Star

A star is a large, self-luminous sphere of hot gas held together by its own gravitational force. There are many billions of stars in each of the more than 200 billion GALAXIES in the universe; yet the SUN (at a distance of 150 million km) is the only star close enough to show directly the details of its surface. Stellar SPECTROSCOPY in Canada was pioneered at the Dominion Astrophysical Observatory near Victoria (established 1918). Canadian astronomers have made important contributions to knowledge about the physical and chemical nature of stars.

The masses of stars range from 0.1 x 1030 kg to 100 x 1030 kg. The sun's mass is 2 x 1032 kg. A star's mass controls its basic structure, the most massive stars being the hottest, brightest and largest. Most stars, including our sun, are called main sequence or dwarf stars. Heat liberated by the conversion of hydrogen to helium inside the star results in high central pressure that prevents gravity from further compressing it. Four hydrogen nuclei produce one helium nucleus. During this nuclear fusion reaction, 0.7% of the mass of the hydrogen nuclei is converted to energy. A small amount of helium is converted to heavier elements.

Since a star's mass is finite, eventually all available nuclear fuel will be expended. More massive stars radiate vast amounts of energy; consequently, their nuclear fuel is used more rapidly, resulting in a shorter lifetime. The reverse is true for low-mass stars. As the fuel is consumed, gravity can no longer be resisted and dramatic changes result. The radius of the star increases 10- to 100-fold (depending on mass) and its surface temperature decreases: the star becomes a giant or supergiant. The brightest supergiants are about 2 million times brighter than the sun.

Sometimes an explosive event occurs, such as Supernova Shelton 1987A, the first to be sighted in modern times near Earth's galaxy, discovered by Ian Shelton of the University of Toronto. Further rapid changes follow in which gravity dominates: the star shrinks, becoming a white dwarf, a neutron star (pulsar) or a BLACK HOLE.

Chemical Composition

Stars and gaseous nebulae can be chemically analysed using the spectral lines present in their light. In most cases, hydrogen is overwhelmingly dominant (up to 99% of a star's mass). The chemical composition of the universe at its origin was mostly hydrogen and helium; however, since stars produce heavier elements out of hydrogen and as some of the stellar material is sent back into interstellar space by stellar winds, novae and supernovae, succeeding generations of stars have more heavy elements. The earliest-formed stars have as little as 1% of the heavy elements present in the sun.

Stars form from clouds of interstellar gas and dust which are pulled together by gravity until collisions between the molecules generate enough pressure to slow the contraction. Internal nuclear reactions have not yet started. The time required for contraction is a small fraction of the star's total lifetime; for example, our sun has a contraction time of about 100 000 years, compared to a main sequence lifetime of about 10 billion years. The contracting star begins to shine during this relatively short stage. Contraction ceases and the star joins the main sequence soon after internal nuclear reactions start.

Rotation of a star arises from the spin-up of the clouds from which it formed. Larger stars rotate rapidly, some near the point of "break-up" (ie, material spinning off their equators). Smaller stars rotate much more slowly because escaping mass gets caught in the magnetic field of the star. The "magnetic brake" reduces the rotation of main sequence stars by a factor of about 2 each billion years.

The Solar Photosphere

The photosphere is the atmosphere from which light leaves the star. Convection columns are clearly visible in the solar photosphere. Other stars are too far away for the surface markings to be seen and, instead, convection is detected from the shapes of spectral lines using D.F. Gray's technique (University of Western Ontario). K.O. Wright of DAO measured chromospheres in supergiants by observing eclipsing binary stars. In the outermost atmosphere (corona), temperatures of several million degrees are found and X-rays are emitted. Beyond the corona, a small amount of mass streams away. J.B. Hutchings (DAO) measured the rates of mass loss in many hot stars. Mass loss from our sun is responsible for auroras on Earth (see NORTHERN LIGHTS).

Cool dark spots are detected on stars. Dark spots on the sun were seen in 1611 when Johannes Fabricius and Galileo Galilei turned the first telescopes on it. The number and sizes of spots, along with explosions (flares) and enhanced emission from the upper atmosphere, are referred to as "activity." Many stars, including the sun, show cyclic changes (about 10 to 20 years) in the amount of activity. Activity is produced by the star's magnetic field, which itself is generated by the interaction of the star's rotation with convection, in a process called a dynamo.

Binaries and Multiple Stars

A binary star is a system of 2 stars that travel in periodic orbits around a common centre of gravity. Multiple systems of 3 or more stars are called binaries or multiple systems. The orbital motions of the stars are determined by the law of gravity. Binaries composed of components so widely separated that they can be seen separately through a telescope are called visual binaries. The orbital periods of visual binaries may be only 2 years; however, the average is several centuries.

A spectroscopic binary is a system with components too close together to be seen separately and with an orbital plane tilted toward Earth. The orbital motion produces a periodic change in the apparent motions of the stars toward and away from Earth. This change is detected as a periodic Doppler shift of the spectral lines. The orbital periods of spectroscopic binaries are usually a few days or weeks, but some are less than 20 minutes and others longer than 20 years.

Many spectroscopic binaries and a few visual binaries have orbital planes which are seen edge-on. During each orbit, first one star and then the other passes in front of (ie, eclipses) its companion. Eclipsing binaries are studied by observing their periodic variations in brightness. Many of the brightest stars in the sky are binaries. Algol (Persei) is an eclipsing system in which the brightness decreases to one-third its normal value every 69 hours, a change visible to the unaided eye. Algol is a triple system; its third component has been detected spectroscopically. Albireo (Cygni) is a visual binary with red and blue components which can be resolved in a small telescope; its orbital period is at least 30 000 years.

Variable Stars

Strictly, the term variable star refers to a star that appears to vary in any sense (eg, light, velocity, spectrum); in practice the term is generally reserved for a star that varies because of some internal cause (usually pulsation) and excludes variations arising externally (eg, eclipses by an orbiting companion). There are about 35 000 variable stars designated in the 88 constellations.

Whether or not a star pulsates is determined mainly by its mass, radius and surface temperature. Stars that have combinations of these near any one of several critical combinations will pulsate to some degree. Variable stars thus tend to form families, all members of which have similar properties. Variable stars are very important in astronomy. The period of pulsation, measured by the cycle of light variation, is related to the star's intrinsic brightness. Once this relation is calibrated, observations of light changes in a distant variable will reveal how bright it really is. When this information is combined with data about how bright it appears to be, the star's distance can be calculated. Thus, by observing variable stars in distant galaxies, we can establish the scale of the universe. Furthermore the pulsation period is a sensitive diagnostic of a variable star's internal conditions and so serves as an important check on theories of stellar constitution and evolution.

J.D. FERNIE

White Dwarfs

A white dwarf is a star typically having about 60% of the mass of the sun, compressed into a sphere with a radius of a few thousand kilometres. It thus has a mass typical for a star but contained in a volume comparable to that of Earth, and is therefore extraordinarily dense, approximately a million times denser than water. Because a white dwarf is both small and massive, its gravitational pull is extremely strong; an object at its surface would weigh roughly 100 000 times more than at Earth's surface.

The interior of the white dwarf is a bizarre kind of solid, somewhat like a very dense metal, usually composed mainly of atoms of carbon and oxygen. Inside the star, electrons liberated from atoms by the very high density of the matter provide the pressure ("degeneracy pressure") that supports the outer layers of the star against the very strong gravitational attraction toward the star's centre. The interior is extremely hot, typically 10 million K (Kelvin). The outermost, visible surface of a white dwarf is a thin atmosphere usually composed of almost pure hydrogen or almost pure helium which tends to float to the surface above the denser atoms of the interior. A few white dwarfs contain extraordinarily strong magnetic fields, ranging from 2 million to 500 million times that of Earth.

A white dwarf is the remnant of a star like the sun that has exhausted its nuclear fuel. The star thus loses its ability to support itself against gravity and collapses. If at the time of collapse the star is less than 1.4 times as massive as the sun, it becomes a white dwarf; if it is more massive the white dwarf structure is not strong enough to support the star, and it collapses still farther to become a neutron star or even a black hole. Once formed, a white dwarf changes little in structure but simply cools, until after more than 10 billion years it becomes too cool to be visible. Cooling of the interior furnishes the energy that we see emitted as light from the atmosphere, which gradually declines in temperature over several billion years from an initial temperature of over 100 000 K to below 5000 K. The hottest observed white dwarfs were formed only millions of years ago; very cool ones have been white dwarfs for billions of years. Because 80-90% of all stars become white dwarfs, white dwarfs are very common in space: about 10% of all stars near the sun are white dwarfs and as time goes on this fraction will steadily increase. The nearest white dwarfs are the faint companions of the bright stars Sirius and Procyon.

The most important group in the world currently studying the structure and evolution of white dwarfs is in the Physics Department of U de M, under the leadership of Gilles Fontaine. Other important members of this powerful team include François Wesemael, Pierre Bergeron, Pierre Brassard and Monique Tassoul. This group combines energetic and varied observational work with a remarkable array of theoretical and mathematical talent. The work of the group has been substantially influenced by Georges Michaud (also at U de M), the leading expert in the study of diffusion atoms inside stars.

JOHN D. LANDSTREET

Star Cluster

Many stars in our Galaxy are found not isolated in space, but in groups called star clusters. The stars in a cluster orbit around the centre of the whole group, rather like a swarm of insects, and the group stays bound together by the mutual gravity of its members. Clusters in the Milky Way are usually classified into 2 types: open and globular.

Study of Binary Stars

The study of binary stars provides the only way of measuring stellar masses and yields information about the sizes and surface temperatures of stars. Many spectroscopic and eclipsing binaries have so small a separation that one is tidally distorted into an ellipsoid by its companion's gravitational field. This physical distortion may produce additional periodic variations in brightness. As stars evolve they expand. In a binary, expansion may result in the transfer of matter from the atmosphere of one star to its companion. This mass exchange is a common phenomenon that can alter the future evolution of both stars, may give rise to radio and X-ray emission, probably accounts for nova explosions and has led to the detection of possible black holes. The DAO, Victoria, and the David Dunlap Observatory at the University of Toronto, have been leading centres of research on binary stars for many years.

DOUGLAS P. HUBE

Open Clusters

Open clusters may contain only a few dozen or as many as several thousand stars and are mainly found in the galactic plane or disc of our Milky Way. Hundreds of open clusters have been identified within about 3000 parsecs of the sun. The obscuring gas and dust spread throughout the galactic plane makes more distant clusters difficult to find, but there are undoubtedly thousands more in the entire Galaxy. Although most stars are not found in clusters, it is possible that most (or all) of the stars in the Galaxy were born in such groups. However, with time, clusters gradually move away from their formation regions and slowly "dissolve" into the general field-star population. Only the most populous and tightly bound clusters are likely to have survived disruption over the galactic disc's 10-billion-year history. The youngest open clusters can still be found near the gas and dust from which they formed and thus make valuable tracers of the spiral structure typical of galaxies such as ours.

Open clusters range in age from those still forming to those more than 5 billion years old. An individual cluster will contain stars of various masses, but the most massive stars emit energy at the greatest rate and evolve through their various stages in life much more quickly than do stars of lower mass. Thus any single cluster provides a snapshot of a gradually changing family of stars: all have the same age, but each star is at a different stage of evolution determined by its mass. Observations of many different clusters permit astrophysicists to test their theoretical models of stellar evolution and thus build up a general picture of the life histories of stars. Open clusters can also provide important clues to the nature and origin of unusual or peculiar stars (binaries, variables and others), which are often found in clusters.

Globular Clusters

Our own Galaxy contains 150 of the objects called globular clusters. These lie scattered throughout the halo of the Galaxy, which is a vast, tenuous region surrounding the disc and central nucleus. Globular clusters tend to have highly eccentric, randomly oriented orbits about the galactic centre, and their stars are older than any other known objects in the Galaxy; current measurements set their ages at roughly 15 billion years. Globular clusters thus provide an important constraint on cosmological models by setting a minimum age of the universe.

Typical globular clusters are far more massive than open clusters; each one may contain anywhere from a few thousand to a million or more stars. Globular cluster stars also have simpler, more primitive compositions than do stars in the galactic disc or nucleus; they contain much lower amounts of "heavier" atoms (ie, elements more complex than hydrogen and helium), which are built up by NUCLEAR FUSION processes inside stars. In total, these characteristics suggest that globular clusters were formed when the Milky Way was still in its "protogalaxy" state as a large, primordial gas cloud, not long after the universal Big Bang (seeCOSMOLOGY).

Globular clusters are known in many large galaxies beyond ours. Even among widely different parent galaxies, these clusters bear many similarities (eg, age, size, composition) which seem to represent a unifying thread in the histories of the galaxies in general. Interestingly, the heavy-element abundances of globular clusters in bigger galaxies are distinctly higher than for Milky Way globular clusters, and some may even have abundances as rich in heavy elements as the sun. Future studies of globular cluster systems in galaxies will help to unravel the mysterious, long-vanished era of galaxy formation and early evolution, just as the star clusters in our Milky Way have told us much about its large-scale history.

Canadian astronomers have made major contributions to star cluster research. Among many who should be cited are H.B. Sawyer HOGG-PRIESTLEY and her later associates, C. Coutts Clement and A. Wehlau (for surveys of the RR Lyrae stars, a type of pulsating variable star typically found in globular clusters); F.D.A. Hartwick, G.G. Fahlman, W.E. Harris, J.E. Hesser, H.B. Richer and P. Stetson (analysis of HR diagrams); and D.A. Hanes, W.E. Harris, R. Racine and S. VAN DEN BERGH (clusters in external galaxies).

GRETCHEN L.H. HARRIS et WILLIAM E. HARRIS

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