Star Formation

Type of physical science: Astronomy; Astrophysics

Field of study: Stars

Star formation occurs as the result of gas and dust collapsing, which is caused by gravity. Heat builds up until temperatures of 10 million Kelvins initiate hydrogen burning. Many examples of star formation are seen in nebulas within the Milky Way.

Overview

Near the end of a star's life, it casts much of its matter out into space by either a slow gentle process, in which the outer layers are gradually expelled, or the violent explosion of supernova. Either way, the interstellar gases in the galaxy become enriched with heavy elements from the dying star's interior, where they have been created. New stars that form from this enriched material have heavy elements for planet formation. All elements, other than hydrogen and helium, were cast off by earlier generations of stars. Stars can create different elements in different amounts. Carbon, oxygen, silicon, and iron are readily produced within the interiors of stars. All elements heavier than iron are produced in supernova explosions.

Many cold clouds of gas and dust are scattered throughout the Milky Way galaxy, which are termed "giant molecular clouds." In some cases, they appear as dark regions silhouetted against glowing background nebulosity. In other cases, they appear as dark areas that obscure background stars. These dark objects are also called Barnard objects, after their discoverer, Edward E. Barnard, near the end of the nineteenth century.

Smaller dark clouds are termed Bok globules, named for Bart J. Bok, who called attention to them in the 1940's. More than two hundred of these Bok globules are known in the Milky Way. These are spherical dark nebulas that are typically 1 to 2 lightyears across and have a mass ranging from 20 to 200 solar masses. Spectroscopic analysis of these globules finds that they have a cosmic abundance of elements. This is 75 percent hydrogen, 23 percent helium, and 2 percent heavier elements, formed from supernova or other star death. Infrared observations have demonstrated that the internal temperature of globules are 5 to 15 Kelvins. Larger globules are warmer, on the order of 20 Kelvins. At these very cold temperatures, the dense regions inside the globule contract gravitationally, and these globules coalesce gradually into lumps called protostars.

At first the protostar is a cool blob of gas and dust several times larger than the solar system. The low pressure that exists inside the protostar is incapable of supporting the cool gas that surrounds it. The protostar will begin to collapse under the influence of gravity. As the atoms within the protostar become more crowded, more collisions take place between the atoms.

These collisions increase the temperature of the protostar. Gravitational energy is thus converted to thermal energy. The gases that make up the protostar begin glowing as they heat up. After only a few thousand years of gravitational contraction, the surface temperature of the protostar reaches 2,000 to 3,000 Kelvins. As this cloud collapses, all solid grains or ice trapped in the cloud vaporize by 2,000 Kelvins. The protostar that is in the state of collapsing is very large.

Because it is so large, therefore, it has a low surface temperature; it has a great luminosity. After a thousand years of contraction, a protostar of 1 solar mass would be twenty times larger and one hundred times more luminous than Earth's sun. Much of this radiation is visible only as infrared light. When this protostar begins to shine at visible wavelengths, it is luminous, red, and cool.

Thus, it is located in the upper right of the Hertzsprung-Russell diagram, which plots stars by their temperature and luminosity.

A protostar of 5 or more solar masses will become hotter without much change in overall luminosity. This is caused by the effect of decreasing surface area, which is counterbalanced by an increase in surface temperature. The evolutionary track of massive protostars is to transverse the Hertzsprung-Russell diagram from right to left, as they age toward the main sequence stage; that is, they get hotter while keeping the same luminosity. Protostars of 100 solar masses, which are very rare, rapidly develop such extremely high temperatures that radiation pressure becomes the dominant force supporting the star against gravitational collapse.

In less massive protostars, the increase in surface temperature is less rapid than massive protostars. This does not compensate fully for the shrinking surface area, resulting in a luminosity decrease. Protostars less than 0.08 solar mass never manage to develop the necessary pressure and temperatures to initiate hydrogen burning. These become planetlike objects. If Jupiter had been ten times more massive, it would have had enough material to begin nuclear fusion. The solar system then would have had a tiny red dwarf star along with the sun.

The more massive the protostar, the faster it evolves. A protostar of 15 solar masses takes ten thousand years to evolve to the main sequence, that point when a protostar's shrinking builds heat to 10 million Kelvins, which is when nuclear fusion begins. For a 1-solar-mass protostar, the journey to the main sequence requires a million years. Nuclear fusion involves hydrogen combining to make helium and release energy. This thermonuclear process releases tremendous amounts of energy. The outpouring of energy creates conditions inside a protostar that finally halt gravitational collapse. Hydrostatic and thermal equilibrium are established, and a star is created.

Astronomers are unlikely to observe the birth of a star in visible wavelengths because the surrounding globule or interstellar clouds hide the protostar. A cocoon nebula, a brilliant nebula embedded in clouds of interstellar grains that absorb visible light, surrounds this protostar. Heated to a few hundred Kelvins, it reradiates infrared radiation; both infrared and radio waves can penetrate these dust clouds.

The most massive stars are the first stars to form. The more massive the protostar, the sooner it develops the necessary central pressure and temperature to initiate fusion reactions. The most massive main sequence stars are of spectral class O and B, which are luminous and hot.

Surface temperatures of O and B stars range from 15,000 to 35,000 Kelvins. Much of their energy is radiated in ultraviolet light. These energetic ultraviolet photons radiated from these newborn massive stars ionize the surrounding hydrogen gas. Most of the hydrogen gas in the galaxy is neutral hydrogen, that is, uncharged, with temperatures at or below 60 Kelvins.

Astronomers refer to these areas as HI regions. The ionization energy has a dramatic effect on the globule or dark nebula in which the cluster of stars is forming. The hydrogen glows in the red portion of the spectrum at 6,563 angstroms. Nebulosities around these newborn stars shine red and are called emission nebula.

The intense ultraviolet radiation of O and B stars can produce atomic excitation of HI regions. If the energy is of very high frequency and very short wavelength and strikes a neutral atom, it may cause the expulsion of an electron, termed photoionization. The free electron may then combine with a free proton. It would likely orbit in a high-energy orbit. As it descends to a lower-energy orbit, it gives off light. The Orion nebula is an example of this.

Because the star-forming clouds are predominantly ionized hydrogen, they are termed HII regions. The few O and B stars at the core of the nebula producing ionizing radiation are called an OB association. With a density of one hydrogen atom per cubic centimeter, one O star may produce complete ionization for a radius of 600 light-years. In HII regions, the ionizing ultraviolet radiation from nearby stars is plentiful, and the electrons are ejected at the time of ionization with sufficiently high speeds to produce temperatures in interstellar gas of 10,000 Kelvins. Less than 10 percent of interstellar gas in the Milky Way is ionized. A newborn HII region buried deep within such a dark cloud would be detectable with radio rather than visible wavelengths. As new stars continue to develop, the HII region grows and eventually becomes visible at optical wavelengths. From the first O and B stars that develop in this dark cloud, a cavity is carved out by stellar winds and ultraviolet radiation. A shock wave forms where the outer edge of the expanding HII region encounters this giant molecular cloud. The shock wave compresses hydrogen gas, stimulating a new round of star birth.

Any mechanism that can compress these interstellar clouds can trigger star formation.

Other mechanisms include the expanding shell of a supernova remnant. As this encounters an interstellar cloud, it triggers star formation. The supernova explosions produce a considerable mass of heavy elements. The tremendous energy from the shock wave, plus the ionization energy from ultraviolet light, would help form stars. It is estimated that 5 to 10 million years after the explosion, an OB association would form. There is strong evidence that the sun was once a member of one of these stellar associations created by a supernova. The shock wave of the explosion caused the collapse of the solar nebula some 5 billion years ago. A collision between two cosmic clouds of gas and dust would produce a compression at the interface between the clouds. These clouds move up to 10 kilometers per second relative to each other and must collide.

Applications

There is ample evidence in the skies that leads astronomers to believe that they have a good understanding of star formation. One of the best-studied areas of star formation is in Orion.

The Orion nebula is a spectacular region of turbulent gas and dust heated by the radiation of hot young stars that have recently condensed from the matter of the cloud. The diameter of the Orion nebula is twenty thousand times that of the solar system and contains 10,000 solar masses. In the heart of the Orion nebula is the Trapezium star cluster, which constitutes the four brightest components of an expanding cluster containing hundreds of fainter stars. These O and B stars make the Orion nebula an HII region. The Orion nebula glows from fluorescence from these stars. Radio observations show a large cloud of neutral hydrogen with a diameter of 320 light-years and 50,000 to 100,000 solar masses. The Orion nebula is a small spot of ionized hydrogen within a larger complex.

The Orion aggregate is an enormous interstellar boiling pot, made of mostly neutral hydrogen. Some of it is condensed apparently into relatively young stars, which ionize some of the gas and cause it to glow. No globules are seen within the Orion nebula, though within the complex are several strong infrared sources which are likely protostar formations. Within this complex are other well-known nebulas, such as the Horsehead. Evidence of a violent past for this complex exists in Barnard's loop, which may be an ancient supernova remnant. At least three O and B stars seem to have been ejected from the Orion aggregate 2.5 million years ago to scattered parts of the sky.

There are many T Tauri stars found near the Orion nebula. These are young stars that are vigorously ejecting gas just before reaching the main sequence. It is theorized that the fusion reactions are preceded by stars propelling their outermost layers back into space. Infant stars in the T Tauri stage can lose 0.4 solar mass before entering the main sequence. The T Tauri stage is the gap between protostars and young stars. A T-association has an abundance of T Tauri-type stars. They occur in groupings, some near O and B associations, others by themselves, but always in regions of the sky where cosmic dust is plentiful. A T-association is formed around the Orion nebula and its surroundings, especially in regions of dark nebulosity.

It is theorized that star formation is presently occuring in the Orion complex. Plates obtained in 1947 and 1954 have shown conspicuous progressive changes in several of the small nebulous knots in the Orion cloud. These are Herbig-Haro objects, which are in the final stages of pre-main-sequence contraction. They are found near T Tauri stars. From year to year, these clumps of glowing gases change slightly in size, shape, and brightness. Even after stars arrive on the main sequence, there is sporadic activity.

The Rosette nebula in the constellation Monoceros is found near the Orion complex, which is a glowing wreath of gas surrounding a dark cluster of young stars. Photographs show many dark tendrils, or globules, of gas, which are most likely stars in formation. These globules are collapsing under gravity to become self-luminous stars eventually.

Other emission nebula show similar evidence of star formation. The Lagoon nebula in Sagittarius contains many T Tauri stars and globules. The North America nebula in the constellation Cygnus contains 12 hydrogen atoms per cubic centimeter. By earthly standards, this would be a nearly perfect vacuum; however, in space, this gas is thick. The North America nebula contains enough gas to create more than one hundred suns.

These nebulas have lanes of obscuring material that obviously overlie the brighter structure of the nebula, and numerous dark spots against the bright background. Some of these spots have a windblown, turbulent appearance, whereas the globules have a rounder appearance.

The smallest of these globules has a diameter of 0.1 light-year and a hydrogen concentration 100,000 times that of interstellar space. A star may be formed by the process of collapse and fragmentation of the original interstellar cloud. This may lead to multiple stars or a star cluster.

Context

The process of star formation follows the well-established path of gravitational contraction. This theory is similar to the idea formulated in the 1800's: The Helmholtz-Kelvin contraction suggested that the sun was powered by a slow contraction. The gravitational collapse is turned into thermal energy. Though it is now known that the sun is powered by nuclear reactions, as observed, the contraction was theorized to create stars.

Observations with radio telescopes have detected regions called giant molecular clouds.

These clouds have a mass of 10,000 to 1 million solar masses with a diameter of 60 to 240 light-years. Inside one of these clouds, the density is 200 hydrogen molecules (2 hydrogen atoms joined) per cubic centimeter. Approximately five thousand such clouds are located in the Milky Way. These giant molecular clouds and globules contribute materially to the total mass of the galaxy. Halfway between the sun and the center of the galaxy is a ring of three thousand molecular clouds with 2 billion solar masses. The estimated twenty-five thousand globules have a mass of 2 to 3 million solar masses. Large, dark nebula that have a radii of more than 12 light-years and a mass of 2,000 solar masses may be slated for fragmented collapse into a cluster of protostars. Some of the larger nebula are observed in the act of breaking into many units. In the Southern Coalsack (a dark nebula found in the constellation Crux), fragmentation is already in progress. The Coalsack is a nearly spherical ball of cosmic dust that is a conglomerate of smaller units, some very dense and headed for star formation.

Star formation is an ongoing process in a spiral galaxy. The spiral form of the galaxy is maintained by density waves. A high-pressure shock wave accompanies each density wave that maintains the spiral form. If clouds of gas and dust of more than average density are present in the interstellar medium, they may be compressed by the shock wave five to ten times the original density. Star formation begins in the densest region of these clouds. Massive stars, the first born, emit ultraviolet radiation, which ionizes the surrounding hydrogen, forming an HII region. This is evident as protostars and newborn stars are along the inner edge of the spiral arm, whereas O and B stars are brilliant and slightly older, marking the center line of the galaxy.

Astronomers now have a good general understanding of stellar formation. By observing various stages of star birth with radio, infrared, and visible light, it is well known that stars form from gravitational collapse. Star formation will be most evident in spiral arms.

Principal terms

ASSOCIATION: a loose cluster of stars whose position, spectra, or motions suggest that they have a common origin

BOK GLOBULES: small units of dark clouds that are nearly spherical and that contain both dust and interstellar molecules

EMISSION NEBULA: a glowing gaseous nebula whose light comes from fluorescence caused by a nearby star

HELMHOLTZ-KELVIN CONTRACTION: a theory that the sun and all stars' energy was created by the star contracting and slowly gaining heat

HERBIG-HARO OBJECTS: clumps of glowing gas that are in their final stages of pre-main-sequence contraction

HI REGION: a region of neutral hydrogen in interstellar space

HII REGION: a region of ionized hydrogen in interstellar space

O AND B STARS: very hot young stars with enormous amounts of energy; because of their rapid burning of fuel, they live for a short time

PROTOSTAR: the embryonic stage of a young star that is in the process of formation

T TAURI STARS: very young stars vigorously ejecting gas prior to burning hydrogen and becoming a main sequence star

Bibliography

Bok, Bart J., and Priscilla F. Bok. THE MILKY WAY. 5th ed. Cambridge, Mass.: Harvard University Press, 1981. Bart and Priscilla Bok have been among the great researchers of the Milky Way galaxy. Bart Bok developed theories of Bok globules, which are a cornerstone of star formation. Describes the galaxy in detail, with intense treatment of the nucleus and of the spiral structure.

Burnham, Robert Jr. BURNHAM'S CELESTIAL HANDBOOK. 3 vols. New York: Dover, 1978. This is the standard reference book for deep-sky objects. In a survey of each of the eighty-eight constellations, Burnham describes how to find deep-sky objects, how they will appear, and historical and modern theories as to their nature. Much of the numerical data on deep-sky objects was derived from Burnham.

Ferris, Timothy. GALAXIES. San Francisco: Sierra Club Books, 1980. This coffee-table book has wonderful, large observatory photographs of nebulas and galaxies. The text briefly covers star formation, especially in the Orion complex. Very clear on the structure of galaxies and galaxy evolution.

Hartmann, William K., and Ron Miller. CYCLES OF FIRE. New York: Workman, 1987. This is a spectacularly illustrated book that helps the reader visualize difficult concepts, such as protostar development. The text is brief, clear, and concise. Many of the illustrations are original paintings by Hartmann.

Kaufmann, William J. UNIVERSE. New York: W. H. Freeman, 1985. This freshman-level textbook covers all aspects of astronomy with some mathematics. The chapters on planetary formation, the sun, and stellar formation are particularly useful. The mathematics and nuclear reactions are well presented. Illustrations clarify more difficult concepts.

Raymo, Chet. 365 STARRY NIGHTS. Englewood Cliffs, N.J.: Prentice-Hall, 1982. Introduces the reader to the night sky on each night of the year. A number of essays cover star formation in nontechnical terms. The reader is left with a clear understanding of astronomical theories.

How stars end their lives largely depends on their mass

Interstellar Clouds and the Interstellar Medium

Nuclear Synthesis in Stars

Protostars and Brown Dwarfs