FEATURE ARTICLE
Protostars
"Stellar embryology" takes a step forward with the first detailed look at the youngest Sun-like stars
Thomas Greene
Baby Steps to Stardom
The process of making a star begins inside enormous clouds of interstellar gas and dust that lie primarily within the plane of the Galaxy (Figure 1). Consisting chiefly of molecular hydrogen (H2), the largest of these giant molecular clouds may be several hundred light-years across and weigh as much as 100,000 solar masses. These clouds have an average density of about 100 H2 molecules per cubic centimeter, but they are far from being homogeneous. Some regions may have a density of 10,000 molecules or more per cubic centimeter, or more than 1,000 times the density of the most rarefied parts of the cloud. A molecular cloud may have many such dense cores (each of which may become a star), but the star-formation process is not very efficient, as a 100,000-solar-mass cloud never yields 100,000 Sun-sized stars. In fact, the conversion efficiency of cloud mass to stellar mass probably averages less than 10 percent.
Giant molecular clouds are supported against their weight by thermal pressure, turbulent gas motions and magnetic fields within, but at some point their dense cores become gravitationally unstable and begin to collapse (Figure 2). The center of the collapsing core becomes a protostar, which, as the name suggests, is the first step toward stardom. The protostar's life (lasting about 100,000 years) is defined by the rapid accumulation of mass from the surrounding envelope of gas and dust (Figure 3). It does so at the rate of a few millionths of a solar mass—equivalent to one Earth-sized planet—every year. Coinciding with the onset of accretion is a steady outflow of material in powerful winds that emanate from the poles of the young star. These bipolar jets are telltale signs of a protostellar system, and it is ironic that protostars are often detected by the jets that shed mass from the system, rather than the process of accretion (Figure 4).
Throughout this period, the protostar progressively increases in density as it shrinks in size. The infalling material, which had been rotating relatively slowly around the dense core, begins to speed up as the radius of the protostar decreases. The angular momentum—which is the product of rotational velocity and radius—of the core material remains constant, so material rotates faster as it gets closer to the protostar. Slowly moving material falls directly onto the protostar, but some of the gas and dust is moving so quickly that it travels in an orbit instead. Since all of the material in the envelope surrounding the protostar rotates in the same direction, the matter falls into orbits of various sizes depending on its velocity, and so forms a circumstellar disk. Most matter eventually flows onto the protostar through the disk, but some of it remains in orbit. As the surrounding envelope of dust disperses, the accretion process stops, and the central globe of gas is no longer considered to be a protostar; it is now a pre-main-sequence (or PMS) star. (Protostars and PMS stars are often grouped under the term young stellar objects, which neatly includes all prenatal stars.)
The pre-main-sequence phase of evolution lasts for tens of millions of years. In its earliest phases, the first few million years or so, these objects are often called T Tauri stars, a name derived from the archetypical star in the constellation Taurus. Having shed their dusty envelopes, T Tauri stars are the youngest objects that can be seen with an optical telescope. They are still surrounded by a disk of dust and gas—often called a protoplanetary disk at this stage—and may continue to eject material in their bipolar jets. After a few million years, much of the dust and gas in the protoplanetary disk dissipates, leaving a bare PMS star in the center. In some instances, a few large bodies may continue to orbit the star in a remnant debris disk (Figure 5). The planets, moons and asteroids of our solar system all had their beginnings in such a disk.
At this stage the internal structure of the star is determined by a balancing act between gravity, which compresses and heats the object, and the pressure of the gas, which acts to expand the star. The gas pressure is proportional to the star's temperature, which is only about 3,000 to 4,000 kelvins (degrees above absolute zero) in the outer regions but nearly one million kelvins in its core. This represents a dramatic increase from the frigid 10 to 20 kelvins typical of the dense cores in a molecular cloud, but one million degrees is still considered to be "cool" for a stellar core. This temperature is just hot enough to fuse a deuterium atom (hydrogen-2) and a proton into helium-3, a process that releases only a modest amount of energy.
It takes tens of millions of years, but eventually the crushing force of gravity wins the battle against the outward thermal pressure of the gas within the star. The compression of material raises the temperature of the star's interior to about 10 million kelvins—hot enough to fuse four protons into one helium-4 atom, which is the primary generator of energy within a true star. The event marks its arrival on the main-sequence phase of its evolution. This period of stellar evolution is extremely stable, and may last for many billions of years. Our Sun is a fine example of a main-sequence star—one that has been steadily burning hydrogen for 5 billion years, and should continue to do so for another 5 billion years. The Sun's relative stability is the key reason that creatures such as ourselves can evolve on the "debris" that continues to orbit the star after it is formed.
» Post Comment