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1503 The most accurate star positions to date are recorded by Bernhard Walther at Nuremberg. 1543 Copernicus introduces the idea of a sun-centered cosmos, improving the prediction of planetary positions. These, however, are still inaccurate


1610 Galileo’s use of the telescope starts a revolution that eventually supersedes naked-eye astronomy. 1620 Johannes Kepler completes his laws of planetary motion. 1670s Major observatories are established in all the capitals of Europe.

The astronomy of Tycho’s time still followed the teachings that Aristotle had laid down nearly 1,900 years earlier. Aristotle had stated that the stars in the heavenly firmament were fixed, permanent, and unchanging. In 1572, when Tycho was 26, a bright new star was seen in the sky. It was in the constellation of Cassiopeia and stayed visible for 18 months before fading from view. Influenced by the prevailing Aristotelian dogma, most observers assumed that this was an object high in the atmosphere, but below the moon. Tycho’s careful measurements of the new object convinced him that it did not move in relation to nearby stars, so he concluded that it was not an atmospheric phenomenon but a real star. The star was later discovered to be a supernova, and the remnant of this stellar explosion is still visible in the sky as Cassiopeia B. The observation of a new star was an extremely rare event. Only eight naked-eye observations of supernovae have ever been recorded. This sighting showed that the star catalogs in use at the time did not tell the whole story. Greater precision was needed, and Tycho led the way.

Precision instruments

To accomplish his task, Tycho set about constructing a collection of reliable instruments (quadrants and sextants, and armillary spheres) that could measure the position of a planet in the sky to an accuracy of about 0.5 arcminute (± 1⁄120º). He personally measured planetary positions over a period of around 20 years, and for this purpose in 1576 he oversaw the building of a large complex on the small island of Hven in the Øresund Strait, between what is now Denmark and Sweden. This was one of the first research institutes of its kind.

Tycho carefully measured the positions of the stars and recorded them on brass plates on a spherical wooden globe about 5 ft 3 in (1.6 m) in diameter at his observatory on Hven. By 1595, his globe had around 1,000 stars recorded on it. It could spin around a polar axis, and a horizontal ring was used so that stars positioned above the horizon at any given time could be distinguished from those below the horizon. Tycho carried the globe with him on his travels, but it was destroyed in a fire in Copenhagen in 1728. Further evidence of a changing cosmos came from Tycho’s observation of the Great Comet in 1577. Aristotle had claimed that comets were atmospheric phenomena, and this was still generally believed to be the case in the 16th century. Tycho compared measurements of the comet’s position that he had taken on Hven with those that had been taken at the same time by Bohemian astronomer Thaddaeus Hagecius in Prague. In both instances, the comet was observed in roughly the same place, but the moon was not, suggesting that the comet was much farther away. Tycho’s observations of the way the comet moved across the sky over the months also convinced him that it was traveling through the solar system. This overturned another theory that had been believed for the previous 1,500 years. The great Graeco-Egyptian astronomer Ptolemy had been convinced that the planets were embedded in real, solid, ethereal, transparent crystalline spheres, and that the spinning of these spheres moved the planets across the sky. However, Tycho observed that the comet seemed to move unhindered, and he concluded that the spheres could not exist. He therefore proposed that the planets moved unsupported through space, a daring concept at the time.

Tycho used his immense wealth to design and build fine instruments, such as this armillary sphere, which was used to model the night sky as seen from Earth

No parallax

Tycho was also very interested in Copernicus’s proposition that the sun, rather than Earth, was at the center of the cosmos. If Copernicus was right, the nearby stars should appear to swing from side to side as Earth traveled annually on its orbit around the sun—a phenomenon known as parallax. Tycho searched hard, but could not find any stellar parallax. There were two possible conclusions. The first was that the stars were too far away, meaning that the change in their position was too small for Tycho to measure with the instruments of the day. (It is now known that the parallax of even the closest star is about 100 times smaller than the typical accuracy of Tycho’s observations.) The second possibility was that Copernicus was wrong and that Earth did not move. This was Tycho’s conclusion.

Tycho Brahe’s observatory complex on the island of Hven attracted scholars and students from all over Europe between its founding in 1576 and its closure in 1597.

The Tychonic model

In reaching this conclusion, Tycho trusted his own direct experience. He did not feel Earth moving. In fact, nothing that he observed convinced him that the planet was moving. Earth appeared to be stationary and the external universe was the only thing that appeared to be in motion. This led Tycho to discard the Copernican cosmos and introduce his own. In his model of the cosmos, all the planets except Earth orbited the sun, but the sun and the moon orbited a stationary Earth.

For many decades after his death in 1601, Tycho’s model was popular among astronomers who were dissatisfied with Ptolemy’s Earth-centric system but who did not wish to anger the Catholic Church by adopting the proscribed Copernican model. However, Tycho’s own insistence on observational accuracy provided the data that would lead to his idea being discredited shortly after his death. His accurate observations helped Johannes Kepler to demonstrate that the planets’ orbits are ellipses and to create a model that would displace both the Tychonic and Copernican models.

Tycho’s improved measurements would also allow English astronomer Edmond Halley to discover the proper motion of stars (the change in position due to the stars’ motion through space) in 1718. Halley realized that the bright stars Sirius, Arcturus, and Aldebaran had, by Tycho’s time, moved over half a degree away from the positions recorded by Hipparchus 1,850 years earlier. Not only were the stars not fixed in the sky, but the changing positions of the closer stars could also be measured. Stellar parallax was not detected until 1838.

The Tychonic model kept Earth at the center of the cosmos as in the Ptolemaic model, but the five known planets were now orbiting the sun. Although he was impressed by the Copernican model, Tycho believed that Earth did not move.


1588 Astronomiæ Instauratæ Progymnasmata (Introduction to the New Astronomy)

Born a nobleman in 1546 in Scania (then Denmark, but now Sweden), Tyge Ottesen Brahe (Tycho is the Latinized version of his first name) became an astronomer after sighting a predicted solar eclipse in 1560. In 1575, King Frederick II gave Tycho the island of Hven in the Øresund Strait, where he built an observatory. Tycho later fell out with Frederick’s successor, Christian IV, over the potential transfer of the island to his children and closed the observatory. In 1599, he was appointed Imperial Mathematician to Emperor Rudolph II in Prague. There, Tycho appointed Johannes Kepler as his assistant. Tycho was famed for his distinctive metal nose, the legacy of a duel he fought as a student. He died in 1601, allegedly of a burst bladder, having refused out of politeness to take a toilet break during a long royal banquet.

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In 1995, two Swiss astronomers, Michel Mayor and Didier Queloz, researching at the Observatoire de Haute-Provence near Marseille, found a planet orbiting 51 Pegasi, a sunlike star 60 light-years away in the constellation of Pegasus. This was the first confirmed observation of a true extrasolar planet, or exoplanet—a planet beyond the solar system. It was
orbiting a main sequence star, and was therefore assumed to have formed by the same process as that which created the solar system.

Mayor and Queloz named the new planet 51 Pegasi b, but it is unofficially known as Bellerophon after the hero who rode Pegasus, the winged horse of ancient Greek myth. Its discovery prompted a major hunt to find more exoplanets. Since 1995, several thousand exoplanets have been found, many in multiple star systems. Astronomers now estimate that there is an average of one planet around every star in the galaxy, although this is probably a very conservative figure. Some stars have no planets, but most, like the sun, have several. 4The discovery of 51 Pegasi b marked the final milestone in a process that has forced astronomers to abandon any lingering notion that Earth occupies a privileged place in the universe.

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Solar system




In the late 1950s, astronomers across the world started to find mysterious, compact sources of radio signals in the sky without any corresponding visible objects. Eventually a source of these radio waves was identified—a faint point of light, which became known as a quasar. In 1963, Dutch astronomer Maarten Schmidt discovered a quasar that was hugely distant (2.5 billion light-years away). The fact that it was so easily detected meant it must be pouring out energy.

Searching for quasars

By the mid 1960s, many radio astronomers were searching for new quasars. One such figure was Antony Hewish, part of a radio astronomy research group at Cambridge University. Hewish had been working on a new technique in radio astronomy based on a phenomenon called interplanetary scintillation (IPS), which is a “twinkling,” or fluctuation, in the intensity of radio emissions from compact radio sources. The twinkling of sources of visible light, such as stars, is caused by disturbances in Earth’s atmosphere that the light has to pass through. The twinkling of radio sources, however, is caused by streams of charged particles emanating from the sun. As radio waves pass through this “solar wind,” they are diffracted, meaning that the waves spread out, making the radio source appear to twinkle.

Hewish hoped that IPS could be used to find quasars. Radio waves coming from a compact source, such as a quasar, twinkle more than radiation from a less compact source, such as a galaxy, and so quasars should twinkle more than other radio sources. Hewish and his team built a large radio telescope designed specifically to detect IPS. It covered an area of nearly 4.5 acres (2 hectares), took two years to construct, and required more than 120 miles (190 km) of cable to carry all the signals.

Members of the Cambridge radio astronomy group built the new telescope themselves. Among them was a Ph.D. student named Jocelyn Bell. When the telescope started operating in July 1967, Bell was made responsible for operating it and analyzing the data, under the supervision of Hewish. Part of her job was to monitor output data from the telescope, made by pen recorders on chart-recorder printouts. Examining about 100 ft (30 meters) of chart paper every day, Bell quickly learned to recognize scintillating sources.

Little Green Man 1

About two months into the project, Bell noticed an unusual pattern of signals, which she described as “scruff.” It looked far too regular and had too high a frequency to be coming from a quasar. Checking back through her records, she found it had appeared in the data before and always came from the same patch of sky. Intrigued, Bell started making more regular chart recordings of the same area of sky. At the end of November 1967, she found the signal again. It was a series of pulses, equally spaced and always 1.33 seconds apart.

Bell showed the signal, dubbed “Little Green Man 1” (LGM-1), to Hewish. His initial reaction was that a pulse occurring every 1.33 seconds was far too fast for something as large as a star, and the signal must be due to human activity. Together, Bell and Hewish ruled out various human-related sources, including radar reflected from the moon, Earth-based radio transmissions, and artificial satellites in peculiar orbits. A second telescope was also found to pick up the pulses, which proved that they could not be due to an equipment fault, and calculations showed that they were coming from well outside the solar system.

Hewish had to revise his opinion that the signals had a human origin. The possibility that they were being sent by extra-terrestrials could not be ruled out. The team measured the duration of each pulse and found it was only 16 milliseconds. This short duration suggested that the source could be no larger than a small planet. But a planet—or an alien civilization living on a planet— was unlikely, since the signal would show slight changes in frequency, called Doppler shifts, as a planet orbited its star.

This image of a pulsar in the Crab nebula, a well-known supernova remnant, was taken in space by the Chandra X-ray Observatory. The white dot at the center is the neutron star.

Publishing dilemma

Hewish, Bell, and their colleagues were unsure how to publish their findings. While it seemed unlikely that the signals were being sent by an alien civilization, no one had any other explanation. Bell returned to her chart analysis, and soon found another “scruff” in a different part of the sky. She discovered it was due to another pulsating signal, this time slightly faster, with pulses every 1.2 seconds. Now she was reassured that the pulses must have some natural explanation—two sets of aliens in different places would surely not be sending signals to Earth at the same time and at nearly the same frequency.

By January 1968, Hewish and Bell had found four pulsing sources in total, which they decided to call “pulsars.” They wrote a paper describing the first source, suggesting that it might be due to pulsed emissions from a theoretical type of superdense collapsed star called a neutron star. Objects of this type had been predicted as long ago as 1934, but up to that time had never been detected.

“My eureka moment was in the dead of night, the early hours of the morning. But when the result poured out of the charts … you realize instantly how significant this is—what it is you’ve really landed on—and it’s great!” Jocelyn Bell Burnell

Explaining the pulses

Three months later, Thomas Gold, an Austrian−American astronomer at Cornell University in the US, published a fuller explanation for the pulsed signals. He agreed that each set of radio signals was coming from a neutron star, but proposed that the star was rapidly spinning. A star like this would not need to be emitting pulsed radiation to account for the pattern of signals observed. Instead, it could be emitting a steady radio signal in a beam that it swept around in circles, just like a beam of light from a lighthouse. When the pulsar’s beam (or perhaps one of its two beams) was pointing at Earth, a signal would be detected, which would show up as the sort of short pulse that Bell had noticed on printouts. When the beam had passed by Earth, the signal would stop until the beam came around again. Challenged about the pulsation rates, which implied extremely rapid spinning, Gold explained that neutron stars could be expected to behave in this way because of the way in which they form—from the collapse of stellar cores in supernova explosions.

A pulsar is a spinning neutron star with an intensely strong magnetic field. It emits beams of radiation from its north and south poles.

Confirming the hypothesis

Initially, Gold’s explanations were not well received by the astronomy community. However, they became widely accepted after the discovery of a pulsar in the Crab nebula, a well-known supernova remnant. Over subsequent years, many more pulsars were found. They are now known to be rapidly rotating neutron stars with intense electromagnetic fields, which emit beams of electromagnetic radiation from their north and south poles. These beams are often, but not always, radio waves and sometimes other forms of radiation, including in some cases visible light. One reason for the excitement regarding the discovery of pulsars was that it increased the likelihood that another theoretical phenomenon—black holes—might also be detected and proven. Like neutron stars, black holes are objects that could result from the gravitational collapse of a stellar core following a supernova explosion.

In 1974, Hewish and Martin Ryle shared a Nobel Prize: “Ryle for his observations and inventions … and Hewish for his decisive role in the discovery of pulsars.” However, Jocelyn Bell Burnell was told that she would not share the award with them because she had still been a student at the time of her work. She graciously accepted that decision.

A spinning neutron star that is emitting radiation beams can be detected as a pulsar on Earth if, as it spins, one or possibly both of its radiation beams recurrently point in Earth’s direction as they are swept around through space. The pulsar will then be detected as a very regular series of signal “blips.”


Jocelyn Bell was born in 1943 in Belfast, Northern Ireland. After earning a physics degree from Glasgow University in 1965, she moved to Cambridge University, where she studied for a Ph.D. There, she joined the team that built a radio telescope to detect quasars. In 1968, Bell became a research fellow at the University of Southampton and changed her last name to Bell Burnell when she married. She has held astronomy and physics-related positions in London, Edinburgh, and at the Open University, where, from 1991 to 2001, she was professor of physics. From 2008 to 2010, she was President of the Institute of Physics. Bell Burnell has received numerous awards for her professional contributions, including the Herschel Medal of the Royal Astronomical Society in 1989. In 2016, she was visiting professor of astrophysics at Oxford University.

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X-rays are a form of high-energy, electromagnetic radiation released by extremely hot objects. In the early 20th century, astronomers realized that space should be flooded with X-rays from the sun. Moreover, the sun’s X-ray spectrum would reveal a lot about the processes at work within the star. However, X-ray astronomy was not possible until the advent of rockets and satellites. Despite their energy, X-rays are easily absorbed, which is why they are so good at imaging the body. Water vapor in Earth’s atmosphere effectively blocks X-rays from reaching the surface—a good thing for life,
because high-energy X-rays can cause damage and mutations when they impact on soft, living cells.

The first glimpse of the sun’s X-rays came in the late 1940s, during a US Naval Research Laboratory (NRL) program to study Earth’s upper atmosphere. A team led by US rocket scientist Herbert Friedman fired German V-2 rockets into space equipped with X-ray detectors—essentially modified Geiger counters. These experiments provided the first incontrovertible evidence of X-rays from the sun. By 1960, researchers were using Aerobee sounding rockets to detect X-rays, and the first X-ray photos of the sun were taken from an Aerobee Hi. Two years later, the first cosmic X-ray source was detected.

“Nothing is going to happen unless you work with your life’s blood.” Riccardo Giacconi

Extrasolar X-rays

Riccardo Giacconi, an Italian astrophysicist then working for American Science and Engineering (AS&E), had successfully petitioned NASA to fund his team’s X-ray experiment. The team’s first rocket misfired in 1960, but by 1961 it had a new, improved experiment ready for launch. This instrument was one hundred times more sensitive than any flown to date. Using a large field of view, the team hoped to observe other X-ray sources in the sky. Success followed a year later: the rocket aimed its camera first at the moon and then away from it. What the camera saw came as a complete surprise to the team. The instrument detected the X-ray “background”—a diffuse signal coming from all directions—and a strong peak of radiation in the direction of the galactic center.

Stars like the sun emit about a million times more photons at visible light frequencies than they do as X-rays. The source of the X-ray signals, by contrast, radiated a thousand times more X-rays than light. Although a small, barely visible point in the sky, the source was pumping out one thousand times more X-rays than the sun. Furthermore, certain physical processes were taking place within the source and these had never been seen in the laboratory. After weeks of analysis, the team concluded that this must be a new class of stellar object.

Search for the source

There was no candidate in the solar system to account for the intense radiation. The most likely source was named Scorpius X-1 (Sco X-1 for short) after the constellation within which it was located. Herb Friedman at the NRL confirmed the result using a detector with a larger area and better resolution than the AS&E instrument. Sco X-1 is now known to be a double star system and is the brightest, most persistent X-ray source in the skies.

Further launches revealed a sky dotted with X-ray sources, both galactic and extra-galactic. In a short space of time, the team had detected a disparate set of celestial oddities emitting X-rays. These included supernova remnants, binary stars, and black holes. Today, more than 100,000 X-ray sources are known.

The Chandra X-ray Observatory was launched by NASA in 1999. It was initially planned to operate for five years, but was still in use in 2016.

Toward Chandra

By the mid-1960s, instruments were becoming ever more sensitive. Detectors were able to record X-rays one thousand times weaker than Sco X-1 just five years after Giacconi’s discovery. Initially proposed by Giacconi in 1963, Uhuru, the first satellite dedicated solely to X-ray astronomy, was launched in 1970. It spent three years mapping X-rays. This all-sky survey located 300 sources, including a bizarre object in the center of the Andromeda galaxy, and it earmarked Cyg X-1 as a potential black hole. Uhuru also found that the gaps in galaxy clusters are strong sources of X-rays. These apparently empty regions are in fact filled by a low-density gas at millions of degrees Kelvin. Although thinly spread, this “intercluster medium” contains more mass than that of all of the cluster’s galaxies combined.

In 1977, NASA launched its High Energy Astronomy Observatory (HEAO) program. HEAO-2, renamed the Einstein Observatory, was equipped with highly sensitive detectors and revolutionized X-ray astronomy. With its fused quartz mirrors, the telescope was a million times more sensitive than that of Giacconi’s 1961 discovery rocket. Einstein observed X-rays emanating from stars and galaxies, and even from planetary aurorae on Jupiter.

Eager to probe the X-ray background further, Giacconi once again proposed an advanced telescope. In 1999, this became the Chandra X-Ray Observatory, the third of the orbiting Great Observatories. Chandra is the most powerful X-ray telescope ever built, tens of billions of times more sensitive than the early detectors. Its phenomenal performance outstripped all expectations and its mission lifetime was tripled from five to 15 years. As of 2016, however, its mission is ongoing. Chandra’s outstanding technical firsts include detecting sound waves coming from a supermassive black hole. The X-ray data, when combined with optical observations from the Hubble Space Telescope and infrared data from the Spitzer Space Telescope, have provided stunning images of the cosmos.

Active regions of the sun are revealed by combining observations from many telescopes. Highenergy X-rays are shown in blue; low-energy X-rays green.

Realm of the X-rays

X-ray astronomy observes the highest-energy objects in space: colliding galaxies, black holes, neutron stars, and supernovae. The energy source behind this activity is gravity. As matter falls toward a massive concentration of material, particles collide and accumulate. They give up their energy by emitting photons, which at these speeds have X-ray wavelengths (0.01–10 nanometers, or billionths of a meter)—equivalent to temperatures of tens of million of degrees. The same mechanism, is at work in a wide range of dramatic phenomena: active stars more massive than the sun, for example, produce strong solar winds and significant amounts of X-rays. “X-ray binary star” systems, in which mass transfers from one star to its partner, also produce intense radiation.

“The universe is popping all over the place.” Riccardo Giacconi

Seeing black holes

When stars explode at the end of their lives, the blast waves from the supernova compress the interstellar medium, causing the gas to release Xrays. Left within what remains of the supernova, the massive star continues life as a neutron star or a black hole. Turbulence generated by material being torn apart as it is sucked into a black hole will also produce X-rays. The radiation being pumped out causes the outer layers of the supernova remnant to fluoresce in a range of colors.

Certain galaxies have centers that outshine all the billions of stars in the galaxy itself, with emissions that are bright at all wavelengths. The center of such an “active galactic nucleus” is assumed to contain a supermassive black hole. Material falling toward the centers of galaxy clusters—the largest structures in the universe—also shines in X-rays, and is not visible in other light frequencies. Chandra has now taken two “deep field” images of the Xray background—23- and 11-day exposures of the northern and southern hemispheres of the sky. X-ray instruments of the future may help scientists see how black holes are distributed.

Observations in the X-ray spectrum reveal hidden structures. The larger blobs in this patch of sky from an ESA X-ray survey are galaxy clusters; smaller dots are black holes.


Born in Genoa, Italy, in 1931, Riccardo Giacconi lived in Milan with his mother, a mathematics and physics high school teacher. She instilled a love of geometry in the young Riccardo. Giacconi’s first degree was from the University of Milan. With a Fulbright Scholarship, he moved to Indiana University in the US, and then to Princeton, to study astrophysics.

In 1959, Giacconi joined American Science and Engineering, a small firm in Cambridge, Massachusetts. AS&E built rocket-borne monitoring equipment for measuring electrons and artificial gamma-ray bursts from nuclear weapons. Giacconi was tasked with developing instruments for X-ray astronomy. He was at the heart of most of the breakthroughs in X-ray astronomy, and in 2002, he was awarded a share of the Nobel Prize in Physics for his contributions to astrophysics. In 2016, he was still working in his mid-80s, as principal investigator for the Chandra Deep Field-South project.

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