Interstellar Spectra

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I often refer to what we call the interstellar medium as the galaxy’s “backstage,” and I do that for a reason: for the most part, we can’t see it.

The backstage of any theater isn’t part of the show. You, as part of the audience, never see it. But you see evidence of it, when new props appear as the play progresses through scene after scene and the actors interact with their backstage.

The same thing happens with the interstellar medium. It’s not the hidden area behind the stars of the galaxy. (Ha, get it? Stars?) In fact, more often than not it’s actually the one hiding stars from view. But we can’t see it…unless we study how stars interact with it.

One way to do that is to look at reflection nebulae—evidence of the light from bright young stars reflecting off the dust of the nebula. That qualifies as interaction.

And in the case of emission nebulae, hot O-type stars ionize the hydrogen gas of the nebula. I’d say that’s interaction, too.

Even dark nebulae can technically be seen, since we see them as shadowy clouds silhouetted against background nebulae or stars.

But sometimes, it’s not that simple. Sometimes, we have to rely on the galaxy’s props to guess at what must be stored backstage. And that means studying stellar spectra. Continue reading

Extinction and Reddening of Starlight

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Take a wild guess: What do you think this image is showing you?

If you said it looks like a giant black hole in space, I don’t blame you. I also don’t blame you if you thought it looks like a giant outer space blob…and the funny thing is, that’s actually closer to the truth.

This isn’t a hole in space. We can’t see any stars in this region, but not because there aren’t any. In fact, there are just as many there as there are flanking the giant space blob.

What you’re seeing is evidence of the vast interstellar medium, the galaxy’s backstage. The interstellar medium is the stuff between the stars, often invisible since it’s not hot enough to produce its own light.

Sometimes we can see it as a pale blue reflection nebula, or a bright pink emission nebula. But in this case, we’re looking at a dark nebula—visible only because it blocks the light from stars beyond it. It appears to be a hole in space.

It’s closer to being an outer space blob. But what exactly is it? Continue reading

The Stuff of Nebulae

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What you see here is the Trifid Nebula, a vast cloud of gas and dust in space.

In my last post, we explored why it looks the way it does. We discovered that the pink hues of emission nebulae are caused when extremely hot nearby stars “excite” the gas of the nebula itself to emit its own light, which our eyes perceive as pink.

The haze of blue to the right, on the other hand, is the result of light from hot young stars nearby getting scattered among the nebula’s dust particles. It looks blue for the same reason the sky looks blue. We call nebulae like this reflection nebulae.

And the black wisps of dark nebulae are hardly as ominous as they look; they’re simply ordinary clouds of gas and dust, ordinary nebulae, that we can only see because they’re silhouetted by brighter objects in the background.

But nebulae, for all their different names, are actually a heck of a lot more similar than you might think. Continue reading

The Average Star

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What the heck is the average star like?

We’ve talked about a lot of stars over the past few weeks. We’ve discovered the vast distances between the stars, looked more closely at what really makes a star bright, and covered all kinds of ways to classify stars—from their spectral type to their luminosity class.

Most importantly, we’ve looked at the H-R diagram, the diagram that classifies stars by their color, temperature, composition, and luminosity…and relates those properties with many other features stars have.

We know what kinds of stars are out there. We know they range from thousands of times smaller than the sun to thousands of times larger. We know they range from desperately faint to incredibly luminous. We know they come in all the colors of the rainbow.

But how many blue stars are there? How many small stars are there? Are most of them small, or are there about the same number of small stars as large ones? Continue reading

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? Continue reading

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. Continue reading

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? Continue reading

How Far Are the Stars?

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Stars don’t look small because they’re really the size of pinholes in a blanket. The smallest are the size of Earth. The largest have 128,865,170 times Earth’s diameter.

They look small in the sky because they’re distant. It’s for the same reason you can tell how far away your surroundings are by how small they appear; you know the mountains on the horizon are far away because they look shorter than your house.

The nearest star to our solar system is 4.3 light-years away. But what exactly is a light-year?

Light seems to travel instantaneously from your flashlight to the nearest surface, but it actually has a finite speed. In one second, it travels 299,792 km—fast enough to wrap itself around Earth’s equator 7.5 times.

In one year, light covers 9,460,730,472,580.8 kilometers, enough to wrap around the sun’s equator 2160.5 times. Four times that is the distance to the nearest star.

But how do we know this? Continue reading

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. Continue reading

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? Continue reading