Variable Stars

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Most people regard the stars as constant and unchanging. A character in one of Shakespeare’s plays refers to a friend “as constant as the Pole Star.” While Shakespeare was probably referring to the constant position of the Pole Star, he did not know about the precession of the equinoxes. Alternatively, if he was referring to the constant light of the Pole Star (Polaris), he was in error there also. Astronomers now know that the light from Polaris varies, visible only with the use of a telescope. Polaris is an example of what astronomers call a variable star. Originally, the term was used only for stars that vary a great deal in brightness, but now it is used to cover a wide range of stars. In fact, all stars probably vary in one way or another. The sun, for example, has very slight variations in its energy output during the life of one of its sunspot cycles. Here, in this article, the term variable star will be restricted to stars that are much more variable than the sun.

Most people today do not observe the stars closely enough to note any of their brightness changes. Ancient people with darker skies did observe some stars changing in brightness. The Arabs noted that a bright star in the constellation Perseus dropped to about half its normal brightness for two hours every three days. They named this star Algol meaning the demon star. Astronomers know today that Algol is an example of an eclipsing binary star system. The change in brightness results when a dimmer companion star moves in front of the brighter star, blocking part of the light of the brighter star.

However, the change in brightness of most variable stars is due to changes taking place within a single star. These stars are known as intrinsic variable stars. Astronomers have identified several classes of intrinsic variable stars. Some variable stars are classed as periodic, meaning that they go through regular changes in brightness every few hours or within a few days or weeks. This group includes such stars as the Cepheid variables and RR Lyrae variables. Other stars are only roughly periodic or non-periodic in their brightness changes and are known as semi-regular and irregular variables, respectively. The variable stars that are most apt to capture the attention of the public are called nova and supernova stars. While these are not often seen, they sometimes become bright enough so that they are easy to observe without the use of a telescope.

Cepheid variables are so named because one of the first of this class studied is found in the constellation Cepheus. These stars are all supergiant stars and are many times larger than the sun. These stars vary because of pulsations within the stars themselves. They go through a cycle of expansion (brightening) and contraction (dimming). Over a period of many weeks the brighter Cepheids may vary by a factor of 300% in their energy output. Polaris is an example of a dimmer Cepheid and it varies only about 7% over a period of four days. A general pattern of brightness variations of Cepheid variables was discovered by American astronomer Henrietta Swan Leavitt (1868– 1921), at Harvard University, early in the twentieth century. She discovered that the brighter cepheids had longer periods of brightness variation and the shorter period cepheids were generally dimmer. This period-luminosity relationship later made it possible for American astronomer Edwin Hubble (1889–1953) to demonstrate that there are galaxies other than the sun’s own Milky Way galaxy.

They are called the RR Lyrae stars because, when studying this class, one of the first stars was found in the constellation Lyra. These stars have very short periods of light variation (usually less than one day) and they are dimmer than the Cepheids. They are usually found in large spherical collections of stars called globular clusters. Astronomers now know that globular clusters uniformly surround the Milky Way galaxy. In the 1920s, American astronomer Harlow Shapley (1885– 1972) used RR Lyrae variables to map the distribution of globular clusters and concluded that the sun was not in the center of the galaxy as was generally believed at the time. He found that the sun and the solar system are located about 30,000 light-years from the galactic center (where one light-year is the distance that light travels in a vacuum in one year).

Some stars vary because they have large spots on their surface. The area covered by these spots changes with time. It affects the output of energy from their surfaces. The sun is one example of a spotted star. The study of how other spotted stars change their energy output has lead American astrophysics and astronomer Sallie Baliunas, of the Harvard-Smithsonian Center for Astrophysics, to suggest that there may be a link between the number of spots on the sun and the climate patterns on the Earth. Records suggest that when the spots are at a minimum for a long period of time, as they were in the late 1600s, there was a significant change in Earth’s climate.

Ancient astronomers noted that once in a while stars would appear where they had not been seen before. These were called nova stars from the Latin word for new. Later, it was noted that some of these stars were much brighter than other nova and the name supernova was coined. Only astronomers usually notice novas (or novae), but historical records indicate that some of the supernovas were so bright that they could be seen by ordinary people. Today, nova is defined as a star that suddenly increases in brightness but eventually, over months or years, returns to its original luminosity. A supernova, on the other hand, is a star that has exploded in the last stage of its evolution.

Astronomers now believe that nova always occur in double-star systems. Over a long period of time, one of the members of the pair evolves and becomes a white dwarf star. Later, the other member of the pair evolves to become a giant star and a portion of its outer layer is drawn onto the surface of the white dwarf. A temporary nuclear explosion, similar to a hydrogen bomb, takes place.

A supernova, on the other hand, usually results when a single, massive star evolves until it has used up most of its hydrogen fuel. The star then collapses on itself with a tremendous explosion, leaving behind small but massive remnants that become either a neutron star or a black hole. If an observer is located in the right orientation, the neutron star may be observed as a object called a pulsar. These objects are some of the most unusual variable stars. They are thought to be highly magnetic, rotating objects only a few miles in diameter. Pulsars release their electromagnetic energy in regular bursts as frequently as a fraction of a second up to five seconds.

In a few seconds a supernova can produce as much energy as 10 billion suns. It becomes a brilliant star and even can be seen in the daytime. In the sun’s own galaxy, astronomers have seen only a few in all of recorded history. The Chinese reported the appearance of supernovas in 1006, 1054, and 1181 AD. The July 4, 1054 supernova was so spectacular that there are rock carvings recording the event by Native Americans of the American Southwest. Later, in 1572 and 1604, two other supernovas were observed

KEY TERMS

Black hole —A supermassive object with such a strong gravitational field that nothing, not even light, can escape it.

Cepheid variable star —Stars that belong to a class of supergiant pulsating stars.

Eclipsing binary star system —A double star system in which the plane of their mutual orbit is nearly parallel to our line of sight. As one star passes in front of the other star it periodically dims the other.

Intrinsic variable star —Any star that changes its energy output because of changes within the star or on the surface of the star.

Light year —A light year is equal to the distance that light would travel in one year within in vacuum, and its value equals about 6 trillion miles or 9 trillion kilometers.

Neutron star —The result of the collapse of a supernova star. It is believed that they are almost completely composed of neutrons, hence their high densities.

Nova —A star that has a temporary increase in energy output on the order of a thousand times its normal energy output.

Period-luminosity relationship —An empirical relationship exhibited by Cepheid variables between the variation in their energy output and the time period for this variation.

Pulsars —Small, dense, rapidly rotating neutron stars whose magnetic fields direct their energy output in much the same way as a lighthouse been that sweeps over the observer.

RR Lyrae variable stars —A class of giant pulsating stars who have periods of about a day.

Sunspot —Cooler and darker areas on the surface of the sun. They appear dark only because they are cooler than the surrounding surface. Sunspots appear and disappear in cycles of approximately 11 years.

Supernova —The final collapse stage of a supergiant star.

Variable star —Any star that has a variation in energy output.

White dwarf —A star that has used up all of its thermonuclear energy sources and has collapsed gravitationally to the equilibrium against further collapse that is maintained by a degenerate electron gas.

and recorded in Europe, and these are the last seen in our galaxy. Nearby, however, in the Large Magellanic Cloud, which is a small satellite galaxy orbiting it, a supernova took place on February 23, 1987. In actuality, the explosion of SN 1987A took place about 170,000 years earlier, but astronomers only saw the light on that date, since it took that long for its light to reach Earth. The star that exploded was a blue supergiant, probably 20 times more massive than the sun, and when it exploded it was visible to the naked eye. Supernova 1987A became one of the most studied events in astronomical history, as observations of the expanding blast revealed numerous exciting results.

Most astronomers believe that the Milky Way galaxy is overdue for a supernova. Life on any planet near a supernova would be instantly vaporized, but humans should not worry. The sun is not massive enough to become a supernova. The closest likely candidate for a supernova is the red supergiant star Betalguese in Orion, and it is located nearly 500 light years away. At this distance, there would likely not be any terrestrial effects if the star did become a super-nova. Antares, the brightest star in Scorpio, is another notable example.

See also Stellar evolution.

Resources

BOOKS

Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.

Aveni, Anthony F. Uncommon Sense: Understanding Nature’s Truths Across Time and Culture. Boulder, CO: University Press of Colorado, 2006.

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Chaisson, Eric. Astronomy: A Beginner’s Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Hoflich, Peter, Pawan Kumar, and J. Craig Wheeler, eds. Cosmic Explosions in Three Dimensions: Asymmetries in Supernovae and Gamma-ray Bursts. Cambridge, UK, and New York: Cambridge University Press, 2004.

Kirshner, Robert P. The Extravagant Universe: Exploding Stars, Dark Energy, and the Accelerating Cosmos. Princeton, NJ, and Oxford, UK: Princeton University Press, 2004.

Mark, Hans, Maureen Salkin, and Ahmed Yousef, eds. Encyclopedia of Space Science & Technology. New York: John Wiley & Sons, 2001.

Stephenson, Francis Richard. Historical Supernovae and their Remnants. Oxford, UK, and New York: Oxford University Press, 2002.

van Putten, Maurice H.P.M. Gravitational Radiation, Luminous Black Holes, and Gamma-ray Burst Supernovae. Cambridge, UK, and New York: Cambridge University Press, 2005.

Darrel B. Hoff