The Dane Tycho Brahe was the last great astronomer of the pre-telescope era. Realizing the importance of trying to record more accurate positions, Tycho built some high-precision instruments for measuring angles. He accumulated an abundance of observations, far superior to those available to Copernicus.
Magnifying the image
The realm of heavenly bodies still seemed remote and inaccessible to astronomers at the time of Tycho’s death in 1601. However, the invention of the telescope around 1608 suddenly brought the distant universe into much closer proximity.
Telescopes have two important advantages over eyes on their own: they have greater light-gathering power, and they can resolve finer detail. The bigger the main lens or mirror, the better the telescope on both counts. Starting in 1610, when Galileo made his first telescopic observations of the planets, the moon’s rugged surface, and the star clouds of the Milky Way, the telescope became the primary tool of astronomy, opening up unimagined vistas.
After Tycho Brahe died, the records of his observations passed to his assistant Johannes Kepler, who was convinced by Copernicus’s arguments that the planets orbit the sun. Armed with Tycho’s data, Kepler applied his mathematical ability and intuition to discover that planetary orbits are elliptical, not circular. By 1619, he had formulated his three laws of planetary motion describing the geometry of how planets move.
Kepler had solved the problem of how planets move, but there remained the problem of why they move as they do. The ancient Greeks had imagined that
the planets were carried on invisible spheres, but Tycho had demonstrated that comets travel unhindered through interplanetary space, seeming to contradict this idea. Kepler thought that some influence from the sun impelled the planets, but he had no scientific means to describe it.
“If I have seen further it is by standing on the shoulders of giants.” Isaac Newton
It fell to Isaac Newton to describe the force responsible for the movement of the planets, with a theory that remained unchallenged until Einstein. Newton concluded that celestial bodies pull on each other and he showed mathematically that Kepler’s laws follow as a natural consequence if the pulling force between two bodies decreases in proportion to the square of the distance between them. Writing about this force, Newton used the word gravitas, Latin for weight, from which we get the word gravity.
Newton not only created a new theoretical framework for astronomers with his mathematical way of describing how objects move, but he also applied his genius to practical matters. Early telescope makers found it impossible to obtain images free from colored distortion with their simple lenses, although it helped to make the telescope enormously long. Giovanni Domenico Cassini, for example, used long “aerial” telescopes without a tube to observe Saturn in the 1670s.
In 1668, Newton designed and made the first working version of a reflecting telescope, which did not suffer from the color problem. Reflecting telescopes of Newton’s design were widely used in the 18th century, after English inventor John Hadley developed methods for making large curved mirrors of precisely the right shape from shiny speculum metal. James Bradley, Oxford professor and later Astronomer Royal, was one astronomer who was impressed and acquired a reflector.
There were also developments in lens-making. In the early-18th century, English inventor Chester Moore Hall designed a two-part lens that greatly reduced color distortion. The optician John Dollond used this invention to build much-improved refracting telescopes. With high-quality telescopes now widely available, practical astronomy was transformed.
WRINKLES IN TIME
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
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.
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.
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.
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
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?
IMPORTANT NEW DISCOVERIES WILL BE MADE WITH FLYING TELESCOPES
In 1946, a full 11 years before Sputnik 1, the first satellite, was launched into Earth’s orbit, a 32-year-old astrophysicist named Lyman Strong Spitzer Jr. conceived of a powerful telescope that would one day operate not on Earth’s surface but in orbit. High above the opaque atmosphere and the light pollution, this space telescope would have a clear and unprecedented view of the universe. It would be more than four decades before Spitzer’s dream was realized, but his patience and tenacity would eventually pay off.
More than light
The discovery of extraterrestrial radio sources by Karl Jansky in 1935 revealed that there were ways of observing the universe other than by visible light. The outbreak of World War II in 1939 interrupted research into this new and exciting field. It was left to an amateur astronomer from Illinois named Grote Reber to take the first steps in radio astronomy. In 1937, Reber had made the first survey of the radio universe using homemade antennae he had built in his backyard. Soon afterward, wartime researchers found that meteors and sunspots produced radio waves of their own, this time in the microwave band used in radars. If it was possible to discover new objects using radio, then it stood to reason that other forms of electromagnetic radiation, such as infrared, ultraviolet (UV), and X-rays, could be harnessed as tools of observation.
There was a problem, however. Earth’s atmosphere, transparent to visible light, is opaque to many of these kinds of radiation. The waves are absorbed by the air’s molecules, reflected back into space, or scattered in all directions into a meaningless hodgepodge. As a result, it is almost impossible to garner information about most kinds of non-visible radiation from terrestrial observatories.
Spitzer’s 1946 paper, entitled Astronomical Advantages of an Extraterrestrial Observatory, highlighted the problem of detecting non-visible radiation. His solution was to put a telescope into space. But Spitzer also highlighted the obstacles to such a proposal: first, the technological challenge of inventing space travel, and second, that of designing an instrument capable of operating in space by remote control from the ground.
“Astronomy may be revolutionized more than any other field of science by observations from above the atmosphere. In a new adventure of discovery, no one can foretell what will be found.” Lyman Spitzer Jr.
Twinkle, twinkle little star
The rest of Spitzer’s paper was focused on solving a problem that had frustrated astronomers for centuries—the sky itself. Viewed from Earth, the stars appear to twinkle. This effect is caused by the star’s light shifting back and forth, and rising and falling in brightness. This is not a property of starlight, but is caused by Earth’s thick atmosphere. The twinkle becomes more marked as magnification increases, making the objects appear shaky and fuzzy in the eyepiece of a telescope or as diffuse smears of light in photographs. The scientific term for twinkling is scintillation. It is caused when light passes through layer upon layer of turbulent air in the atmosphere.
The turbulence itself has no effect on the light, but the density and temperature differences that are making the air churn and swirl do have an effect. As the starlight passes through one pocket of air to another of a different density, it refracts slightly, with some wavelengths bending more than others. As a result, the straight beam of light that traveled to Earth across the cosmos starts to follow an ever-changing and haphazard zigzagging path through the air. A telescope or a naked eye focused on it will see a fluctuation in brightness as some of the light is directed in and out of that line of sight.
The impact twinkling has on capturing sharp astronomical images is called “seeing.” When the atmosphere is very still and seeing is good, the image of a distant star in a telescope is a small steady disk. When seeing is poor, the image breaks up into a squirming cluster of dots. An image taken over a period of time is smeared out into a larger disk. The effect is similar to the telescope being out of focus.
Improving the view
Observational conditions change constantly with the atmosphere. Before the 1990s, observers simply waited until distortions were at a minimum. For instance, high winds clear away turbulence, creating near-perfect viewing conditions. In the late 1940s, astronomers started to use movie cameras to film the sky in the hope that, among the thousands of frames filmed over time, there would be the odd “lucky image” that captured the sky in crystal clarity. Another solution was to go higher. Today, the world’s most effective terrestrial observatories are invariably built at the top of high, arid mountains, where cloud cover is minimal, and the air above is generally calm.
With the advent of powerful computers in the 1990s, earthbound astronomers began using adaptive optics (AO) to correct the problems of astronomical seeing. AO measures distortions in the arriving light and evens it out, just as a distorted mirror might be used to correct a deformed image to make it look like the original image prior to deformation. AO systems use minutely adjustable mirrors and other optical devices, but they also rely heavily on computers to filter out the atmospheric “noise” from images.
Despite the dramatic improvements brought about by AO, however, a large telescope in orbit, which could observe in multiple wavelengths of the spectrum, including visible light, was the ultimate goal for astronomy.
“Our knowledge of stars and interstellar matter must be based primarily on the electromagnetic radiation which reaches us.” Lyman Spitzer Jr.
The road to Hubble
As the leading voice in the field, Spitzer had been made head of NASA’s task force for developing the Large Space Telescope (LST) program in 1965. In 1968, NASA scored its first space-telescope success with the Orbiting Astronomical Observatory (OAO-2), which took high-quality images in ultraviolent (UV) light, doing much to raise awareness of the advantages of space-based astronomy.
Spitzer’s LST aimed to achieve more dramatic results than the OAO-2, observing near and far objects with the visible light spectrum. His team settled on a 10-ft 5-in (3-m) reflecting telescope and a launch was scheduled for 1979. However, the project became too expensive for its budget. The aperture was reduced to a less costly 100 in (2.4 m), and LST was postponed to 1983. As that year came and went, no launch occurred, but Spitzer persisted and the project continued. In the meantime, LST was renamed the Hubble Space Telescope (HST) after Edwin Hubble, who had first grasped the true scale of the universe. By now, the telescope’s mirrors had been constructed. To help reduce weight, a top layer of low-expansion glass sat on a honeycomb support. The shape of the mirrors was crucial. During construction, they were held on a support that emulated weightlessness to ensure they did not warp in space. The glass had to be polished into a curve with an accuracy of 10 nanometers. This would make it possible for HST to view everything from UV light to the upper end of the infrared spectrum. Further delays pushed the launch of HST to 1986, but then tragedy struck with the explosion of Space Shuttle Challenger on January 28, 1986, with the result that NASA’s shuttle fleet was grounded for two years.
Finally, on April 24, 1990, Space Shuttle Discovery hauled the 11-ton HST to its orbit 335 miles (540 km) above Earth. Spitzer had finally realized the dream of his career—a telescope in space unencumbered by the problems of poor seeing and an atmosphere partly opaque to ultraviolet and infrared rays.
The problems that had beset the mission on the ground, however, continued in space. The first images sent back by HST were so badly distorted that they were almost worthless. Was HST going to be a worse observational tool than a ground-based telescope? Analysis of the images revealed that the mirror was the wrong shape around the edge. The error was tiny—about 2 millionths of a meter—but enough to send the light captured by the outer part of the primary mirror to the wrong area of the secondary mirror, creating serious aberrations in the images. This was a worrying moment for Spitzer and his team, as it seemed as if HST might be about to prove an embarrassing failure.
If Hubble was to fulfill its potential, it needed corrective elements added to its optical system. In effect, it was given a pair of eyeglasses. The problem with the primary mirror was precisely calculated by analyzing the telescope’s images. The solution was to add carefully designed mirrors in front of Hubble’s instruments so that the light entering them from the main mirror was correctly focused. Two sets of these mirrors were fitted during a crucial service mission to HST in 1993. They worked perfectly. HST could at last be put to work, and the results were astonishing.
Astronauts serviced HST four more times after 1993 and for the last time in 2009, in one of the final shuttle missions. The shuttles were retired in 2011, after which it would not be possible to service HST again. However, that final service added significant upgrades, which mean that HST may remain in use until 2040.
Ultra deep, ultra clear
Despite its shaky start, HST has surpassed all expectations. The telescope has made 1.2 million observations to date during its 3-billion-mile (5-billion-km) journey around Earth. Despite traveling at 17,000 mph (27,000 km/h), it can pinpoint a position in space to an accuracy of 0.007 arc seconds—which is like hitting a penny coin from 200 miles (300 km) away. It can resolve an object that is 0.05 arc seconds. NASA likened this to standing in Maryland and viewing two fireflies in Tokyo, Japan. Astronomers worldwide began booking HST’s time to see objects of interest. The archive of everything it has seen—totaling 100 terabytes and counting—can be viewed on a public website.
Many of HST’s observations have looked deep into space—and far back in time. In 1995, the Deep Field image focused on an empty patch of space, one 24-millionth of the total sky. Combining 32 long exposures revealed a number of unknown galaxies that were 12 billion light-years away—light that began its journey just 1.5 billion years after the Big Bang. In 2004, the Ultra Deep Field showed objects 13 billion light-years away, and in 2010, HST used infrared radiation to make the eXtreme Deep Field of objects that existed just 480 million years into the history of the universe. To see farther than this will require the infrared James Webb Space Telescope in 2018.
“Nature has thoughtfully provided us with a universe in which radiant energy of almost all wavelengths travels in straight lines over enormous distances with usually rather negligible absorption.” Lyman Spitzer Jr.
Spitzer in space
HST is the most famous of the four great observatories that are Lyman Spitzer Jr.’s legacy. Between 1991 and 2000, the Compton Gamma Ray Observatory looked at gamma-ray bursts, energetic events that occur at the edge of the visible universe. The Chandra X-ray Observatory was launched in 1999, and is tasked with finding black holes, infant solar systems, and supernovae. The final member is the Spitzer Space Telescope, which entered space in 2003. One of its tasks was to peer into nebulae to pick out the hot zones where stars are forming. In 2009, the liquid helium that kept its heatsensitive detectors cool finally ran out.
Observatories can be placed in orbit around the sun rather than Earth, where it is easier to shield them from the sun’s heat and light and they have a wide, unobstructed view of the sky. Today, there are about 30 observatories in orbit, sending back images. NASA’s Kepler, which searches for extrasolar planets, and two ESA missions, Herschel and Planck, are examples. All were launched in 2009. Herschel was the largest infrared telescope ever put into space, while Planck studied the cosmic microwave background. In 2015, ESA launched the LISA Pathfinder to test the technology for a space observatory that would detect not electromagnetic waves, but gravity waves. Not even Lyman Spitzer Jr. could have predicted such an advance.
LYMAN SPITZER JR.
Lyman Spitzer Jr. was born in Toledo, Ohio, in 1914. He received a Ph.D. in astrophysics from Princeton under the supervision of Henry Norris Russell. After World War II, he became head of the astrophysics department, and began his 50year devotion to space telescopes.
As an expert in plasma, Spitzer invented the stellarator in 1950. This device contained hot plasma within a magnetic field and started the search for fusion power that continues today. In 1965, Spitzer joined NASA to develop space observatories, but that year he triumphed in another field entirely. With his friend Donald Morton, Spitzer became the first to climb Mount Thor, a 5,495-ft (1,675-m) peak in the Canadian Arctic. In 1977, his campaigning for a space telescope paid off, and funding was granted to the Hubble Space Telescope. He lived long enough to see his dream become a reality in 1990.
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