Physics 320
Astrophysics I:  Lecture #9
Prof. Dale E. Gary

Formation of Protostars

A. What Is a Star and Where Do They Come From?

You may wonder how we decide between various objects such as stars and planets. Here are some possible statements that may seem reasonable to distinguish them:

In fact, none of these properties is sufficiently unique to use to classify when an object is a star or planet. The single distinguishing factor that defines a star is the occurrence of nuclear fusion. Nuclear fusion is the fusing (combining) of two atomic nuclei to form a heavier atomic nucleus, giving off a tremendous amount of energy. This is the same process that occurs in the hydrogen bomb (H-bomb), and requires tremendous pressure and temperature. In an H-bomb, the release of energy cannot be contained, resulting in an explosion, but in the center of a star the gravitational force due to the outer layers of the star keep the energy from escaping immediately, so the fusion process continues steadily over billions or even 10's of billions of years. An sufficiently large object that is too small to sustain nuclear burning of any kind is called a planet, but if the object is large enough that nuclear burning of some type occurs, it is called a star. Brown dwarfs, intermediate between a planet and a true star, do have nuclear burning, but of deuterium rather than hydrogen.

To make a star, then, we have to collect enough mass in a small enough region of space that the temperature and pressure grows high enough to begin nuclear fusion. The material to make a star comes from the gas and dust of the interstellar medium (ISM). It is gravity that provides the force needed to keep the object in one piece and cause it to shrink over time until it "ignites." An object that has enough mass to be a star, and is on its way to becoming dense enough and hot enough for nuclear fusion, is called a protostar. It is not yet a star, but given enough time it will become one.

Planetary systems are born during or just after the period of collapse of the protostar, and it is this process that we will examine in detail in this lecture.

B. Basic Picture of the Collapse

Clouds of gas and dust collapse and form Young Stellar Objects (YSOs), sometimes visible and sometimes enshrouded in dust. The relevant physics of the collapse includes

Thus, we can expect a cloud of typical size 0.1 pc to collapse over perhaps 1 million years, and when it reaches stellar size it should be spinning very rapidly and be highly magnetized. This is exactly what is observed, except that the spin rate and magnetic field strength are somewhat less than predicted by these simple arguments. Our best understanding of the reason for the discrepancy is that the strong magnetic fields themselves slow down the rotation rate by coupling the spinning core to the outer disk, and dynamical processes cause the proto-stellar object to shed some of its mass and much of its magnetic field. Observational Evidence

Difficulty of making these observations: The following three images show the star field around Fomalhaut, a zoomed view of the star field with an apparent circular gap in stars, and the same view with an actual image taken from the Deep Sky Survey (DSS). You can see why there are no stars cataloged near Fomalhaut--the extreme brightness of Fomalhaut makes seeing any stars nearby completely impossible. Imaging trying to see the protoplanetary disk--it requires clever techniques of occulting (covering up) the star in order to make the ring visible. Fomalhaut overview, Fomalhaut zoom, Fomalhaut DSS Image