The Herschels, a family of astronomers

   Sir William Herschel (1738-1822) was an eminent astronomer. He was born at Hanover, Germany. He served as a musician in the army. He came to England in 1757 and obtained employment first as a bandmaster. He employed his leisure time in studying mathematics and astronomy. In 1774 he made himself a reflecting telescope. During the American Revolutionary war, Herschel was enthusiastically engaged in mapping the heavens. George III gave him a pen¬sion enabling him to devote his entire time to astronomy. In 1787 he completed a tele¬scope, the largest then known. It was forty feet in length and four and one-half feet in diameter. It weighed over a ton. With this instrument he discovered an extinct volcano on the surface of the Moon, two satellites of Saturn, and the planet Uranus. Herschel established the fact that Saturn's rings revolve, and made many other minor observations and discoveries. He mapped no less than 5,000 nebulae and clusters of stars new to astronomy. His sister Caroline worked with him. She herself discovered five new comets. She also drew up a catalog of the stars, and deserves to shine by reason of her own light. Sir William's work was continued by his only son, Sir John. The latter had the advantage of excellent instruments, and began work where his father left off. His first contribution to astronomical science was the study of the stars in the southern hemisphere—stars never seen by residents of the north. He spent four years near Capetown. He was made president of the Royal Astronomical Society. At his death in 1871 he was honored with burial in Westminster Abbey. The Herschels are credited with laying the foundation of modern astronomy. They began the work of mapping the heavens accurately.

Sir Frederick William Herschel

F. W. Herschel

   Sir Frederick William Herschel (1738-1822) was an English astronomer, born in Hanover, Germany. He went to England in 1757; became a teacher of music, organist and conductor. He continued his study of mathematics and astronomy and built bis own telescopes with which to study the heavens. 

   In 1781 he discovered the planet Uranus for which he became fa¬mous. Herschel then devoted all of his time to astronomy and with the help of his sister Caroline carried on important scientific explorations of the sky. 

   He set up a powerful telescope at Slough, 1789, that had a 48-inches mirror and a focal length of 40 ft. With this he explored beyond the solar system. Herschel dis¬covered 2,500 new nebulae as well as nebulous stars and planetary nebulae and also was the discoverer of the 6th and 7th satellites of Saturn. 

   His studies convinced him that the entire solar system moved through space and he determined a point toward which he thought it to be moving. Herschel died in Slough, his work being carried on by a son, Sir John Frederick William, who set up an observatory at the Cape of Good Hope.

The Wayward Red Planet —the Mars problem

   The Red Planet and the motions challenged every astronomer from antiquity onward. By the late 1500s, the Danish astronomer Tycho Brahe (1546-1601) had amassed decades of observations of Mars and the other planets. His observations were by far the most accurate to date, yet he failed to weld them into a coherent system of the universe. Then Tycho hired a young Austrian mathematician-astronomer named Johannes Kepler (1571 -1630) and put him to work on the orbit of Mars. When Tycho died shortly afterward, Kepler inherited his position and, more importantly, his observations.
   Kepler battled with Mars for much of a decade, finally emerging with the, first two of his three laws of planetary motion. These include the conclusion that planets orbit in ellipses, with the Sun at one focus. (Every prior astronomer had insisted on combinations of circles.)
   Kepler's work paved the way for Isaac Newton's Principia (1687), the cornerstone of all modern physical science.

Retrograde motion of Mars

How did the big bang theory get its name?

   The big bang theory, ironically, got its name from an off-hand remark made by a steady-state theory proponent, British astronomer Fred Hoyle, speaking on a radio show in England. Hoyle has made detailed studies of the nuclear reactions that take place in the core of a star and has also researched the gravitational, electromagnetic, and nu¬clear fields of stars and the various elements formed within them. Hoyle is the author of several books on stars, both technical and for general readers, as well as a number of science fiction stories and even a script for an opera. In 1948, Hoyle joined the debate between steady-state and big bang theorists on how the universe began. He wrote sev¬eral books siding with steady-state proponents Thomas Gold and Hermann Bondi. Hoyle was not very happy about the popularity of his "big bang" remark and avoided the term from then on.

What is an Open Cluster?

   A galactic cluster, or open cluster, is a group of associated stars that travel together through Earth's galaxy, the Milky Way. About 1,100 such clusters, each with a dozen to several hundred stars, have been catalogued. Almost all of them are located close to the plane of the galaxy. Open clusters range from about 5 to about 70 light-years in diameter. Often a thin, misty light, caused by reflection off cosmic dust and gas, envelops the entire cluster.
   Open clusters are classified by the number of stars in them and by the degree to which the stars are concentrated toward the center of the cluster. All clus¬ters in the same class are roughly the same size.
A study of galactic clusters was carried out in 1930 by R. J. Trumpler of the Lick Observatory. His investi-gations provided the first clear evidence of the existence of cosmic dust and gas throughout the galaxy.

Open Star cluster Pleiades

The Leonid Meteor Storm

   One of the most famous meteor storms on record occurred on 17 November 1833, during the Leonid shower. All along North America's east coast,—including Niágara Falls, New York—stunned observers saw hundreds of meteors per minute. They described the meteors as falling like snowflakes or heavy rain. Estimates of the hourly rate range from 50,000 to 200,000 meteors.
   Every 33 years, the Earth passes through an especially dense' portion ofthe Leonid stream, resulting in a brief storm. Following the 1833 event, thousands of meteors again rained down in 1866. The Leonid stream can be perturbed by Jupiter's gravity, whtch may explain the poor performances ín 1899 and 1932. The storm returned in full force, however, on 16 November 1966, when an estimated 150,000 meteors were seen a few hours before dawn. And in 1999 the Leonids "roared" again, although changes in the meteor stream reduced the numbers greatly compared to 1966.

How near is the nearest star?

  Our Sun is really a star, and it is closer to us than any other. But that probably isn't what you mean when you ask the question. So the answer is this: Beyond the Sun, the nearest star, Proxima Centauri, is more than twenty-five trillion miles (40 trillion km) away.
  Distances in space are very great. It is a nuisance to figure them in kilometers or miles, because there are too many zeros. And so astronomers measure distance by light years. A light year is the number of miles that light can travel in a year. Light travels very fast — about 186,000 miles a second. In a year it goes about six trillion miles.
  Let's see what that means. When you look at the nearest star you see a twinkling light. The light you see has been traveling for almost 4.2 years! If you are in the fourth grade, the light that you see now left that star when you were still in kindergarten. It has been speeding toward the Earth, night and day, ever since then.
If that star seems a long way off, think about this: Astronomers have discovered galaxies so far away that it takes light 13 billion years to travel from there to the Earth!

Proxima Centauri

How is an optical telescope different from other types of telescopes?

  An optical telescope is the type with which most people are familiar, the kind one looks through in the backyard. The only type of radiation it detects is visible light, meaning it sees what the human eye sees except magnified many times. Other types of telescopes are used to observe radiation from other regions of the electromagnetic spectrum. For example, infrared telescopes detect infrared radiation, and radio tele¬scopes detect radio waves. Other telescopes, placed onboard satellites, study ultraviolet radiation, X-rays, and gamma rays in space. The two main types of optical tele¬scopes are refractors and reflectors.

How powerful was Galileo's first telescope?

  Galileo made his first telescope in 1609. It employed two lenses and was strong enough for astronomical viewing by magnifying objects to thirty-two times their original size. By today's standards, that level of magnification is not very impressive. A relatively inexpensive telescope today features a magnification fifty to five hundred times that of Galileo's.

Supernova I987A

Astronomers around the world were excited when, on 24 February 1987, a new naked-eye star suddenly appeared within the Large Magellanic Cloud. It was the first supernova seen with the unaided eye since 1604, reaching magnitude 2.9 at its peak and remaining visible for several months. The object was originally a massive blue star called Sanduleak -69 202.
The star actually exploded some 165,000 years ago, the light having taken that long to reach us. The light was accompanied by a burst of neutrinos—tiny, elusive particles. Scientists had predicted that these would be produced in very large numbers during a supernova explosion.
The study of SN 1987A continues today, with rings of light, shown in the illustration of the Hubble Space Telescope image (right), being the most recent surprises in the ongoing tale of the supernova's discovery.

Classifyng the stars

  Seen through the telescope, the silvery stars turn into jewels of every color known. Since the glow of a star is from heat, te color depends on its temperature. Red stas are relatively cool, with surface temperatures of about 6,000 °F. Yellow stars, like our Sun, are hotter by thousands of degrees, and blue stars are still hotter. The hottest ultraviolet star may be more that 100.000 °F.
  Relationships exist between tbe color and size of stars and their age and Iocation in tbe galaxies. Stars tend to fall into two great categories called Population I and Population II. Population I consists of stars in the arms of spiral galaxies and in irregular galaxies like the Malleganic Clouds. Population II consists of stars in the nuclei of spiral galaxies, in elliptical galaxies, and in glo-bular clusters.
  The biggest, brightest stars in Population I are blue giants, which spread a blue radiance around them. The biggest, brightest stars of Population II are red giants, which give their surroundings a reddish-orange tint.
In Population I, the smaller stars are red and relatively cool — that is, cool for stars. The larger ones are blue and hot. Until a few decades ago, astronomers believed that the bigger stars are, the hotter they are. Then, as telescopes probed deeper into space, new populations of stars were found. Out in remote globular clusters and still more distant galaxies, the giant stars were red and cool. And even in our own galaxv some stars have been discovered that change periodically in size and brightness. These are the so-called pulsating, or variable, stars. Some change regularly; some are irregular.
  As nuclear physics developed, astronomers learned more about processes that go on in the stars. Then it became clear that different types of stars represent different stages in star evolution. Apparently Population I stars are in earlier stages of their evolution, and Population II stars are probably in later stages.

Is the story of the apple falling really true?

Shortly after finishing college, while living on his family's farm, English mathematician Isaac Newton watched an apple fall from a tree and wondered if the force that caused it to fall, gravity, also applied to orbiting bodies in space, such as the Moon. Why did they not fall away, instead of remaining in an orbital path? Prior to this time, gravity had been thought of as a force that only functioned on Earth. Newton explained the movement of orbiting planets as the result of motion along a straight line combined with the gravitational pull of the Sun. Newton put aside his notes for seventeen years, until astronomer Edmond Halley convinced him to write up his results. Three years after that, Newton's book was published.

Apparent magnitude of well-known stars and other heavenly bodies

In the second century B.C., the Greek astronomer Hipparchus arranged the stars in six grades or classes, of brightness, or apparent magnitude. (As applied to a star, the word "magnitude" has to do with brightness and not with size.) The brightest stars were put in the first grade, the next brightest stars in the second grade and so on. Hipparchus' classification was adopted and improved by Ptolemy of Alexandria in the second century A.D. Our present system of apparent magnitudes is based on the work of these men, though the light values assigned to the different magnitudes have been greatly refined. In the case of the heavenly bodies that are brighter than the stars of the first magnitude, each increasing stage of brightness above 1 is indicated by the appropriate numeral (0, 1, 2, 3 and so on) preceded by a minus sign.

• Sun —26.7
• Moon —12.5 at brightest
• Venus —4.3 at brightest
• Mars —2.8 at brightest
• Sirius A —1.5
• Júpiter —1.3
• Canopus -0.9
• Alpha Centauri A 0.33
• Vega 0.1
• Capella 0.2
• Arcturus 0.2
• Rigel 0.3
• Procyon A 0.5
• Betelgeuse 0.9 (variable)
• Altair 0.9
• Saturn 1.
• Mercury 1.
• Aldebaran 1.1
• Antares 1.2
• Spica 1.2
• Pollux 1.2
• Fomalhaut 1.3
• Deneb 1.3
• Regulus 1.3
• Castor 1.6
• Bellatrix 1.7
• Mira Ceti 2.2 (variable)
• Shedir 2.3
• Polaris 2.3 
• Mizar 2.4
• Alcyone 3.
• Alcor 4.
• Uranus 6.

Cassiopeia A

  When Cassiopeia A (3C 461) exploded as a supernova some 9,700 years ago, a gigantic, circular shell of gas raced out into space. Today, this still-expanding supernova remnant is too faint to see with amateur equipment, but its powerful energy is detectable at radio wavelengths. The radio output emission is created by high-speed electrons spiraling around magnetic field lines as the expanding cloud collides with thin gas between the stars.
   Radio images of the cloud, the brightest radio source outside of the Solar System, show gas racing away from the spot where the star exploded. By calculating this speed and the distance traveled by the gas since the explosion, astronomers estímate that the light from the explosion reached Earth around 1680, creating a 5th magnitude star. No record exists of anyone noticing this short-lived supernova in Cassiopeia.

What are Seyfert galaxies?

Seyfert galaxies are spiral-shaped galaxies characterized by an exceptionally bright nucleus that produce spectral line emission from highly ionized gas. Like the Milky Way, they consist of a central disk of stars with starry arms that extend outward and wrap around the disk like a pinwheel, but Seyfert galaxies display very faint arms and a very bright nucleus. The nuclei, in addition to emitting radiation in vis¬ible light wavelengths, also give off infrared radiation, radio waves, and X-rays. They contained very hot gases: hydrogen, ionized oxygen, nitrogen, neon, sulphur, iron, and argon. These gases are prone to explosions, which cause the nucleus to rotate much faster and more violently than the rest of the galaxy. Seyfert galaxies greatly outshine the other galaxies in a cluster. Some even approach the brightness of quasars, the brightest and most distant objects from Earth. The nuclei of Seyfert galaxies are also similar to quasars in that both types of objects emit radiation from all across the electromagnetic spectrum. This pattern has led some astronomers to theorize that the nuclei of Seyfert galaxies may be faint quasars. Another recent theory about Seyfert galaxies is that they are a stage of development through which all giant spirals pass. If this is true, our own Milky Way galaxy may spend 10 percent of its existence as a Seyfert galaxy.
Seyfert galaxies were named after Carl Keenan Seyfert, the astronomer who first identified the class in 1943.

Double stars

   The first double star ever recorded—Mizar in the Big Dipper— was discovered accidentally in 1650 by the Italian astronomer Giovanni Battista Riccioli. Subsequent discoveries by other astronomers were also accidental. By 1779, enough observations had been compiled to inspire the indefatigable William Herschel (1738-1822) to begin a systematic search for these stellar curiosities. Two years later, he had discovered more than 800 new double stars, assessing each pair with a filar micrometer, a device that allowed him to precisely measure the separation and orientation of the components. Later measurements of these stars by Herschel and others revealed that some of them were, in fact, orbiting each other.
   The American astronomer S, W. Burnham (1838-1921) kicked off a new age of double-star discovertes in 1873 when he published a list of 81 new pairs he had found with his 6 inch (150 mm) refractor. Over the next four decades, this tireless, sharp-eyed observer discovered an additional 1,340 double stars using telescopes of various sizes. In 1906, his observations were collected in the General Catalogue of Double Stars.

Transit of Venus

   Every One-hundred years or so, observers on Earth can watch Venus at inferior conjunction pass across the face of the Sun. These rare events are called transits of Venus, and they occur in a pair, 8 years apart. The last two transits were in 1874 and 1882; the next two take place on 8 June 2004 and 6 June 2012. In 2004, Venus crossed the southern part of the Sun, and in 2012, the northern part.
   In earlier times, transits of Venus let astronomers measure the distance from Earth to Venus, and, by extension, the scale of the Solar System. After the telescope was invented, a handful of astronomers made individual efforts for the 1639 transit. But for the 1761 and 1769 events, the British and French sent out expeditions all over the globe, among them the famous exploring voyage of James Cook, which went to Tahiti for the 1769 transit.
   Better distances for the Solar System did emerge from these efforts, but the most notable finding was about Venus itself. Observing the 1761 transit, the Russian scientist Mikhail Lomonosov discovered that Venus has an atmosphere. He noted the halo it produced around the black dot of the planet as it slipped onto the solar disk and off again.

Transit of Venus in 2004

Waht is Apparent Magnitude?

Sirius A and B

   Apparent magnitude refers to the brightness of stars. Hipparchus, a Greek astronomer of the second century B.C., made a scale of apparent magnitude for comparing the bright¬ness of stars, and catalogued about 1,000 stars. He called the brightest stars "first magnitude stars." The faintest stars visible to the naked eye, he called "sixth magnitude stars."
   The luminosity, or brightness, of a star depends upon its temperature and size. Its apparent brightness, or visual magnitude, depends not only upon its luminosity, but also upon its distance from Earth. Today, it is known that some bright stars are brighter than first-magnitude stars. The scale has now been extended to include zero magnitude and also negative magnitudes. Sirius, the bright¬est star, has been assigned a magnitude of —1.4. A zero-magnitude star is 100 times brighter than a fifth-magnitude star.
   The scale was extended beyond the sixth magnitude to take in the numerous stars seen only with the aid of a telescope. Certain stars in the North Polar Sequence are used as a standard for finding the magnitude of a star. The comparison is made on photographic plates. For greater accuracy, a photoelectric cell is used to measure a star's light, which can then be compared to the light from the standard star.

Absolute magnitude is another scale used in astronomy. It refers to the total amount of light radiated by a star without reference to the amount received on earth. Absolute magnitude is the apparent magnitude the star would have if it were a fixed distance (10 parsecs) away. On the absolute scale, Sirius has a magnitude of —1.5.

The New General Catalogue (NGC)

Galaxy NGC 1300

  In 1874, the largest telescope in the world was owned by an amateur astronomer. At his ancestral home of Birr Castle, Ireland, the third Earl of Rosse, William Parsons, built what became known as the "Leviathan of Parsonstown". For its day, it was 'a monster-'—a 72 inch (1.8 m) diameter telescope slung on cables between two massive brick walls. With this unwieldy instrument. Rosse discovered that many "nebulas" had a spiral shape. Today we know them to be spiral, galaxies. Rosse himself rarely used the telescope, but many other keen observers did. Among them was Johann Louis Emil Dreyer. Dreyer served as Rosse's assistant from 1874 to 1878, recording the mysterious spiral nebulas, among others, with sketch pad and notebook.
  By 1886, Dreyer and other observers" had discovered so many nebulas and star clusters that a new catalog was needed. The Royal Astronomical Society assigned Dreyer the task. Published in 1888, the New General Catalogue (NGC) contained entries for 7,840 objects. The two Index Catalogues (IC) later added another 5,386 objects, some discovered with the newly invented process of photography. A century later, the 13,000-plus, objects-of the NGC and IC lists form the core of all of today's comprehensive databases of deep-sky wonders.

How do astronomers measure the diameter of a star?

Since stars are too remote for any telescope to measure their diameters directly, how do astronomers know that Sirius is 1.8 times the diameter of our Sun or that Aldebaran is 45 times wider? The answer is indirect measuring techniques, which have yielded the diameters of several hundred stars.
The first method involves the precise electronic monitoring of a star when it is blocked out by the Moon. As the Moon makes its monthly orbital trek around Earth, it passes in front of many stars, but only occasionally are the stars bright enough for the detection equipment to complete the experiment. Although the technique is little more complicated than observing the length of time it takes for the star to disappear (which is almost instantaneous), it has revealed accurate diameters for several dozen stars.
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