October 13

A Whirlpool Among the Stars

1773 — Charles Messier Discovers the Whirlpool Galaxy

Observation time not established

On October 13, 1773, French astronomer Charles Messier was searching the sky when he noticed a faint patch of light in the constellation Canes Venatici. It did not look like a star. Through his telescope, it appeared only as a dim, hazy object. Messier described it as a very faint nebula without stars. It would eventually become the 51st object in his famous catalog. Today we call it Messier 51—the Whirlpool Galaxy. And it is one of the most recognizable galaxies in the night sky. Messier could not possibly have understood what he had found. He was not primarily searching for distant galaxies. He was hunting comets. Comets could appear as faint fuzzy patches in eighteenth-century telescopes. Unfortunately for comet hunters, the sky contained many other objects that looked similarly hazy but remained fixed among the stars. Messier began recording these troublesome objects so that he would not repeatedly mistake them for comets. The resulting Messier Catalogue became one of astronomy's most enduring observing lists. Its objects include star clusters, nebulae, supernova remnants, and galaxies. But those categories were not yet understood as we understand them today. When Messier discovered M51, even the existence of galaxies beyond the Milky Way was unknown. To him, it was a nebula. Eight years later, in 1781, Messier's colleague Pierre Méchain identified a second nebulous object close beside M51. We now know it as NGC 5195, a smaller galaxy interacting gravitationally with the much larger Whirlpool Galaxy. But another major discovery remained. In 1845, William Parsons, the Third Earl of Rosse, turned his enormous 72-inch reflecting telescope—the “Leviathan of Parsonstown”—toward M51. Rosse saw something Messier's smaller telescope could not reveal. The nebula had a spiral structure. His drawing showed curling arms extending around a central region. M51 became the first object recognized as what astronomers then called a spiral nebula. That discovery created another mystery. What were spiral nebulae? Some astronomers thought they might be planetary systems forming within the Milky Way. Others suspected they might be enormous systems of stars at immense distances. The argument became one of the great astronomical questions of the nineteenth and early twentieth centuries. Only in the 1920s did observations of Cepheid variable stars in the Andromeda “nebula” demonstrate convincingly that objects such as Andromeda lay far beyond the Milky Way. The universe suddenly became vastly larger. Spiral nebulae were not clouds inside our galaxy. Many were galaxies themselves. Messier's faint smudge was therefore an enormous island of stars. M51 lies roughly 31 million light-years from Earth. Its graceful spiral arms contain vast lanes of gas and dust, clusters of young stars, and regions of active star formation. Its smaller companion, NGC 5195, is not merely nearby by coincidence. The two galaxies are interacting gravitationally. That interaction helps explain why the Whirlpool's spiral structure is so striking. As NGC 5195 passes near M51, its gravitational influence disturbs the larger galaxy. Gas within the spiral arms becomes compressed. Compressed clouds can collapse. New stars are born. The beautiful spiral structure is therefore not simply decorative. It records physical processes occurring on a galactic scale. Modern observatories examine M51 across the electromagnetic spectrum. Visible-light images reveal stars and dark dust lanes. Infrared observations penetrate dusty regions and trace material associated with star formation. Ultraviolet observations highlight populations of hot young stars. X-ray telescopes reveal energetic phenomena, including compact objects such as neutron stars and black holes in binary systems. At the center of M51 lies a supermassive black hole. The object that appeared to Messier as a faint blur has become an astrophysical laboratory. And its story illustrates something remarkable about scientific discovery. Messier found M51 for one reason. Astronomers value it today for entirely different reasons. He wanted to know: Is this a comet? The answer was no. But the object he cataloged to prevent future confusion became far more interesting than the comet he was seeking.

Why It Matters: Charles Messier's discovery of M51 added what would later become one of astronomy's most important galaxies to the observational record. M51 subsequently became the first nebula recognized as having spiral structure and, after astronomers learned that spiral nebulae are galaxies beyond the Milky Way, became a classic laboratory for studying galactic structure, gravitational interaction, and star formation. Its history demonstrates how an observation made for one purpose can acquire entirely new significance as scientific understanding advances.

 

Also on This Day

1994 — Magellan Ends Its Exploration of Venus

Contact lost after 10:05 UTC

On October 13, 1994, a spacecraft that had transformed humanity's knowledge of Venus entered the planet's atmosphere for the last time. Its destruction was deliberate. NASA's Magellan spacecraft had accomplished its principal mission. Now engineers intended to learn something more from its final moments. Venus presents an unusual challenge to anyone attempting to map it. Its surface is permanently concealed beneath thick clouds. Ordinary cameras in orbit cannot simply photograph the landscape below. Magellan therefore explored Venus primarily with radar. Radio waves could penetrate the clouds, reflect from the surface, and return information from which scientists could reconstruct the terrain. Magellan had launched aboard Space Shuttle Atlantis in May 1989 and entered orbit around Venus on August 10, 1990. The spacecraft then began systematically mapping the planet. What emerged was a world of extraordinary geological complexity. Magellan ultimately mapped about 98 percent of Venus's surface with radar. The images revealed enormous volcanic plains. There were great shield volcanoes. There were long lava channels. There were unusual circular volcanic features. There were broad highlands and mountain belts. There were distinctive volcanic structures sometimes called pancake domes because of their wide, flattened appearance. Impact craters were present, but their distribution indicated that the visible surface was geologically much younger than the solar system itself. The images transformed Venus from a world hidden beneath clouds into a world whose landscapes could be studied almost globally. Magellan also measured the planet's gravity field. Gravity measurements provide another way of examining a planet. Small variations in gravitational pull can reveal differences in the distribution of mass below the surface. By comparing topography with gravity, scientists can investigate the structure of the crust and mantle and ask how mountains, volcanic regions, and other large features are supported. To improve those measurements, engineers attempted something bold. They used the atmosphere of Venus itself to change Magellan's orbit. The technique is called aerobraking. Instead of using large quantities of rocket propellant to reshape an orbit, a spacecraft can repeatedly dip into the upper atmosphere. Atmospheric drag slows the spacecraft slightly on each pass. Over time, its orbit changes. In 1993, Magellan became the first planetary spacecraft to demonstrate aerobraking extensively as a means of changing its orbit. The technique subsequently became important for missions to Mars and elsewhere because it can reduce the amount of propellant a spacecraft must carry. By 1994, Magellan had vastly exceeded its original objectives. Mission controllers decided to use its final days for one more experiment. The spacecraft was commanded deeper into Venus's upper atmosphere. Its solar panels were deliberately oriented in opposite directions, somewhat like the blades of a windmill. By monitoring how the spacecraft responded to aerodynamic forces, engineers could learn about the atmosphere and about spacecraft behavior during atmospheric flight. Eventually, Magellan descended too deeply to survive. NASA's later mission chronology records that contact was lost after 10:05 UTC on October 13. The spacecraft continued descending and was destroyed in Venus's atmosphere roughly ten hours later. There would be no landing. Magellan had never been designed to survive the crushing pressures and furnace-like temperatures of the Venusian surface. Its final destination was the atmosphere of the world it had spent four years revealing. By the time it disappeared, Magellan had completed more than 15,000 orbits of Venus. NASA later calculated that the spacecraft had returned about 1,200 gigabits of data—more data than all previous NASA planetary missions combined up to that time. Its maps became fundamental reference works for the study of Venus. And decades later, scientists continue returning to Magellan's observations with new techniques and new questions. The spacecraft is gone. Its scientific life is not.

Why It Matters: Magellan transformed Venus from a planet largely concealed by clouds into a mapped geological world. Its radar observations covered about 98 percent of the surface, its gravity measurements provided clues to the planet's interior, and its pioneering use of aerobraking demonstrated a technique later employed by other planetary missions. Even its deliberately destructive finale was turned into an experiment.

 

The Bigger Picture

October 13 brings together two objects that human eyes could not readily understand. One was hidden by distance. The other was hidden by clouds. When Charles Messier looked toward M51 in 1773, his telescope showed only a faint patch. The galaxy was there in all its enormous complexity—spiral arms, star-forming regions, dust clouds, stellar populations, and a central black hole. Messier simply lacked the technology and scientific framework necessary to see what the object truly was. Venus presented a different problem. By the twentieth century, astronomers knew exactly where the planet was. It was nearby in astronomical terms. But its surface remained concealed beneath an opaque atmosphere. Again, technology changed what could be known. For M51, increasingly powerful telescopes gradually revealed structure. Lord Rosse saw its spiral form. Photography recorded details the eye could miss. Spectroscopy revealed motion and composition. Twentieth-century astronomy established that it was a separate galaxy. Space telescopes eventually examined it at wavelengths inaccessible from Earth's surface. For Venus, the breakthrough was radar. Magellan effectively looked through the clouds. The connection between the two events is therefore deeper than their difference in scale. Science is often limited not by the absence of information but by our inability to retrieve it. Sometimes the information has traveled millions of light-years and arrives as faint electromagnetic radiation. Sometimes the information lies beneath a nearby planet's clouds. The task is to devise an instrument capable of extracting it. There is another connection. Neither Messier nor Magellan reached the end of the story. Messier did not know M51 was a galaxy. Magellan did not answer every question about Venus. Science does not usually produce a final map and declare a world understood. Each improvement in observation reveals new questions. What drives M51's star formation? How exactly has its encounter with NGC 5195 altered both galaxies? Why did Venus develop so differently from Earth? How active is Venus today? Did it once possess oceans? Could its geological history help us understand why apparently similar terrestrial planets evolve in radically different directions? Those questions require new observations. October 13 therefore reminds us that exploration does not always mean physically standing somewhere. Sometimes exploration means finding a way to see what nature has hidden.

At a Glance

1773 — Observation time not established — Charles Messier discovers the object later cataloged as M51, the Whirlpool Galaxy, while observing the sky for comets.

1994 — After 10:05 UTC — NASA loses contact with Magellan during the spacecraft's deliberately commanded final descent into the atmosphere of Venus, concluding a mission that radar-mapped about 98 percent of the planet's cloud-hidden surface.