quickly.  I mean, things in the universe don't happen quick, usually.  And so you can see here in the second picture in the gallery is an example of a supernova from 2008.  You can see a first picture here from January 6, and then by the time we get to the right-hand side, just one month later, that supernova has gotten much fainter.  So over the course of like thirty days, the supernova goes from its brightest spot to fainter, fainter, fainter, and just keeps fading, getting fainter and fainter.  So why is this important?  Well, it's important because we need to measure the brightness of that supernova.  Here's how it can help us.  

There's a relationship in the brightness of the supernovas.  So this is a crazy graph.  Stick with me.  The graph is showing, on the vertical axis, it's showing basically the luminosity… like the intrinsic brightness, how bright really is this supernova if you were standing in front of it.  And, on the right-hand side is the amount of time in days.  You see this covers about two months.  And what you notice is that the intrinsic brightness, the luminosity of these supernovas, regardless of where they're happening, that luminosity, that peak point is basically exactly the same.  

So when a star goes supernova it gets insanely bright but it reaches about the same ceiling.  And we know what that luminosity is.  We can calibrate this and figure out that luminosity.  That means that anytime there's a supernova anywhere in the universe, we know how luminous that supernova is; how much energy it's giving up.  And so, if we can compare how bright it looks in our sky, how bright it looks to us, to how bright we know it actually is, then we can use that inverse square relationship and say, well then the supernova must be this far away, because the light gets dimmer and dimmer in the time it takes to get to us.  We call these standard candles, because it's this candle, it’s this source of light in deep space, but we know how bright it is; we know its luminosity.  And that's the key; we need to be able to know the luminosity in order to be able to tell how far away it is.  

One of the things you'll notice about the supernova, we call this a light curve, its brightness and how it changes with time, one of the things you'll notice is that it starts a little lower, and then comes to a peak, and then goes down, down, down.  And so the real challenge is, if you start, if you notice a supernova, you want to try to catch it in just those first couple of days because you really want to measure its brightness when it's at the peak.  That's where we know what its luminosity is.  

So what will happen sometimes at observatories is that someone, an astronomer, will be looking at something else that they want to observe and they'll all of a sudden get a notification there's been a supernova; you need to turn your telescope and go immediately to look at the supernova, because it needs to happen now, as soon as possible.  So these supernovas are one of the very few things in astronomy where you need to respond very quickly to be able to collect the data that you need.  As you'll notice, as you get further and further away from the supernova, it scatters more and more, it's less precise of a measurement.  

So you can see how important, you know, we've talked before about how important photometry is -  photometry, measuring the brightness of objects - and how digital cameras allow us to do that so well.  Well, this is an example of how important that is.  Because when we measure the photometry, the brightness of the supernovas, we can actually use that to measure the distance to these galaxies.  And we saw before the supernovas are really very rare.  In any given galaxy, you'll only see a supernova... well, it's not super common... so discovering the supernovas is an area of ongoing effort in astronomy where we want to see a new supernova and find it as it's happening.  You’ve got to be looking during a window where it's bright enough that you can see it.  Okay.  So supernovas are one way of… I’m just making sure I'm not missing anything… and that curve (indistinguishable), okay, yep.  

But they're not the only way that you can find the distance to galaxies.  There's another awesome technique that was discovered early on by a female astronomer who was doing lots of work inside the Milky Way Galaxy with variable stars.  So these stars are called Cepheid variablestars, and they were discovered by a woman named Henrietta Leavitt.  And this is one of the most important discoveries in all of astronomy – Cepheid variablestars -  because what she discovered is that these stars, which vary in brightness, you can see here in the fourth picture of the gallery, from April 23, this is over the course of about a month, you can see this star in the middle gets fainter, it's almost disappeared on May 9, and then it gets brighter again.  So it's continuously like pulsing over the course of days and weeks; brighter, fainter, brighter, fainter.  And some stars do this.  Variablestars can do this.  

So a Cepheid variable does this kind of pulsation, and there's a really important relationship that Henrietta Leavitt noticed which is that the rate at which one of these variable stars pulses, we call that the period of its oscillation, so it might be four days, it might be four weeks that it takes it to go from bright to faint and back to bright again, that time is related to how luminous the star is.  So it's what we call a period luminosity relationship.  And here's what the graph of that looks like.

Now, it gets a little complicated, but here we go.  On the horizontal axis is the period of variability.  How many days does it take for it to go from bright to faint to bright again?  That’s something we can measure.  If you measure the brightness and just graph it, you'll actually see a little curve and you just measure how many days was it that it took to get bright again.  It's not very hard to measure, especially with digital cameras.  You know, Henrietta Leavitt was doing this like when photographic film was just invented, so it was difficult, much more difficult than it is now.  So you can measure that period.  And then the vertical axis is the luminosity - how bright are these stars; like their actual brightness.  

And you can see how well stars fall along this line.  And that's true of stars both in our Milky Way and in other galaxies; they all do this.  So this relationship is a very good relationship because if you can measure the period, which is really easy to measure, then you know the luminosity.  And if you know the luminosity you can find the distance because you compare how bright you know it is to how bright it looks in the sky, and you can use that inverse square law of light to say, well, light gets dimmer and dimmer, and I can tell how far away it is.

Okay, so the trick then is can you find Cepheid variables in other galaxies?  And in fact, we can.  With large telescopes, good cameras; you can find Cepheid variable stars in other galaxies; you can measure their period, determine their luminosity, and measure the distance.  And when this was done what they found is that the distances were way bigger than anyone ever expected.  None of these astronomers thought these galaxies would be millions of light years away.  In fact, they almost didn't believe the results because it was just uncanny that it could be possible. 

So that's where we find ourselves.  We have several independent ways of measuring the distance to these galaxies.  And they're powerful measurements; it didn't have to be that way.  It didn't have to be that there's any way to measure; they're far too far away for parallax or anything like that, but it's almost as if God has left these little breadcrumbs for us, little ways that we can actually measure these things and we can determine how far away these galaxies really are.  And in the process, determine how big our universe really is.

Okay, we'll see you next time.