Andromeda Galaxy Also known as Messier 31, the nearest galaxy to our own (the Milky Way), apart from a number of smaller ‘companion galaxies’ that are satellites of the Milky Way. The Andromeda Galaxy is a spiral galaxy situated about two-and-a-half-million light-years away in the Andromeda constellation and contains 1 trillion (1,000,000,000,000) stars.
Cepheid variable Type of star that varies between a compressed and an expanded state. The stars (which have a mass of between 5 and 20 times that of our Sun) expand when pressure builds up, then contract because pressure is lower in the star’s expanded state. Cepheids are used by astronomers to establish extragalactic distances.
comet An icy body with a coma (temporary atmosphere) and a tail in orbit around the Sun that becomes visible when sufficiently close to the Sun. The tail points away from the Sun, while the curved coma follows the comet’s path.
diffuse star formation nebulae Higher density clouds in the interstellar medium, in which stars are formed.
elliptical galaxy Galaxy in the form of an ellipsoid (a three-dimensional ellipse). One of three galaxy types identified in 1936 by American astronomer Edwin Hubble, along with lenticular and spiral galaxies.
galaxy A system bound together by gravity, containing stars, clouds of dust and gas in the interstellar medium and dark matter.
globular cluster Group of stars, tightly bound in a spherical shape by gravity, that orbits the core of a galaxy. There are between 150 and 160 globular clusters in the Milky Way, containing some of the oldest stars in the Galaxy.
interstellar medium The matter that fills the space between the star systems within a galaxy. Also known as ISM, it consists principally of gas and dust; from these materials, new stars are formed, and light generated by their birth heats the remaining gas atoms and makes pink-red nebulae visible in the vicinity of the new stars.
lenticular galaxy Type of disc galaxy with a central group of stars (called a bulge), similar to a spiral galaxy, but lacking the spiral arms of the spiral galaxy shape.
nebula (pl. nebulae) Visible cloud of dust or gas in interstellar space. Emission nebulae are visible because gas atoms within them have been heated by ultraviolet light emanating from a nearby star; reflection nebulae can be seen because they are reflecting light from a local star or star group; dark nebulae are identifiable because they block the light from a star or star group behind them.
open cluster of stars Loosely bound group of stars held together by mutual gravitational attraction and in orbit around the centre of a galaxy. The Pleiades group is an example of an open cluster. There are believed to be more than 1,100 open star clusters within the Milky Way Galaxy.
Orion Nebula Also known as Messier 42, a vast region of gas and dust – 13 light-years across – situated just to the south of Orion’s belt in the constellation of Orion.
Pleiades cluster A group of stars, also known as Messier 45, another of the ‘Messier objects’ identified by the eighteenth-century French astronomer Charles Messier. The cluster is about 425 light-years’ distance from the Earth, and it contains hundreds of stars, although only a few are visible to the naked eye. These bright blue stars are also known as ‘the Seven Sisters.’
spiral galaxy Type of galaxy with a central group of stars (a bulge), and with spiral arms containing stars, gas and dust, extending outwards into a rotating disc-like structure.
standard candle Astrophysical object with known luminosity that can be used to calculate the distance from Earth of a formation in which it appears. Cepheid variables are used as standard candles.
starburst Period of intense star formation. In a starburst phase, stars form at a rate of up to 100 times that of regular stars.
Triangulum Galaxy Also known as Messier 33 or the ‘Pinwheel Galaxy,’ a spiral galaxy in the constellation Triangulum. Part of the Local Group of galaxies that includes the Milky Way and the Andromeda Galaxy, the Triangulum lies 3 million light-years from Earth but can be seen with the naked eye in very good conditions, making it one of the most distant objects that can be viewed without a telescope.
Virgo cluster of galaxies Group of galaxies in the constellation Virgo. The group contains up to 2,000 galaxies and is at the heart of an even larger Virgo supercluster. The Local Group of galaxies, which contains our galaxy (the Milky Way) and the Andromeda Galaxy, belongs to the Virgo supercluster and is orbiting around the Virgo cluster.
The different stars we can see in each constellation are very rarely physically associated with each other, but just happen to be in the same line of sight from the Earth. Constellations are grid-like segments around recognizable patterns of stars called ‘asterisms.’ For example, the asterism known as the ‘Big Dipper’ corresponds to the seven brightest stars in the larger Ursa Major constellation. The majority of the constellations take their names from the astronomical treatise Almagest, written in the second century AD by Egyptian astronomer Claudius Ptolemy. The apparent motion of stars in the sky is due to the Earth’s rotation on its axis. We see them rotating around the Earth’s rotation axis, which is practically aligned with the North (or Pole) Star – Polaris. The planets and the Moon all orbit the Sun on roughly the same plane, which is named the ecliptic. Consequently, the Sun, the other planets and the Moon all appear as moving objects on the same circle in the Earth’s celestial sphere, in a background of stars that scarcely move during a human lifetime. The constellations crossed by this circle are the 13 ‘zodiacal’ constellations.
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The celestial sphere is divided into 88 constellations – regions of the sky around recognizable patterns of stars as perceived from the Earth.
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Billions of people believe that human behaviour is influenced by the position of planets in the sky, the phase of the Moon, or the ‘zodiacal sign’ (based on the constellation in which the Sun used to rise on their birth date some 2,000 years ago). However, there is not the least clue that any physical property related to astronomical objects (such as gravity or light) could have an influence on people.
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CLAUDIUS PTOLEMY
C. AD 100–C. AD 170
Egyptian astronomer
NICOLAS LOUIS DE LACAILLE
1713–62
French astronomer who catalogued 10,000 stars
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François Fressin
The seven stars that form the ‘big dipper’ in the constellation Ursa Major are not related, and all are at different distances from the Earth.
The space between the stars is not completely empty but teems with atoms and molecules of gas, making up the interstellar medium. The combined mass of all the gas in the Milky Way is only one-tenth of that contained in stars, yet it is clumped into diffuse clouds that extend out way beyond the spiral arms of a galaxy. At temperatures of tens to hundreds of degrees above absolute zero, the matter in the clouds is so cold that it is mostly in the form of neutral hydrogen atoms, and is thus transparent at visible wavelengths. Traces of heavier elements, such as carbon, oxygen and iron, pepper the clouds, which have been recycled from the hearts of massive stars by the explosions that mark the end of the stars’ lives. The coldest, densest pockets within these clouds provide the ideal conditions for starbirth. Stars newly formed from the interstellar medium flood the surrounding clouds with energetic light, heating the gas atoms and causing them to glow. The gas then becomes apparent as the distinctive, pinky-red nebulae that accompany young blue star clusters lining the spiral arms of a galaxy.
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Interstellar gas clouds are the fundamental reservoir from which new stars form, along with their planetary systems – and any life forms living on those planets.
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Tiny solid particles (‘dust grains’) are mixed with the gas. The densest concentrations within a nebula form opaque silhouettes against background light. The dust protects the core of such clouds from heat and light; temperatures plummet to a few degrees above absolute zero, and the atoms can form complex molecules. These molecular clouds have typical sizes between 3 and 50 light-years, and can contain up to 1,000 Solar masses of matter.
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BART BOK
1906–83
Dutch-born American astronomer
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Carolin Crawford
The gas atoms in a diffuse cold cloud of interstellar gas are heated in the vicinity of newly formed clusters of stars; they then emit visible light to glow as nebulae.
Charles Messier was one of the first ‘comet hunters’, dedicated to the discovery and observation of new comets. These would appear through his telescope as indistinct, faint and fuzzy patches of light, whose nature was revealed only by their motion from day to day against the fixed backdrop of stars. Messier became frustrated in his searches by the presence of other faint structures, which – unlike the comets – were permanent features of the sky. To aid against confusion, he compiled a catalogue of such nebulae that could be mistaken for comets; many were original discoveries, although some were either visible to the unaided eye (such as the Orion Nebula, or the Pleiades cluster) or taken from the work of other astronomers, such as Edmond Halley. Ironically, Messier is remembered today less for the comets he did discover, and more for his catalogue of objects that were not comets. The modern Messier catalogue collects 110 diverse objects, from globular and open clusters of stars to supernovae remnants, planetary nebulae and diffuse star formation nebulae. The catalogue also includes 40 galaxies (identified as nebulae before their true nature was established), 16 of which comprise the nearby Virgo cluster of galaxies.
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The eighteenth-century French astronomer Charles Messier created a catalogue of nebulae that lists some of the most interesting objects in the sky.
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Messier objects are often among the closest and best-known examples of their type. Messier’s original observations were carried out with a telescope comparable in power to simple instruments available today, so his catalogue provides a selection of targets suitable for amateur astronomers. A ‘Messier marathon’ is an attempt to observe as many Messier objects as possible during one night; only during late spring and from low northern latitudes can a person observe them all.
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EDMOND HALLEY
1656–1742
English astronomer
CHARLES MESSIER
1730–1817
French astronomer
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Carolin Crawford
Comet hunter Charles Messier compiled a catalogue of some of the brightest, best-known astronomical objects in the sky.
All of the stars visible to the unaided eye are contained in our home galaxy, the Milky Way. They are flattened into a disc-like structure 100,000 light-years across, and their light merges to form a band of diffuse light that arches across the night sky. Clusters of blue stars, glowing nebulae, and dark rifts of star dust trace out the spiral arms. The Sun lies within this disc, halfway between the centre and its outer edge. A central bulge of older stars contains a dormant supermassive black hole at its core, with a mass 4 million times that of our Sun. Along with all the stars in the disc, the Sun rotates around the centre of the Galaxy in response to the gravitational pull of all the material closer in; travelling at 220 km/s (135 miles/s), it takes 240 million years to complete one orbit. The outer stars rotate too fast to remain bound to the Galaxy, but do not – as would be expected – spin off from their orbits. This suggests that there is much more matter providing gravitational attraction for the stars than is observed in the stars and gas, and is evidence for the presence of ‘dark matter.’
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The Sun is just one of 100,000 million stars that trace a vast spiral-shaped galaxy we call the Milky Way.
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The Milky Way is a member of the ‘Local Group’ of about 30 galaxies, along with the Andromeda and Triangulum spiral galaxies, and many dwarf galaxies in orbit around each of them. We are being steadily pulled by gravity towards our twin, the Andromeda Galaxy, which lies some two-and-a-half-million light-years away. The two galaxies are expected to merge in about 6 billion years’ time, forming a new, much larger galaxy.
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HEBER CURTIS
1872–1942
American astronomer
HARLOW SHAPLEY
1885–1972
American astronomer
JAN OORT
1900–92
Dutch astronomer
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Carolin Crawford
The solar system is contained in one of the spiral arms of our local galaxy, the Milky Way.
The founder of modern stellar astronomy, discoverer of binary stars and the first person to realize that the solar system itself moves was not, amazingly, an astronomer by trade or training. Friedrich Wilhelm Herschel was born in Hannover to a family of musicians; he emigrated to England with his brother Jacob at the age of 19, and he spent four years playing and teaching the oboe, cello, harpsichord, violin and organ. In 1766 (his name now anglicized to William Herschel), he was appointed organist at the Octagon Chapel in Bath and settled into a musical career, composing 24 symphonies. It wasn’t until he was 35 years old that he began to examine the skies. A musician’s interest in English mathematician Robert Smith’s Harmonics (1749) had led him to Smith’s book A Compleat System of Opticks (1738), which piqued an interest in lenses and telescopes. Herschel made significant improvements to the Newtonian reflecting telescope of the day, quickly gaining an international reputation for his craftsmanship, building more than 400 models and developing a lucrative sideline in making and selling them. When he discovered Scottish astronomer James Ferguson’s Astronomy Explained on Sir Isaac Newton’s Principles (1756), and began looking through his telescopes to while away the long winter nights, the musician became the most significant astronomer of his age.
Noting his observations meticulously – with the help of his sister Caroline (herself no cosmic slouch; she found eight comets and at least four nebulae) – Herschel created an exhaustive catalogue of nebulae, multiple star clusters, single stars and deep-sky objects that is still in use today; by constantly revising, comparing and pruning his observations, he was, over a 25-year period, able to list more than 2,500 celestial objects, theorize (correctly) on the gravitational orbit of binary stars (of which he found 800) and work out that the solar system is moving, and in which direction (towards Lambda Herculis, a star in the constellation Hercules). On a March night in 1781, he discovered what would become known as the planet Uranus; he called it Georgium Sidus (‘George’s star’) to honour the Hanoverian king of England, George III. His other achievements were many. He found two moons for Uranus in 1787 (later named Titania and Oberon), and two more moons for Saturn (Mimas and Enceladus), as well as demonstrating that the Milky Way must be disc-shaped. And while trying to find lenses suitable for studying the Sun, he discovered infrared radiation.
Born in Hannover, Germany
1757
Emigrated to England
1766
Became organist at Octagon Chapel, Bath
1774
Began building telescopes and observing the skies, beginning with Orion Nebula
1780
Became director of the Bath orchestra
1781
Elected to the Royal Society
13 March 1781
Discovered what will become known as the planet Uranus
1782
Gave up music to become Court Astronomer
1782
Observed Messier objects; discovered the hitherto unseen Saturn Nebula
1783
Started regular sky sweeps
1783–1802
Observed and cataloged around 2,500 new nebulae and star clusters
1783
Published observations leading to the discovery of solar motion (theory that the solar system is moving through space)
1789
Finished his largest telescope, with a 1.2 m (48 in) aperture
1800
Discovered infrared radiation
1801
Met Napoleon Bonaparte and Charles Messier
1802
Published Catalogue of 500 new Nebulae, nebulous Stars, planetary Nebulae, and Clusters of Stars; with Remarks on the Construction of the Heavens; theorized that some double stars may be binaries orbiting each other
1803
Published Account of the Changes that have happened, during the last Twenty-five Years, in the relative Situation of Double-stars; with an Investigation of the Cause to which they are owing
1820
Cofounded Astronomical Society, which would receive its Royal Charter in 1831
25 August 1822
Died at Slough, Berkshire
Although early telescopic observations of nebulae revealed that some displayed a curious spiral structure, it wasn’t clear that they were not part of the Milky Way. Only in the early 1920s, when American astronomer Edwin Hubble determined the distance to the Andromeda nebula, was it proved that the spiral nebulae were separate, and, therefore, that the Milky Way was not the entirety of the Universe. There is a huge variety of galaxies, from dwarf to giant – ranging from one-thousandth to a thousand times the mass of the Milky Way, and from one-hundredth to ten times the Milky Way’s size. The galaxies are generally classified according to their optical shape and contents. Spiral galaxies reveal bright lanes of active star formation that trace out a spiral pattern within the flattened disc surrounding a central bulge. The more common elliptical galaxies are ball-shaped structures rich in hot X-ray gas, but relatively devoid of the cold interstellar gas and dust required for massive star formation. Galaxies without any regular structure are known as ‘irregular’; these are often the end-product of a gravitational interaction between two galaxies, and are what remains after a short-lived but dramatic ‘starburst’ phase.
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The Milky Way is only one of about 100 billion galaxies scattered throughout the observable Universe – and the average galaxy contains about 100 billion stars.
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Measurement of a galaxy’s distance from us relies on identification of an object within it for which we know the inherent brightness. Comparison of this expected luminosity to that observed yields the distance to the object’s host galaxy. Such ‘standard candle’ objects include variable stars and supernovae. The furthest galaxies known are more than 13.2 billion light-years away and have a blobby appearance, suggesting they have assembled through the merging of smaller systems.
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HEBER CURTIS & HARLOW SHAPLEY
1872–1942 & 1885–1972
American astronomers who publicly debated in 1920 about the size of the Universe
EDWIN HUBBLE
1889–1953
American astronomer
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Carolin Crawford
Galaxies vary in size and shape, from loose fluffy spirals to dense massive ellipticals. Ones that appear tiny (such as those arrowed) are usually the most remote.
Many galaxies are tethered by their combined gravity to form clusters of galaxies. Hundreds, even thousands, of galaxies are packed into a volume of space a few tens of millions of light-years across. The first cluster to be noticed was included in the catalogue of nebulae prepared by French astronomer Charles Messier, which includes 11 ‘nebulae’ towards the constellation of Virgo. Clusters were not systematically catalogued until the advent of detailed photographic plates in the 1950s, and were identified from overdensities of galaxies by eye. Most of the galaxies in a rich cluster are elliptical in shape, with a few blue spirals present at the outskirts of a cluster. The core is dominated by giant ellipticals, some of which are the most massive galaxies known. All the galaxies are bathed in a hot gaseous atmosphere that contains ten times more mass than that of the stars, but which is visible only at X-ray wavelengths. The observed physical properties of this gas, along with the motions of the galaxies in the cluster, and gravitationally lensed mirages of background sources, all point to most of the gravitating mass of a cluster being in the form of dark matter.
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Galaxies are not distributed randomly across the sky, but collect together to form a tapestry of ever-larger structures.
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Many galaxy clusters are clumped into even larger structures called superclusters, which are usually flattened into slab-like concentrations known as walls. On a large scale, we see that the superclusters surround under-dense regions of comparable size, known as voids. Galaxy surveys show that this cobwebby pattern only repeats itself as we move outwards, giving the Universe a distinctly cellular appearance on the largest scales.
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HARLOW SHAPLEY
1885–1972
American astronomer
GEORGE O. ABELL
1927–83
American astronomer who catalogued clusters of galaxies
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Carolin Crawford
On the very largest scales of the Universe, galactic structures are distributed into long filaments that surround equally large voids in space. Massive clusters of galaxies are thought to form where the filaments intersect.
