Cosmic spectra- Beatrice Gross
In the course of their study on the evolution of planets and the formation of stars (the age of a star is generally measured by its color and brightness, both expressions of its energetic intensity and chemical composition), a team of researchers decided to take on the rather unorthodox and speculative mission of calculating the average color of the universe. The scientists wondered: if all the light emitted by 200,000 galaxies – the greatest astronomic sample ever analyzed – were uniformly combined and made visible to the naked eye, where would the resultant chromatic composite made visible to the naked eye lie on the stellar spectrum?
At first, the average color of the universe was identified as a turquoise green. But that result turned out to be false, the consequence of a calculation error due to inaccurate white levels in the software the researcher used. After correction, the average color of the universe was officially declared a particular shade of beige which, after heated debate among the image-making astronomers, was named “Cosmic Latte.”
In the early days, back when the universe was young and “green,” it was, actually, a bluer shade. The younger a star is, the colder (or bluer) its color, meaning its temperature is higher. As it ages, a star’s temperature drops, but its color warms (or reddens) – the inverse of our standard codification of blue and red. The current color of the universe, then, is the result of a gradual loss of energy, a fading in the electromagnetic fields that surround us and yet, to our eyes, remain invisible.
Indeed, human vision can only access a tiny sliver of the spectrum of light, a narrow band of wavelengths from violet to red. The light we see, called “white light”, was initially considered largely homogeneous, until 1671, when Isaac Newton demonstrated that solar radiation could be broken down into an entire chromatic range. In a controlled experiment, Newton refracted a beam of sunlight with a glass prism and successfully reproduced a prodigious phenomenon common in nature yet almost always haphazard: the apparition of a rainbow and its colors. Soon after, we learned that the spectrum of light extends beyond those visible colors; it actually deploys a much wider palette of invisible rays.
In 1800, by placing a thermometer just beyond the visible part of a refracted sunbeam, the astronomer William Herschel discovered the presence of
“radiative heat,” later called infrared radiation, since it refers to the thermal load of rays above the red on the spectrum. Because its rays cannot be blocked by interstellar gas clouds, infrared radiation allows us to see into the darkest corners of the universe by illuminating those parts that absorb white light. Thanks to infrared we can see previously invisible objects, from newborn stars in the Orion constellation to massive black holes.
The symmetrical opposite of infrared is ultraviolet, that slice of the electromagnetic spectrum commonly called “black light,” which can only be seen indirectly through the phenomenon of fluorescence. Ultraviolet radiation – discovered in a process similar to the one used in silver chloride photography, a process sensitive to white light – led us to a better understanding of the composition of the hottest celestial bodies, including the massive suns that dominate galaxies. With the help of ultraviolet rays, we were also able to detect aurora borealis on other planets, like Jupiter.
In 1895, Wilhelm Röntgen identified another piece of the invisible spectrum and called it the X-ray to underscore its unknown nature. Emitted during violent explosions in space, X-rays can also be found among cannibal star systems, when one star feeds off the energy of its neighboring star, forcing the expulsion of an extremely high-temperature gas. Since cosmic X-rays are blocked by the Earth’s atmosphere, they can only be captured in and studied from outer space.
And then, there are the gamma rays, highly penetrating electromagnetic rays that cannot be detected by conventional optical instruments. Gamma rays give off the highest levels of light energy ever observed and tend to accompany the most violent events in outer space, like the formation of supernovae or the activity of supermassive black holes.
The term “black hole” did not emerge until the 1960s; before that, these voracious matter-eating entities were called “invisible stars” or “dark stars,” the expression that Pierre Simon de Laplace used when he first proved their existence in his 1796 treatise The System of the World. No type of matter, not even photons (the building blocks of light) can escape the gravitational force of a black hole, stronger and even faster than light, which travels in the cosmic void at the vertiginous speed of 300,000 kilometers per second. Because of their massive density and tendency to attract and devour everything nearby, it is practically impossible to get close to black holes. To study them, astrophysicists are left to work with approximative representations, “artists’ images” based on computer simulations. Only recently, some of these have been confirmed by gravitational-wave detection.
In 1840, John W. Draper created the first astronomical photograph, a daguerreotype of the Moon. Soon
after, the first photochromatic images were made, using processes that were rather unstable (like photosensitive plates that had to be kept in the dark) but later perfected to make the images last longer (thanks to Lipmann’s inferential process, for example). It is interesting to note that, alongside all the landscape and Still-life depictions, some of those first images were of the solar spectrum.
The second half of the 19th century marked the emergence of photography as the preferred medium for scientific visualization. Yet, well into the 1950s, the majority of planetary photos were still lower in quality than many of the earlier drawings based on observations made with elementary telescopes. Hence, Eugène Michel Antoniadi’s watercolors of Mars, the pastels of the aurora borealis by Etienne Leopold Trouvelot Milliers, and, more recently, drawings of the SL9 comet’s collision with Jupiter by Audouin Dollfus rival the quality of digital images produced by the Hubble space telescope and its array of powerful photographic sensors.
For optimal effectiveness, astronomic observation needs to be multi-wave; in other words, it needs to employ as many wavelengths of light as possible and render them clearly – even if that means arbitrarily assigning “false colors” to the electromagnetic data and chemical components our eyes cannot naturally see. This kind of augmented vision also has to overcome other obstacles, in particular visibility issues that arise from bad weather conditions and atmospheric turbulence. (This is the primary reason why we put observation satellites and sensor probes on rockets and send them to extremely high altitudes.)
But before they were sent to outer space, observational telescopes were first affixed to a variety of aerial craft with ranges that were relatively modest compared to the scale of the universe (thanks in large part to the efforts of Johannes Kepler in the latter part of the 16th century we only became aware of the cosmos’s depth. Until then, space was understood as a grand celestial vault upon which the stars were hung.)
In 1959 for instance, a mathematician, astronomer and aeronaut by the name of Audouin Dollfus rode to a record 14,000 meters above the earth in a pressurized capsule lifted by a gigantic cluster of hydrogen balloons. Armed with a telescope and a spectrometer capable of resisting stratospheric pressure, Dollfus directed his attention to Venus and to the water vapor content of the terrestrial and lunar atmospheres.
It was in part water vapor – measuring its quantity and particular composition – that led the Swiss scientist Horace-Bénédict de Saussure to climb all the way up to the summit of Mont Blanc in 1787. The instigator of a heavily publicized race to conquer the highest peak in Europe, Saussure actually took second place, when he reached the top of that “accursed mount” almost a year
to the day after the unprecedented ascension of guide Jacques Balmat and Dr. Michel-Gabriel Paccard.
At the top of Mont Blanc, Saussure conducted 20 or so experiments and made observations using the instruments that had been carried up by the 18 guides who accompanied him. He took samples of snow, collected air in glass tubes, gathering as much data as he needed to complete his meticulous study of the color of the sky. Saussure had already found fame as the inventor of the hair tension hygrometer, a device that uses a horse hair or a human hair to measure the relative humidity of the air. (In the process, he discovered that blond hairs tend to be more sensitive than brown ones.) Saussure also invented the diaphanometer, for measuring the clarity of the atmosphere, and the cyanometer, an instrument for judging the blueness of the sky. The first cyanometer consisted of a simple sheet of cardboard that Saussure marked with 16 graduated shades of blue – rendered in watercolor and diluted ink – and an equal number of cut-out windows through which he could see the actual color of the sky. Pointed vertically toward the sky, at high noon, the same day, the device enabled Saussure to take a chromatic reading from the peak of Mont Blanc. He noted a value between 1 and 2, while his son, down the mountain at Chamonix, registered a tone between 5 and 6; and a friend in Geneva marked down a shade value of 7.
Now we know that the blue of the sky is the result of diffraction, the bending of sunlight as it passes through an atmosphere that tends to absorb green, yellow and red rays. We are left with blue, diffused by molecules suspended in the air – without them, the sky would appear black.
And it is the color black, both literally and metaphorically, that may predominate.
After all, the universe, or something like three-quarters of it, is believed to be made of dark energy, that primal energy about which we still know close to nothing aside from its tendency to accelerate the expansion, potentially infinite and eternal, of the great cosmological whole.