Observations of Star Birth


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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:

  1. When a human is a mere fetus developing within its mother, a star is a protostar.
  2. 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.
  3. 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.
  4. An “adult” star is one that has begun to fuse atomic nuclei in its core for fuel. At this stage, the star has reached the main sequence.
  5. When a star runs out of fuel, it leaves the main sequence. We’ll cover this evolution in depth very soon.

I explained this process in depth in my last post. But I also posed the question: how do astronomers know all this? Where’s the evidence?

One such piece of evidence is T Tauri stars.

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T Tauri stars are found in most nebulae containing young stars. They are stars surrounded by dust clouds or disks. Astronomers also have evidence—in the form of Doppler shifts—that gas is flowing away from many of these stars.

But what are they?

T Tauri stars match our predictions for what protostars and YSOs should look like. The gas flowing away from them indicates that they are in the process of blowing away their dust disks, just as stars approaching the main sequence should.

Here’s another example of star birth in progress in the form of the Elephant Trunk, a dark nebula in the constellation of Monoceros (the unicorn).

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Notice how this dark dust cloud has been shaped by the stars around it. It’s like seeing little rilles in the sand of a riverbed.

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You can tell, because you’re familiar with what untouched sand should look like, that water has had an effect on this terrain.

The same goes for space. Dust clouds aren’t shaped the way they are just because. There’s always a reason, and these clouds are rarely undisturbed.

Here’s a closer look at the Elephant Trunk…

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There’s no water in space to shape dust clouds, but there are stars. Our own sun puts out a solar wind of ionized particles that constantly bombards every planet in the solar system. In fact, that wind is the main reason why Mars has very little atmosphere—and it’s the reason for Earth’s northern and southern lights.

If our sun puts out such a wind, so do stars. We call these stellar winds, and it seems that those exhaled by hot, young stars are particularly strong. Such winds are more than strong enough to compress and twist the Elephant Trunk.

Stellar winds aren’t the only culprit. These winds are composed of solid particles (that is, ionized atoms). Stars don’t just emit a steady stream of ionized particles. As we all know, they also produce radiation—visible light, heat, the UV rays that cause sunburns, and much more.

electromagnetic spectrum

Visible light is just a small portion of the spectrum of radiation—all of which stars emit.

Radiation exists as both a wave and a particle. Don’t ask me to explain it; it makes about as much sense to me as the idea in quantum mechanics that particles exist in two separate states right up until they’re actually observed. One of these days I’ll figure it out and let you know what I find.

Anyway, as I was saying…particles of radiation are known as photons, something you’ve probably heard of before—especially if you watch Star Trek and know all about photon torpedoes. And photons exert something called radiation pressure.

Together, stellar winds and radiation pressure—especially from newborn stars—can eat away at dust clouds and affect their shapes. Yeah, that’s one way that stars are very different from humans. I’m pretty sure newborn humans aren’t that powerful. If they were, I’d be scared.

T Tauri stars help assure astronomers that their predictions for protostars and YSOs are correct, and the shapes of dust clouds are evidence for the strong winds of the young stars nearby. But how do we know how the evolution process to become a main-sequence star works?

Below is an H-R diagram of stars in the star cluster NGC 2264, plotted according to their temperatures and luminosities. Open circles represent T Tauri stars and closed circles represent other types of protostars and YSOs. As I covered in my previous post, the birth line is the point when a star becomes visible.

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This H-R diagram is showing you a single snapshot of these star’s lives. All the data plotted here is from this one star cluster, from a single moment in time.

It shows that stars with more mass—and therefore more gravity—contract faster and have already begun to fuse atomic nuclei in their cores, earning them the distinction of “main sequence.” It also shows that stars with less mass have not yet finished contracting.

While most of the cluster’s more massive stars have reached adulthood, the lower-mass stars have some time yet before they “grow up.”

We’ve also made observations of very strange pairs of nebulae that remind me a bit of sundogs.

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Sundogs are atmospheric phenomena that are like little rainbows and usually occur in pairs. So too do Herbig-Haro objects.

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The dusty disk you see in the center of the image is that of a protostar or YSO; it’s easy enough to explain. But what’s going on with those weird plumes of color near the star’s poles?

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Apparently, jets of gas are the culprit.

These color plumes are individually called Herbig-Haro objects—that is, each one is a Herbig-Haro object, and newborn stars normally have two.

This is an example of a circumstance where observation both confirms our ideas and invites new questions. Nothing in science is ever so simple as question, answer, problem solved. Our answers usually bring up new questions—which is what I find so exciting.

Herbig-Haro objects do confirm what we already believe. Young stars produce powerful winds, and it makes sense that if they had a thick dust disk surrounding them, those winds would escape through the poles.

But…why aren’t the jets more uniform?

You’d think, if a star emitted a steady stream of gas and radiation, these jets would appear…well…smooth. But apparently that’s not the case. Something is going on with these stars that we don’t quite understand yet.

Yet more evidence of stellar youth occurs in the form of associations. These are basically star clusters, but the difference between an association and, say, the brilliant Double Cluster, is that associations aren’t permanently bound by their own gravity.

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the Double Cluster

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a T association

I know what you’re going to say. That’s a constellation, not a star cluster.

Exactly my point. An association is much more widely distributed than a star cluster. There are T Tauri stars all throughout the constellation Orion, and altogether, they form an association—specifically, a T association. There are also OB associations—associations of O and B stars.

By now, we’ve covered how stars form, how they spend their adolescence, and how they eventually grow up—within millions to billions of years. (Yeah, these are some long-lived kids.)

But what keeps them stable? How does a giant blob of gas stick around for millions—and in most cases, billions—of years?

That’s a topic we’ll explore coming up.

P. S.

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