Astronomy

Astronomy is the study of objects and phenomena in the universe beyond Earth’s atmosphere. Cosmology is a related science devoted to studying the universe as a whole. There are two main branches of astronomy, observational and theoretical, and several subfields of each, as well as specializations with both observational and theoretical components.

Astronomy is a historical discipline dating back to ancient times. Early civilizations used astronomy to mark the passage of time for both agricultural and religious purposes. Around 3000 BCE, if not earlier, Sumerian astronomers were recording the positions of the stars and identifying constellations in the night sky; the earliest record of a lunar eclipse comes from the Sumerian city-state of Ur in 2094 BCE. What knowledge of Sumerian astronomy still exists was preserved by the Babylonians, who controlled much of Mesopotamia several centuries after the Sumerians and who were the first known astronomers to recognize the periodic nature of certain astronomical phenomena. Other ancient cultures that left important astronomical records include China, Egypt, and Greece; both geocentric and heliocentric models of the universe were first proposed by astronomers of ancient Greece.

Brief History

The foundations of modern astronomy date back to ancient civilizations such as Sumer, Babylon, China, and Egypt. Ancient Babylon was one of the first civilizations to have written records, using the writing system developed by the Sumerians sometime before 3000 BCE, and archaeologists have discovered Babylonian clay tablets dating back as far as the seventeenth century BCE that record astronomical observations, including the movements of the sun, the moon, Mercury, and Venus. Babylonian astronomers kept such careful records of solar and lunar eclipses that they were eventually able to predict lunar eclipses, and later solar eclipses, with some degree of accuracy. The ancient Egyptians observed the night sky and constellations and understood moon cycles; by the thirteenth century BCE, they knew of at least forty-three constellations and five planets, including planetary movements. Ancient Chinese astronomers also recorded solar and lunar eclipses, as well as the movements of the stars. Over the course of 2,600 years, starting around 1300 BCE, they recorded about six hundred lunar and nine hundred solar eclipses.

In later antiquity, ancient Greek astronomers made a concerted effort to apply systematic and rational thought to the study of the cosmos. The Greco-Egyptian astronomer, geographer, writer, and astrologer Ptolemy developed the geocentric model of the universe, also called the Ptolemaic system, which posits that Earth sits at the center of the universe and the sun and moon rotate around it. Aristarchus of Samos calculated the size and distance of the moon and sun; about a century later, Hipparchus compiled a catalog of 1,020 stars.

Overview

There are several different subfields of modern astronomy. These subfields are often divided into two main branches, observational astronomy and theoretical astronomy, although some have elements of both. Observational astronomy is the most familiar type of astronomy, in which celestial bodies and other space phenomena are studied through telescopes and other data-gathering equipment. It can be further subdivided into optical astronomy, which studies objects within the visible wavelengths of electromagnetic radiation, and nonoptical astronomy, which studies wavelengths beyond the visible spectrum. Within nonoptical astronomy are subdisciplines such as radio astronomy, infrared astronomy, ultraviolet astronomy, and x-ray and gamma-ray astronomy, each of which focuses on a specific range of electromagnetic wavelengths.

While observational astronomy is primarily concerned with recording data about astronomical objects, theoretical astronomy, also called theoretical astrophysics, uses the principles, models, and theories of physics and chemistry in order to investigate the nature of those objects. Theorists tackle subjects such as general relativity, astroparticle physics, string cosmology, and cosmic rays. Astronomic fields not based on the electromagnetic spectrum include neutrino astronomy, gravitational wave astronomy, and celestial mechanics.

Observing the Universe

The scale of the universe is vast. Distances in outer space are measured in light-years—the distance light travels in a year, about 9.46 trillion kilometers (5.88 trillion miles)—or in parsecs, units of about 3.26 light-years each. The nearest star to Earth other than the sun is Proxima Centauri, about 4.2 light-years away; a spacecraft traveling at the same speed as the National Aeronautics and Space Administration’s (NASA’s) Juno probe, about 265,000 kilometers (165,000 miles) per hour, would take 17,157 years to reach it. As such, observations of outer space must be made from a great distance.

Modern astronomers use telescopes or remote-sensing apparatuses and probes such as Juno. Until the early seventeenth century, even basic telescopes were not available, and astronomers had to make their observations using only the human eye. While Galileo did not invent the telescope—most scholars credit that achievement to Dutch lens crafter Hans Lippershey—he did improve on the invention, and he was the first person to use one to make observations of the night sky. Over time, telescopes became bigger and more powerful, but they still only operated within the wavelengths of visible light—understandably so, as these are the only wavelengths that the human eye can see. Yet the visible wavelengths represent only a very small fraction of the information to be gleaned by observing outer space. As long as humans could only observe phenomena in the visible spectrum, their understanding of the universe would necessarily be limited. This began to change somewhat in the early nineteenth century, when German-born British astronomer and composer William Herschel discovered the existence of infrared radiation coming from the sun, but true advancement in telescope technology would not come until more than a century later.

In 1931 Karl Jansky, a physicist at Bell Telephone Laboratories, was searching for sources of radio interference when he discovered a mysterious signal he could not identify. After more than a year spent studying the signal, Jansky determined it was coming from outer space, specifically from the direction of the Milky Way galaxy. Although Jansky’s employers did not authorize him to build the equipment necessary to study the radio signal further, another scientist, radio engineer Grote Reber, read about Jansky’s discovery and decided to investigate. In 1937, in his free time and at his own expense, Reber built a parabolic reflecting antenna in his backyard—the first radio telescope. Using this homemade telescope, he made a radio-frequency map of the sky, confirming Jansky’s observations that the intensity of the signal was greatest toward the center of the Milky Way. He also pinpointed the location of several other sources of strong radio emissions for the first time, including Cygnus A, a radio galaxy, and Cassiopeia A, a supernova remnant. Jansky and Reber’s discoveries marked the beginning of radio astronomy as a discipline; new telescopes were subsequently developed to observe other parts of the electromagnetic spectrum.

The Big Bang

The prevailing cosmological model of the universe’s origins is the big bang theory, although the so-called big bang was not an explosion but rather a sudden and massive expansion. Essentially, the theory states that in an instant, all of the matter and energy that make up the universe expanded from a single point of near-infinite density—a gravitational singularity—to a width of at least a hundred billion light-years across, and it has continued expanding to this day, growing gradually less dense.

The idea of the big bang was first proposed in the 1920s by Belgian priest and physicist Georges Lemaître, but it was not really acknowledged in astronomy until after World War II. Until the early twentieth century, astronomers had believed the universe to be static, neither expanding nor contracting. This theory was challenged by two discoveries of the 1920s: Russian physicist Alexander Friedmann’s equations that he derived from Albert Einstein’s theory of general relativity, later known as the Friedmann equations, and Edwin Hubble’s observations that more distant galaxies are moving away from Earth at greater speeds than those closer to the Milky Way. Both the Friedmann equations and Hubble’s observations suggested that the universe is in fact constantly expanding.

A number of new theories were proposed, with two eventually emerging as the most likely: the steady-state theory, championed by English astronomer Fred Hoyle, and the big bang theory, further developed from Lemaître’s ideas by Russian-born American physicist George Gamow. Foyle’s steady-state theory posited that the universe had no beginning and no end, and although it was constantly expanding, its density remained unchanged as a result of the steady production of new matter. This theory was generally favored in the astronomical community until 1965, when radio astronomers Arno Penzias and Robert Wilson, also of Bell Laboratories, were attempting to identify the source of a persistent background noise they had picked up with an extremely sensitive radio telescope. Unlike the radio signal discovered by Jansky, this signal did not seem to have a single source but instead came from everywhere at once, constantly and without variation. Penzias and Wilson had accidentally discovered cosmic microwave background radiation (CMB)—radiation left over from the big bang, the existence of which had been predicted by Gamow’s colleagues Ralph A. Alpher and Robert Herman in 1948. The discovery of CMB was the final piece of evidence necessary to make the big bang theory the dominant theory of the origin of the universe.

The 1990s saw the launch of the Hubble Space Telescope (HST), which began producing high-resolution images of planets, stars, galaxies, and nebulae in 1993. In the twenty-five years following its launch in 1990, the HST made more than 1.2 million observations, including observations of astronomical phenomena that occurred more than 13.4 billion light-years from Earth. In 1998, HST observations of very distant supernovae led to the discovery that the universe was expanding more slowly in the distant past than it is today. These observations challenged the belief that the expansion of the universe would slow due to gravity and provided the first evidence of the existence of dark energy, which is thought to be the dominant component of the universe.

The 1990s also saw the completion of telescopes with unprecedented resolutions, such as the Keck Observatory telescopes, which are the largest optical and infrared telescopes in the world and which began making observations in 1993 and 1996. The Keck telescopes aided in the detection and observation of extrasolar planets (also called exoplanets).

The Laser Interferometer Gravitational-Wave Observatory (LIGO), located in Livingston, Louisiana, and the Hanford Site, Washington, began operation in 2002 with a goal to detect cosmic gravitational waves. In September 2015, the project detected evidence of gravitational waves, a discovery that was announced in February 2016.

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