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. Or, 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 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, we will restrict ourselves 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. We 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.
But 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 semiregular 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 a Harvard astronomer, Henrietta Leavitt, 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 Edwin Hubble to demonstrate that there are galaxies other than our 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. We now know that globular clusters uniformly surround our galaxy. In the 1920s, Harlow Shapley used RR Lyrae variables to map the distribution of globular clusters and concluded that the Sun was not in the center of our galaxy as was generally believed at the time. He found that our Sun and solar system are located about 30,000 light years from our galactic center.
Some stars vary because they have large spots on their surface. The amount of area covered by these spots changes with time and this effects 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 another Harvard astronomer, Sallie Baliunas, 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 our climate.
Ancient astronomers noted that once in awhile 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. Nova are usually noticed only by astronomers but historical records indicate that some of the supernova were so bright that they could be seen by ordinary people.
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 neutronstar 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 our own galaxy we have seen only a few in all of recorded history. The Chinese reported the appearance of supernovas in A.D. 1006, 1054, and 1181. 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 supernova were observed and recorded in Europe, and these are the last seen in our galaxy. Most astronomers believe that we are overdue for a supernova in our own galaxy. Life on any planet near a supernova would be instantly vaporized, but we should not worry. The Sun is not massive enough to become a supernova. The closest likely candidate for a supernova is the 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 supernova.
See also Stellar evolution.
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Darrel B. Hoff