Chapter 16

Molecular clouds are favorable locations for star formation for two reasons: low temperature and high density. Their low temperature keeps their pressures about the same as other interstellar clouds, despite the higher density. But the higher density means that gravity is stronger in molecular clouds, so it is able to overcome the pressure in molecular clouds. This increased gravitational attraction allows collapse, leading to star formation.

The heat generated as the clouds contract due to gravity is lost as photons. The photons are generated by the molecules' rotational and vibrational energy levels, which are excited by collisions between molecules. Since the heat can be radiated away effectively, it does not build up and star formation is not stopped.

Stars tend to form in clusters because more massive clouds are better able to overcome pressure and collapse. As these larger clouds collapse, the density increases and gravity gets an increasingly large advantage over pressure. So smaller and smaller parts of the cloud are able to collapse on their own, leading to fragmentation of the cloud. Each fragment becomes a star or a system of stars

We think that the first generation of stars must have been more massive than the Sun because there were no elements heavier than hydrogen and helium to form the molecules that cool molecular clouds. Since the clouds would have been warmer, the clouds would have had to form larger fragments to collapse. The larger fragments would have gone on to form larger stars.

When the cloud's thermal energy can no longer escape in the form of photons, the cloud heats up and the pressure rises. This resists the pull of gravity and slows the contraction.

A protostar is a clump of gas that will become a new star. It forms when a collapsed cloud fragment can no longer emit its thermal energy via photons so that the core heats up, raising the pressure and resisting the gravitational contraction. Protostars grow in mass because material around the protostar, which feels a weaker pull of gravity and does not collapse as quickly as the protostar, continues to contract. This material rains down on the protostar, gradually increasing the mass.

A protostellar disk is a spinning disk of gas around a protostar. Because of friction in the disk, material gradually spirals inward and eventually falls onto the protostar.

Jets are streams of gas that are fired out from a protostar into interstellar space at high speeds. We think that they are related to the protostar's rotation because they are fired out along the protostar's rotation axis. The jets help clear away the cocoon of gas that surrounds a forming star. They also pump a significant amount of kinetic energy into the surrounding molecular cloud, causing some of the turbulent motion that we see in the clouds.

The four stages are: (1) formation of a protostar, during which time it is clearing away dust and gas and is primarily convecting energy from its interior, so it increases in luminosity and temperature; (2) convective contraction, during which its surface remains at 3000 K, and it shrinks, dropping nearly straight down in the H-R diagram; contraction leads to decreasing luminosity but the temperature remains constant. (3) The decreasing luminosity reverses during radiative contraction, when the gas in the protostar's core becomes ionized enough so that radiation is the primary means of transporting energy. Its surface gets a little hotter too. (4) Fusion increases to balance the rate of energy escaping the surface: self-sustaining fusion is achieved and the star is now on the main sequence.

Degeneracy pressure is a quantum mechanical effect that halts the contraction of protostars with masses less than 0.08 solar mass. Unlike thermal pressure, degeneracy pressure depends only on density and not on temperature. Since degeneracy pressure does not weaken as the core cools, it will continue to support a core even when the core becomes cold.

The minimum mass for a star is 0.08 solar mass. Below this mass, degeneracy pressure halts the collapse of the core before it gets hot enough to start fusion. A brown dwarf is an object in which degeneracy pressure halted the collapse of the protostar before fusion began, making it a "failed star."

The maximum mass for stars is around 150 solar masses. This limit is set by radiation pressure, the pressure exerted by light. In stars larger than 150 solar masses, energy is generated so furiously that gravity cannot resist the force of radiation pressure and the extra mass is blown away into space.

Low-mass stars greatly outnumber high-mass stars, with a few hundred stars less than 0.5 solar mass for every star greater than 10 solar masses.

This statement is sensible. An infrared telescope can see both the visible stars and the stars shrouded in dusty molecular clouds.

This statement is not sensible. If a protostar trapped all of its energy, no photons would escape and we wouldn't be able to see it. This situation is physically impossible because even if the cloud is opaque, it will emit thermal radiation.

This statement is sensible. A colder cloud requires less mass to overcome thermal pressure; a denser cloud has more gravitational pull in a smaller volume.

This statement is not sensible. A protostar gains mass as matter continues to rain in on it during the protostar phase. A star can also lose mass in various ways during its lifetime.

This statement is not sensible. The rotation of a protostar is determined by the amount of angular momentum it has. A disk forms because the angular momentum of the gas particles prevents them from raining directly onto the protostar. A protostar's rotation will speed up as it collapses because angular momentum is conserved, and it can slow only if the protostar can shed its angular momentum somehow (jets, winds, etc.).

This statement is sensible. The most massive stars have main-sequence lifetimes of less than 10 million years, which is a shorter time than stars like our Sun spend on the pre-main sequence life track.

This statement is not sensible. Ultraviolet light is very easily absorbed by the molecular gas clouds surrounding protostars; also, the surface of protostars tends to be maintained at a constant 3000 K because of the absorption of light by hydrogen ions.

This statement is not sensible. Degeneracy pressure is not related to the temperature of a material. It is important when the electron density is so high that the laws of quantum physics prevent the electrons from getting any closer together.

This statement is sensible. 0.08 solar mass is 80 times the mass of Jupiter, so if Jupiter were 100 times more massive, it could ignite hydrogen in its core. If it were 10 times more massive, we'd probably consider it to be a brown dwarf companion of the Sun, although this classification could be debated.

This statement is not sensible, since (a) the main-sequence turnoff point for the stars born at the same time as the Sun is more massive than the Sun, and (b) many more low-mass stars form in any cloud than high-mass stars. Therefore the majority of the stars born from the same cloud as the Sun are still alive and well.

Which of these colors of light passes most easily through interstellar clouds?

Molecular clouds stay cool because their molecules emit photons. Which of these molecules produces the largest number of photons in a molecular cloud?

What happens to a cloud's thermal pressure if its temperature falls while its density rises?

What happens within a contracting cloud in which gravity is stronger than pressure and temperature remains constant?

Why are the very first stars thought to have been much more massive than the Sun?

What slows down the contraction of a star-forming cloud when it makes a protostar?

Which kind of pressure prevents stars of extremely large mass from forming?

Which kinds of stars are most common in a newly formed star cluster?