Stellar Nurseries

Author: Cindy Hu

From: West Orange, NJ, USA

Orion Nebula (image credit)

Intro

Twinkle, twinkle, little star,

I don’t wonder

What you are.

For by spectroscopic ken

I know that you are hydrogen.

Molecular Clouds

As far as we know, the universe is fairly empty. But if we took a trip through space, we would eventually come across clouds of gas and dust, among other celestial objects and phenomena.

The coldest of these clouds, known as molecular clouds, are home to potential baby stars. Stars form when gravity becomes strong enough to pull a cloud of gas inward, so there are a few characteristics shared among all molecular clouds:

First, these clouds are typically very cold (between 10-30° Kelvin) because only then will gravity be able to overcome the gas pressure. Think about it this way: heating a gas will make its particles move faster. Faster-moving particles lead to high levels of pressure. So cold gases will have slower-moving particles and a lower level of pressure.

Secondly, these clouds have to be dense enough for their particles to combine into larger molecules. If the cloud was too spread apart, gravity wouldn’t be strong enough to pull it together.

Lastly, at least until its first stars are formed, these clouds will be dark. The cloud’s dust particles can also block out light from outside sources.

I guess molecular clouds aren’t the most comforting nurseries.

Protostars

Hubble image of a protostar (image credit)

Here’s the first step in the birth of a star. As previously mentioned, gravity will cause the coldest, densest regions of a molecular cloud to contract and shrink. While this happens, these regions of dust and gas will heat up due to the conservation of energy. (In this case, “heat up” is equivalent to reaching a temperature of 100° Kelvin, or a whopping -280° Fahrenheit. The universe is cold!)

This heated clump of dust and gas emits energy as infrared radiation, something we can’t see but can feel as heat. As the dust and gas continue contracting, it becomes harder and harder for radiation to escape because the particles of the cloud are so close together. Eventually, this radiation begins heating its center.

Since we already know that higher temperatures lead to more pressure, this means that the gas pressure (which pushes outward) here will begin to work against gravity (which pulls inward). The contraction of this clump of dust and gas will begin to slow down and has officially become a protostar.

Main Sequence

Remember how we said that molecular clouds had to be between 10 and 30 degrees Kelvin? Well, to become an “official,” or main-sequence star, the clump of dust and gas (previously a protostar) has to reach 10 million degrees Kelvin.

This is because a star has to be able to fuse hydrogen into helium efficiently, which only happens at insanely high temperatures. In fact, many elements essential to life on Earth today were made on stars that died long ago. So we’re all walking dead star corpses.

But I digress. Once the protostar has become a main-sequence star, its size stabilizes (at least until it dies). This is known as “energy balance.”

The amount of time it takes for stars to go through this formation process varies significantly depending on how massive they are. And in this case, “massive” means “having a lot of mass” rather than “really big,” which is the more colloquial definition.

For example, our sun spent about 30 million years evolving from a protostar to a main-sequence star. For more massive stars, this can happen in (only) about a million years. (They also die a lot faster.) For less massive ones, this can take over 100 million years! Slow to form and slow to die, less massive stars are the most common in our universe.

Failure

Brown dwarf (image credit)

Evidently, not all star births will be successful. The most massive stars, ones over 150 times the mass of our sun, will essentially blow themselves up.

On the other hand, “stars” with masses lower than 0.08 times the mass of our sun won’t get hot enough to reach 10 million degrees Kelvin. (Honestly, who blames them?) Without a high enough temperature, the “star” won’t be able to fuse hydrogen into helium. So instead of being classified as a main-sequence star, these failed stars are deemed brown dwarfs,

Conclusion

Let’s just take a moment to look up at the night sky and appreciate all the stars we can see. No matter how minute or far away they may seem to us, they’re all powerful pressure cookers that took millions of years to form. Going from a clump of dust and gas in a molecular cloud to a protostar to finally a main-sequence star, it’s undeniable that star formation is complicated.

Think about this the next time you hear “Twinkle, twinkle.”

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Author: Cindy Hu

Cindy is a high school junior in NJ who loves math, physics, and biology, among many other subjects. Her hobbies include reading, playing the piano, and learning to crochet.

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Citations:

1. Bennett, J. O., Donahue, M., Schneider, N., & Voit, M. (2018). The Essential Cosmic Perspective. New York, NY: Pearson.

2. The Star. (n.d.). Retrieved January 16, 2021, from https://www.tf.uni-kiel.de/matwis/amat/admat_en/kap_3/advanced/b3_3_1.html