61 Cygni Binary star in the constellation Cygnus. It was the first star to have its parallax measured, in 1838, by Prussian astronomer Friedrich Bessel, and when Bessel calculated its distance at 10.4 light-years, this was the first estimate of distance to a star other than our Sun. The actual distance is 11.4 light-years.
accretion The capturing and drawing in of gas by a massive body. As the gas is captured and spirals inwards towards a black hole, temperatures of millions of degrees are generated and the gas gives off X-ray radiation. Astronomers are able to identify the presence of black holes, which are invisible, from this radiation. Accretion also describes the capturing of gas or other matter by a small star (or remains of a star) from its partner in a binary star system.
asteroid A rocky body in our solar system that is smaller than a planet and in orbit around the Sun. The majority of identified asteroids are found in the asteroid belt between the orbits of Mars and Jupiter. Research in 2012, led by the Carnegie Institution in Washington DC, suggested that landings on Earth of asteroids – and not comets, as previously thought – were the original source of Earth’s water.
blueshift Compression of light’s wavelength towards the blue end of the spectrum, caused by the fact that the object emitting light is moving towards the observer. For example, light from the Andromeda Galaxy is blue-shifted because the Andromeda Galaxy is moving towards our own Milky Way Galaxy within the Local Group of galaxies. Blueshift also describes the shortening of wavelengths outside the visible spectrum – for example, of radio waves and X-rays – because their source is moving towards the observer.
ellipse A flattened circle. The shape is created by a point moving in a closed curve in which the sum of the point’s distance from two fixed points is always constant. Orbits in space – of a satellite around its primary, the planets of the solar system around the Sun, and of stars around one another or the centre of a galaxy – are elliptical.
gravity Force that attracts physical bodies towards one another. In space, gravity acts with a strength proportional to the product of the mass of two bodies and inversely proportional to the square of the distance separating them. On Earth, gravity causes an object to have weight and to drop to the ground when released. In space, gravity has many effects – for example, keeping the Earth and the other planets in orbit around the Sun or keeping the Sun in orbit around the centre of the Milky Way Galaxy. Gravity is also the force that creates a black hole, when in a region matter becomes so compressed and mass so large that gravity is sufficiently powerful to draw everything in the vicinity into the black hole.
Local Group Group of more than 30 galaxies, with a diameter of 10 million light-years, that contains our own galaxy (the Milky Way), our nearest spiral galaxy (the Andromeda Galaxy) and the Triangulum Galaxy. The Group’s gravitational centre lies between the Milky Way and the Andromeda Galaxy.
Proxima Centauri Red dwarf star in the constellation Centaurus, the closest star to our own Sun at a distance of around 4.3 light-years. The two stars of Alpha Centauri, our second and third nearest stars, are only 0.2 light-years from Proxima Centauri. At one time, astronomers classified Alpha Centauri as a binary star system, but they now generally view Proxima Centauri and the two stars of Alpha Centauri as a single entity, a triple star system.
redshift The stretching of light’s wavelength towards the red end of the spectrum, caused by the fact that the object emitting light is moving away relative to the observer. Light from distant galaxies that are moving away from us is ‘redshifted’. As with blueshift (see above), redshift also describes the lengthening of wavelengths outside the visible spectrum because their source is moving away from the observer.
spacetime Continuum of space and time, proposed by German-Swiss-American theoretical physicist Albert Einstein. While, in the conventional view of the Universe, the three dimensions of space and the single dimension of time were viewed as separate, spacetime is a four-dimensional continuum.
Before 1543, most astronomers argued that the Earth does not move because the stars do not change position. In that year, Nicolaus Copernicus showed that the Earth does move – it orbits the Sun; since stars do not move relative to each other, it followed that stars must be at great distances. In fact, stars do appear to move by tiny amounts; the apparent movement of a star due to the Earth’s motion is called ‘parallax’. The first astronomer to measure a star’s parallax was Friedrich Bessel, who in 1838 measured the parallax of the star 61 Cygni to be ⅓ of 1 second of arc (⅓ of 1 second of arc is approximately 1/10,000 of a degree), roughly as thick as a needle seen from 135 metres (150 yards). The distance of a hypothetical star whose parallax is 1 second of arc is defined as 1 parsec – equivalent to 30.6 trillion kilometres (19 trillion miles) – although the nearest star is not quite this close. Bessel illustrated the distance of 61 Cygni by calculating that light travel time from the star is 11 years. A ‘light-year’ – the distance light travels in one year – is 9.5 trillion kilometres (5.9 trillion miles).
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A star’s distance from Earth is expressed as the time light takes to travel to us, and its shift of position as Earth orbits Sun.
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Conjuring up the stars’ distance in seventeenth-century terms, the natural philosopher Francis Robartes wrote in 1694 that ‘Light takes up more time in Travelling from the Stars to us, than we in making a West-India Voyage (which is ordinarily performed in six Weeks).’ The nearest star, Proxima Centauri, is at a distance of 4.3 light-years – 40.9 trillion kilometers (25.4 trillion miles) – and has a parallax of 0.7 of a second of arc.
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OLE RØMER
1644–1710
Danish astronomer who first measured the speed of light
FRIEDRICH BESSEL
1784–1846
Prussian mathematician and astronomer
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Paul Murdin
Light travels from the nearest star to the Earth in 4 years, but from the Sun to the Earth in only 8 minutes.
Astronomers up to the sixteenth century believed that the planets moved around the Sun in circles, or combinations of circles called epicycles. Tycho Brahe set up a pre-telescopic observatory to measure how they really moved, and in 1605 his pupil, Johannes Kepler, used Brahe’s measurements to show that the planets actually orbit the Sun in ellipses. Why they do this was a mystery, until Isaac Newton demonstrated that it was a consequence of his theory of gravitation; in particular, that the force of gravity between the Sun and a planet was proportional to the inverse square of the distance separating them. The orbit of one star around another or of a satellite around its parent planet are also elliptical. Newton took pride in the fact that his theory was ‘universal’ – and it was a further triumph when Newton’s friend, Edmond Halley, used the theory to show that the orbit of a particular comet around the Sun was a flattened ellipse, and that it would return in 74 years. (We now know the comet as ‘Halley’s Comet’.) More typical comets orbit the Sun in a parabola; this geometric figure can be looked at as an extreme ellipse, very long and thin.
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Gravity is the most important force determining the motions of celestial bodies, and Isaac Newton’s discovery of how it works launched the scientific era.
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An ellipse is a good approximation to the orbit of a planet in the solar system because the gravitational attraction of the Sun is dominant and that of the other planets negligible. In the long run, however, the orbits of two or more planets around a star are chaotic, the planets deflecting each other and looping in unrepeating orbits, which eventually become incalculable. Gravitational orbits are not as deterministic as is often thought.
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TYCHO BRAHE
1546–1601
Danish astronomer
JOHANNES KEPLER
1571–1630
German discoverer of three laws of planetary motion
EDMOND HALLEY
1656–1742
English astronomer
EDWARD LORENZ
1917–2008
American discoverer of the theory of chaos
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Paul Murdin
The planets and the asteroids orbit the Sun in ellipses. Some comets orbit in ellipses, some in parabolas.
Light travels as waves and the wavelength, or size, of the waves determines their colour. All light we see is a combination of many wavelengths of visible light, from 400 to 750 nanometres in wavelength, or from blue/violet to red. Astronomers use ‘spectrometers’ to split light from an object into a rainbow and measure its spectrum, its brightness at each individual wavelength. Our eyes are spectrometers, too, but coarse ones. They lump the multitude of wavelengths of a spectrum into three broad groups, so we perceive colours simply as a mixture of ‘red’, ‘green’, and ‘blue’. Incandescent lightbulbs (which have a smooth distribution of wavelengths) and fluorescent bulbs (which have only a few distinct wavelengths) appear the same to our eyes, although their spectra are different when viewed through spectrometers with higher resolution. Spectrometers enable astronomers to analyze distant objects without visiting them. Different atoms and molecules emit or absorb in different sets of wavelengths; by observing these spectroscopic fingerprints, astronomers can determine the mineralogy of asteroids, the composition of stars, the gravity of white dwarfs, the motions of galaxies, the dynamics of accreting black holes, and more – all from the comfort of a telescope control room.
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There is more to light than meets the eye! Astronomers explore the cosmos by splitting light from celestial objects into millions of colours.
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Light waves are similar to sound waves. When a car with a siren comes towards you and zooms past, you hear the pitch of the siren change – the car’s motion is compressing and stretching the sound waves. Likewise, the wavelengths of light from objects can be blueshifted (compressed) or redshifted (stretched) due to motion. High resolution spectroscopy can detect motions even as minuscule as 1 meter (40 inches) per second, like those from planets tugging on their stars.
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ISAAC NEWTON
1642–1727
English physicist
CECILIA PAYNE-GAPOSCHKIN
1900–79
English-American astrophysicist
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Zachory K. Berta
The spectrum of starlight is mostly a smooth, continuous rainbow of light, but it is missing light at very specific wavelengths, due to absorption by the star’s atoms and molecules.
In the seventeenth century, English physicist and mathematician Isaac Newton introduced our basic view of gravity as an invisible pull acting on an object from a distance. His law of gravity states that each body in the Universe exerts a force on any other: a stronger pull is produced by larger masses, and its strength drops with their increasing separation. Gravity gives a mass its weight, and dictates in which direction a supported object moves when released. The simple principle of gravity accounts for much of the observed behaviour of the nearby Universe. It accounts precisely for almost all the orbital motion of the planets and their moons, enabling space agencies successfully to send robotic probes to explore the solar system. Gravity dictates the motion of the stars in our galaxy, and the galaxies within clusters, such as our Local Group. Our understanding of gravity has been enhanced by German-Swiss-American physicist Albert Einstein’s General Theory of Relativity (1915), which supersedes Newton’s ideas when the velocities involved approach the speed of light. We understand more about what gravity does than what it is – unifying gravity with quantum theory remains a major unsolved problem in physics.
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Gravity is fundamental to our understanding of the Universe because it is the force that dictates the motion and interactions of all astronomical bodies.
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Any change in the strength of the gravitational pull experienced across a cosmic object creates tidal forces. The large difference in force experienced by Earth’s oceans both closest to and furthest from the Moon creates two high tides a day. Tidal forces between pairs of merging galaxies rip out long streams of stars and gas; and stars passing too close to a black hole can be completely shredded by the tidal forces.
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ISAAC NEWTON
1642–1727
English physicist
ALBERT EINSTEIN
1879–1955
German-Swiss-American theoretical physicist
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Andy Fabian
When galaxies pass close to each other at relatively slow speeds, the gravitational pull of one on another tears out ‘tidal’ tails of gas and stars, distorting their spiral shape.
In his Special Theory of Relativity (1905), Albert Einstein stated that measurements of length and time intervals are affected by the relative speeds of a person’s observations and the event or object being observed. Observations by a stationary observer of a rapidly moving clock show it to run slow, and measurable lengths are observed to be shorter than when remaining at rest. Despite this, the speed of light and the laws of physics remain unchanged for all observers, no matter how fast they move. This leads to Einstein’s famous equation, E=mc2, which expresses how any mass can also embody energy – this concept explains how the mass released through atomic reactions gives stars energy. In 1915, Einstein extended his ideas to a General Theory of Relativity, which encompassed accelerations between the observer and object being observed. This gave a new account of gravity as being caused by the curvature of space and time in the presence of a mass. Thus the distribution of matter in the Universe influences the overall shape of space. Prominent also is the principle of equivalence, which states that it is impossible on small scales to distinguish between the downward pull of gravity and upward acceleration of an observer.
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Relativity describes how the relative speed of an observer and object, and changes in speed, alter measurements of distance and time, and the workings of gravity.
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The predictions of relativity have so far been proved correct by each experiment designed to test them. Unlike Newton’s theory of gravity, relativity can account for anomalies in Mercury’s orbit, mirages caused by gravitational lensing, and the slowing of time in a stronger gravitational field. It also explains the spiralling-in of very close binary stars, due to the emission of gravitational waves – ripples of spacetime propagating outwards at the speed of light.
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HENDRIK LORENTZ
1853–1928
Dutch physicist
ALBERT EINSTEIN
1879–1955
German-Swiss-American theoretical physicist
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Andy Fabian
The shape of space is curved into deep wells around massive objects such as planets, stars and galaxies.
Magnifying lenses on Earth work by focusing light that passes through them, using refraction to bend light as it passes between air and glass. Out in the vacuum, light travels along straight paths through space, never deviating from its initial trajectory. But what happens if light passes by a very massive object? The strong gravity associated with the object’s mass will – according to Albert Einstein’s General Theory of Relativity – cause space itself to bend; if space bends, light travelling through it will appear to bend, too. Galaxy clusters, weighing 1,000,000,000,000,000 times the mass of our Sun, can act as powerful ‘gravitational lenses’, magnifying light from background galaxies, sometimes distorting their appearance into beautiful, slender arcs on the sky. A few rare gravitational lenses happen to be positioned just perfectly to provide a natural (if immovable) zoom lens in front of our telescopes, allowing astronomers to peer at remarkably detailed features of young galaxies still in the early stages of formation out at the edge of the observable Universe. In addition to galaxy clusters, many other astronomical objects can act as gravitational lenses on smaller scales, from powerful supermassive black holes all the way down to tiny little planets.
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The Universe is scattered with gravitational lenses, enormous astrophysical magnifying glasses that focus or distort background starlight long before it reaches our telescopes.
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Astronomers monitoring the brightness of millions of stars have found rare sparkles of light caused by gravitational ‘microlensing’ from stars and/or planets in our galaxy. Stars and planets drifting through space sometimes align with distant background stars, catch their light, focus it to Earth, and dramatically magnify them for a brief time. To date, more than a dozen planets have been discovered with the microlensing magnification technique.
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FRITZ ZWICKY
1898–1974
Swiss astronomer
BOHDAN PACZYNSKI
1940–2007
Polish astronomer and leading researcher into gravitational lensing and microlensing
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Zachory K. Berta
A massive galaxy cluster bends light from background objects, creating multiple, magnified images of them that we can observe with our telescopes.
Best known as the father of dark matter, which he identified and named, Fritz Zwicky was born in Bulgaria to a Swiss father and a Czech mother, educated at the prestigious Eidgenössische Technische Hochschule in Zürich and spent all his working life in the United States, most of it at the California Institute of Technology. He became Caltech’s (and the world’s) first astrophysicist, simply by deciding to marry his physics training with astronomy. He was a maverick and original thinker, many of whose early ideas and theories, mocked by some contemporaries, have developed into orthodoxy: the existence of dark matter, neutron stars, gravitational lenses, and supernovae; those that didn’t (nuclear goblins, shifting and/or customizing the solar system, creating artificial meteors) still loiter in the realms of science fiction. On the practical side is Zwicky’s indispensable six-volume catalogue of galaxies and his pioneering work on jet propulsion during and after the Second World War.
Zwicky did not just think outside the box, he developed his own box for thinking in. This was morphological analysis (MA), the refinement of a scientific investigative tool introduced by Goethe. It works by aggregating all data, however apparently unrelated, into a matrix, and looking at all possible outcomes to a problem, even the most startling solutions. An apocryphal story has him ordering an assistant to fire a revolver through the slit of a telescope to clear turbulence; it didn’t work, but is a fine example of his radical, left-field turn of mind.
Zwicky’s glittering scientific reputation has been rather compromised by his attitude to peers and students; although a principled humanitarian, he was not at his best around individuals; he was famously unable to suffer fools at all, let alone gladly, and was convinced that he was always right and everyone else was stupid – even Robert Oppenheimer. At his death, he was still locked in confrontation with some of his students over an excoriating introduction he wrote to his Catalog of Selected Compact Galaxies (1971), in which he styled himself as a heroic lone wolf and all others as scatter-brained, date-bending sycophants and fawning apple-polishers.
Born in Varna, Bulgaria
1904
Sent to Switzerland for education; eventually studied in Zürich at the Swiss Federal Institute of Technology, ETHZ
1925
Emigrated to the USA to work at Caltech on Rockefeller fellowship
1933
Inferred the existence of dark matter
1934
Coined term ‘supernova’; published (with Walter Baade) Cosmic Rays from Super-Novae
1935
Pioneered (with Baade) use of Schmidt telescope
1937
Posited that galaxy clusters and nebulae could act as gravitational lenses, as predicted by Einstein
1942
Appointed professor of astronomy at Caltech
1943–1961
Research consultant/Director at Aerojet Engineering Corporation
1946
Published On the Possibility of Earth-Launched Meteors
1949
Awarded Presidential Medal of Freedom for work on rocket propulsion
1961–1968
With colleagues compiled six volume Catalog of Galaxies and Clusters of Galaxies. Appointed Professor Emeritus at Caltech (1968)
1969
Published Discovery, Invention, Research through Morphological Analysis
1971
Self-published Catalog of Selected Compact Galaxies
1972
Awarded Gold Medal of the Royal Astronomical Society
8 February 1974
Dies in Pasadena; buried at Mollis, Switzerland
Albert Einstein’s General Theory of Relativity allows for the existence of black holes that create a bridge to a different place in the Universe, or to another universe. This bridge takes the form of a tube (wormhole) that links separate points in spacetime. If space is simplified as a two-dimensional sheet curved back on itself, a wormhole can be visualized as a hollow tunnel making a ‘shortcut’ between the two layers. The time taken to pass through a wormhole would be far less than a journey on a path through normal space, and so the wormhole could potentially provide a means of faster-than-light travel. If one mouth of the wormhole were greatly accelerated and the other kept stationary, then the stationary mouth would age less rapidly according to the theory of time dilation in Einstein’s Special Theory of Relativity. Thus it might be possible to crawl into the moving end of the wormhole only to emerge prior to one’s time of entry, and thus travel through time. However, wormholes are so far a purely theoretical concept, and do not arise naturally from the type of black hole created by the collapse of a massive star.
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A wormhole is a hypothetical tunnel connecting different regions of space and time – either within our own universe, or leading to a parallel universe.
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Wormholes are predicted to be unstable, limiting their potential to provide a means for space and time travel. They would collapse so rapidly that no matter could make a journey through before the exit route was pinched off. Some scientists suggest that the mouth of a wormhole could be kept open were it filled with exotic matter that produced ‘antigravity’, such as that speculated to fulfill the role of dark energy in the Universe.
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ALBERT EINSTEIN
1879–1955
German-Swiss-American theoretical physicist
NATHAN ROSEN
1909–95
American-Israeli physicist
JOHN ARCHIBALD WHEELER
1911–2008
American theoretical physicist
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Andy Fabian
A wormhole can be visualized as a potential connection between different times and places in the Universe.
