astronomical unit One astronomical unit (AU) is the mean distance between the Earth and the Sun: roughly 150 million kilometres (93 million miles). Towards the edge of the solar system, the Kuiper Belt (containing dwarf planets) is 30–55 AU from the Sun, while the far more distant Oort Cloud (containing icy objects) is 5,000–100,000 AU from the Sun.
coma Very thin cloud of gas and dust surrounding the heart or nucleus of a comet. The heart is a ball of ice and rock particles, described by American astronomer Fred Whipple as an icy conglomerate, or ‘dirty snowball’. As the comet nears the inner Solar System and is more strongly heated by the Sun, some of the ice and dust is vaporized, creating the coma.
convective zone In the Sun, an area between the radiation zone (nearer the core) and the solar photosphere, through which energy passes by convection. Hotter material rises from the bottom, carrying energy, then sinks again after cooling; the cooling material heats up again as it sinks and then rises once more in a rolling process.
Halley’s Comet Officially known as 1P/Halley, a short-period comet named after English astronomer Edmond Halley, who correctly calculated in 1705 that the comets seen in 1531, 1607 and 1682 were one returning comet and that this comet would return in 1758. Halley is the brightest short-period comet visible to the naked eye and is visible every 75–76 years. It has been known since at least 240BC, was seen during the Norman Conquest of England in 1066, and was represented in the Bayeux Tapestry that recorded the conquest. Last seen in 1986, it will appear again in 2061.
Kuiper Belt Doughnut-shaped region in the outer solar system, billions of kilometers from our Sun, containing small bodies and dwarf planets, including Pluto. Because their orbit lies beyond that of Neptune, they are often called ‘trans-Neptunian objects’.
nuclear fusion The combination (fusion) of two atomic nuclei to form a heavier nucleus, accompanied by the release of energy. Nuclear fusion powers the Sun and other active stars.
Oort Cloud Spherical cloud in the outer solar system, far beyond the Kuiper Belt, that could contain up to two trillion frozen bodies. The further reaches of the Oort Cloud mark the limit of the Sun’s gravitational attraction – and so are at the boundary of our solar system. Astronomers believe that most comets originate in the Oort Cloud.
orbital period Time taken for an object to make a complete orbit around another. The Earth’s orbital period around the Sun is one year, or 365.256363 days.
Perseids The meteor shower occurring annually from 23 July to 20 August, so called because the area from which the meteors appear to fall lies in the constellation Perseus. The dust and debris come from the comet Swift-Tuttle. The Perseids are mostly visible in the northern hemisphere.
protoplanetary disc Rotating disc of gas and dust surrounding a newly formed star in a developing solar system. Planets form from the gas and grains of dust.
short-period comet Comet with an orbital period around the Sun of less than 200 years.
solar corona The outer atmosphere of the Sun, not normally visible because it is one million times less bright than the visible solar photosphere. The corona can be seen during a total solar eclipse, when the brightness of the solar disc is blocked by the Moon, or using a coronagraph instrument, which blocks the light coming from the solar disc in order to enable study of the solar atmosphere.
solar photosphere The visible outer layer of the Sun, only about 100 kilometres (60 miles) thick. Sunspots, faculae (bright areas) and granules (cellular features) are visible on the photosphere.
star A vast ball of gas of great mass (held together by gravity) that generates heat and light through nuclear fusion reactions at its core.
sunspots Dark spots on the solar photosphere that result when magnetic activity limits convection, creating areas where the very high temperature is partly reduced.
Very high temperatures and pressures deep in the core of the Sun squeeze hydrogen into helium, converting a fraction of the atoms’ mass into pure energy through nuclear fusion. This energy radiates outwards and bubbles, like water in a boiling pot, up through the Sun’s convective zone, riding on buoyant plumes of ionized gas (plasma). Finally, having travelled 700,000 kilometres (435,000 miles) from the centre of the Sun (100 times further than the distance from the Earth’s core to its surface), the energy escapes as bright white light from the solar photosphere (the visible outer layer of the Sun), then radiates into the darkness of space. Just one-billionth of this energy actually lands on the Earth, where it drives our weather and keeps us warm. Although it is the coolest layer of the Sun, the solar photosphere is so hot – at 5,500°C (10,000°F) – that it would vaporize any solid material. Strong magnetic fields, generated by swirling eddies of plasma deep in the convective zone, pierce the photosphere and leave behind dark sunspots that mottle the Sun’s surface. Sunspots are most prevalent close to times of maximum solar magnetic activity, which occur once every 11 years.
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Our star, the Sun, is a 100 trillion terawatt nuclear furnace, the source of energy for almost every living organism on Earth.
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The Sun is a star, just like the 100,000,000,000 other stars in our Galaxy. Because the Sun is so close, we can study it better than any other star. In ‘helioseismology’, scientists use slow oscillations seen on the Sun’s surface to measure its internal structure and composition. Using underground detectors, they can even observe weakly interacting fundamental particles (neutrinos) that are by-products of the nuclear reactions taking place in the Sun’s core.
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3-SECOND BIOGRAPHIES
JOSEPH VON FRAUNHOFER
1787–1826
German optician
JOSEPH NORMAN LOCKYER
1836–1920
English scientist and astronomer
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Zachory K. Berta
The Sun and the Earth are shown with their relative sizes (but not distances) to scale. A million Earths could fit inside the Sun.
Above the solar photosphere hovers a tenuous billow of even hotter plasma, known as the solar corona. What heats this corona is still an active area of research, but the process may involve magnetic or acoustic waves crashing above the Sun’s surface. This corona exuberantly launches billions of tons per hour of energetic particles (electrons, protons and heavier ions) out into the vacuum of space, blowing a ‘solar wind’ outward at millions of kilometers per hour. At these speeds, when a solar flare causes an outburst in the solar wind above usual levels, it can travel from the Sun to the Earth in a few days. Fortunately, our intrinsic magnetic field shields us from this dangerous wind, safely deflecting it and preventing it from destroying our satellites or obliterating our biosphere. Deflected charged particles from the solar wind spiral along the Earth’s magnetic field towards the poles, where they interact with our atmosphere to create the colourfully glowing aurora borealis (‘Northern Lights’) or aurora australis (‘Southern Lights’). Aurorae grow brighter and extend further towards the equator when the Sun is most active and the solar wind most intense. The solar wind also sculpts the straight plasma tail of many comets.
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In addition to emitting light, the Sun also spews out the solar wind – a supersonic blast of charged particles constantly buffeting the Earth’s magnetic field.
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In 1859, solar astronomer Richard Carrington witnessed a bright flash of light on the Sun’s surface. This massive solar flare launched a solar wind gust that smashed into Earth’s magnetic field one day later, creating colourful night-time aurorae bright enough to read the newspaper by and shocking telegraph operators’ fingers with electrical sparks. Another geomagnetic storm as intense as this ‘Carrington event’ could have crippling effects on our modern telecommunications and power grids.
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RICHARD CARRINGTON
1826–75
English astronomer
KRISTIAN BIRKELAND
1867–1917
Norwegian scientist who identified the cause of the Northern Lights
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Zachory K. Berta
Particles from the solar wind interact with Earth’s magnetic field atmosphere, creating beautiful aurorae that are more commonly seen closer to Earth’s north or south poles.
Beyond the main planets lies a zone of smaller ones called trans-Neptunian objects. They orbit the Sun in an outer region of the solar system called the Kuiper Belt. They are a motley collection of planetary scraps ejected from the inner solar system by the combined and repeated tug of Jupiter and Saturn early in the solar system’s development. Eris and Pluto, two of these trans-Neptunian objects, are respectively the ninth and tenth most massive planets in the solar system: Eris is 2,325 kilometres (1,445 miles) in diameter and Pluto is 2,320 kilometres (1,440 miles) in diameter. Pluto was considered the ninth planet when it was identified in 1930. However, the discovery of Eris in 2005 provoked astronomers to rethink the definition of a planet, and in linked decisions in 2006 and 2008 Eris and Pluto were reclassified as ‘dwarf planets’. Ceres (the largest asteroid in the main belt of asteroids between Mars and Jupiter, discovered in 1801) was also reclassified as a dwarf planet. Further trans-Neptunian objects were discovered and classified as dwarf planets between 2002 and 2007: Haumea, Makemake, Orcus, Quaoar and Sedna, all named after mythological creatures from creation myths of less well-known civilizations.
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Dwarf planets could be called the adolescent offspring of the main planets – miniature versions of their seniors, without dominance over the space they inhabit.
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Planets were shaped by gravity as their material settled down over time – the interior of each planet supports its own weight. This is true both of the main planets and dwarf planets that are larger than about 560 kilometres (350 miles) in diameter, depending on composition and rotation. The main planets have additionally cleared their neighbourhood either by absorbing or ejecting anything that has strayed too close. Dwarf planets, by contrast, do not dominate their orbital zone.
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GERARD KUIPER
1905–73
Dutch-American astronomer and planetary scientist
CLYDE TOMBAUGH
1906–97
American astronomer who discovered Pluto
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Paul Murdin
Pluto and its four small satellites go around the Sun scarcely further out than planet Neptune, and in an inclined, highly elliptical orbit that suggests its erratic journey from the inner solar system.
Our solar system hosts only eight major planets, but is swarming with a multitude of smaller rocky asteroids; hundreds of thousands have been observed so far. These airless chunks of rock and metal come in all sizes, from misshapen specks of dust to the dwarf planet Ceres, almost 1,000 kilometres (600 miles) in diameter. Asteroids exist throughout the solar system, from inside Mercury’s orbit to outside Neptune’s, but they only survive for more than a short period where the gravitational forces exerted by the more massive planets do not sweep them away or suck them into a collision course. Asteroids are more common in the main asteroid belt, just beyond the orbit of Mars. The spaces between asteroids are so huge, even in this asteroid belt, that asteroids collide with each other quite infrequently. Except for these rare collisions, many asteroids have remained unchanged since they condensed out of the primordial protoplanetary disc 4.5 billion years ago, during the birth of the solar system. They preserve a record of ancient conditions that we can study with telescopes, by visiting them with spacecraft, or when chunks of asteroids fall to Earth as meteorites.
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Rocky asteroids litter our solar system and hold clues to its beginnings and evolution.
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Asteroids whose orbits bring them close to our planet are known as Near Earth Objects (NEOs). The probability of a large, life-threatening asteroid colliding with the Earth is small, but astronomers carefully track NEOs because the consequences of a collision would be so catastrophic. The Minor Planet Center of the International Astronomical Union compiles data and calculates orbits for all known asteroids, so that it can predict any potentially hazardous collisions before they occur.
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GIUSEPPE PIAZZI
1746–1826
Italian astronomer who discovered the asteroid Ceres
DANIEL KIRKWOOD
1814–95
American astronomer who identified the ‘Kirkwood gaps’ in the asteroid belt
KIYOTSUGU HIRAYAMA
1874–1943
Japanese astronomer who first found groups of asteroids sharing almost identical orbits
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Zachory K. Berta
Asteroids are tiny in size compared to the vast distances that lie between them.
Nicolaus Copernicus – the man who turned the world upside down, or at least turned the solar system inside out – was an unlikely revolutionary. His paradigm-shifting heliocentric theory stated that the Sun, not the Earth, was at the centre of the Universe. This challenged the orthodoxy of the Ptolemaic system, named after the Egyptian astronomer Ptolemy of Alexandria (c. AD 100–c. AD 170), which stated that the Sun, Moon and planets revolved around the Earth. Copernicus’s theory was painstakingly put together over a long period (no one knows when he started, but it was probably in about 1510) and was the result of early pickings at mathematical threads dangling from the edges of the Ptolemaic system. Why did the planets not follow uniform concentric orbits? And why was the theory of the equant (a point Ptolemy had invented to make everything work) so unsatisfactory? Once he had started picking, Copernicus carried on until he had unravelled the whole thing, and he came to the conclusion that the maths only worked if the Sun, not the Earth, were at the centre of everything.
Multitalented Renaissance polymath – scholar, linguist, translator, mathematician, astronomer, physician, artist, economist, diplomat and cleric – Copernicus was above all a dutiful family man. His education and career were paid for and directed by his maternal uncle Lucas Watzenrode, the powerful Bishop of Warmia, who had determined that Copernicus should achieve high office in the Church. This the young man did: after studies at the University of Cracow, he was elected canon at Frombork (Frauenberg), in northern Poland, where he stayed for the rest of his life, taking several leaves of absence for further study at Padua, Bologna and Ferrara. His administrative duties and the medical care of his benefactor took up much of his time, and he had to fit astronomical observations in whenever he could.
Copernicus wrote three astronomical works: Commentariolus (The Little Commentary), a 40-page précis of what was to become his heliocentric hypothesis, written before 1514 and circulated among peers and friends; the ‘Letter against Werner’ (1524), a critical drubbing of the work of mathematician Johann Werner; and his great De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres), published in the year of his death, 1543. According to legend, Copernicus was on his deathbed when an advance copy of the printed book arrived: it was placed in his hands just before he died.
Born in Torun (Thorn), now in Poland
1491–95
Studied mathematics, astronomy, and natural science at the University of Cracow
1495
Elected canon, but his installation postponed
1496–1501
Studied canon law at the University of Bologna; became assistant to Italian astronomer Domenico Maria de Novara
1497
Formally appointed canon
1501–03
Studied medicine at the University of Padua
1503
Received a doctorate of law from the University of Ferrara
1503–10
Secretary and physician to his uncle, the Prince-Bishop of Warmia
c. 1514
Wrote the Commentariolus, an initial outline of his heliocentric theory
1512–15
Made observations of Mars, Saturn and the Sun
1532
De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres) is almost finished, but Copernicus is reluctant to publish for fear of scornful reaction
1533
Johann Albrecht Widmanstetter lectured on the Copernican theory and was heard by Pope Clement VII; Copernicus was urged to publish but still reluctant
1539
Mathematician Georg Joachim Rheticus visits Copernicus and becomes his pupil and secretary
1540
Rheticus writes and publishes Narratio Prima, a description of the Copernican theory, which tested reactions to the idea
1542
Rheticus takes the manuscript of De Revolutionibus to Nuremberg
1543
De Revolutionibus is published
24 May 1543
Dies at Frombork
1566
Second edition of De Revolutionibus is published
Far from the Sun, a ball of ice and rock, only a fewkilometers across, drifts slowly through the frozen dark. After spending most of its life as an inert, dirty snowball, it gradually accelerates inwards. The increasing warmth of the Sun heats its outermost surface, vaporizing ice to form a diffuse, gaseous ’coma’ tens of thousands of kilometers in size. In this spewing state, it is an active comet and will release increasing amounts of material as it plummets further into the hot inner solar system. Two tails stretch millions of kilometers out from the coma: a curved, yellowish tail, from sunlit dust blasted off the nucleus and now drifting slowly behind; and a straight, bluish tail pointed away from the Sun, from plasma trapped in the magnetized solar wind. Comets can occur from any ice-rich bodies, be they once-in-human-history visitors from the distant Oort Cloud or Kuiper Belt objects on shorter eccentric orbits, such as Halley’s Comet that returns every 75–76 years. The latter may eventually become inactive asteroids, as outbursts over successive visits past the Sun strip them of their comet-fuelling volatiles.
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The beautiful comets that glitter across the night sky are not static, stable or constant objects but rather transient events, evolving processes – celestial happenings.
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Giant collisions during Earth’s formation may have blasted the oceans and atmosphere off our young planet. Outgassing of volatile molecules trapped in Earth’s mantle could provide much of the missing water and gas, but astronomers also believe that bombardment by water-rich comets may have contributed to Earth’s hydrosphere. Without this substantial surface water or a protective atmosphere, it would have been difficult for life to arise on our planet.
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EDMOND HALLEY
1656–1742
English astronomer who first computed the orbit of Halley’s Comet
JAN OORT
1900–92
Dutch astronomer who gave his name to the Oort Cloud of comets
FRED WHIPPLE
1906–2004
American astronomer who first described the nuclei of comets as icy conglomerates, or ‘dirty snowballs’
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Zachory K. Berta
The nucleus of a comet is tiny, just a millionth the comet’s total size.
Be it a fleck off an asteroid, a pebble from a comet’s tail, or even man-made trash, a ‘meteoroid’ is any small body out in space that will eventually collide with our Earth. Once it enters the Earth’s atmosphere, typically with speeds of 10–70 km/s (7–44 miles/s), it becomes a ‘meteor’. Air friction slows its rapid descent and heats it to the point of incandescence, creating a burning streak across the sky. A chunk of rock or metal, a nugget of the original meteoroid, may survive all the way to the ground to become a ‘meteorite’. Meteors can be seen on clear nights, but occur most often during annual meteor showers (for example, the Perseids in early August), when the Earth’s orbit brings it through the meteoroid-rich debris clouds left by long-gone comets. Those meteors that we see flash across the sky are typically from meteoroids about 1 cm(½ in) diameter. ‘Micrometeors’ (10–100 microns diameter) are vastly more common, but fall unnoticed because they are so small. The largest meteors are extraordinarily rare. An example is ‘Cretaceous-Paleogene impactor’, 10 kilometres (6 miles) diameter, whose collision with the Earth 65 million years ago probably obliterated the dinosaurs.
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If you have seen a shooting star, you have witnessed a meteor: the fiery final plunge of a chunk of space debris into Earth’s atmosphere.
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Meteorites found on Earth, typically from bodies initially 1–10 metres (3–30 ft) across, are among the few tangible ambassadors that we have from outer space. Other astronomical objects are studied by the light they reflect or emit, but meteorites can be dissected in exquisite detail using sophisticated equipment. For example, radioactive dating of certain types of meteorites place their age, and that of our solar system, at precisely 4.56 billion years.
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LUIS & WALTER ALVAREZ
1911–88 & 1940–
American scientists who found evidence that a massive meteor impact could have caused the extinction of the dinosaurs, and other species
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Zachory K. Berta
When the Earth crosses through the eccentric, rubble-strewn orbits of comets, we experience more meteors than usual, a repeating event known as a meteor shower.
