We’ve already mentioned that nebulas provide the raw material needed to form planetary systems. Now, let’s talk about the first few steps of the nebular theory in more detail.

2.4.1 Step 1: Start with a nebula

Nebulas are some of the most beautiful objects that have been photographed in space, with vibrant colors from the gases and dust they contain, and brilliant twinkling from the many stars that have formed within them . The very first nebulas in the universe consisted only of hydrogen and helium, with smaller amounts of lithium and beryllium. Thanks to stellar nucleosynthesis, the quiet deaths of low mass stars, supernova explosions from the death of high mass star, and collisions of white dwarfs or neutron stars, each successive generation of nebulas contained more and more heavier elements. In addition to gas, nebulas contain dust that is made up of tiny mineral grains, ice crystals, and organic particles. (Keep in mind that most of the visible mass of the universe still consists mostly of hydrogen and helium in proportions of 74% and 24%, respectively.)

Like most things, nebulas come in many types that can vary in chemistry, spectral characteristics, density, and/or origin. A “single” nebula, especially if it is really large, can be heterogeneous, containing regions of different types of nebulas. If the temperature is hot enough (for example, because the gas was recently released by a supernova), the gases in a nebula will be forced to exist in an ionized state, such H⁺. What’s cool about ionized gases is that they emit light at various wavelengths producing a “glow.” We call these glowing clouds of dust and gas emission nebulas (Figure 2.13). In other nebulas, gases exist in the neutral state and, instead, the dust reflects light from nearby stars; these are called reflection nebulas (Figure 2.14).

Over time, if unaffected by external sources of heat, nebulas cool and contract due to gravitational attraction between matter. If the temperature gets cold enough and the density of the material gets high enough, the hydrogen atoms in a nebula will bond together to form molecular hydrogen (H₂). This forms what we call a molecular cloud (Figure 2.14). (In other types of nebulas, hydrogen exists in an ionized or neutral state, without bonding to other atoms.) The temperature of molecular clouds is ~10-20 K which is very cold for the interstellar medium. Some regions of molecular clouds are so dense that they block (through absorption) the passage of visible light, giving them a very opaque appearance. These regions are called dark nebulas. Molecular clouds are often called star nurseries because it is at this nebular stage that stars are able to form from gravitational collapse of dense clumps.

2.4.2 Step 2: Grow clumps

To create a planetary system, the cold dust and gas in a molecular cloud must be brought closer together to form higher density pockets called clumps. These clumps have masses 50 to 500 times the mass of the Sun. Exactly how clumping starts isn’t clear; it might be triggered by the violent behavior of nearby stars as they progress through their life cycles, a shockwave from a black hole, or collisions with other molecular clouds or galaxies. All of these possible disturbances release a wave of energy that compresses the gas and dust in nearby neighborhoods within the cloud, allowing gravitational attraction to more effectively pull the material together into a smaller region. Once triggered, the clumping of gas and dust continues in almost a runaway-effect. As the clump grows in mass, its gravity gets stronger, allowing it to pull more material onto itself and increase its mass even more. In this way, clumps get bigger and denser over time.

2.4.3 Step 3: Collapse the core of a clump and put a disk around it

Within clumps, at their center of mass, there are even denser, smaller regions called cores. Cores are considered the embryos of star. The conditions in these cores—low temperature and high density—are just what is required trigger gravitational collapse and eventually make stars. (When we use the word “collapse,” we are talking about an object shrinking in radius and, therefore, increasing its density and temperature.)

Remember that the essence of the life story of any star is the ongoing competition between two forces: gravity and pressure. The force of gravity, pulling inward, tries to make a star collapse. Internal pressure produced by the motions of the gas atoms, pushing outward, tries to force the star to expand. When a star is first forming from a clump core, low temperature (and hence, low pressure) and high density (hence, greater gravitational attraction) both work to give gravity the advantage.

Gravity works to pull more matter into the core from the surrounding clump and ambient molecular cloud. Eventually a critical threshold is reached where the gravitational force of the infalling gas and dust becomes strong enough to overwhelm the pressure exerted by the cold material that forms the dense cores. The material then undergoes a rapid collapse under its own weight, and the density of the core increases greatly as a result. During this time when a dense core is contracting to become a true star, but before the fusion of protons to produce helium begins, we call the object a protostar.

The process of gravitational collapse also generates heat. When gas and dust particles fall inward, their gravitational potential energy is converted into kinetic energy. This kinetic energy is converted into thermal energy when individual gas and dust particles inevitably collide with one another during contraction. A protostar must continue to collapse and heat up to become an actual star (specifically a main sequence star).  Its internal temperature be exceed 10 million Kelvin for hydrogen to be able to fuse into helium. The amount of time it takes for a star to form from the initial collapse of a nebula is dependent on the star’s mass. We think it takes around 50 million years to create stars like our Sun.

The gas and dust that does not become part of the central protostar settles into a flat, rotating disk called a protoplanetary (or accretionary) disk. This disk is where planets will eventually form. Figure 2.16 shows an artist’s impression of a protoplanetary disk, and Figure 2.17 shows an actual protoplanetary disk surrounding the star HL Tauri.

When you watch videos summarizing the nebula theory, you will notice that both the protostar and the accretionary disk spin faster than the original nebula from which they formed. This is a result of the conservation of angular momentum. When gravitational attraction pulls gas and dust inward, it effectively decreases the size of the system which, in turn, causes the material to spin more rapidly. Furthermore, because the protostar formed from the contraction of a greater amount of material, it spins more quickly than they surrounding accretionary disk.

The Orion Nebula located in the “sword” of the Orion constellation is a active star forming region located relatively close to us. It is favorite study area of the Hubble Space Telescope and has allowed scientists to better understand the process of planetary system formation. Watch the first video listed below for a summary of what the Orion Nebula has taught us and the second video for additional information about the collapse of molecular clouds.

Text Attributions

The text of this chapter is adapted from:

• Section 21.2 of OpenStax’s Astronomy 2e (2022) by Andrew Fraknoi, David Morrison, and Sidney Wolff. Licensed under CC BY 4.0. Access full book for free at this link.
• Section 22.3 by Karla Panchuk in Physical Geology – 2nd Edition (2019) by Steven Earle. Licensed under CC BY 4.0, except where otherwise noted.

Media Attributions

• “Hubblecast 32: Born in Beauty: Proplyds in the Orion Nebula.” YouTube, uploaded by HubbleWebbESA, 24 Jun 2010, https://www.youtube.com/watch?v=Ey9OiGwAfQc&t=206s. Licensed under CC BY 4.0.
• “Classroom Aid – Giant Molecular Cloud Collapse.” YouTube, uploaded by David Butler, 18 Oct 2019, https://www.youtube.com/watch?v=d9sZn3KbR9k. Licensed under CC BY 4.0.