From Cold Cloud to Hot Protostar


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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.

But they do heat up eventually. How?

Remember how I said that stars need little else but gravity to form?

Well, that’s the short answer.

For this post, we’re going to need to consider two different types of energy: gravitational energy and thermal energy. I discussed thermal energy in my previous post. It’s the total energy of all the moving particles within an object—in this case, a giant molecular cloud.

Gravitational energy, on the other hand, is much like kinetic energy.

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Kinetic energy is the energy a particle possesses due to its motion. Think about walking across your bedroom, versus jogging around your neighborhood, versus running a marathon. The faster you’re moving, the more energy you need to have, and the more you need to eat.

Humans actually need energy from food to move for the same reason, but I’ll elaborate on that when I finally move on to writing about the life sciences. Right now, remember that moving particles have energy—and the faster they’re moving, the more energy they have.

Gravitational energy is a similar concept.

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When the gravity of a molecular cloud’s dense cores begins to pull material inward, that material is now falling. Think about it: on Earth, “falling” means motion toward the Earth, or to be more precise, toward the Earth’s center of gravity.

In a giant molecular cloud, for any one particle, “falling” means moving in the direction of the gravitational force of one of the cloud’s dense cores. These falling particles have “gravitational energy.” And, like kinetic energy, this is energy due to motion.

Let’s focus on what’s happening to just one of these dense cores of material. As the cores grow denser, the GMC fragments into smaller but denser clouds. Material falls inward and picks up speed, just like an object falling on Earth.

The cosmological principle, which tells you that the laws of physics here on Earth are the same anywhere else in the universe, is a well-tested and accepted theory. It means that if objects accelerate—pick up speed—as they fall on Earth, then so do the particles that fall toward the center of a dense cloud.

This state of picking up speed as they fall inward is known as free-fall collapse. Towards the outer reaches of a core’s gravity, particles may be moving slowly, but by the time they reach the center, they are moving very rapidly.

The particles trapped in the gravity of this core have gravitational energy. But do they have thermal energy?

You can say that they have kinetic energy—the energy of movement. But that doesn’t mean they have thermal energy. While both concern the movement of the particles, thermal energy requires the particles to be moving in random directions, and right now, they’re all falling in toward the center.

When particles begin to accumulate at the center of the core, they can’t fall any further. They begin colliding with one another in the central region of the cloud. Now their motion becomes randomized and jumbled.

At this point, the cloud begins to grow hotter, and we can say that gravitational energy has been converted to thermal energy.

I would liken it to converting potential energy to kinetic energy.

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Here’s that diagram of potential and kinetic energy again. Potential energy isn’t so much energy as an object’s potential to have energy. If it’s going to be dropped, then the higher up it is, the more time it’ll have to accelerate and the more kinetic energy it will have.

So when dense clouds are contracting, they have gravitational energy. This is just the potential to have thermal energy, as once the material gets to the center and begins to collide, thermal energy will be generated.

There are many cases in astronomy where gravitational energy gets converted to thermal energy. Interestingly enough, we see one such case when stars nearing the end of their life cycle begin to expand and contract. We see this conversion from gravitational to thermal energy both at the beginning and at the end of a star’s life.

Now here’s the million-dollar question. Before star formation begins, giant molecular clouds resist the forces pushing them to contract simply with the energy of motion of their particles colliding and repelling one another.

So in a dense cloud, when the material begins to heat up, will this stop the contraction?

It won’t—not if it has a way to rid itself of the thermal energy.

I’ll bet I know what you’re going to ask next. What’s the point of all this, if the newly forming star has to get rid of its energy? How can a star ever form, if it can’t contract without losing what little energy it’s got?

Because it still has gravitational energy, being constantly converted to more thermal energy. The cloud has not finished contracting, and it is essential that it continue to contract. It needs to gain enough mass so that pressures in its core will be high enough to ignite hydrogen fusion.

And in order to continue to contract, the cloud must radiate away the thermal energy as it is converted. What I find incredible is the sheer perfection of this process.

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The core that will form a new star is still ensconced deep within a thick cloud of gas and dust. Only the longer wavelengths of radiation can penetrate the cloud. And by chance, the cloud’s low thermal energy means it can only radiate those longer wavelengths.

If that didn’t work, stars couldn’t form. Heat would get trapped inside the contracting cloud, and it would cease to contract. A star could never form there.

Star formation is possible simply because of a quirk of the electromagnetic spectrum—that cool objects emit long wavelengths, and the longer the wavelength, the better the radiation can penetrate thick clouds.

How do astronomers know all this? Because longer-wavelength radiation must escape the cloud in order for it to contract, we can look for that radiation with telescopes—or, specifically, with a spectroscope.

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As I’ve described in many posts, a spectroscope separates out the many wavelengths of radiation an object produces and shows us which wavelengths are being emitted the most intensely—and which wavelengths are being completely blocked by certain atoms.

An emission spectrum, specifically, is produced by the excited atoms of a hot gas, like that of a contracting cloud.

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If we aim a spectroscope at a suspected region of star formation, we observe emission lines in the far infrared, dubbed cooling lines. I imagine they’re called “cooling lines” because they are evidence of a cloud regulating its temperature by cooling off.

But this can’t last forever. Remember, the dust in the cloud is opaque to the shorter wavelengths of radiation—which carry more energy. And as the core continues to contract and get hotter, it will emit exactly that. These wavelengths won’t be able to escape and let the cloud cool off.

At this point, the cloud’s contraction slows. We can track its path on the H-R diagram

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The H-R diagram—named Hertzsprung-Russel for its creators—is a plot of all stars according to their temperature and luminosity. Temperature, as you can see, corresponds directly to color, and luminosity is a measure of the total energy emitted by the star—which corresponds directly to its surface area.

Meaning, a star could be very cool and still very luminous, as long as it is very large. Conversely, a star could be very hot and very faint, as long as it is very small. However, this graph only shows the main-sequence, the part of a star’s life cycle where temperature corresponds almost directly to size and luminosity.

You can see on the H-R diagram that a giant molecular cloud starts out very cool and very faint, fainter than most stars. When it breaks up into dense contracting clouds, the clouds are hotter and still very large. As the clouds contract further, they are also accumulating mass, so they grow hotter and more luminous.

But once a protostar is born, it stops accumulating mass. It continues to contract within its cocoon of dust and gases, causing it to shrink, and its luminosity drops as a result. And roughly at this point, a star is born—which we’ll explore in posts coming up.

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