Planets, Stars, Galaxies, and the Universe

The Process of Star Formation


Additional reading from

Giant molecular clouds (GMCs) are truly giant. In fact, it is difficult to choose a representative image to show you because, in most cases, you can't see the cloud (you can only see part of the cloud). For example, the famous pillars in the Eagle Nebula image are only a small part of a vast giant molecular cloud. GMCs can contain gas with a mass as much as several million times the mass of the Sun. So, you should keep in mind that a single cloud in the ISM can perhaps create thousands or millions of stars. As we discussed previously, dark clouds absorb and scatter light. As a consequence, they are quite cold, with 10 kelvin being accepted as a typical value for their average temperature. Their internal density is quite high compared to typical values for interstellar space, but still many orders of magnitude lower than the pressure in Earth's atmosphere at sea level.

Try this!

You may be familiar with the properties of gases, but, as a reminder, you can run the Colorado PhET simulation on gas properties.

  1. Fill the box with a light gas.
  2. Use the heat control to lower the temperature to 10 K, the typical temperature of a molecular cloud.
  3. Observe the motion of the particles in the gas.
  4. Use the slider on the right hand side to increase gravity.
  5. Again, observe the motion of the particles in the gas.

If you consider a spherical cloud of gas, you can write out the mathematical relationship for the force of gravity pulling the cloud together and the radiation pressure of the gas in the cloud resisting the inward pull of gravity. (The gas properties simulation above is an excellent demo you can use to demonstrate how gas exerts an outward pressure on the cloud.) You can then write the expression for the mass of a cloud that has these two forces exactly in balance. This quantity is usually referred to as the Jeans mass, and it can be thought of as the minimum mass for a cloud to have its internal pressure balanced by gravity. So, what happens to a cloud that exceeds the Jeans mass? The cloud will be unstable to gravitational collapse, meaning that if some event can cause the cloud to begin to collapse, its internal pressure will not be strong enough to resist the collapse. Depending on the density and temperature, clouds of thousands of solar masses in size are generally above the Jeans mass. Once the collapse begins, the cloud's density and temperature will increase.

Interstellar clouds are not very uniform, and we expect that as they collapse, they will fragment into a number of clumps. During the collapse phase of an individual clump, a few additional effects occur. First, the clump in the cloud will attract nearby gas particles, causing the clump to grow more massive. Early in the process, the cloud is still thin enough that the photons generated inside the cloud (remember, any warm gas is emitting some light) can easily escape, so that the cloud radiates light as it collapses. (It should be noted here that even though the clump is radiating light from its core, the surrounding GMC is still so dense and dusty that the light from the core does not escape, and we are not able to see these cores with telescopes that detect optical light.) At some point, though, the core of the collapsing clump becomes so dense that the radiation being generated deep inside the clump becomes trapped (it has become opaque), causing the temperature of the core to increase quickly. At this point, the core can be referred to as a protostar.

We should consider for a moment size scales. A typical interstellar cloud is of order 1014 km, or roughly 10,000 times larger than the size of the Solar System. A cloud fragment which forms one or a few protostars is of order 1012 km, or roughly 100 times larger than the size of our Solar System. By the time the core of the fragment has become a protostar, its size will be approximately 1010 km, and its temperature of order 10,000 kelvin.

The temperature in the core of the protostar is hot enough that the thermal pressure becomes strong enough to slow the collapse down to a much slower contraction. The continued contraction of the protostar converts gravitational potential energy into thermal energy, causing the object to radiate as much light as 1,000 Suns. During this same time period, conservation of angular momentum causes the initially very slowly rotating cloud to spin much more rapidly. There will be significant centripetal force to resist further gravitational collapse along the equator of the rapidly rotating protostar, but the material near the poles will not feel this same resistance. Because of this, a disk will form around the central protostar. This disk is called either a protoplanetary disk or a proplyd. We will see in later lessons that this material is perhaps the location of the origin of planets that orbit stars.

Watch this!

I have personally found it quite challenging to teach the physics behind how rotating, roughly spherical clouds of gas collapse down to form protoplanetary disks. The detail in the previous paragraph is sparse for exactly this reason; it is not easy to describe conceptually. However, the team behind "Minute Physics" has posted an excellent short video that does quite a good job at explaining how these spherical clouds collapse to form disks. I highly recommend watching their video:

Here is an excellent image of a protoplanetary disk seen in silhouette around a protostar in the Orion star forming region:

Hubble Space Telescope image of a protoplanetary disk silhouetted against a background of light from the larger nebula.
Figure 5.6: A Protoplanetary Disk Silhouetted Against the Orion Nebula
Credit: Hubblesite

As the contraction phase of the protostar continues, the core temperature begins to reach more than 1 million kelvin. The protostar undergoes some violent changes during this time period; the outer parts of the clump are radiating an enormous amount of light, but the amount of light varies by large amounts on short time scales. This is usually called the “T Tauri” phase, after the first such object discovered. Stars in the T Tauri phase are also sometimes observed to be emitting bipolar outflows of material. These bipolar outflows have their own name; they are usually referred to as Herbig-Haro or HH objects. Some examples can be seen at Hubblesite: HH objects.

Also, during this T Tauri / HH phase, the protostars somehow shed some of the outer layers of material leftover from the GMC, revealing them for the first time to telescopes that can detect optical light. If you recall the discussion of the Hubblesite: The Eagle Nebula image, photoevaporation is one example of a process that may strip the outer layers from the protostars in a GMC. In a molecular cloud like the one that includes the Eagle Nebula, we see that the brightest stars form first, and their intense radiation evaporates the outer layers of material hiding the smaller protostars.