spectrograph

What is a Nebula?

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What’s a nebula?

Well…you’re looking at one.

Okay, okay, I know. You want to know what that actually is. You want to know why it’s there. You want to know why there are colors in space…and why you’ve never noticed such a thing in your own night sky before.

Nebulae are the stuff between the stars. They’re the galaxy’s backstage. They’re the only visible evidence of a vast expanse of gas and dust between the stars, completely invisible to the human eye, called the interstellar medium.

Nebulae are the sites of star birth. Planets form from the dusty particles present in these glowing space clouds. They’re the galaxy’s way of replenishing itself. And they’re pretty cool to look at, too.

But how come they look the way they do?

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Spectroscopic Binary Stars

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Consider a solar system far different from our own. A solar system governed by two suns, and consisting of planets we can only dream of.

Would it surprise you to hear that, based on recent discoveries, that might actually be the norm?

The surroundings we grow up in determine our outlook on the world, and this is never more true than with our solar system. Our eight planets (though some would vehemently insist upon nine) and their parent star are all we know.

But what if I told you that most of the stars you see when you look up at the night sky have companions? And often, these companions are impossible to detect by visual means.

So how do we know they exist?

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Star Types Demystified

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By now, I’ve introduced you to a lot of different ways to classify stars.

Months ago, I talked about the different spectral classes—O, B, A, F, G, K, and M. Even before that, I told you about apparent visual magnitude, our ranking system for how bright stars appear to the naked eye.

More recently, we explored absolute visual magnitude and the related absolute bolometric magnitude and luminosity. All these are related to a star’s actual brightness, not just how bright they seem to be from Earth.

And last but not least, we talked about the H-R diagram and how to rank stars by their luminosity classification.

In short, it may seem like sorting stars is a complicated business. But it’s not really. And here, I intend to give you an overview to put all this together.

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What Makes a Star Blue?

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Albireo is the distinctive double star in the head of the constellation Cygnus. You can find it yourself if you look for the Summer Triangle amid the dusty trail of the Milky Way across the night sky.

The brighter, orange star of Albireo is a K3-class bright giant. That means it’s just a few thousand Kelvins (Celsius degrees plus 273) cooler than the sun. But it’s also larger—70 times the sun’s radius—and that makes it brighter than you would expect.

The blue star, on the other hand, is a B8-class dwarf. It has only about 3.5 times the sun’s radius, although it’s hotter by about 7422 Kelvins.

Neither star in Albireo is particularly unusual. There are doubtless millions, even billions, of other stars similar to each one. But Albireo certainly offers us the most striking contrast. Bright blue and red stars don’t often appear so close together.

But what exactly gives these stars their distinctive colors?

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Star Luminosity Classes

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What do you think it would mean for a star to be in a specific luminosity class? I mean…does that mean they go to school to learn how to be bright?

(Ha, ha…yeah, I know, bad astronomy pun.)

Well…not quite.

Stars can be sorted in a lot of ways—and a good thing, too, because there are literally trillions upon trillions of them. Astronomers would be lost if we couldn’t sort them into groups to study.

They can be sorted according to spectral type (composition and temperature), apparent visual magnitude (how bright they look to the naked eye from Earth), and absolute visual magnitude (how bright they would look to the naked eye from ten parsecs away).

They can also be sorted according to their absolute bolometric magnitude (how bright they would look from ten parsecs away if the human eye could see all types of radiation).

And…they can even be sorted according to their luminosity.

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The H-R Diagram

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There are 250 billion stars in our galaxy alone. Many are much like the sun, labeled with the Latin sol for “sun” in this diagram. But many more are not quite what we might expect stars to be like, after living under the light of a white G2 star our whole lives.

Wait a second. White G2? Since when is the sun white? And what the heck does G2 mean?

I’m talking about its spectral type—a classification system that organizes stars by their temperatures, determined by what they’re made of. The sequence is O, B, A, F, G, K, and M, in order from hottest to coolest. The sun is a fairly cool star.

But the thing is, the spectral types don’t actually tell you anything about how bright the star is, how big it is, how luminous it is…I could go on.

So how can we make things easy for ourselves and classify stars according to spectral type, size, and luminosity all at the same time?

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Spots on the Sun

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Have you ever looked at the sun, and seen something like this?

Now, before you decide to look at it right now and see what you see, it’s my responsibility as an amateur astronomer to remind you of the safety risks. Focusing your eyes on the sun is dangerous—there’s a reason our eyes automatically flinch away.

How dangerous, you ask? Dangerous enough to burn and even scar your retinas, permanently damaging or even destroying your vision.

Yes, I’m serious.

Now, all this is not to turn you off solar observing entirely. There are safe—and cheap—ways to look at the sun, and see its spots.

But what exactly are sunspots?

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Our Sun: The Chromosphere

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This diagram is a tiny bit misleading.

Here, it looks like the chromosphere is the visible surface of the sun, with the photosphere just below. Really, we never see the chromosphere. If you ever look through a solar telescope at the sun, the photosphere is the surface that you see.

The sun is structured a lot like the Earth, just in that it has a core, a dense region between the core and the surface, a “surface” layer, and a few atmospheric layers. The chromosphere is part of that solar atmosphere. And you never see it.

Well…almost never.

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Types of Stars

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Meet the sun: a G2 class star towards the middle of its lifespan.

Wait a second…G2? What does that even mean?

It’s all part of a way astronomers break down the billions of stars in the sky and organize them by temperature. They can use a star’s spectrum to figure out what it’s made of, and that helps them figure out how hot it is.

But really…being able to read stellar spectra (plural for spectrum) is only so helpful. There are billions. It helps to have an organizational system.

That way, if an astronomer sees a stellar spectrum that looks a certain way, they can know immediately that it’s a certain class of star.

So…how exactly are stars classified?

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The Building Blocks of the Universe

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“The Building Blocks of the Universe.” When you put it that way, atoms sound less like a topic specifically for a chemistry class and more like something astronomers might discuss.

They really are. I’ve got a fantastic reason to include atoms under astronomy, and its name is stellar spectra.

We’ve encountered stellar spectra before in these astronomy posts. When I wrote about the spectrograph, an instrument astronomers use to study data, I talked about spectral lines. I also promised we’d come back to elaborate on that later.

We’re not actually going to talk about the spectrograph in this post. I’m saving that for another time. For now, I’m going to cover atoms in a little more detail.

That way, we’ll have a better understanding of how they interact with light later on—and that will help us understand the spectrograph.

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