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American astronomer Annie Jump Cannon was the early 20th-century’s leading authority on the spectra of stars. When she died in 1941, Cannon was described as “the world’s most notable woman astronomer.” Her great contribution was to create the basis of the system for classifying the spectra of stars that is still in use today.

Cannon worked at the Harvard College Observatory, as part of the team of “Harvard Computers,” a group of women employed by the director Edward C. Pickering to help compile a new stellar catalog. The college’s catalog, begun in the 1880s with funding from the widow of astrophotographer Henry Draper, used new techniques to collect data on every star in the sky brighter than a certain magnitude, including obtaining the spectra of as many stars as possible. In the 1860s, Angelo Secchi had set out a provisional system for classifying stars according to their spectra. Pickering’s team modified this system. By 1924, the catalog contained 225,000 stars.

“Each substance sends out its own vibrations of particular wavelengths, which may be likened to singing its own song.” Annie Jump Cannon

Early approaches

Williamina Fleming, the first of Pickering’s female computers, made the earliest attempt at a more detailed classification system, by subdividing Secchi’s classes into 13 groups, which she labeled with the letters A to N (excluding I), then adding O, P, and Q. In the next phase of the work, fellow computer Antonia Maury, working with better data received from observatories around the the world, noticed more variety in the detail. She devised a more complex system of 22 groups designated by Roman numerals, each divided into three subgroups. Pickering was concerned that applying such a detailed system would delay the task of compiling the catalog. However, Maury’s approach to stellar classification proved a crucial step toward the creation of the Hertzsprung–Russell diagram in 1910, and consequent discoveries about stellar evolution.

Cannon joined the Harvard College Observatory staff in 1896 and began working on the next part of the catalog, which was published in 1901. With Pickering’s approval, to make classification clearer and easier, she reverted to Fleming’s spectral classes designated by letters, but she changed the order.

Maury had realized that stars of similar colors have the same characteristic absorption lines in the spectra. She had also deduced that a star’s temperature is the main factor affecting the appearance of its spectrum and made her classes a temperature sequence from hotter to cooler. On this, Cannon followed Maury’s lead. Some of Fleming’s letters were dropped because they were unnecessary, so the final sequence became O, B, A, F, G, K, M, based on the presence and strength of certain spectral lines, especially those due to hydrogen and helium. Students of astronomy still learn it by remembering the mnemonic, “Oh Be A Fine Girl, Kiss Me,” attributed to Henry Norris Russell.

The seven main classes of star, categorized according to spectra and temperature, are, from left to right: O, B, A, F, G, K, and M, with O the hottest and M the coolest.

Harvard system

Cannon’s 1901 system laid the foundations for the Harvard Spectral Classification system. By 1912, she had extended it to introduce a range of more precise subclasses, adding 0 to 9 after the letter, with 0 the hottest in the class and 9 the coolest. A few new classes have been added since. The Harvard system essentially classifies stars by temperature and takes no account of the luminosity or size of the star. In 1943, however, luminosity was added as an additional dimension, creating the Yerkes classification system, otherwise called the MKK system after William Morgan, Philip Keenan, and Edith Kellman, the astronomers based at the Yerkes Observatory in Wisconsin who formulated it. This system denotes luminosity with Roman numerals, although a few letters are also used.

The advantage of the MKK system is that it gives a star a size as well as a temperature, so that stars can be described in colloquial terms such as white dwarf, red giant, or blue supergiant. The main sequence stars, including the sun, are all small enough to be called dwarfs. The sun is a G2V star, which indicates that it is a yellow dwarf with a surface temperature of about 5,800 K.

Classes and characteristics

The hottest class of star, O-types have a surface temperature in excess of 30,000 K. Most of the radiation these stars emit is in the ultraviolet part of the spectrum and appear blue when viewed in visible light. O stars are mainly giants, typically 20 times as massive as the sun and 10 times as wide. Only 0.00003 percent of main sequence stars are this hot. O-type stars burn their fuel very quickly and release huge amounts of energy. As a result, they have a short life expectancy, which is measured in tens of millions of years, compared to billions for cooler stars. Members of this class have weak lines of hydrogen in their spectra, and strong evidence of ionized helium, which is present because of the high temperature.

With a surface temperature of between 10,000 and 30,000 K, B-type stars are brighter in visible light than O-types, despite being cooler. This is because more of the radiation is emitted as visible light, making them “blue-white.” Again, B-type dwarfs are rare, making up less than 0.1 percent of main sequence stars. When they do occur, they are perhaps 15 times more massive than the sun. B-type stars have non-ionized helium in their spectra and more evidence of hydrogen. Because they live for only a short time, B-type stars are found in molecular clouds or star-forming regions, since they have not had time to move far from the location in which they formed. About twice as large as the sun, main sequence A-type stars have a surface temperature of between 7,500 and 10,000 K. They have strong hydrogen lines in their spectra and emit a wide spectrum of visible light, which makes them look white (with a blueish tinge). As a result, they are some of the most easily seen stars in the night sky, and include Vega (in Lyra), Gamma Ursae Majoris (in the Big Dipper), and Deneb (in Cygnus). However, only 0.625 percent of main sequence stars are A-type stars.

“The prism has revealed to us something of the nature of the heavenly bodies, and the photographic plate has made a permanent record of the condition of the sky.” Williamina Fleming

Cooling stars

As dwarf stars cool, the hydrogen in their spectra becomes less intense. They also exhibit more absorption lines due to metals. (To an astronomer, everything heavier than helium is a metal.) This is not because their composition is different from that of hotter stars but because the gas near the surface is cooler. In hotter stars, the atoms are too ionized to create absorption lines. F-type stars have a surface temperature of between 6,000 and 7,500 K. Called yellow-white dwarfs, they make up 3 percent of the main sequence and are a little larger than the sun. The spectra of these stars? contain mediumintensity hydrogen lines and strengthening lines for iron and calcium.

The strengths of the absorption lines of different elements vary according to the surface temperature of the star. Lines of heavier elements are more prominent in the spectra of cooler stars.

The sun’s class

Type-G yellow dwarfs, of which the sun is one, make up 8 percent of the main sequence. They are between 5,200 and 6,000 K on the surface and have weak hydrogen lines in their spectra, with more prominent metal lines. TypeK dwarfs are orange and make up 12 percent of the main sequence. They are between 3,700 and 5,200 K on the surface and have very weak hydrogen absorption lines but strong metallic ones, including manganese, iron, and silicon. Type-M are red dwarfs. These are by far the most common main sequence stars, making up 76 percent of the total, although no red dwarf is visible to the naked eye. They are just 2,400–3,700 K on the surface and their spectra contain absorption bands for oxide compounds. The majority of the yellow, orange, and red dwarfs are believed to have planetary systems.

A white dwarf sits at the heart of the Helix planetary nebula. When its fuel ends, the sun will become a white dwarf.

Extended classification

Stellar spectral classes now cover even more types of stars. Class W are thought to be dying supergiant stars. Class C, or carbon stars, are declining red giants. Classes L, Y, and T are a diminishing scale of colder objects, from the coolest red dwarfs to the brown dwarfs, which are not quite large or hot enough to be classed as stars. Finally, white dwarfs are class D. These are the hot cores of red giant stars that no longer burn with fusion and are gradually cooling. Eventually they should fade to black dwarfs, but it is estimated it will take a quadrillion years for that to happen.


Born in Delaware, Annie Jump Cannon was the daughter of a state senator, and was introduced to astronomy by her mother. She studied physics and astronomy at Wellesley College, an all-women’s college. Graduating in 1884, Cannon returned to her family home for the next 10 years. On the death of her mother, in 1894, she began to teach at Wellesley and joined Edward C. Pickering’s Harvard Computers two years later.

Cannon suffered from deafness, and the ensuing difficulties in socializing led her to immerse herself in scientific work. She remained at Harvard for her entire career, and is said to have classified 350,000 stars over 44 years. Subject to many restrictions over her career due to her gender, she was finally appointed a member of the Harvard faculty in 1938. In 1925, she became the first woman to be awarded an honorary degree by Oxford University.

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In 1916, as he worked on his theory of relativity, Albert Einstein predicted that, as a mass moved, its gravity would cause ripples in the fabric of spacetime. Every mass would do this, although larger masses would make bigger waves, in the same way that a pebble dropped in a pond makes an ever-increasing circle of ripples, while a meteor impacting the ocean creates tsunami-sized waves.

In 2016, 100 years after Einstein’s predictions, a collaboration between scientists working under the name LIGO announced that they had discovered these ripples, or gravitational waves. Their decades-long search had revealed the gravitational equivalent of tsunamis created by two black holes spiraling around each other and then colliding.

It is hoped that the discovery of gravitational waves will provide a new way of observing the universe. Instead of using light or other electromagnetic radiation, astronomers are hoping to map the universe by the gravitational effects of its contents. While radiation is obscured in many ways, including by the opaque plasma of the early universe up to 380,000 years after the Big Bang, gravitational waves pass through everything. This means that gravitational astronomy could see back to the very beginning of time, a trillionth of a second after the Big Bang.

Within a period of 20 milliseconds, the two black holes LIGO had detected increased their orbital speed from 30 times a second to 250 times a second before colliding.

Wave behaviors

LIGO stands for Laser Interferometer Gravitational-Wave Observatory. It is a remarkable set of instruments for measuring expansions and contractions in space itself. This is no easy task. A ruler cannot do it because, as space changes in size, so does the ruler, so the observer measures no change at all. LIGO succeeded using the benchmark that remains constant whatever space is doing: the speed of light. Light behaves like a wave, but it does not require a medium through which to travel. Instead, light (and any kind of electromagnetic radiation) is an oscillation of an electromagnetic field: in other words, light is a disturbance in a field permeating all of space.

Gravitational waves can be understood as disturbances in the gravitational field that permeates the universe. Einstein described how these disturbances are caused by the mass of objects curving the space around them. What is understood as the “pull of gravity” is a small mass appearing to alter its motion and “fall” toward a larger mass as it encounters a region of warped space.

All masses are in motion—planets, stars, even galaxies—and as they move, they leave a trail of gravitational disturbances in their wake. Gravity waves propagate in a comparable way to sound waves, by distorting the medium through which they travel. In the case of sound waves, that medium is made of molecules, which are made to oscillate. In the case of gravity, the medium is spacetime, the very fabric of the universe. Einstein predicted that the speed of gravity would be the same as the speed of light, and that the ripples in spacetime would move outward in all directions. The intensity of these ripples diminishes rapidly with distance (by a square of the distance), so detecting a distinct gravity wave from a known object far out in space would require a very powerful source of waves and a very sensitive instrument.


As its name suggests, LIGO employs a technique called laser interferometry. This makes use of a property of waves called interference. When two waves meet, they interfere with one another to create a single wave. How they do this depends on their phase—the relative timing of their oscillations. If the waves are exactly in phase—rising and falling perfectly in sync—they will interfere constructively, merging to create a wave with double the intensity. By contrast, if the waves are exactly out of phase—one rising as the other falls—the interference will be destructive. The two waves will merge and cancel one another out, disappearing completely.

LIGO’s source of waves is a laser, which is a light beam that contains a single color, or wavelength, of light. In addition, the light in a laser beam is coherent, which means that its oscillations are all perfectly in time. Such beams can be made to interfere with one another in very precise ways.

The laser beam is split in two and the resulting beams are sent off perpendicular to one another. They both hit a mirror and bounce straight back to the starting point. The distance traveled by each beam is very precisely controlled so that one has to travel exactly half a wavelength farther than the other (a difference of a few hundred billionths of a meter). When the beams meet each other again, they are exactly out of phase as they interfere, and promptly disappear—unless a gravitational wave has passed through space while the beams were traveling. If present, a gravitational wave would stretch one of the laser tracks and compress the other, so the beams would end up traveling slightly altered distances.

With no gravitational waves, LIGO’s light waves cancel one another out when they are recombined. Gravitational waves stretch one tube while compressing the other, so that the waves are no longer perfectly aligned, and a signal is produced

Noise filter

The laser beams are split and sent on a 695-mile (1,120-km) journey up and down LIGO’s 2.5-mile (4-km) long arms before being recombined. This gives LIGO the sensitivity to detect minute perturbations in space that add up to a few thousandths of the width of a proton. With the distances put very slightly out of sync, the interfering beams would no longer cancel each other out. Instead, they would create a flickering pattern of light, perhaps indicating a gravity wave passing through LIGO’s corner of space.

The difficulty was that such a sensitive detector was prone to distortions from the frequent seismic waves that run through Earth’s surface. To be sure that a laser flicker was not an earth tremor, two identical detectors were built at opposite ends of the United States: one in Louisiana, the other in Washington state. Only signals registering on both detectors were gravitational waves (the signals are in fact 10 milliseconds apart—the time it takes for light, and gravitational waves, to travel from Louisiana to Washington). Ligo operated from 2002–2010 with no success, then started up again in 2015 with enhanced sensitivity.

LIGO’s precision instruments must be kept completely clean. Maintaining the purity of the laser beams is one of the project’s biggest challenges.

Colliding black holes

On September 14, 2015, at 9:50:45 GMT, two black holes a billion lightyears away collided and unleashed huge warps in the fabric of space. In fact, this event occurred a billion years ago but it had taken that long for the ripples they had released to reach Earth—where they were detected by both LIGO detectors. The researchers took another few months to check their result and went public in February 2016.

The search is now on for more gravitational waves, and the best place to do it is from space. In December 2015, the spacecraft LISA Pathfinder was launched. It is headed to an orbit at L1, which is a gravitationally stable position between the sun and Earth. There, the spacecraft will test laser interferometry instruments in space, in the hope that they can be used in an ambitious experiment called eLISA (evolved Laser Interferometer Space Antenna). Provisionally scheduled for 2034, eLISA will use three spacecraft triangulated around the sun. Lasers will be fired between the spacecraft,
making a laser track 2 million miles (3 million km) long that is many times more sensitive to gravitational waves than LIGO.

The discovery of gravitational waves has the potential to transform astronomers’ view of the universe. The patterns in the fluctuations in the light signals from LIGO and future projects will produce new information, providing a detailed map of mass across the universe.

“Gravitational waves will bring us exquisitely accurate maps of black holes—maps of their spacetime.” Kip Thorne

LIGO splits one beam of laser light and sends beams down two tubes at 90° to each other. To prevent unwanted interference, the tubes are vacuums at one trillionth of the pressure of Earth’s atmosphere. LIGO also has to make adjustments to allow for the tidal pull of the sun and the moon


LIGO is a collaboration between Caltech and MIT, and also shares its data with a similar experiment called Virgo, which is running in France and Poland. Hundreds of researchers have contributed to the discovery of gravitational waves. However, there are two people, both Americans, who stand out among them all: Rainer “Rai” Weiss (1932–) and Kip Thorne (1940–). In 1967, while at MIT, Weiss developed the laser interferometry technique used by LIGO, working from the initial ideas of Joseph Weber, one of the inventors of the laser. In 1984, Weiss cofounded LIGO with Thorne, a counterpart at Caltech, who is a leading expert on the theory of relativity. LIGO is the most expensive science project ever funded by the US government, with a current cost of $1.1 billion. After 32 years of trying, in 2016 Weiss and Thorne announced their discovery of gravitational waves at a news conference in Washington, D.C.

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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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The Cosmic Microwave Background, or CMB, was discovered in 1964. This is the afterglow of the Big Bang and it is as near as scientists can get to observing the event that brought the universe into existence, 13.8 billion years ago. Linking the structures observed in the universe to the features discerned in the CMB remains a key challenge for cosmologists.

“I always think of space-time as being the real substance of space, and the galaxies and the stars just like the foam on the ocean.” George Smoot

Wrinkled time

The first great breakthrough came from the Cosmic Microwave Background Explorer, known as COBE, a NASA satellite launched in 1989. The detectors on COBE, designed and run by George Smoot, John Mather, and Mike Hauser, were able to find the oldest structures in the visible universe, described by Smoot as “wrinkles in time.” These wrinkles in otherwise uniform space were once dense regions containing the matter that would form stars and galaxies. They correspond to the large-scale galaxy superclusters and great walls seen in the universe today, and add weight to the inflationary model of the early universe proposed by American Alan Guth.

The CMB is a flash of radiation that was released about 380,000 years after the Big Bang, at the time the first atoms formed. The expanding universe had cooled enough for stable ions (positively charged nuclei) of hydrogen and helium to form, and then, after a little more cooling, the ions began to collect electrons to make neutral atoms. The removal of free electrons from space led to the release of photons (particles of radiation).

Those photons are visible now as the CMB. The CMB comes from the whole sky, without exception. It has redshifted (the wavelengths have stretched), and it now has wavelengths of a few millimeters, while the original radiation’s wavelengths would be measured in nanometers (billionths of a meter). One of the key observations of the CMB came in the 1970s, and removed any doubt that it was an echo of the Big Bang. This was the discovery that the thermal spectrum of radiation from the CMB tallied very closely with that of a theoretical black body.

The Cosmic Microwave Background Explorer (COBE) spent four years in space collecting information about the CMB, scanning the celestial sphere every six months

Black bodies

Black bodies do not really exist—they cannot be made and no object observed in the universe functions as black bodies do in theory. However, the CMB is the closest match that has ever been found.

A black body absorbs all radiation that hits it. Nothing is reflected. However, the absorbed radiation adds to the thermal energy of the object, and this is released as radiation. In 1900, German Max Planck, the founding figure of quantum physics, showed that the spectrum of radiation released by a black body is entirely dependent on temperature.

In an everyday example of radiation varying with temperature, an iron bar glows red when first heated. Heating it more makes it orange, and eventually the bar will glow “blue hot.” Metalworkers learn to roughly judge the temperature of iron by its color. The metal is not particularly close to a black body in the theoretical sense, but stars and other astronomical objects are a much closer match to a black body, and so the color, or wavelengths of their emissions, can be compared to the thermal spectrum of a theoretical black body to give a relatively precise temperature.

The temperature of the CMB today is a chilly 2.7 K. The thermal spectrum at that temperature contains no visible light, which is why space looks black to human eyes. However, the spectrum has redshifted (stretched) over time as the universe has expanded. Extrapolating back to the moment the CMB was emitted gives an original temperature of about 3,000 K. The color of radiation at this temperature is orange, so the CMB started out as a flash of orange light that shone out from every point in space.

Smooth signal

The early observations of the CMB suggested that it was isotropic, which means that its spectrum is the same everywhere. In cosmology, the terms density, energy, and temperature are somewhat synonymous when discussing the early universe. So the isotropic nature of the CMB suggested that, in those early days, space had a uniform density, or spread of energy. However, this did not tally with the developing theories of the Big Bang, which demanded that matter and energy were not evenly spread through the young universe, but had been concentrated together in places. These denser areas, or anisotropies, were where the stars and galaxies formed. COBE was sent into space to take a close look at the CMB to see if it could find any anisotropies, to find out whether the CMB changed, however slightly, depending on where it looked.

The full-sky map produced by WMAP in 2011 showed many fine details of the isotropy of the CMB. Colder spots are blue, while hotter spots are yellow and red.

COBE’s mission

A mission to study the CMB from space had been in the planning stages since the mid-1970s. Construction of COBE began in 1981. It was initially designed to enter polar orbit (its orbit passing over both poles). However, the Challenger disaster of 1986 grounded the shuttle fleet, and the COBE team had to look for another launch system. In 1989, the satellite was launched using a Delta rocket, and it was placed in a sun-synchronous geocentric orbit —orbiting in a way that saw it pass over each place on Earth at the same time of day. This worked just as well as a polar orbit in that it allowed COBE to point away from Earth and scan the entire celestial sphere, strip by strip.

The spacecraft carried three instruments, all protected from the sun’s heat and light by a cone-shaped shield, and chilled to 2 K (colder than space itself) using 100 gallons (650 liters) of liquid helium. George Smoot ran the
Differential Microwave Radiometer (DMR), which mapped the precise wavelengths of the CMB, while John Mather was in charge of FIRAS, the Far-InfraRed Absolute Spectrophotometer, which collected data on the spectra of the CMB. These two experiments were looking for anisotropies. The third detector on COBE had a slightly different goal. The Diffuse Infrared Background Experiment, run by Mike Hauser, found galaxies that were so ancient and far away that they are only visible by their heat radiation (or infrared).

COBE’s instruments created the most accurate map of the CMB to date. However, it was not a simple surveying job. Smoot and Mather were interested in primary anisotropies—that is, the density differences that were present at the time the CMB formed. To find these, they needed to filter out the secondary fluctuations caused by obstacles that lay between COBE and the edge of the universe. Dust clouds and the effects of gravity had interfered with the radiation on its long journey to Earth. The data from the three instruments were used to detect and correct these so-called secondary anisotropies.

“[COBE has made] the greatest discovery of the century, if not of all time.” Stephen Hawking

In addition to mapping the CMB, WMAP measured the age of the universe as 13.77 billion years, dark matter as 24.0 percent of the universe, and dark energy as 71.4 percent.

Tiny fluctuations

After 10 months in space, COBE’s helium ran out, which limited the function of the two infrared detectors, but the DMR continued working until 1993. By 1992, the COBE team’s analysis had shown what they were looking for. The CMB, and thus the early universe, was not a uniform blob of energy. Instead it was riddled with tiny but significant fluctuations. The differences were minute, with density variations of about 0.001 percent. However, the pattern was enough to explain why the contents of the universe are clustered together, while the rest of space is made from vast empty voids.

Since COBE, two subsequent missions have added detail to the picture of the CMB. Between 2001 and 2010, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) mapped the CMB to a higher resolution than COBE. Then, from 2009–2013, the ESA’s Planck Observatory produced the most accurate map to date.

Every wrinkle on the map is the seed from which an entire galaxy formed about 13 billion years ago. However, no known galaxy can be seen forming
in the CMB. The CMB radiation detected today has traveled from near the edge of the observable universe over the course of most of the age of the universe. Astronomers can only see 13.8 billion light-years away, but most of the universe now lies farther away than that. The galaxies forming in the CMB are now far beyond what can be observed, and are receding faster than the speed of light.


After a childhood in Florida and Ohio, Smoot began his career as a particle physicist working at MIT. His interests switched to cosmology and he moved across the country to the Lawrence Berkeley National Laboratory. It was there that Smoot studied the CMB and developed ways of measuring its radiation.

Smoot’s early work involved fitting detectors to high-altitude U2 spyplanes, but in the late 1970s, he became involved in the COBE project to take his detector into space. After his success with COBE, Smoot cowrote Wrinkles in Time with Keay Davidson to explain the discovery. Smoot won the Nobel Prize in 2006, along with John Mather, for his work on COBE. He reportedly gave his prize money to charity. However, three years later, Smoot won an even greater sum when he bagged the $1 million jackpot on the TV game show Are You Smarter Than a 5th Grader?

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