A Star is Born

For about ten years of my life, I was really into celebrity gossip. Given that I’m also an astronomer, my mom used to say I was interested in the stars and the stars. Now, I don’t follow the gossip accounts as much, and my research never really hewed towards actual stars.

But both astronomical and earthly stars do genuinely have something in common: they both shine brightly.  In this newsletter, I’m going to be giving you an introduction to some of the coolest things in astronomy—the things that I taught in my classes and the things that I think are just neat. Stars seemed like a fun place to start.

Stars form in the clouds of gas and dust that permeate galaxies.  These clouds are called the interstellar medium, and they are made up of mostly hydrogen, with a little bit of helium and other stuff (aside: the astronomy periodic table of the elements is really easy—it’s just hydrogen, helium, and metals, where everything that’s not hydrogen or helium is a metal. Chemists love us.).

Astronomers subdivide the interstellar medium further into hot gas, warm gas, cold gas, and interstellar clouds, also known as giant molecular clouds.  At 10-20 Kelvin (which is a unit of temperature--the coldest anything in the universe can ever get is 0 Kelvin, or absolute zero.) these giant molecular clouds are the coldest and most dense part of the interstellar medium Don’t be fooled, though. These giant molecular clouds are still very low density compared with anything we’re used to here on Earth.  Because of their low temperature and high density, giant molecular clouds are where more complicated molecules start to form.

At the cores of these clouds, stars are born.

As with all things stars (human and celestial), there is a constant competition.  In the stars in our night sky, the collapsing pull of gravity competes with the outward push of pressure.  This pressure is caused by the motion of the atoms inside the star and the photon radiation that interacts with them (this is why we call it radiation pressure). Inside the giant molecular cloud, the low temperature and high (relatively speaking!) density work together to give gravity the edge, which causes the gas to start to collapse and form a star. This is because as the gas collapses, the density increases, kicking off a runaway process to start gravitational collapse.  As this happens, the internal temperature of the cloud increases.  Eventually the density and temperature are both high enough that hydrogen starts fusing into helium. At this point the pressure from the motion of the atoms is enough to balance the collapse from gravity and we have a full-fledged star, ready for its close-up.

The cloud has to reach a specific mass before a star is born, however. This mass differs from cloud to cloud and depends on the density and temperature of the cloud. We call this mass the Jeans mass, named after astronomer James Jeans, who was the first person to do this calculation.  If the cloud is at the Jeans mass stars will form almost immediately.  All cold, dense parts of the interstellar medium are unstable to gravitational collapse, meaning that they will eventually form stars. 

You may wonder why stars always form in clusters. That’s because the Jeans mass is inversely proportional to the square root of the density of the cloud.  This is a mathematical way to say that as the cloud collapses and the density goes up, the Jeans mass decreases, and the cloud will fragment into lots of stars. This is also why we have more low mass stars—they are more likely to form because it’s easier to make smaller fragments (our Sun is a medium-size star of average mass).  Eventually the temperature starts to increase during the collapse, and the fragmentation stops, stopping the formation of new stars.

Just like celebrities attract hangers-on, all stars eventually give rise to planets.  The turbulence that occurs during fragmentation also imparts a spinning motion to the cloud. Conservation of angular momentum says that as the cloud shrinks in size, the cloud will spin faster. As the protostar rotates, it is easier for material to fall onto the poles of the star (rather than on the equator). This creates a thin accretion disk around the star, which is where planets eventually form. I’ve crudely illustrated this here:

So, what direct evidence to we have for that stars and planets form in this way? Unfortunately, we can’t directly observe the process, because it takes place on such short time scales, astronomically speaking.  The collapse phase takes on order thousands of years, which to us seems like a long time, but astronomically speaking is a blink of the eye to the universe. Because the collapse phase is such a small fraction of the star’s life, very few stars are in this phase at any given time, making it very difficult to observe. Astronomers have pieced together the process described above through the rare observations that we do have combined, with theoretical calculations. 

While I’ve described the process for a single star just to make things easier, most stars actually form in binary or triple systems. Depending on how close to each other they are, they may or may not share the same accretion disk.  Our sun is one of the unusual single stars out there!

Once the stars have formed, they all follow the same trajectory, more or less, until they use up all the hydrogen that they have been converting into helium.  Once this happens the end of a star’s life takes a very different path depending on how massive the star is.  I’ll talk about this later, but for now, just know that the biggest stars in our sky, live the biggest stars in Hollywood from James Dean to Amy Winehouse, live fast and die young.