spectra

What the Heck is a Quasar?

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I’ll give you a hint: this is not an image of a quasar.

This image is from a first-season Star Trek: The Original Series episode, “The Galileo Seven”–the original footage, before it was remastered (and before astronomers had clear images of actual quasars).

According to the story, the crew of the USS Enterprise had standing orders to investigate any quasar they came across. A landing party spent the episode struggling to survive after they crashed on a primitive planetary body within a strange nebulosity that they called a “quasar.”

At times like these, Star Trek provides a fun glimpse into the past. TOS aired in the 1960s, just as very strange “quasi-stellar objects” were being discovered. Dubbed “quasars” for short, they were a new frontier of research and discovery.

The remastered footage uses an actual (if substantially edited) image of a quasar. But the “quasar” portrayed in the story…well…yeah, not even close.

So…what are quasars, really?

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What are Seyfert Galaxies?

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Meet NGC 1566, an intermediate spiral galaxy in the constellation Fornax.

To the human eye, this galaxy looks almost like any other spiral: It has a central nucleus and spiral arms, and it’s full of gas and dust. As an “intermediate” spiral, its nucleus has a bit of a barred structure, but not a strong one. All that’s pretty normal, as galaxies go.

But if we look a little closer, we see that this galaxy’s nucleus is unusually bright, especially for its small size.

As it turns out, NGC 1566 isn’t so normal after all…

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What is the Hubble Law?

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Last week, I teased you with the idea that it’s actually easy to estimate distances to galaxies.

I do mean estimate–and distance indicators are still important.

The Hubble Law is named for Edwin Hubble, the astronomer who was first able to settle the debate over what galaxies were–using the new Hale Telescope, the largest in the world at the time. But the Hubble Law is undoubtedly what he’s most famous for.

In order to understand the Hubble Law, though, we first need a little review of the Doppler effect…

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Unique Neutron Stars

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Neutron stars—the compact remains of massive stars that have gone supernova—are some of the most extreme objects in the universe, narrowly beaten by black holes (and, as we’ll talk about in future posts, active galaxies and such).

Dense balls of pure neutron material with diameters barely larger than Los Angeles, neutron stars have strong magnetic fields that produce beams of radiation at the magnetic poles. Their speedy rotation makes these beams sweep across the sky like a lighthouse.

When one of their beams crosses directly over Earth, human astronomers observe rapid pulses of light called pulsars.

These objects are whacky, to say the least. And there’s more…

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Binary Neutron Stars

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Way back when we spent a number of posts surveying the stars, we covered binary systems. These are star systems that contain multiple stars. Imagine if our sun had a companion, and two stars rose and set in our sky over the cycle of day and night.

It might surprise you that the majority of stars in the universe are actually in binary systems. Our solar system seems to be an outlier in that regard. Most stars have a companion or two or six…

…and so do some neutron stars.

Remember that neutron stars are the collapsed remnants of massive stars that have gone supernova. If most stars are part of binary systems, then naturally, some of these stars will evolve into neutron stars and still be part of their birth system.

Not all neutron stars are still part of their birth system. As I covered in my last post, many neutron stars rocket through space at incredible velocities, leaving their birth system behind.

Those that stay, though, provide astronomers with fascinating insight into the nature of neutron stars.

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What Exactly are Supernovae?

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This is one topic I bet you guys have been looking forward to since I first started posting about stellar evolution. Well, I won’t disappoint you!

In my last post, we covered how a massive star gets to the point of supernova. When it exhausts all the nuclear fuel in its core, iron ash is left behind—which can’t be fused or split for energy. That’s a dead end for the star, and the core begins to freely collapse…

Until a shockwave, originating in the center of the star, pushes outward. It’s stalled at first, but convection as in-falling material bounces off the dense core gives it a boost, and the star bursts apart.

Now, we’ll cover all the ins and outs of these spectacular explosions.

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What are Planetary Nebulae?

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Meet the planetary nebula, one of the universe’s most gorgeous phenomena.

If you’ve ever looked through a telescope, you may have seen one of these before. Through a small telescope, one might look like a little planet—hence the name. But make no mistake, these nebulae have nothing to do with planets, and everything to do with stars.

Up until now, we’ve covered how stars form, evolve, and eventually meet their end. They form out of a giant molecular cloud, or GMC. Eventually one cloud fragments and the cores condense into multiple stars, forming a star cluster.

The star then evolves across the main sequence, runs out of hydrogen fuel, expands into a giant, and begins to fuse helium in its core, which causes the star to contract a little and get hotter.

Then, as the star runs out of helium fuel in its core, it expands into a giant a second time. This is the last time a medium-mass star will expand. It’s also the end of the line for the fuel in its core, since it can’t get hot enough to fuse carbon.

At this point, the star is so big that gravity at the surface is too weak to hold onto its atmosphere, especially in the face of the superwind of radiation pressure from the still-collapsing core.

The result is a planetary nebula…but what exactly is a planetary nebula? What is it made of? Why does it look the way it does?

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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.

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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?

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What is a Nebula Made of?

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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.

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