Stardust We Are. A Short History of Astronomy

Humanity has stared at the night sky for ages trying to comprehend the cosmos. From the very first celestial models to modern astrophysics, spatial exploration, and multi-messenger astronomy, every stage has altered our conception of the Universe and of our place within it. Along the way, Astronomy has played a crucial part in the development of science and technology while benefiting from their findings. And, as any human endeavor, it has always been subject to the ebb and flow of politics, society and religion, even war.

The first astronomical observations measured the motion and position of astronomical objects to anticipate the changing of the seasons and to predict phenomena such as eclipses. The geocentric model of the universe—with the Earth as its core—prevailed until the 16^th century, when Nicolaus Copernicus reformulated the heliocentric model—with the Sun at the center—that Aristarchus had proposed in the third century BCE. 

The Copernican Revolution was supported by the invention of the telescope and Galileo Galilei’s observations of Jupiter’s moons and of the phases of Venus in 1609, and it enabled the dismantling of the geocentric model. Almost at the same time, Johannes Kepler developed his Laws of Planetary Motion within the heliocentric model. These could be explained with Isaac Newton’s later Law of Universal Gravitation, which were then applied and validated through the calculations of the orbit of the Great Comet of 1680. Despite what Aristotle postulated, terrestrial and celestial physics were indeed one. 

A century later, in 1781, William Herschel discovered Uranus—the first planet beyond Saturn––which enlarged the known limits of the Solar System. Herschel was also the first to explain the motions of a binary star, which established that the laws of physics were equally valid outside the Solar System. The Earth finally stopped being the center of the Solar System in the 1830s when Thomas Henderson, Friedrich Bessel, and Friedrich von Struve, respectively, measured the parallax of a star0—the change of its apparent position when observed from opposite spots of the terrestrial orbit. Given that the inverse of the parallax provides the distance to these stars—a few dozens of light-years—the scale of the known universe grew to around 30 thousand times the size of the Solar System. 

At the same time, our understanding of light deepened significantly. Newton proved that white light can be split into a spectrum of colors. Herschel discovered infrared light, the first kind that is outside of the optical range that humans can perceive with our naked eyes—thus opening a whole new world of possibilities to see the Universe. Joseph Fraunhofer developed the diffraction grating, invented the spectroscope and discovered absorption lines in the Sun’s and other stars’ spectra. James Clerk Maxwell’s equations explained the wave-like nature of light in the 1860s, allowing us to understand phenomena like interference, diffraction, and polarization. A bit later, Dr. Henry Draper—an amateur astronomer—developed astrophotography and stellar photographic spectroscopy along with his wife, Mary Anna Palmer Draper. 

Astronomy evolved away from being mainly positional and started concerning itself with the physical properties of astronomical objects. Women astronomers at Harvard College made fundamental contributions (see Omaira González [texto 09], pp. XX in this number). Beginning in 1896, Annie Jump Cannon classified 350 thousand stellar spectra. Henrietta Leavitt discovered the relation between the pulsation period and the intrinsic brightness of the stars known as Cepheids. These “standard candles” would later allow us to measure distances to other galaxies. Cecilia Payne used the spectra from the Henry Draper Catalogue and Meghnad Saha’s equation to prove that stars are mostly made of hydrogen and established the fact that differences in their spectra are due to different temperatures and not because of their chemical composition. This set the groundwork for stellar astrophysics that led to Arthur Eddington’s explanation for nuclear fusion as the energy source for stars in 1926, as well as to the realization in 1957 by Margaret and Geoff Burbidge, William Fowler, and Fred Hoyle that elements heavier than hydrogen and helium are produced through reactions in stellar cores: we are stardust. 

Based on the relation found by Leavitt in 1924, Edwin Hubble derived the distance to Andromeda and confirmed it was another galaxy, and not a nebula within our own Milky Way. Our galaxy, then, is not the center of the Universe either, whose size at that moment was already 200 thousand times larger than the distance measured a century earlier to the closest stars. In 1929, Hubble discovered that galaxies race apart from one another at increasing speeds, accelerating while getting further frome one another. Based on the Einstein field equations, Georges Lemaître proposed in 1931 that this happens because of the expansion of the Universe. Thus, in reverse, this must have originated in a “Big Bang”. The static universe was replaced by a dynamic and evolutive cosmos. 

In the 1930s, Fritz Zwicky noticed that the galaxies in clusters move at rates of speed so high that the inferred mass from the light of its stars would not be able to contain them. This led him to hypothesize the existence of “dark matter”, an idea reinforced in the 1970s by Vera Rubin, who found that stars in spiral galaxy’s discs also move at very high speed. Adam Riess, Saul Perlmutter and Brian Schmidt made a surprising finding in 1998—the expansion rate of the Universe had accelerated around five thousand million years ago—eight thousand million years after the Big Bang—, which is attributed to the action of the so called “dark energy”, opposed to gravity and now considered the most abundant component of the Universe. This “dark sector”, which includes both dark energy and dark matter, is frequently invoked and appears to represent 90 percent of the Universe, but, however, remains undetected directly. 

Karl Jansky detected radio waves coming from the center of the Milky Way in 1932. Radioastronomy revealed a universe invisible for optical telescopes and led to the discovery of quasars in 1963—at least a thousand times farther than Andromeda—as well as the Cosmic Microwave Background (CMB) radiation, the remnant glow of the Big Bang. A race started here to study the distant universe, which because of the finite speed of light, is also the early universe. 

In the mid-20^th century, the Space Age began in the midst of the Cold War and allowed us to transcend the Earth’s atmosphere to make observations in every region of the electromagnetic spectrum. Space telescopes revolutionized observational astronomy and ushered in the era of precision cosmology. The Hubble Space Telescope—launched in 1990—provided us with unprecedented views of deep space and cosmic evolution. Other missions like COBE, WMAP, and Planck mapped the CMB with exquisite detail. High-energy telescopes like Chandra and Fermi revealed the extreme physics of black holes, neutron stars, and gamma ray bursts. On another highly important note for modern astrophysics and our place within the Universe, Michel Mayor and Didier Queloz discovered the first extrasolar planet around a Sun-like star—51 Pegasi. Today, thousands of these are known. 

The 21^st century marked the beginning of multi-messenger astronomy in which cosmic phenomena are studied through various types of signals: light, neutrinos, cosmic rays, and gravitational waves. Neutrino observatories like IceCube and Super-Kamiokande have provided information about supernovas and the solar core. The detection of gravitational waves by LIGO and Virgo in 2015 confirmed Einstein’s century-old prediction, offering a new way to study cosmic mergers of black holes and neutron stars. Meanwhile, the images obtained with the James Webb Space Telescope are enabling the study of the first stars and the first stages in the evolution of galaxies. 
From ancient celestial maps to modern multi-messenger astronomy, our understanding of the Universe has been shaped by paradigm shifts and technological developments. Every new finding in the future will continue to expand our cosmic horizon and will help reveal a much more dynamic and complex Universe than we ever imagined. 

---