supernovae

The Composition of the Milky Way

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What is our home galaxy made up of?

In the broadest sense, it’s made up of stars, clouds of dust and gas, and the mysterious dark matter.

We could also get a little more detailed. We could say that it is a great wheel of stars, made up of a thin disk component, a central bulge, and a broader spherical halo that surrounds the disk.

We could even build on that, and say that the thin disk is where all the youngest stars are found. We could say that within the thin disk are spiral arms, where the star formation actually happens. We could say that the oldest stars are found in the central bulge and the halo, where there is very little dust and gas to make new stars.

But…what about its chemical composition? If we could explore our galaxy and bring home test tubes of “star stuff,” what would we find? And what can that tell us about our galaxy’s history?

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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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Pulsars as Neutron Stars

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For those of you who missed my last couple of posts, allow me to introduce the neutron star: a stellar remnant similar to a white dwarf, but much denser, so dense that its protons and electrons have combined to form a neutron soup.

A neutron star forms from the collapsing core of a star between 10 and 20 M☉ (solar masses). Its collapse produces powerful magnetic fields and extremely high temperatures, but because it becomes so small—less than the size of Los Angeles—it is very faint and radiates away its heat very slowly.

The exception to that rule comes in the form of two powerful beams of radiation that blast away from the object’s magnetic poles. As a neutron star spins—at around a hundred times per second—these radiation beams sweep across the sky like the the beams of a lighthouse.

If these beams happen to sweep over Earth, human observers see regular, rapid pulses of light. This visual phenomenon produced by neutron stars is called a pulsar.

Now that we have a basic understanding of neutron stars and pulsars, let’s explore some of the details of how these extreme objects work.

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Why Neutron Stars Should Exist

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Above is a theoretical rendering of a white dwarf, the collapsed husk of a low-mass or medium-mass star. Interestingly enough, these strange cosmic objects—which begin their existence as intensely hot balls of carbon the size of the Earth—may eventually cool off and crystalize into giant space diamonds.

White dwarfs are made up of free-floating hydrogen and helium nuclei and degenerate electrons—and their mass is supported by the nature of these electrons.

But degenerate electrons, like any other material, have a specific material strength. What happens if they’ve, well…just got too much stuff to support?

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How Massive Stars Die

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When people think of star death, they most often think of supernovae (plural for supernova). So why haven’t I spent the past bunch of posts on star death talking about them?

Because supernovae are not actually the most common fate to await a star. Only a small fraction of the stars in our universe are massive enough to go supernova. Most stars die fairly quietly, gently expelling their outer layers and contracting to form white dwarfs.

No such gentle fate awaits the most massive stars.

But why do massive stars go supernova?

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What is Coronal Gas?

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Stars are hot. Space is cold. We’re all familiar with that, right?

Ok, good.

Technically, it’s more complicated than that. Space isn’t completely frigid—absolute zero, the temperature at which there is no heat whatsoever, is purely theoretical and not thought to exist in the universe. But it is pretty darn cold.

And stars aren’t always very hot—there is one newly discovered star that’s only as hot as fresh coffee. (It’s a brown dwarf, and if you go by the definition of a star as an object that’s ignited hydrogen fusion in its core, then it doesn’t actually count.)

In general, though, stars are pretty darn hot. Some special types of stars reach up to 200,000 K—that’s 359,540.33℉. Our own sun is about 5,778 K, which much cooler, but still almost ten thousand degrees Fahrenheit.

As a rule, we can think of stars as being much hotter than the space in between…except in the case of coronal gas.

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