Breve descripción de los EPIS's y protecciones colectivas que se utilizarán en prevención

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away from the MAIN SEQUENCE, many becoming cool giants; those that remain main-sequence stars are small and cool. As a result the average colour of these stars is yellow, and they are collectively referred to as POPULATION IIstars. By contrast the disk that spreads even beyond the Sun’s orbit, and contains the spiral arms, is a site of star formation, as seen in the ORION NEBULAand similar loca-tions. The integrated light of young stars is dominated by the hottest, brightest specimens, and is therefore blue.

These stars are referred to as POPULATION Istars. Inter-spersed among the stars, and especially between the curv-ing spiral arms, are dense clouds that will continue to form new stars for thousands of millions of years.

Just like planets round the Sun, the stars of our Galaxy orbit the central bulge. Each star does so at a speed dic-tated by the mass of material (stars, gas and any unseen objects such as BLACK HOLES) within its orbit. The stellar orbits are not quite circular, so stars shuffle relative to one another. In the bulge, some orbits are quite elliptical whereas others swing up and down in complex gyrations that may be visualized by drawing a sine wave around the surface of a slightly squashed cylinder. Our Sun circles the galactic centre at about 250 km/s (160 mi/s), taking some 200 million years per orbit.

If the motions of the outermost stars of the Galaxy are measured, the total mass can be calculated. This measure-ment is only partially feasible, because velocities can only be determined along the line of sight (using the DOPPLER EFFECT) and not across it. A further complication is our own motion, itself not very accurately known. Various sta-tistical analyses of the measured motions suggest that the Galaxy’s mass is nearly one million million times as much as the Sun, which is nearly ten times the mass of the visi-ble constituents. This extra mass forms a spheroidal halo around the Galaxy some 150,000 l.y. across, which is in turn embedded within a spherical corona, which may be up to 600,000 l.y. in diameter and which includes the

MAGELLANIC CLOUDS, the GLOBULAR CLUSTERSand some other dwarf galaxies. The Milky Way may have formed the halo and corona through the accretion and tidal break-up of other small galaxies.

galaxy cluster Group of ^GALAXIES. Galaxies are usually found as members of clusters. Rich clusters can have thousands of members and poor clusters may have only dozens of galaxies. The clustering of galaxies is an important constraint on cosmological models and the degree of clustering in the Universe today is related to the anisotropies in the matter distribution of the early universe.

Margaret GELLERand colleagues at Princeton University have mapped a portion of the Universe and attempts are currently being made to characterize the degree of galaxy clustering using CORRELATION FUNCTIONS.

Galilean satellites Collective term for the four largest

SATELLITES of JUPITER, discovered in 1610 by GALILEO GALILEI. They were the first proof that not all motion in the Solar System is centred on the Earth, and it paved the way for the acceptance of the HELIOCENTRIC THEORY.

Little was known about the Galilean satellites until they were seen from close range during flybys by the two

VOYAGER probes in 1979. More detailed images were obtained during the 1995–2003 orbital tour by the

GALILEOspacecraft, and repeated encounters cast light on their internal density distributions and detected their magnetic fields. The Galilean satellites show a progres-sive decrease in density, corresponding to an increase in the ratio of ice to rock, with distance from Jupiter. This observation reflects the higher temperatures that pre-vailed in the inner part of Jupiter’s PROTOPLANETARY DISK

while the satellites were accreting. The innermost satel-lite, IO, has the same structure as a TERRESTRIAL PLANET, with an iron-rich core surrounded by a rocky mantle.

The next satellite, EUROPA, has a similar structure overlain by an outer layer of ice. GANYMEDE, the largest of the four, is composed of a roughly 40:60 ice-rock mixture, and its interior appears to be fully differentiated (seeDIF

-FERENTIATION), with an iron-rich inner core, a rocky outer core and an icy mantle. CALLISTO, the outermost Galilean satellite, is slightly less dense and appears to be only weakly differentiated. TIDAL HEATINGis responsible for present day volcanic activity on Io and the recent break-up of Europa’s surface, where there may well be an ocean below the ice. It may also account for the substan-tially different evolution of Ganymede and Callisto.

Galilean telescope REFRACTING TELESCOPE having a plano-convex objective and a plano-concave eyepiece. It forms an erect image and gives a small field of view. This type of instrument, first employed by GALILEOto make astronomical observations, is no longer used in astronomy, but the same optical system is still used in opera glasses.

Galileo First spacecraft to orbit the giant planet JUPITER

and to deploy an atmospheric probe into the Jovian atmosphere, in 1995 December. It was launched by the

NATIONAL AERONAUTICS AND SPACE ADMINISTRAION

(NASA) in 1989 October aboard the Space Shuttle and deployed initially in Earth orbit. The launch had originally been planned for 1986 May but was delayed by safety concerns about flying Galileo with a cryogenic upper stage on the Shuttle after the Challenger accident. Soon after deployment, it was discovered that the spacecraft’s large, high-gain antenna had not deployed fully, jeopardizing the success of the mission’s ability to transmit data.

Corrections to software and systems meant that the spacecraft’s loss could be compensated in part, although transmission of data took longer and contained less information. Galileo flew to Jupiter via a gravity assist flyby of Venus in 1990 February and two flybys of the Earth in 1990 December and 1992 December. The spacecraft’s path allowed it also to visit two asteroids, marking a space first in 1991 October with a flyby of GASPRA. The asteroid

IDAwas explored in 1993 August. Close-up images of both asteroids at a minimum distance of 1600 km (1000 mi) and 2410 km (1500 mi) respectively were taken.

In 1995 July, the atmospheric probe was deployed, some 80 million km (50 million mi) from Jupiter, and both galaxy cluster

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Galileo Galilei

spacecraft headed towards a rendezvous with the planet.

The probe entered the Jovian atmosphere on 1995 December 7 and transmitted data for 57 minutes during a 152-km (95-mi) descent under a single parachute, trans-mitting data to Galileo as it was preparing to enter orbit the following day. Transmission of the probe data to Earth was completed by 1996 April. The data revealed an intense radiation belt 50,000 km (31,000 mi) above the clouds, a Sun-like hydrogen–helium ratio, some organic compounds, one cloud layer and 640 m/s (2000 ft/s) winds below the cloud deck. Thunderstorms many times larger than Earth’s result from the vertical circulation of water in the top layers, leaving large areas where air descends and becomes dry and other areas where water rises to form thunderstorms.

Galileo began its tour of the Jovian satellites, which continued until 2002, with a flyby of GANYMEDE, fol-lowed by several further encounters with this and the other main moons. Galileo established that the ring sys-tem is made of small grains blasted off the satellites’ sur-faces by meteoroid impacts. IOwas found to be the Solar System’s most volcanically active body, while EUROPAhas a possible salty ocean containing more water than does Earth. Ganymede has a magnetic field and CALLISTOmay also have a water ocean. In 2000 December, Jupiter was for the first time in space history explored by two space-craft simultaneously, with Galileo’s observations being complimented by those from CASSINIen route to Saturn during the ‘Millennium Flyby’.

Galileo Galilei (1564–1642) Italian astronomer, physicist and mathematician. He was one of the first to use the telescope for astronomical observations, discovering mountains and craters on the Moon, and four satellites of Jupiter now known as the Galilean satellites. He observed the phases of Venus, and studied sunspots, from whose motion he deduced that the Sun rotates. Galileo concluded that ARISTOTLE’s picture of the world, still widely believed in his time, was wrong and he championed COPERNICUS’ heliocentric theory. This brought him into conflict with the Catholic Church and led to his trial and house arrest for the last eight years of his life.

Born in Pisa, the son of a lawyer and musician Vincenzio Galilei, Galileo was educated at the universities of Pisa and Padua. At first he studied medicine, but abandoned it for mathematics. Early in his career, he developed the fondness of controversy that would play a major part in precipitating his condemnation in 1633. One body of opinion in the uni-versities of his day, which Galileo came first to criticize and then to openly ridicule, was the interpretation of all natural phenomena in terms of the philosophy of Aristotle, with its axiomatic assumption that the Earth stood fixed at the cen-tre of the Universe (seeGEOCENTRIC THEORY). It is all too easy for us to sympathize with Galileo’s attacks upon Aris-totelianism, and take his polemical language at face value when he called his Aristotelian contemporaries ‘simple-tons’. Yet even by the time of his death in 1642, let alone in 1600, there was no known physical proof of the Earth’s motion in space, and Copernicus’ theory hinged more upon mathematical elegance than upon observed fact.

Galileo’s youthful anti-Aristotelianism was confirmed by a series of experiments in physics. The first of these, dating from his time in Padua in 1583, made him realize that when a pendulum swings, the determining physical factor is not the heaviness of the bob, but the length of the pendulum itself. Likewise, the results of his research-es in Padua, where he became profresearch-essor of mathematics in 1592, further contradicted Aristotle, for it was clear that the rate of acceleration of a falling body is governed by a fixed mathematical law that has nothing to do with the weight of the body. In Aristotle’s philosophy there was no place for such laws. Terrestrial events were caused by an intermixing of earth, water, air and fire;

only the heavens displayed mathematical regularities. Yet Galileo had shown that terrestrial as well as celestial motions conformed to exact mathematical laws, and he can be said to have established physics as a discipline founded on mathematical theory and experiment.

But it was his astronomical work that made Galileo famous, to become by the end of 1610 Europe’s first scien-tific celebrity. Even before 1600, he was convinced of the truth of the heliocentric COPERNICAN SYSTEM. His convic-tion derived from Copernicanism’s mathematical elegance in explaining the retrograde motions of planets, combined with his own anti-Aristotelian conclusions in physics. But when he first used the newly invented telescope to observe the night sky over the winter of 1609/10, he saw things that he felt secured the Copernican case. He found that the Moon’s terrain was mountainous, not the smooth surface Aristotle had predicted. He also found that even under the

30 magnification of his most powerful telescope, the Milky Way fragmented into millions of individual stars, and that instead of a few thousand stars being attached to the inside of a black dome, as the ancients had taught, there seemed to be countless millions extending throughout space. And then, in 1610 January, he discovered that the classical ‘wandering star’, Jupiter, was a spherical world, with four tiny satellites orbiting it. (Simon MARIUS later claimed to have observed them first.) Very obviously, the Earth was not the only centre of rotation in the Universe.

These and other telescopic discoveries by Galileo were published in 1610 March as Siderius nuncius, ‘The Starry Messenger’, and won him renown across Europe. All these discoveries, along with that of the phases of Venus (which showed that Venus revolved around the Sun) and, in 1611, of sunspots (which showed that the Sun was not an unblemished golden sphere), added to the weight of evidence against Aristotle’s ideas, yet failed to provide physical proof that the Earth was in motion. For that, a stellar parallax or some similar phenomenon was needed, and instruments would not become sufficiently accurate to measure such a parallax until 1838.

Galileo’s advocacy of Copernicanism thus rested not upon being able to prove the heliocentric theory, but upon his skill in undermining the objections of conserva-tive astronomers who still adhered to Aristotle and Ptole-my. And as he could be very mocking to his opponents, Galileo made enemies, especially in the Jesuit order, whose Christoph SCHEINERhad co-discovered sunspots, and yet had interpreted them in accordance with Aris-totelian criteria. In the wake of his 1610 discoveries, however, Galileo had won the patronage of the Medici family, who had invited him to their Grand Ducal Court in Florence. The now famous Galileo was elected to the prestigious Accademia dei Lincei (‘Academy of Lynx-es’) and left Padua to become a scientific courtier and holder of a research chair at Pisa. In Florence, he was

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Galileo Repeated passes close to the four large satellites of Jupiter by the Galileo orbiter since 1996 have revealed these in more detail than ever before.

This image shows the volcano Zal Patera on Io.

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lionized by the Medici and moved on easy terms amongst ambassadors, cardinals and bishops.

In 1616, however, Galileo was officially prohibited by Cardinal Bellarmine from teaching Copernicanism as the truth, as opposed to a hypothesis. Wisely, Galileo – who was a sincerely devout Catholic – complied, but when his friend Cardinal Barbarini became Pope Urban VIII in 1623, he attempted unsuccessfully to have the 1616 prohi-bition lifted. Nonetheless, in 1632, when the Church granted permission for his DIALOGUESto be published, he believed that he had the right publicly to discuss Coperni-canism. However, Galileo’s ensuing condemnation in 1633 was not for a serious heresy (the Church then had no offi-cial line for or against Copernicanism) but was discipli-nary, for breaching the prohibition of 1616. Even so, the elderly Galileo’s condemnation to house arrest at his villa at Arcetri, Florence (where he also discovered the lunar libra-tion) shocked scientists across Europe. But no further Copernicans were punished, and one wonders how far Galileo’s provocative personal style, the enemies he made in the Jesuit order, and the undoubted pride of Pope Urban VIII (who felt personally slighted by Galileo), rather than the science itself, were responsible for his fate. The Catholic Church re-opened the Galileo affair in 1822, and in 1992 Pope John Paul II proclaimed Galileo’s complete exoneration, and offered the Church’s apologies.

Galileo National Telescope (TNG) In Spanish, the Telescopio Nazionale Galileo, a 3.5-m (138-in.) optical/infrared telescope completed in 1997 as an Italian national astronomy facility. It is located at the ROQUE DE LOS MUCHACHOS OBSERVATORYon the island of La Palma at an elevation of 2358 m (7740 ft). It is operated for the Consorzio Nazionale per l’Astronomia e l’Astrofisica (CNAA) by the Centro Galileo Galilei. The TNG is close-ly modelled on the ESO’s NEW TECHNOLOGY TELESCOPE. Galle, Johann Gottfried (1812–1910) German astronomer who, while at the Berlin Observatory, was the first to identify Neptune, on 1846 September 23, though others saw the planet before him but failed to recognize it as such. Galle, who was aided in his search for the new planet by Heinrich D’Arrest, used orbital calculations by

Urbain LE VERRIER. He also discovered three comets and Saturn’s semi-transparent Crêpe Ring (the C Ring) (1838), though George Phillips BONDand William Rutter

DAWES often receive credit for re-discovering this ring years later. Galle made the first reliable distance estimates for asteroids by measuring their parallaxes, first with minor planet (8) Flora in 1873

Gamma Cassiopeiae star (GCAS) Rapidly rotating

VARIABLE STAR of class Be III–V; it occasionally sheds material into surrounding space, generally from the equator, at apparently random intervals, in what are termed ‘shell episodes’. Gamma Cassiopeiae stars may be subdivided into BE STARS, where the ejected material forms a ring around the star, and SHELL STARS, where it forms a circumstellar shell. They are related to, but less extreme than, the S DORADUS STARS.

gamma-ray astronomy Research covering the energy range of the ELECTROMAGNETIC SPECTRUMhigher than 30 keV (wavelength less than 0.04 nm), essentially the last part of the spectrum to be explored. The extreme penetration of gamma rays makes them valuable for probing regions of the Galaxy and beyond where other radiation is absorbed. COSMIC RAYSwere first thought to be a form of gamma radiation, and many gamma rays are produced by interactions of cosmic rays with the gas in the interstellar medium. However, generally gamma rays are associated with the most energetic mechanisms in the cosmos. There are several places where gamma rays are produced: cosmic rays interacting with gas in molecular clouds; SUPERNOVA REMNANTS, where cosmic rays accelerated in the supernova explosion interact with nearby gas; massive stars producing gamma rays in their winds; and PULSARS.

There were some early balloon experiments, but gamma-ray astronomy did not really start until 1967, with the launch of the American satellite OSO III. GAMMA-RAY BURSTSwere first discovered around this time, by satellite-borne detectors meant to monitor violations of the Nuclear Test-Ban Treaty. In the 1970s SAS II and COS B were launched and they found a few tens of sources. The subject was revolutionized with the launch of the NASA

COMPTON GAMMA RAY OBSERVATORY (CGRO) in 1991, which operated until 2000. New satellites are planned –

INTEGRALis due for launch by ESA in 2002 and GLAST is due for launch by NASA in 2006. At the highest gamma ray energies (around 0.2 TeV and above), ground-based telescopes can detect CERENKOVlight from the passage of gamma rays in Earth’s atmosphere. There are now several Atmospheric Cerenkov Imaging Tele-scopes (ACITs) which record the flashes (some ACITs have several dishes). Most of their results have been received in the 300 GeV to 30 TeV range.

The gamma-ray view of the sky in this energy range looks rather similar to the visible picture, in that there is a general concentration of emission in the galactic plane and an increase in the general direction of the galactic centre. Galactic gamma-ray radiation will be produced mostly by cosmic rays interacting with the gas in the interstellar medium. The lower-energy gamma rays (below about 100 MeV) are produced by electrons, and the electron intensity is higher in the inner Galaxy than locally and in the outer Galaxy. However, the poor angu-lar resolution of the gamma-ray telescopes has led to many problems, particularly with the nature of the ‘hot

The gamma-ray view of the sky in this energy range looks rather similar to the visible picture, in that there is a general concentration of emission in the galactic plane and an increase in the general direction of the galactic centre. Galactic gamma-ray radiation will be produced mostly by cosmic rays interacting with the gas in the interstellar medium. The lower-energy gamma rays (below about 100 MeV) are produced by electrons, and the electron intensity is higher in the inner Galaxy than locally and in the outer Galaxy. However, the poor angu-lar resolution of the gamma-ray telescopes has led to many problems, particularly with the nature of the ‘hot