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British astronomer Arthur Stanley Eddington (1882-1944) was the first scientist to propose that the tremendous heat production at a star's core is what keeps a star from collapsing under its own gravity. Eddington, the most highly respected astronomer of his time, took as his main field of research the structure and life cycle of stars. The temperature at a star's core, Eddington said, reaches millions of degrees, creating an outward pressure to balance the inward pull of gravity. He outlined these concepts in a book, Internal Constitution of the Stars, that was later used by German-born American physicist Hans Bethe in his description of nuclear fusion.
Stars produce their energy by a process called nuclear fusion.
German-born American physicist Hans Albrecht Bethe (1906- ), after joining the faculty at Cornell University in Ithaca, New York, went to work on the problem of how stars produce their energy. To arrive at the answer, Bethe combined what he knew about subatomic physics with theories of the high temperatures of stars.This approach led him to understand the process of nuclear fusion. In May 1938 Bethe announced his answer.
In describing his discovery of the source of a star's energy, German-born American physicist Hans Albrecht Bethe suggested that deep in a star's core, where the temperature is in the millions of degrees, nuclear fusion takes place. He suggested two ways this can happen. In very hot stars, the nuclei of hydrogen atoms can fuse with carbon nuclei. This process begins a complex chain reaction, ending with the fusion of four hydrogen nuclei into a helium nucleus. Also produced are one recycled carbon nucleus, plus a tremendous amount of energy. In slightly cooler stars, hydrogen nuclei do not fuse with carbon, but fuse together to produce helium and energy.
Best known for his work on nuclear fusion and the big bang theory, Russian-born American physicist George Gamow (1904-1968) discovered that as a star grows older and uses up its hydrogen in the fusion process, it actually becomes hotter, not cooler as previously thought. Carried one step further, this fact means that when the sun reaches the end of its life in four or five billion years, the Earth will burn up rather than freeze.
British astronomer Arthur Stanley Eddington (1882-1944) proposed a theory that every star ends its life by collapsing to a small, dense, glowing object known as a white dwarf. A star the size of the sun would thus end up as a white dwarf the size of the Earth, yet so dense that a teaspoonful would weigh at least 5.5 tons (5 metric tons). This theory was later amended by Indian-born American astronomer Subrahmanyan Chandrasekhar who determined that Eddington's calculations did not hold true for stars with a mass greater than one-and-a-half times that of the sun. Chandrasekhar showed that a more massive star would be crushed by its own gravity and become either a neutron star or a black hole.
Subrahmanyan Chandrasekhar (1910-1995) was born in Lahore, a part of India that is now in Pakistan. His interest in astronomy was furthered when he read Sir Arthur Eddington's book Intemal Constitution of the Stars. In 1930 Chandrasekhar used theories from Eddington's book as well as Albert Einstein's theory of relativity to calculate that a star greater than a certain size would not undergo the evolution that as-characteristics of a population of stars. Russell published his work in 1913 only to discover that ten years earlier Danish astronomer Ejnar Hertzsprung had come to the same conclusion. The chart, therefore, was given the name Hertzsprung-Russell diagram, in honor of both of its creators. It is still considered one of the most famous diagrams in astronomy.
See also: Black hole,- Neutron star; Nova and supenova; Red giant star, Dwarf star
If you were to look up at the same stars night after night for years, you would probably never see them change. In reality, however, stars are constantly changing. The reason these changes are not apparent to the observer is that a star's life lasts for billions of years. Thus its changes occur very, very slowly. Since astronomers cannot observe the entire life cycle of a single star, they learn about stellar evolution by observing many different stars at various stages of life.
A star is created when a hot cloud of gas and dust in space condenses. Depending on the size of the cloud, it may become a single star, a binary star (a system of two stars that orbit around a common center of gravity), or a star cluster. When the cloud gets hot and dense enough, fusion of hydrogen into helium begins to occur, producing starlight. The fusion process is taken as evidence that a star has been created.
As long as a star has plenty of hydrogen fuel, fusion within the star will continue and the star will keep shining. When a star's hydrogen supply runs low, it enters the final stages of its life, and changes begin to occur. What happens next is determined by the size of the star.
An average-sized star, like our sun, will spend the final 10 percent of its life as a red giant. In this phase of stellar evolution, a star's surface temperature drops to between 3,100 and 6,700 degrees Fahrenheit (1,700 and 3,700 degrees Celsius) and its diameter expands to ten to one thousand times that of the sun. The star takes on a reddis color, which is how this stage of a star's life gets its name. Buried deep inside the star's atmosphere is a hot, dense core, about the size of the Earth. Helium left burning at the core eventually casts off the atmosphere, which floats off as a planetary nebula. The glowing core, called a white dwarf, is left to cool for eternity.
Once a star at least eight times as massive as our sun runs out of fuel, it will go supernova, shedding much of its mass. In most cases the star will then end up as an extremely dense neutron star. For the most massive stars (at least ten or twenty times the mass of our sun), the gravitational collapse of the supernova is so complete that only a black hole remains. A black hole is a single point in space where pressure and density are infinite. Anything that gets too close to a black hole gets pulled in, stretched to infinity, and remains forever trapped.
Capella, a binary star system. Just 45 light-years from Earth, the stars are so close together that conventional telescopes view them as a single star (also called Alpha Aurigae), the seventh brightest in the sky.
A binary star is a star system in which two stars orbit each other around a central point of gravity. Binaries are further described by their appearance. A visual binary is a pair in which each star can be seen distinctly, either through a telescope or with the naked eye. In an astrometric binary, only one star can be seen, but the wobble of its orbit implies the existence of another star in orbit around it. When the plane of a binary's orbit is nearly edgewise to our line of sight, each star is partially or totally hidden by the other as they revolve. This system is called an eclipsing binary. Sometimes a binary system can be detected only by using a spectroscope (a device for breaking light into its component frequencies). If a star that appears to be a single star gives two different spectra, it is actually a pair of stars, called a spectroscopic binary. These classes of bi- naries are not mutually exclusive. That is, a binary may be a member of one or more classes. For instance, an eclipsing binary may also be a spectroscopic binary if it is bright enough that its light spectrum can be photographed.
Before the nineteenth century, astronomers thought that binary stars were an optical illusion. An observer might see two stars that appeared to be side by side, but assumed that one was actually behind the other, and that they just appeared in the same line of sight. William Herschel made the first discovery of a true binary system in the 1700S. At the time, he was studying the parallax of stars, the apparent change in their position due to the Earth's motion around the sun. Herschel observed the motion of a pair of stars and concluded that they were in orbit around each other. Herschel's discovery provided the first evidence that gravity exists outside our solar system. Herschel never was able to measure parallax (that was achieved by Friedrich Besse] in 1848), but he did discover over eight hundred double stars. He called these star systems binary stars. His son, John Herschel, continued the search for binaries and catalogued over ten thousand systems of two or more stars.
In 1841 German astronomer Friedrich Bessel (1784-1846) noticed that the bright star Sirius wobbled in its path. The motion was unlike that due to parallax, which would be smooth. He theorized that the wobbling was caused by the gravitational tug of an invisible companion star in orbit around Sirius. Bessel's theory was shown to be correct in 1862 when Sirius' companion star was found by telescope-maker Alvan Graham Clark. Clark observed a bright, small, dense star known as a white dwarf. Since both stars were visible, the pair from then on has been considered a visual binary system.
The only accurate way to determine a star's mass is by studying its gravitational effect on another object. Binary stars have proven invaluable for this purpose.
See also.- Cepheid variables; Redgiant star
Variable stars are stars that vary in brightness over time. In most cases, these changes occur very slowly, over a period of months or even a couple of years. In some cases, however, the changes take place in a matter of hours. The category "variable stars" encompasses several different types of stars that vary in brightness for entirely different reasons. Some types of variable stars are red giants, eclipsing binaries, RR Lyrae, and cepheid variables.
The most common variable stars, with the longest bright-dim cycles, are red giants. Red giants are stars of average size, similar to our sun, in the final stages of life. For the last several million years of its multi-billion-year lifetime, one of these stars will puff up and shrink many times. It becomes alternately brighter and dimmer, generally spending about one year in each phase until it completely runs out of fuel.
The apparent variable behavior of eclipsing binary stars is caused by a very different process than that operating with red giants. A binary star is a double star system in which two stars orbit each other around a central point of gravity. An eclipsing binary occurs when the plane of a binary's orbit is nearly edgewise to our line of sight. Each star is then eclipsed by the other as they complete their orbits. Thus their actual brightness doesn't vary, but our ability to perceive their brightness does.
A special class of variables, discovered by American astronomer Henrietta Swan Leavitt, consists of blinking yellow supergiants called cepheid variables. The pulsation of these stars seems to be caused by the expansion and contraction of their surface layers. They become brighter and dimmer on a regular cycle (lasting three to fifty days), the period of which is related to their true brightness. Astronomers use these stars as a way of measuring distances in space.
The group of variable stars known as RR Lyrae stars are similar to cepheid variables but older. These stars are usually found in densely packed groups called globular clus- ters. Because of their age, RR Lyrae stars are relatively dim. They also have very short cycles, lasting less than a day.
While attending graduate school at Radcliffe College in Cambridge, Massachusetts, American astronomer Helen Sawyer Hogg (1905- ) began working with Harvard professor Harlow Shapley on a study of globular clusters, clusters of stars within a galaxy. While at Harvard, Sawyer met her future husband, Frank Hogg, who was researching stellar spectrophotometry, the spectra of light given off by stars. She combined his specialty with her own work on globular clusters and spent countless hours making long time-exposure photographs of globular clusters. In the process she discovered 142 new variable stars. In 1939 Helen Sawyer Hogg created the first complete listing of the known 1,116 variable stars in our galaxy. In 1955, she updated this catalogue, adding 329 new variables, one-third of which she had discovered herself.
See also. Variable stars
Variable stars are stars that vary in brightness over time. In most cases, these changes occur very slowly, over a period of months or even years. However, one class of variables changes in brightness much more quickly, on a regular cycle lasting three to fifty days. These stars are called cepheid variables. Cepheids are blinking yellow supergiant stars, the pulsation of which seems to be caused by the expansion and contraction of their surface layers.
Harlow Shapley, an astronomer at Mount Wilson Observatory in Pasadena, California, used Henrietta Swan Leavitt's findings to measure the size of the Milky Way. He discovered many new cepheid variables within globular star clusters and attempted to calculate the distance to those clusters. Shapley concluded that our galaxy was three hundred thousand light-years across. This size was so drastically different from previous estimates of fifteen to twenty thousand light-years, that Shapley's colleagues had difficulty believing it. Shapley further changed our concept of the galaxy by estimating that the sun was fifty thousand light-years from the center, whereas before it had been assumed that the sun was at the galactic center. It turns out that Shapley's estimate of the size of the Milky Way was about three times too large. The reason for Shapley's error is that the variable stars he used as "astronomical yardsticks" were really smaller and dimmer than he had thought and hence not as far away. Accordingly, he positioned the sun too far from the center of the galaxy, but only by about twenty thousand light-years.
In 1924 Mount Wilson Observatory astronomer Edwin Powell Hubble undertook a study of nebulae, clouds of gas and dust. He was most interested in whether they were part of our galaxy, as was commonly believed, or whether they were extragalactic objects. To answer this question, Hubble identified twelve cepheid variables in one nebula in a region of space called Andromeda. Hubble, like Henrietta Swan Leavitt and Harlow Shapley before him, used the cepheids as distance markers and learned that the nebula was at least eight hundred thousand light-years away. This distance was much greater than the farthest reaches of the Milky Way, meaning that the Andromeda was a separate galaxy.
Population I stars have a chemical composition similar to that of the sun. They contain 1-2 percent, by mass, elements heavier than hydrogen and helium on the periodic table of elements. They have been created over long periods of time from materials expelled by other stars. Population 11 stars are older, formed long before elements heavier than hydrogen and helium had built up in the universe. Thus they contain only about one hundredth of these elements as are found in Population I stars.
One night during World War II, Los Angeles suffered a power outage and the skies were exceptionally dark. Astronomer Walter Baade took advantage of this event to study the stars, which appeared particularly bright in contrast. He was able to make a detailed study of the Andromeda galaxy and became the first person to distinguish the stars near its core in fine detail. Previously, only the stars in the galaxy's spiral arms had been resolved, by Edwin Powell Hubble. Baade found that, in contrast to the whitish-blue outer stars, the core stars were reddish. Baade classified the stars into two groups, naming the stars in the spiral arms Population I and the core stars Population II. The distinction between these two classes of stars had never before been noticed, mainly because the stars at the core of our own Milky Way galaxy are hidden by clouds of dust and gas.
After World War II, astronomer Walter Baade used the new telescope at Palomar Observatory to identify over three hundred cepheid variables in the Andromeda galaxy. Cepheid variables are pulsating stars that can be used to determine distance. Using only Population 11 stars, Edwin Powell Hubble had earlier estimated that the Andromeda galaxy was eight hundred thousand light-years away and that the universe was about one billion light-years in size. Baade, using pulsating cepheid variables in both Population I and Population II, determined that the distance to the Andromeda galaxy was actually more like two million light-years. And he calculated that the universe was twenty times larger than previously thought. These changes were significant for several reasons. First, in order to have expanded to the size Baade calculated, the universe had to be much older than scientists had previously estimated. And second, if other galaxies were farther away than previously thought, this meant they had to be bigger and brighter, in order to be seen over the greater distances.
A constellation is one of eighty-eight groups of stars in the sky, named for mythological beings. Although some constellations may resemble the figures they are named for, others were merely named in honor of them. The constellations encompass the entire celestial sphere, the imaginary sphere that surrounds the Earth. The celestial sphere provides a visual surface on which scientists can plot the stars and other objects in space and chart their apparent movement caused by the Earth's rotation.
A constellation does not represent a scientific grouping of objects. Two objects in the same constellation may or may not have anything in common or any influence on one another. They may even be separated by a greater distance than objects in different constellations. To say that a particular star, planet, or nebula (cloud of gas and dust) is located "within" a given constellation does not take into account the actual distance of that object from Earth or from any other object in the constellation-it merely means that it can be found by looking in one general area in the sky, in relation to Earth.
Originally the constellations were not delineated by fixed boundaries. It was not until 1930 that the International Astronomical Union defined limits for the constellations that are still accepted today. These boundaries are imaginary lines, running northsouth and east-west across the entire celestial sphere, so that every point in the sky belongs to one constellation or another. recording the Positions of 381 stars. His work was warmly received when he returned to England. Halley was awarded an honorary master's degree from Oxford University and was elected to England's Royal Society, an elite science club.