Algol Eclipsing binary star (pair of stars) in the constellation Perseus. Once every 69 hours, one star eclipses the other for about 10 hours. This means that its light appears to dip, noticeably enough to be seen by the naked eye. In many cultures, the star is associated with evil: Al Gol is Arabic for ‘the demon,’ Hebrew tradition calls the star ‘Satan’s head,’ and the ancient Greeks saw it as the winking eye of a Gorgon (female monster) held by the hero Perseus.
black hole Region in which matter has been highly compressed, and as a result gravity acts with such force that everything in the area, even light, is drawn powerfully in. Black holes can come into existence when a massive star is dying.
blue giant star The most massive and hottest known type of star; it gives off bright blue light, often seen in regions of spiral galaxies where stars are being born.
Butterfly Nebula Also known as NGC 6302 and situated about 3,800 light-years distant in the constellation Scorpius, a planetary nebula given its descriptive name because its vast gas clouds resemble the wings of a butterfly. The clouds of gas, ejected by a dying star from a binary star system, have been made to glow by ultraviolet radiation also emitted by the star.
gamma ray burst Flash of high-frequency electromagnetic radiation, typically released during a supernova.
giant star One with a significantly larger luminosity and radius than a main sequence star, typically as much as 1,000 times as luminous as our Sun and with 10–100 times our Sun’s radius. Even larger, more massive and more luminous stars are labelled ‘supergiants’ and ‘hypergiants.’
main sequence stars Those on the main sequence of the Hertzsprung-Russell diagram of colour and brightness.
Mira Also known as Omicron Ceti, red giant star between 200 and 400 light-years away in the constellation Cetus. Mira is an example of a pulsating variable star. Its brightness varies on a regular cycle 332 days in length.
nebula (pl. nebulae) A cloud of dust or gas in interstellar space.
neutron star Extremely dense star created from exhausted nuclear fuel following the final explosion (supernova) of a massive star at the end of its life.
nova explosion Explosion in a white dwarf star caused when the star takes on matter from a twin in a binary star system and reignites, leading to runaway nuclear fusion on the surface of the white dwarf. The explosion is less powerful and less bright than a supernova. The name comes from the Latin word for ‘new,’ because previously invisible white dwarf stars reappear when a nova occurs and may be taken for a new star.
planetary nebula A cloud of gas expelled into space by a red giant star. The term is derived from the German-born British astronomer William Herschel who, when he identified the phenomenon in 1785, thought the nebulae or clouds he viewed were similar to the ‘gas giant’ planet Uranus. Astronomers still use the term, although these nebulae form around dying stars and have no connection to planets.
red giant star A cooler star, of lower mass compared to a blue giant star.
Ring Nebula Also known as Messier 57, a planetary nebula in the constellation of Lyra. It consists of a cloud of ionized gas expelled into space by a red giant star.
supernova (pl. supernovae) Explosion at the end of a star’s life, when the core of a massive star collapses to form a black hole or a neutron star. A particular type of supernova, type 1a, occurs when a white dwarf star takes in material from a companion in a binary star system until, passing a critical mass (1.4 times that of our Sun), it explodes.
supernova remnant Structure created by a supernova explosion, containing the material of the star that exploded and any interstellar material swept along with it.
white dwarf star Highly dense remnant of a star, created after a red giant star swells and gives issue to a vast nebula, exposing the star’s core – which cools and grows dim to form a white dwarf.
We perceive colour in stars when they have an uneven spectrum of light. Some emit more blue light than red, some the reverse – like the colour of hot irons in a fire, the stars’ colour indicates their surface temperature, with blue stars being hottest (20,000°C/36,000°F) and red stars coolest (3,000°C/5,500°F or less). Astronomers code colour in most stars with a sequence of seven letters – O, B, A, F, G, K and M, running from hot to cool; stars are also ranked by brightness – supergiants are very bright, giants less bright, and dwarfs less bright still. The Sun is middle-ranking: a G-type dwarf. In about 1910, astronomers Ejnar Hertzsprung and Henry Russell, each working independently, plotted the brightness of a number of stars against their temperature, creating the Hertzsprung-Russell (H-R) diagram. Brightness and colour are properties of a star’s surface, but the H-R diagram reveals what is happening inside. Most stars sit on the main sequence from blue/bright to red/faint – and a star’s mass determines where it sits, with the most massive stars at the bright end and the least massive ones at the faint end.
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The surface brightness and colour of stars is plotted on the Hertzsprung-Russell diagram – the key to how stars live and die.
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Stars on the main sequence are changing their hydrogen into helium, releasing nuclear energy. As a main-sequence star uses up the hydrogen in its core, it builds a shell of helium around the core, and uses this as its next source of nuclear fuel. The star grows brighter, but also cooler, and becomes a giant star, even a supergiant. Supergiants finally explode, but giants contract again, becoming dimmer white dwarfs, fading gently to blackness.
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EJNAR HERTZSPRUNG
1873–1967
Danish astronomer
HENRY RUSSELL
1877–1957
American astrophysicist
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Paul Murdin
The main sequence of dwarf stars runs diagonally across the Hertzsprung-Russell diagram; white dwarfs occupy the lower left corner and super giants the upper right.
When stars form from massive gas clouds, there is often enough gas to make two stars. Astronomers estimate that about half the stars we see are actually pairs of stars orbiting one another – binary stars. If the planet Jupiter had been born 100 times more massive, then along with the Sun, it, too, would have been a star, and we would be living in a binary star system. There are many types of binary star, because the two stars involved can differ greatly, depending on their mass at birth. Massive stars live fast and die young, becoming black holes, neutron stars or white dwarf stars, while its companion is still the stellar equivalent of a teenager. Sometimes binary stars are so close that one star can strip material off its companion. Other binaries are more peaceful. Some eclipse, where one star hides its companion as they orbit each other. As the companion reappears, unique clues as to the makeup of these systems are revealed. One of the best-known binary stars is Algol. Every 69 hours, it fades by a factor of 3 for almost 10 hours, while the fainter member of the pair hides the brighter star.
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Stars often form in pairs, so if you gaze upon a lonely-looking star, for about half the time it will have a fainter, invisible companion.
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Astronomers can learn a huge amount from binary stars. By watching how quickly the stars orbit one another, we can accurately determine the masses of those two stars, and so establish the mass of all similar stars. Astronomers have also seen stars orbiting black holes in binary star systems, and watching the speed of the orbiting star is the best evidence we have for the existence of black holes.
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WILLIAM HERSCHEL
1738–1822
German-born British astronomer who coined the term ‘binary star’ in 1802
ÉDOUARD ROCHE
1820–83
French astronomer and mathematician who calculated how binary stars could affect each other
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Darren Baskill
This artist’s impression of a binary system shows the stars so close together that gas is flowing from the sunlike star onto its smaller, white dwarf companion.
Variable stars vary in brightness in different ways and for many reasons. Pulsating ones vary in size and brightness in a fairly predictable and regular way. Gravity makes such stars shrink, causing an outer layer of helium to block light escaping from beneath; the energy of this blocked light is then absorbed by the helium, causing the gas and the entire star to expand again. Once it has expanded to a large size, the helium becomes transparent again, and the heat can escape into space, causing the star to cool and collapse – and the cycle repeats itself. The star Mira (in the constellation Cetus) is an example of a pulsating variable star: its regular variation was discovered in 1638. Every 332 days, Mira varies from being visible with the unaided eye to requiring a telescope to see it. Cataclysmic variable stars vary dramatically and unpredictably. These include: dwarf novae, in which a massive avalanche of gas falls through a disc surrounding a star as frequently as every few weeks; novae, in which the outer surface of a white dwarf star suddenly explodes; and supernovae, in which the whole of either a white dwarf or massive star explodes.
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Most stars vary in brightness. Some alter to a degree that is hardly noticeable, others – which we call variable stars – change significantly.
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Variable stars are an active area of research involving professionals and amateurs working together. While professional astronomers scrutinize individual stars, amateurs can scan the entire sky for new or unusual behaviour. Amateurs who spot a variable star behaving unusually can contact variable star organizations, whose members inform professional astronomers. Within hours, the largest telescopes on Earth – or even space telescopes – can follow up an amateur’s observation to see in detail what that star is doing.
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JOHANNES HOLWARDA
1618–51
Frisian astronomer who discovered in 1638 that Mira was a variable star
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Darren Baskill
Astronomers commonly observe a rise and fall in the brightness of stars over hours, decades, or even longer. One, SCP O6F6, discovered by the Hubble telescope in 2006, grew steadily brighter for 100 days, then dimmed back to oblivion after another 100 days.
The wide range in colour and brightness of stars arrayed in the Hertzsprung-Russell diagram demonstrates that there are many with ages, sizes, luminosities and masses very different from those of our Sun. The very rarest kind are the giant stars. Blue giants are the hottest and most massive stars. They produce energy at prodigious rates to withstand the inward pull of gravity, and thus consume the available fuel very rapidly to have lifetimes lasting only a few million years. Their characteristic bright blue light dominates the open clusters of stars that trace the recent star formation arms of spiral galaxies. Red giants, in contrast, are more common. They are lower-mass stars that have evolved beyond creating energy through simple fusion of hydrogen to helium at the centre. In a red giant, the core begins to compress under gravity and heats up to trigger subsequent, and more complicated, phases of nuclear burning. The resulting increase in luminosity inflates the giant’s outer layers, so that the surface of the bloated star is then at a cooler temperature, and appears much redder in colour. Such stars end their lives when they blast themselves apart as a planetary nebula or a supernova.
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Giant stars are those that are between 10 and 100 times larger than the Sun, and up to 1,000 times brighter.
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Stars that are even larger and brighter are known as supergiants and hypergiants. The current record holder for the largest known star is held by VY Canis Majoris, a red hypergiant about 2,000 times larger than our Sun, and 500,000 times more luminous. If placed at the centre of the solar system, VY Canis Majoris’s surface would extend beyond the orbit of Jupiter.
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3-SECOND BIOGRAPHIES
EJNAR HERTZSPRUNG
1873–1967
Danish astronomer
HENRY RUSSELL
1877–1957
American astrophysicist
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Andy Fabian
The term ‘giant star’ is no mere hyperbole: Betelgeuse, for example, is an evolved star with a radius about 1,200 times greater than that of the Sun.
Stars, such as the Sun, eventually become giant stars and swell up; this gives them a reduced surface gravity and their outer layers escape, forming a nebula. Such a nebula often has a beautiful circular or bilateral symmetry, sometimes expressed in its name; examples are the Ring Nebula and the Butterfly Nebula. As the nebula forms, the hot inner core of the red giant star becomes exposed at its centre, energizing the nebula and giving it wonderful colours. The naked core is spent nuclear fuel; inside the star it has been heated to become very hot indeed, and it radiates powerfully. Now exposed, it cannot long stay hot. It quickly cools and dims, while the nebula fades away and dissipates. The star becomes an isolated ‘white dwarf’ – faint, small (Earth-sized), and dense, a cooling, inert stellar cinder, eventually completely dark. White dwarf stars are so dense that they have a strong gravitational field that works to make them collapse. Holding white dwarf stars up is a kind of pressure revealed by quantum mechanics, unknown before 1925, called ‘electron degeneracy pressure.’ It is surprising that a phenomenon of the really small is needed to prop up a star.
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White dwarfs are the cinders of dead stars – although abundant, they are faint or even invisible, and therefore difficult to find in interstellar space.
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An unexpected feature of the theory of white dwarfs is that electron degeneracy pressure is only effective in propping up a white dwarf star if its mass is less than 1.4 times the mass of the Sun. When the core of a star with more mass than this tries to form a white dwarf, it collapses and becomes a black hole.
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SUBRAHMANYAN
CHANDRASEKHAR
1910–95
Indian-American astrophysicist who studied white dwarfs
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Paul Murdin
A red giant star loses its outer layers, which surround the star with a beautiful nebula. The star becomes a small, white dwarf.
Like white dwarfs, neutron stars are stellar cinders, created from the cores of exhausted nuclear fuel inside some massive stars when, at the end of their lives, they explode as supernovae. The core collapses to an extremely dense star made of neutrons, about the mass of the Sun but typically only 15–25 kilometres (10–15 miles) in diameter; the density is comparable to that of a mountain compressed so small it will fit into a teaspoon. During the collapse, the rotation of the core is greatly speeded up, much as the spinning of an ice skater speeds up as she draws her outstretched arms to her sides. The star may have been rotating once every day or month; compressed to a neutron star, the core rotates faster than once every second. At the same time, any magnetic field that was threaded through the core is greatly intensified. The magnetic field generates a broad spectrum of radiation, including radio radiation, that beams into space. If the star’s rotation sweeps the beam towards the direction of Earth, the star appears to us to pulse, like a lighthouse. The phrase ‘pulsating radio star’, used to describe neutron stars, was contracted to ‘pulsar’.
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Formed in supernova explosions, pulsars are neutron stars that reveal their presence as pulsating radio stars.
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Some pulsars exist in binary stars; some of these are a neutron star in orbit around a more usual star; some of them two neutron stars. Binary neutron stars lose energy and approach one other, and – although this has not been observed – are thought eventually to merge with a huge explosion, causing a gamma ray burst and a black hole.
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ANTHONY HEWISH
1924–
British radio astronomer, Bell Burnell’s supervisor, and codiscoverer of pulsars
JOCELYN BELL BURNELL
1943–
British astronomer, the discoverer of pulsars
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Paul Murdin
A dense, small pulsar energizes the swirling central gases of the Crab Nebula – a supernova remnant and pulsar wind nebula.
Susan Jocelyn Bell was born in 1943 in Belfast, Northern Ireland, into a Quaker household. At the age of 11, she failed to gain the necessary qualifications for entry to a grammar school, a type of publicly run British school reserved for more academically able children. This failure was a setback, and she has said that her determination to overcome it spurred her on through her career in astronomy, at the time a difficult and lonely course for a woman.
While studying for her PhD at Cambridge under the supervision of Anthony Hewish she made the discovery that changed the way we think of the Universe. Bell’s field is radio astronomy: as part of her PhD, she helped to build and maintain the Interplanetary Scintillation Array, a huge (1.6-ha/4-acre) radio telescope at the Mullard Radio Astronomy Observatory, Cambridge. Part of Bell’s work was to interpret the mass of data that the telescope generated every 24 hours, and in November 1967 she noticed what has become famous as a ‘scruff’ on the graph. This was a tiny detail and could have easily been missed, but Bell followed it up. It was finally identified as a hitherto unobserved phenomenon, a pulsating star (‘pulsar’), later named CP 1919. The discovery thrilled the astronomical world, and there was jocular talk of ‘little green men’, because one of the theoretical explanations of the regular pulsating radio waves was that there might be ‘somebody out there’ signalling into the void. Eventually, it was determined that pulsars were neutron stars that emitted regular waves of radiation. Bell – known as Jocelyn Bell Burnell after her marriage to Martin Burnell in 1968 – discovered three more pulsars, opening up a whole new field in astrophysics.
Although Bell Burnell’s name is now indissolubly linked with this breakthrough discovery, it caused a controversy beyond the world of astrophysics that is still unresolved. Bell Burnell’s name was second on the paper announcing the discovery, but the Nobel Prize that it won went to Hewish and Martin Ryle (leader of the research team), with no mention of Bell Burnell. It is not usual for research assistants to go uncredited, despite their doing most of the grunt work; however, many considered that pulsars had been recognized only through Bell Burnell’s persistence and attention to detail, and her omission created an outcry – with Sir Fred Hoyle, in particular, championing her cause.
Born in Belfast
1954
Attended a Quaker school in York
1965
Graduated in physics from University of Glasgow
1967
First observation of what will become known as the first pulsar, CP 1919
1968
First use of the word ‘pulsar’
1969
Completed PhD at Cambridge University
1974
Anthony Hewish and Martin Ryle shared the Nobel Prize for Physics; Bell Burnell was not cited
1978
Won Robert Oppenheimer Memorial Prize
1979
Published ‘Little Green Men, White Dwarfs or Pulsars?’ in Cosmic Search Magazine
1987
Won Beatrice M. Tinsley Prize, American Astronomical Society
1989
Awarded Herschel Medal, Royal Astronomical Society, London
1991
Made Professor of Physics at the Open University, and Visiting Professor at Princeton
1999
Awarded British order of chivalry, CBE (Commander of the Order of the British Empire), for services to astronomy
2001–04
Dean of Science at University of Bath
2002–04
President of Royal Astronomical Society
2003
Fellow of the Royal Society
2008
Created Dame
2008–10
The first-ever female president of the Institute of Physics
During the late stages of its evolution, a massive star produces energy through the manufacture of progressively heavier elements deep in its core. The end point of this process for stars above 8 Solar masses occurs at the creation of iron, beyond which energy is no longer released by fusion. The star then abruptly runs out of fuel. The core implodes under gravity to form either a neutron star or a black hole; as it contracts, it becomes denser and hotter, releasing so much energy that the supernova explosion can temporarily outshine its host galaxy. The rest of the star is ripped apart, and hot debris is propelled outwards by the blast to form a shell of material expanding away at some 15,000 m/s (16,000 ft/s), sweeping up any interstellar gas before it. The material is compressed into a filamentary structure known as a supernova remnant. An intense flood of neutrons released in the explosion allows the creation of the very heaviest elements. Both these, and those originally forged at the core of the star, are blasted out to mix with surrounding clouds of gas and dust, where they will eventually be recycled into new generations of stars and planets.
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The end of a massive star’s life is heralded by one of the largest explosions in the Universe, known as a supernova.
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A separate type of supernova explosion can be triggered if a white dwarf steadily accretes matter from a large companion star in a binary system. Once its total mass crosses the threshold of 1.4 Solar masses, it will explode completely to form a supernova Type 1a. Such supernovae are, however, rare events: it is estimated that one occurs in an average galaxy only about once a century.
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WILLIAM FOWLER
1911–95
American astrophysicist
FRED HOYLE
1915–2001
British astronomer
MARGARET & GEOFFREY BURBIDGE
1919– & 1925–2010
British-American astronomers
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Andy Fabian
A giant star ends its life in a supernova explosion, leaving a neutron star or black hole surrounded by a rapidly expanding shell of hot gas.
The existence of black holes was first suggested by eighteenth-century English philosopher John Michell, who in 1783 wondered if stars could exist with a mass so large and a gravity so strong that nothing, not even light, could escape them. He called them dark stars, which aptly describes black holes. We now know that they come in many sizes. Stellar black holes squeeze the mass of ten Suns into an area the size of London, and we know of dozens within our Milky Way Galaxy; supermassive black holes have a mass between 1 million and 10 billion times that of our Sun, and are found at the heart of many galaxies, including our own; and a few intermediate black holes are known with masses in between these extremes. While black holes have a reputation for immensely strong gravity, their effects are felt only at close quarters. If an object is close to a black hole, its nearer parts feel so much more gravity than its further parts that it is stretched into long, thin strands – a process known as ‘spaghettification’. Fortunately, the nearest black hole to us lies more than 3,000 light-years away.
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A black hole is a region in which matter has been extremely compressed, making gravity so powerful that everything in the vicinity is sucked in.
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Looking for dark stars against the blackness of space is extremely difficult. Astronomers do not look for black holes directly, but for their effects on their surroundings. When dismembered stars fall in towards a black hole, they are heated so much that they emit X-rays; by seeing how quickly a star orbits a black hole, astronomers can calculate the mass of both – clues that indicate the presence of black holes.
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JOHN MICHELL
1724–93
English philosopher who first proposed that black holes exist
KARL SCHWARZSCHILD
1873–1916
German physicist who solved Einstein’s General Relativity equations to describe conditions around a black hole
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Darren Baskill
A star that wanders too close to a black hole is spaghettified – vertically stretched and horizontally compressed – due to the hole’s very strong gravitational field.
