Protostars And Brown Dwarfs

Type of physical science: Astronomy; Astrophysics

Field of study: Stars

A protostar represents an early stage of star formation, prior to ignition of the nuclear core. Brown dwarfs represent an aborted stage in the process, in which insufficient mass results in the failure of the core to ignite.

Overview

A protostar represents an early stage of star formation in which a core has formed but is not yet emitting visible light. A brown dwarf represents a stage in star development in which the interior is insufficiently hot for the hydrogen core to undergo nuclear fusion.

The formation of both stars and their precursors, protostars, results from the condensation of massive interstellar molecular clouds or gas, consisting predominantly of hydrogen. The precise details of the process are not well understood. Nevertheless, it is known that the initial stages of star formation result from the gravitational attraction among the molecular particles in the cloud.

Star formation is a slow event, the culmination of a process covering a time span measured in millions of years. Since this period is beyond the span of human existence, it is logical to address first the question of whether the formation of new stars can be observed.

The lifetime of any star is dependent on its mass: The more massive the star, the hotter it becomes, and the more rapidly it consumes the hydrogen fuel in its core through nuclear fusion. High-mass stars are thus short-lived, with life spans measured in millions of years.

Because of the spectrum exhibited by these objects, they are termed "type O" stars. Relatively speaking, O stars are rare, but because of their high luminosity, they are among the most conspicuous stellar objects. By comparison, relatively low-mass stars such as the sun, a G star (of mass less than 20 percent that of type O), have life spans on the order of billions of years. Within the Milky Way galaxy, which contains the solar system, are large numbers of type-O stars, relative newborns on the cosmological scale. Since our galaxy is currently billions of years old, in order for type-O stars to exist in large numbers, such stars must continually be in the process of formation. The current estimate is that numerous stars equivalent in size to the sun are formed in the Milky Way each year.

The young type-O stars are found in the vicinity of clouds of molecular gas, generally in clusters centered within the spiral arms of galaxies. Newly formed stars within these regions of space illuminate gas and dust in these arms, providing visibility. As pointed out by astronomer Rudolph Schild, the ionized hydrogen clouds associated with these nascent stars often provide an excellent means for tracing the spiral structure associated with galaxies. Within the Milky Way, illuminated gas clouds such as the Trifid and Orion nebulas are, in effect, stellar nurseries. At the typical velocities associated with distant stars, the short lifetimes of O stars prevents their moving far from their birthplace before they burn out. Any type-O stars that are seen must still be in the vicinity of their birth. Astronomers have therefore concluded that groups of stars are continually being formed in these regions.

These clouds of interstellar gas are under the influence of two competing forces: gravitational forces that result in attraction and possibly collapse of the cloud, and thermal pressure resulting from the temperature of the cloud, which results in a tendency to expand.

Since the mass of these gas clouds is many times that of a single star, if collapse does occur, a process termed "fragmentation" may result in the formation of multiple protostars.

The theory behind this process was first developed by Sir James Hopwood Jeans, in 1926. Jeans calculated that an interstellar cloud can collapse only if it contains a certain minimal mass, the exact value of which is dependent on the temperature. Any cloud with sufficiently high mass and relatively low temperature will condense under the influence of its own gravity.

Low-density, relatively hot clouds will simply continue to expand. The critical mass, termed the "Jeans mass," is proportional to the square root of the temperature. While stars vary by a factor of millions in their luminosity, the variation in their mass is less than a factor of one thousand. This variation suggests that certain masses of gas clouds are more likely to condense into protostars than others.

One significance of the Jeans mass is its ability to account for fragmentation during the condensation process. When the process begins, the material associated with the molecular cloud may occupy a spherical volume as much as 0.05 parsec in radius, approximately 2 trillion kilometers. (For comparison, this approximates the distance from the sun to the planet Pluto.) As the cloud condenses to a fraction of this size, its density increases and the Jeans mass is reduced.

This results in the collapse of portions, or fragments, of the cloud itself. Eventually, as the density increases, the temperature increases, and fragmentation is stopped. The fragments now have the potential to evolve into protostars.

The precise mechanism of gravitational collapse, with subsequent evolution of the protostar, is not fully understood. A molecular cloud of sufficient mass will collapse from gravitational attraction. As long as the temperature is sufficiently low, gravity remains the major influence on the movement of the gas molecules, and contraction continues. As contraction occurs, gravitational energy is converted to heat. It is at this stage, in which the cloud is undergoing contraction (and heating), that the nascent structure is called a protostar. Most of the energy associated with the protostar results from the accumulation of infalling gas molecules; rather than from ignition of the hydrogen core or outware pressure from increasing temperature.

Collapse of the molecular gas cloud is an uneven process; central regions under a stronger gravitational influence collapse at a more rapid rate than the outer envelope.

Temperatures in the core increase more rapidly than in the outer region, or envelope. The envelope also serves to absorb much of the radiation from the blue, or higher-energy, portion of the spectrum, reradiating it in the infrared region. It is for this reason that protostar formation is observed using infrared detectors and photography.

Eventually, the core reaches a state of hydrostatic equilibrium, a situation in which outward pressure resulting from the increase in temperature balances gravitational attraction. For a nascent star equivalent to the sun, the timescale to reach this stage would be approximately 1 million years. The core will potentially become a star. The precise evolutionary sequence, however, is in part dependent on the mass and chemical composition of the protostar. Three arbitrary sequences can be defined: very low-mass objects, consisting of protostars of mass less than 25 percent that of the sun; solar-mass protostars; and high-mass protostars.

Most of the very low mass objects are well above the minimal mass level necessary for ignition of nuclear fusion reactions in the hydrogen core (somewhat on the order of 10 percent the mass of the sun). Yet, if the mass that accumulated during gravitational collapse is not approximately 10 percent of the solar mass, the temperature of the core will be insufficient to begin nuclear fusion, approximately 1 million degrees Celsius. These substellar objects are referred to as brown dwarfs. Energy released by these objects is generated only by their collapse, which is analogous to the energy associated with the noise made by an object falling on the ground.

The first reported observation of a brown dwarf occurred in 1983. Several additional prospects for brown dwarfs have since been reported but have not been confirmed. The difficulty in detection is inherent in the nature of the objects. Brown dwarfs are larger than planets but insufficiently massive to ignite the hydrogen in their cores as occurs in "true" stars. In effect, these objects could be considered "super" Jupiters. Some astronomers even consider the planet Jupiter as a type of brown dwarf. Since the mass of a distant, nonluminous object cannot be directly measured, the presence of possible brown dwarfs can be detected only indirectly.

Generally, this involves observation of very red (cool) images of low mass or photographic plates, using infrared detectors. Water vapor or carbon monoxide, formed during condensation, should also be found associated with brown dwarfs. Repeated observations of suspected brown dwarfs, however, have failed to confirm their existence.

Solar-mass or high-mass protostars undergo an alternative evolutionary sequence.

Evolution is more rapid. The nascent star continues to contract, and the core temperature continues to increase. Eventually, the temperature is sufficient for thermonuclear reactions to occur, and nuclear fusion of hydrogen to helium begins.

Applications

In the broadest sense, since human existence is dependent on a star--the sun--an understanding of protostar formation and evolution reflects on the study of the formation and destiny of humans. The force that results in condensation of the molecular gases, gravity, is the same as that which causes an apple to fall from a tree and serves to bind humans to Earth. The mathematical principles that are applied to the rotation of the cloud as it collapses are similar to those that explain why the speed of a skater spinning on ice changes as the arms are extended or contracted.

The physical law that governs the rotation of the cloud deals with conservation of angular momentum. As illustrated in the example above, the law requires that the speed of the rotation of the cloud should increase as it collapses. Collapse, on the other hand, should be opposed by forces generated by the rotation. One might compare this with the example of the skater, who now holds a rope tied to a pail of water. Rotational forces oppose the gravitation attraction for the water, which remains in the pail as it is spun. Therefore, unless the angular momentum is reduced, such as through radiated energy, the cloud cannot collapse. One method by which a massive protostar may circumvent this problem is to divide into a binary system, in which the two protostars revolve around each other.

A galaxy is not a rigid, uniform structure, such as a rotating pail of water attached to a rope. The collapse of the molecular cloud is uneven, with the greatest accumulation of mass occurring in the core. Similarly, the greatest accumulation of mass in a galaxy is in its core. Since the strength of gravitational force is proportional to the mass of the object from which it is generated, it follows that the force of gravity should be stronger, the closer any molecule is to the core of the nascent star. Furthermore, as the force of gravity decreases the farther one travels from the core, the speed of rotation should likewise decrease. Therefore, since gravitational attraction would be weakest in the farthest reaches of the spiral, molecules, even stars, should fly from the spinning structure into space, much as if the skater suddenly let loose of the rope. (A similar problem is seen in observations of the relationships within clusters of galaxies and even between clusters on a cosmological scale.) The fact that this does not occur requires one to search for an explanation.

Assuming the laws of gravity developed by Sir Isaac Newton and Albert Einstein are correct, the easiest explanation is that which suggests that additional, nonluminous matter is scattered throughout the galaxy. The presence of this material would account for the mass needed to keep the galaxy intact. Since this material cannot be observed directly, it has been referred to as "dark matter." To resolve the problems outlined above, it has been calculated that as much as 90 percent of the matter in the universe should be in the form of dark matter.

Brown dwarfs may play a role in the resolution of the problem of dark matter. If dark matter indeed exists, astronomists need to determine its form. Interstellar gas or dust, while obvious candidates, would have been easily detected. "Black holes," enormous gravitational sinks formed by the collapse of supermassive stars, would be a possibility. There is no evidence, however, that black holes exist in numbers sufficient to account for the missing mass. One is left with at least some candidates in the form of stellar objects such as brown dwarfs. Though the mass contributed by brown dwarfs would be insufficient, in itself, to account for the missing mass in galactic clusters, this mass could be significant within galaxies.

The desire for a fuller understanding of the cosmos in general, and galaxy or star formation in particular, led to a revolution in technology. Many objects generate energy in the infrared region of the spectrum. The Infrared Astronomical Satellite (IRAS), launched in 1983, mapped approximately 96 percent of the sky over a period of three hundred days. The discovery of numerous protostars within gas clouds led to further understanding of the formation of low-mass stars. While IRAS failed to detect any candidates within the category of brown dwarfs, it was instrumental in establishing an upper limit to the density of these objects in space.

Context

The first evidence for nonluminous matter, or dark matter as it is now known, dates to 1844. At that time, Friedrich Wilhelm Bessel suggested that the star Sirius was in orbit with a companion of similar mass, but which was of low luminosity. In 1862, the companion star was resolved as a white dwarf, the dense remnants of a star that has collapsed to a planet-sized object.

Firmer evidence for the existence of dark matter was supplied by Jan Hendrik Oort in the 1930's.

While analyzing the movement of stars in the Milky Way galaxy, Oort calculated that only half the amount of gravitational matter necessary to account for this movement could be found in visible stars. More precise calculations during the 1970's confirmed this early work and led to the suggestion that as much as 90 percent of the mass in the universe is in the form of dark matter.

Until the mid-1980's, it was believed by many astronomers that while details of the origin of the universe remained to be worked out, a basic outline of events could be described.

As theorized in the 1940's by George Gamow and others, the universe began with a big bang, an indescribable explosion of a massive primordial atom. The products of this explosion included hydrogen, which was to form the molecular clouds. The laws of gravity, initially calculated by Newton in the seventeenth century and refined by Einstein in the twentieth century, seemed to account for the formation of nascent stars, protostars, and the galaxies in which they are found.

According to these laws, the size distribution of protostars should resemble a bell-shaped curve, with the most numerous being of some arbitrary average size. Brown dwarfs would fall in the region of the curve depicting objects of very low mass.

With more precise calculations of the motions and rotational properties of stars and galaxies, it is clear that present knowledge of the possible makeup of brown dwarfs is inadequate. Confirmation of the existence of large numbers of brown dwarfs could answer some of these questions. The problem is that the existence of brown dwarfs remains theoretical.

Clearly, new ideas and more advanced technologies are necessary to account for these observations.

Much also remains to be learned about star formation in general. The concept by which protostars form by progressive fractionation was first outlined by Fred Hoyle in 1953. The bulk of star formation, however, appears to occur within giant molecular clouds, several orders of magnitude larger than those used by Hoyle in his calculations. Details of this process at the molecular level remain to be worked out.

Principal terms

GALAXY: a luminous band in the sky consisting of millions of stars

INFRARED: invisible rays of light just beyond red on the visible spectrum

MASS: the physical property of matter that gives it weight

NEBULA: masses of luminous gaseous matter and stars in space

NUCLEAR FUSION: the energy-generating system in the core of stars, which results from fusion of hydrogen atoms into helium

PARSEC: a unit of measure of astronomical distance, equal to approximately 30 trillion kilometers

SPECTRUM: a series of colored bands of light rays, arranged from violet (shortest wave visible) to red (longest wave visible)

VERY LOW MASS OBJECTS: two classes of objects that include brown dwarfs and very low-mass stars

Bibliography

Abell, George O., David Morrison, and Sidney C. Wolff. EXPLORATION OF THE UNIVERSE. 5th ed. Philadelphia: Saunders College Publishing, 1987. This textbook provides a good introduction to the study of stars and galaxies. Included are discussions of spectroscopy and instrumentation, stellar spectra, and the spectral sequences for star classification.

Degani, Meir H. ASTRONOMY MADE SIMPLE. Garden City, N.Y.: Doubleday, 1976. Although dated, it remains a good introduction to various aspects of astronomy. The text is written at a level that is easily understood.

Gamow, George. THE BIRTH AND DEATH OF THE SUN. New York: Viking Press, 1940. Gamow was a popular and prolific science writer during the 1940's. Though the specifics covered have undergone significant updating, the topic is still covered in a lively, easily understood manner. An overview of star formation and stellar evolution is provided.

Hawking, Stephen. A BRIEF HISTORY OF TIME. New York: Bantam, 1988. Long on the best-seller list, Hawking's view of the origin of the universe is written in a surprisingly lucid fashion. Though not an account of star formation per se, the book presents a description of cosmology discernible to anyone with some science background.

Lightman, Alan, and Roberta Brawer. ORIGINS: THE LIVES AND WORLDS OF MODERN COSMOLOGISTS. Cambridge, Mass.: Harvard University Press, 1990. Consists of interviews with twenty-seven cosmologists on their views of the universe. Both scientific and nonscientific topics are covered. Included are discussions on views of religion in the light of scientific studies.

Riordan, Michael, and David N. Schramm. THE SHADOWS OF CREATION: DARK MATTER AND THE STRUCTURE OF THE UNIVERSE. New York: W.

H. Freeman, 1991. Provides an easily digestible review of views on the origins of the universe, particularly the big bang theory. Also highlighted are discussions on origins of matter and elements, stars, and galaxies.

Rowan-Robinson, Michael. OUR UNIVERSE: AN ARMCHAIR GUIDE. New York: W. H. Freeman, 1990. While an outline of astronomy, the highlight of the text is its photographic essay of stellar objects. Included are large numbers of color photographs of nebulas, galaxies, and infrared photography of developing protostars. Discussions of myths and legends associated with the stars are also of interest.

Roy, A. E., and D. Clarke. ASTRONOMY: STRUCTURE OF THE UNIVERSE. 3d ed. New York: Adam Hilger, 1989. Provides a good discussion on the observational data of stars, stellar motion, and evolution. Topics include stellar properties and the spectral sequence of stars. Though not a basic text, one need not be an expert in the field to follow the material.

Tucker, Wallace, and Karen Tucker. THE DARK MATTER: THE QUEST FOR THE MASS HIDDEN IN OUR UNIVERSE. New York: Quill, 1988. The Tuckers, a husband and wife team, have produced a comprehensive discussion on the presence of dark matter, that is, the hidden mass in the universe. Major emphasis is placed on candidates for the missing mass, including the suspected role of brown dwarfs. A nonmathematical approach, with numerous photographs and diagrams.

Interstellar Clouds and the Interstellar Medium