Our sun is undoubtedly the star we know the best. It’s only 93 million miles away—which might seem far, but isn’t that large a distance when you realize that the nearest neighboring star is a whole 4.3 light-years away.
As in, it takes light—yeah, that same stuff that hits the ground from your flashlight in a split second—a whole 4.3 years to get here.
We’re pretty familiar with our star’s interior. We know it produces most of its energy in its core, a relatively small but very hot region at its center. We also know that energy then radiates outward until it hits the convective layer.
There, the energy gets stuck in circulation for a bit until it finally manages to leave the sun’s surface.
But…how normal is that? Is it the same for all stars, or just the sun?
All life as we know it has to maintain homeostasis—that is, keep internal goings-on regulated. Body temperature is just one example. Mammals can maintain a stable body temperature with no trouble. Reptiles have to bask in the sun to keep warm.
You’re probably familiar with this idea. When you sweat, your body is trying to cool down. When you shiver, it’s trying to warm up. These are all examples of your own body maintaining its own homeostasis.
And then there’s blood pressure, heart rate, hormones, and pH—not that I have any real idea how all that works, but I know they’re all things that your body regulates on its own. Homeostasis is an important thing. Basically, when it fails, things go wrong.
Astronomers have a pretty solid idea of how stars are born. They begin within the dense, cold dust of an interstellar cloud such as this one. They heat up and get more luminous as they contract, and then drop in luminosity as they continue to contract steadily toward the main sequence.
I’m going to spend at least the next ten or so posts talking about the main-sequence portion of a star’s life cycle. Basically, we’re talking about a star’s adulthood.
You know what, while we’re at it, why don’t I draw up an analogy between a star’s life cycle and that of a human:
When a human is a mere fetus developing within its mother, a star is a protostar.
We say a star has been “born” when it crosses the birth line—basically, satisfies certain expectations for its temperature and luminosity for its specific mass—and becomes visible.
After that, a star steadily approaches adulthood. A “child” star is referred to as a Young Stellar Object (YSO) or a pre-main-sequence star.
A protostar forms when one dense core of an interstellar cloud condenses enough so that gravity can overcome the repulsive forces between the particles, and collapse the cloud. A very cool object then forms in the cloud’s depths, visible only at infrared wavelengths—known as a protostar.
A protostar is compressed enough to be opaque no matter the wavelength—that is, no radiation can pass through it due to its density. However, what separates it from a “true” star is that it’s not compressed enough to generate energy by nuclear fusion.
Astronomers also define a protostar specifically as a young star that’s not yet detectable at visible wavelengths. In other words, protostars emit only longer-wavelength light—that is, infrared and radio waves.
You’d think that becoming a true star would be the next step for a protostar. But that’s not quite how it happens… Continue reading →
Yeah…we’re talking about the Orion Nebula again. I know, we already took a tour through the Orion constellation in my last post…but there’s still more to cover about how stars come to life, and Orion is still the best case study I know.
So…hold up a second. Contagious star formation? What’s that supposed to mean? I mean, usually, when you think about “contagion,” you think of catching diseases from others around you. So…can stars get sick?
Well, no. Stars are pretty good at maintaining their own homeostasis, something I’ll explain in a later post. By “contagious” star formation, I mean that star formation can trigger more star formation.
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One of my favorite objects to show people at astronomy outreach events is the Orion Nebula. Not only does it reside within a fairly well-known constellation, but it’s a gorgeous sight to see with a good telescope.
There’s no time like the present up here in the northern hemisphere. Orion is a winter constellation and rises high in the sky this time of year. Not to mention, as a stellar nursery, talking about the Orion Nebula follows on perfectly from my last couple posts on star formation.
If you’ve ever seen the Orion Nebula through a small telescope, you’re probably wondering what all the rage is about. It mostly just looks like a bluish haze around a star—like the telescope operator didn’t tune the focus quite right.
Paradoxically, stars begin in the galaxy’s coolest places: the dense giant molecular clouds (or GMCs).
This is not quite the paradox it seems, as in the beginning, stars require little else but gravity to form. And that’s really quite lucky, because one thing they do need is regions of high density, and high density is unlikely to occur where temperatures are high.
And so stars begin in perhaps the most surprising of ways: as a high-density bundle of very cool gases within an equally cool interstellar cloud.
Astronomers have discovered that the clouds of gas and dust—the interstellar medium (ISM)—found between the stars are made of the same materials as the stars themselves. In fact, hydrogen is the most common element in both stars and the ISM, followed closely by helium.
But it would be more accurate to say that stars are made of the same material as the ISM, not the other way around.
This is because all of the stars formed out of material in the ISM at some point millions to hundreds of billions of years ago. And when they die, they return that material—what’s left of it—to the ISM.
Contrary to popular belief, space is not empty. The space between the stars is filled with clouds of dust and gas. And this space—the interstellar medium—is incredibly beautiful and fascinating.
I often refer to the interstellar medium as the galaxy’s “backstage.” Why? Because it’s not the part of the universe that astronomy enthusiasts usually think about. And yet, there are whole studies devoted to studying this natural wonder of the universe.
Also, the interstellar medium is largely hidden from us. There are ways we can detect it—when light from a distant star passes through it, for example. And with our eyes, we can see nebulae, the visible evidence of this interstellar expanse.
The backstage of a theater is similar—it’s not the main part of the show, but you sometimes see evidence of it in the forms of new costumes donned as the play progresses and new props brought into play. The audience often forgets about it entirely.
Nevertheless, it’s beautiful. Stars are born out of giant molecular clouds, triggered by compression from expanding bubbles of coronal gas. The interstellar medium spells our beginning.