3-Introduction

The nebular hypothesis and the formation of stars in dark molecular clouds is one of the most interesting stories in science.

Outline and Introduction

This introduction to the chapter begins with a review of Immanuel Kant’s nebular hypothesis. I moved stellar classification up because it is important to understand how temperature and mass govern the behavior of stars prior to learning about natural history of stars. The first stars in the universe were massive and did not form in galaxies. Instead, they became the black holes that are the cores of galaxies. Once galaxies formed, all stars in the universe formed in spiral galaxies, such as our own Milky Way. The formation of the sun, as well as other stars began with the collapse of a molecular cloud and formation of the protosun. The sun  entered the main sequence phase when the sun became hot enough to cause nuclear fusion. Evidence of supernovae in meteorites indicates that the sun was born in a dense cluster of stars. Spectral chemical evidence points to old open cluster M67, but models indicate that the sun could not have exited M67 so it is now rejected as the origin of the sun. I included star death in the life cycle of the sun. This history of how religions and philosophers have viewed the nebular hypothesis is quite interesting.

One of the most amazing things about the universe is that dark molecular clouds with vacuous density eventually collapse and form stars, solar systems, and planets. The black areas in Figure 3-1 are the tentacles of a dark molecular cloud blocking the light from the stars behind them. Some people question whether such complexity with highly organized energy and planetary motion can come from the disorder of a vacuous cloud; however, the second law of thermodynamics (entropy) allows complexity to come from disorder in this system because during cloud collapse, gravitational energy in the enormously large cloud is converted to heat and pressure in the relatively small protostar, which then triggers nuclear fusion and the subsequent release of radiation by the star for billions of years. The rotation of the cloud stretches out the disk.

Stellar classification

Scientists became interested in stellar classification (Section 3-4) in the 19th century. In 1894, Edward Pickering at Harvard hired Annie Jump Cannon, Henrietta Leavitt, and Ida Woods as human computers (they were called computers) to calculate the positions and temperatures of stars from photographic plates. All three ladies became famous astronomers. Annie Jump Cannon, the hero of this story, was stricken with scarlet fever when she was about 30 and lost her hearing, and thereafter dedicated herself to astronomy. Possibly because of her lack of hearing, she did not socialize very much, and she never married or had children, which left her plenty of time to look at stars. She and Edward Pickering developed the Harvard Classification Scheme in which they classified stars based on their color (spectra) and temperature. Subsequently, the Hertzsprung-Russell diagram was developed independently by two different astronomers in 1911. It shows the relationship between temperature and luminosity of stars, which are also related to their mass. Scientists have used various techniques to evaluate millions of stars, such as analysis of globular clusters, to learn about the processes in stars. 

Stars in the early universe

In the early universe, the temperature of the universe cooled after a few hundred million years, and stable atoms formed. Clouds of molecular hydrogen and helium formed stars when they collapsed on themselves by gravity. Section 3-2 describes how the gravity of dark matter facilitated the gathering of clouds in the young universe, which then collapsed and formed giant blue stars. These stars exploded as supernovae and became the black holes at the core of galaxies.

Star formation in molecular clouds

As the universe expanded, molecular clouds became less dense, and stars could no longer form in intergalactic space; however, dark molecular clouds continue to form in the Milky Way and other spiral galaxies. They are up to hundreds of light-years in length and block out the stars behind them (Figure 3‑1). Dark molecular clouds exist between 10 to 100 million years prior to cloud collapse and star formation. Section 3-3 describes how the clouds in spiral galaxies gather by gravity and then collapse and form stars in beautiful stellar nurseries.

Birth environment of the sun

Section 3-6 describes the birth environment of the sun. One cluster of stars, M67, has stars with chemical signatures that match the sun’s chemical signature, which indicates that the sun formed in M67. Another indication of the sun’s birth environment is the presence of certain isotopes in meteorites that indicate supernova exploded near the young sun, which would also take place in a cluster; however, early in its life, the sun left the cluster and became a lone star orbiting in the Milky Way. This isolation from the gravitational effects and radiation of other stars allowed the planets in our solar system to form with relatively circular orbits.

Star death

Section 3-7 describes star death, which is one of the most interesting fields of study in astronomy. They have spectacular deaths, and depending on their classification, end as red giants, supernovae, or planetary nebulae and produce many heavy elements that will be incorporated in future stars and planets. The sun formed 4.6 billion years ago, and scientists predict that the sun will exist for another 5 billion years before it dies.

Section 3-8 Excursus

This section is optional. It compares the Mosaic description of a dark chaos followed by light to the formation of the sun in a dark molecular cloud.