Top 10 Most Important Astronomers - Listverse Top 10 Most Important Astronomers Eduardo Jaramillo September 8, 2011 Since the very dawn of humankind, our species has looked out upon the cosmos in awe of the glory of the heavens above. While the spectacular views offered by our night sky haven’t changed a great deal over the eons, our understanding of them has. Knowledge of our universe has gradually expanded over the last few thousand years thanks to the efforts of astronomers through the ages. We know what we do about the universe today thanks to the discoveries of all of these astronomers, the ten most important of whom are listed here. 10 Charles Messier Charles Messier was a French astronomer who was obsessed with discovering and studying comets and their orbits. However, his search for the elusive comets ended up leading him to create one of the most famous catalogues of deep sky objects. He realized these deep sky objects could distract other comet-chasers, and so he decided to distinguish them as immobile objects in the night sky.

The resulting catalogue, published in 1774 when the astronomer was 44 years old, contains over one hundred deep sky objects, including nebulae and galaxies. Being among the most beautiful objects in the night sky, you most likely have seen images of many of them taken by high aperture telescopes, such as the Hubble Space Telescope. Along with being among the first to catalogue these wonderful objects, Messier was also successful in discovering thirteen comets. 9 Ptolemy Ptolemy was an Egyptian astronomer, geographer and mathematician of Greek ancestry. He lived from 90 to 168 AD, and was the last great astronomer before the known world’s descent into the Dark Ages. He is important for preserving the Greek astronomer Hipparchus’ star catalogue, which he included in his astronomical magnum opus, Amalgest. The Amalgest was the most important astronomical text in existence for close to fifteen hundred years after the death of Ptolemy, and because of this, its author gained a near legendary status.

He also included in his work a set of tables (known almost comically as the Handy Tables), which made it much easier to calculate the positions of the planets, the sun and the moon, the rising and setting of the stars, and the dates of lunar and solar eclipses. Without Ptolemy’s preservation of Hipparchus’ work, and his own advances in the field of astronomy, much of the astronomical work done during the Renaissance and the Scientific Revolution may not have been possible. 8 Tycho Brahe Tycho Brahe, whose defining physical characteristic was no doubt his metallic nose (he lost his real one in a duel), was a famed Danish astronomer. Up until his observations, which occurred largely in the late 1500s, no other astronomer had tallied as many, or as accurate of observations as Brahe. He catalogued hundreds of objects, and aspired to a level of accuracy such that each star was catalogued within one arc-minute of its real celestial location. To put that into perspective, hold your little finger at arms length – it is 1 degree in width. Now divide it into 60 parts; take one of those parts, and you have an arc-minute.

While he didn’t always achieve this level of accuracy, the fact that he set the standard so high with the technology available at that point in history is commendable. Brahe is probably best known for his work with new stars, or novae, which in his day was cutting-edge astronomy. In 1572, he observed a bright star that appeared to be a newcomer onto the celestial scene. While some argued that this was an atmospheric phenomenon, Brahe showed through use of parallax that the object was much too far away to be inside the Earth’s atmosphere. Although other supernova events had been observed in the past, Brahe was the first to scientifically observe and acknowledge one. This was a strong argument against the, then, predominant belief that the heavens were fixed on colossal dome-like structures, the idea of which Brahe openly contradicted. 7 Arno Penzias and Robert Wilson These two astronomers come in one package, because their main contribution to the astronomical field was a mutual effort. This important contribution was the discovery of the Cosmic Microwave Background Radiation or CMB. Basically, the CMB was an aftershock of the explosive birth of the Universe – the Big Bang.

This aftershock had been theorized before its discovery by Penzias and Wilson in the 1960s, but it’s exact value hadn’t been pinpointed until the two scientists began experimenting with the Holmdel Horn Antenna at Bell Labs. As they experimented, they realized they had an ever-present background radiation in their data, and after cleaning their equipment of pigeon droppings, they deduced that the radiation was not coming from anywhere on Earth – or even in the galaxy – but outside the Milky Way… It wasn’t until later that the two realized that their discovery had any significance, when it dawned on them that they had discovered the elusive aftershock of the Big Bang. In 1978, Penzias and Wilson were awarded the Nobel Prize for their discovery. The significance of the discovery lies in the fact that at that time there was still a good deal of dispute over whether or not the Big Bang had even occurred. The main opposing theory, known as the Steady State theory, was virtually abandoned by astronomers following the momentous discovery.

Some important outcomes of the discovery include evidence for the inflationary model of the universe, the suggestion of a Dark Age of the Universe, advancements in interferometry and countless other repercussions in the astronomical field. 6 Nicolaus Copernicus Copernicus was a European scientist born in 1473, and one of the most important of the Renaissance. He is extremely significant because he is credited as the first astronomer to put forward a comprehensive heliocentric version of the solar system. Although some ancient astronomers had pondered a heliocentric theory, their works had either been lost to the ages or largely ignored. However, in 1543 when his book, On the Revolutions of the Celestial Spheres, was published, the theory could simply not be brushed aside any longer. The idea that the Earth revolved around the sun (and not vice versa) went directly against the teachings of the church, and this publication was in a time when the church controlled most of society. Although Copernicus died in the year of his great work’s publication, he still no doubt feared persecution from religious authorities and realized that even after death his name and the reputation of his work could be sullied.

Interestingly enough, On the Revolutions of the Celestial Spheres is actually dedicated to the Pope at the time, Pope Paul III, almost definitely to avoid disfavor with the church. For his courageous introduction of the true nature of the solar system to European scholars he is remembered as a monument to the truth in a society largely unwilling to accept it. 5 William Herschel William Herschel was an English astronomer born in Germany, in 1738. Interestingly enough, he was a talented musician (he composed twenty-four symphonies), and as a youngster, his passion for music led him into mathematics. This interest in math eventually led him to astronomy, the field in which he is most famous for working. A fascinating side-note to his astronomical career was the fact that Herschel built his own reflecting telescopes. He used his self-made telescopes to observe binary systems of stars, in which two stars orbit around a common center of gravity in a bound system. These were important star systems because many believed that distances to them could be more easily discovered than single star systems, and because other information on the nature of stars could be gleaned from these binary stars. Herschel is credited with discovering over eight hundred of these binary systems.

However, his massive observational output didn’t stop there– Herschel also discovered over twenty-four hundred deep sky objects that he termed nebulae. Herschel had a good deal of other accomplishments as well. His observations led him to the discoveries of the planet Uranus, two of its moons, as well as two moons of Saturn. He also was the first to realize that the solar system was moving through space, and was able to estimate as to the direction of the movement. His most accidental discovery however, was that of infrared radiation– while experimenting with dividing light through a prism, he measured the temperature of the air next to the red in the rainbow of light produced. Where he expected a lower reading than that of the visible light, he saw a spike on the thermometer. He deduced that there must be some sort of invisible radiation just outside that of the color red– or infrared radiation! 4 Johannes Kepler Kepler was a German astronomer and was the first to fully explain the motion of the planets of our solar system. He described their motion with three laws, which he published in 1609 AD.

What allowed him to basically unlock the mystery was to imagine the planets as having elliptical orbits rather than circular ones, which is how other astronomers would view the solar system. In fact, his first law of planetary motion is the simple statement that planets travel in ellipses. Like Copernicus, Kepler firmly believed in a heliocentric solar system. However, the church was still very opposed to the idea when he was alive. Despite this, Kepler championed the idea like no other astronomer had and brought it to the forefront of the scientific revolution. Interestingly enough, Kepler was himself a very religious man– he had planned on becoming a priest before ultimately deciding on pursuing science. Kepler also happened to work quite closely with Tycho Brahe, although their relationship is known to have been quite strained– Brahe was most likely afraid of being shown-up by his assistant. This is basically what happened when Kepler discovered the laws of planetary motion. However, Kepler also made other important discoveries. He was the first to explain how the moon influenced tides for example, and he also influenced mathematics by forming some of the groundwork for integral calculus.

3 Edwin Hubble Edwin Hubble, who did most of his major work in the first half of the twentieth century, had perhaps the most momentous discovery of all of the astronomers on this list. Hubble is credited with discovering galaxies outside of our own Milky Way. Although the race to solve this mystery had contributions from many different scientists, it was Hubble’s observations through the Hooker telescope, around the year 1923, that proved to the scientific community that there was more to outer space than the Milky Way. In essence, with one finding, Hubble ballooned the Universe from a galaxy of about a hundred thousand light years across, with approximately one hundred billion stars, to an indefinite expanse of intergalactic space, billions of light years across, and with a seemingly infinite amount of stars. However, Hubble didn’t stop there. He showed that these separate galaxies were moving away from each other by observing there redshifts, an effect caused by light being stretched out over vast distances. His observations led him to the discovery that the farther away a galaxy was from the Milky Way, the faster it was moving away from us. This is known as Hubble’s Law.

A cool side-note is that Einstein himself thanked Hubble personally for making this discovery, because it verified some of the work that Einstein had done years before that hadn’t been fully accepted. 2 Hipparchus Hipparchus, an astronomer dating back to ancient Greece, comes in at a very close second on this list. Widely believed to be the greatest astronomer of antiquity, Hipparchus can easily be viewed as a sort of founding father of astronomy. His most important contribution to the field was the first known star catalogue, which historians think he was inspired to construct after viewing a supernova. This star catalogue was later used extensively by Ptolemy in his astronomical observations. His other contributions include important findings on the positions and motions of the moon and Sun. Using trigonometry – a subject for which he is basically credited for inventing – he was able to measure the distance to the moon during a solar eclipse. He is also known for creating the method by which a star’s brightness is measured, a system still in use today. 1 Galileo Galilei The most important astronomer of all time turns out to be the Italian spearhead of the Scientific Revolution, Galileo. Galileo was, in a sense, a lucky astronomer.

To put it simply, he was fortunate to be alive when the telescope was invented (around 1607 AD). He caught wind of this amazing new device, and quickly made his own refracting telescope. This gave him absolutely unprecedented access to information on the heavens– and he was the first to capitalize on it. While military leaders across Europe were using the spyglasses to watch their enemies at sea, Galileo turned his telescope to the sky and discovered secrets that had lay waiting for millennia. Because Galileo lived and worked at such an opportune time, he is considered by most to be the father of modern observational astronomy (not to mention the father of modern physics). Many aspects of his life lend themselves to this title. He was the first to lay eyes on the Rings of Saturn (though they looked more like handles from his perspective), and he also discovered and named various moons of Jupiter. He was also the first to observe sunspots, which was rather significant, because it was then believed by the church that the sun was perfect and without blemishes of any kind. Probably what Galileo is most well known for is his staunch defense of the idea of a heliocentric solar system, regardless of the religious persecution he was subjected to.

Unlike Copernicus however, he was able to provide hard evidence for the fact. One of his famous experiments included observing the planet Venus over a certain period, and recording the various phases it would go through (with a telescope, one can see that Venus goes through phases, not unlike our moon). Galileo used his data and made certain calculations, and was able to show that due to Venus’s phases, it had to orbit the sun. Despite this and other pieces of amazing science however, the church still argued that the sun orbited the Earth. Galileo was eventually placed under house arrest for his heretical views, and lived out the last eight years of his life in his villa near Florence. Honorable Mentions: Subrahmanyan Chandrasekhar, Isaac Newton, Aristarchus, Eratosthanes. More Great Lists Edwin Powell Hubble: Biography Edwin Powell Hubble: Biography By Nola Taylor Redd, Space.com Contributor | May 12, 2012 11:41am ET MORE Often lauded as the father of modern cosmology, Edwin Powell Hubble made several significant discoveries that changed how scientists viewed the universe.

Born in 1889, Hubble began his professional life as a lawyer, but returned to school after only a few years to obtain a doctorate in astronomy. [See also our overview of Famous Astronomers and great scientists from many fields who have contributed to the rich history of discoveries in astronomy .] Here is a brief biography of Hubble: Edwin Hubble, credited with the discovery of the expansion of the universe, at the Mount Wilson Observatory Credit: : Mt. Wilson Archive, Carnegie Institution of Washington After graduation, Hubble was invited to work at the Mount Wilson Observatory in California, but had to delay his acceptance while he served as a soldier in World War I. Upon return to civilian life, he took a position at the observatory, where he was able to work with the two largest telescopes in the world, the 60-inch and 100-inch Hooker reflectors. He left the observatory again in 1942 to serve in World War II, where he was awarded the Medal of Merit.

Before his death in 1953, at the age of 63, he oversaw the construction of the 200-inch Hale Telescope on Palomar Mountain, which would be the largest telescope on Earth until the Russian BTA-6 was built in 1976. Galaxies outside the Milky Way In the 1920s, the small, diffuse patches in the sky were termed nebulae, and were thought to exist within the Milky Way. While examining images of NGC 6822, M33, and M31 individually, Hubble noticed a pulsating star known as a Cepheid variable inside each one. Cepheids are special because their pulsation allows for precise measurements of distance. Hubble calculated how far away each Cepheid lay — and thus how far to each nebula — and realized they were too distant to be inside of the Milky Way. Astronomers realized that these nebulae were in fact galaxies like the Milky Way, each containing billions of stars. The universe, once thought to be contained by the Milky Way, expanded significantly in the eyes of astronomers.

Around the same time, Hubble published a standard classification system to use for the galaxies. At the time, a descriptive system existed, and two other systems were proposed soon after, but they were insufficient. Hubble's clear method of organizing the various classes focuses on three galactic types: ellipticals, spiral and barred spirals, and irregulars. Known as the “tuning fork” diagram (due to its resemblance to that musical piece), the method organizes ellipticals by their ellipticity (how stretched out they are from a perfect circle), while spirals and barred spirals become less tightly wound as they progress. Hubble originally thought that galaxies evolved from ellipticals to spirals, but scientists now know that each galaxy's shape is determined in its early life. The expanding universe In studying the various galaxies, Hubble was able to determine that they did not sit stationary in space. Instead, virtually every galaxy seems to be rushing away from the Earth (the Andromeda Galaxy is instead rushing towards us and will collide with the Milky Way in about five billion years).

Astronomers rushed to test his calculations on other galaxies, and found that some were moving as quickly as 90 million mph (40,000 kilometers per second) in the opposite direction. The calculation to determine the rate at which the universe is expanding is known as Hubble's law, though it was originally proposed by Georges Lemaître in 1927. According to the calculation, the universe is expanding at a constant rate, known as the Hubble constant. Over a decade before Hubble published his work, renowned scientist Albert Einstein 's theory of General Relativity called for an expanding universe. But Einstein removed the equations because they contradicted the evidence of the day. After Hubble had shown the universe was, in fact, expanding, Einstein visited him at Mount Wilson, calling his decision to change the equations, "the greatest blunder of my life." A lifetime contribution Though Hubble is most well known for these major discoveries, he also made a number of other contributions to the field of astronomy, and numerous awards. But he never received the Nobel Prize, despite his role in improving the existing understanding of the universe. During his lifetime, astronomy was considered a field of physics for the world-renowned Nobel Prize.

Hubble labored in vain for a change that would allow astronomers such as himself to be recognized. Unfortunately, it didn't happen until 1953, the year Hubble died. Since the Nobel Prize cannot be awarded posthumously, Hubble was ineligible. In 1990, 101 years after Hubble's death, NASA launched the Hubble Space Telescope into orbit around the Earth. The telescope, named for Edwin Hubble, has provided a wealth of information about the cosmos, transmitting hundreds of thousands of images to scientists on Earth. It has allowed for more precise calculations of the age of the universe, shown galaxies in all stages of the universe, and played a key role in the discovery of dark energy, the force causing the universe to expand. [ Celestial Photos: Hubble Space Telescope's Latest Cosmic Views ] — Nola Taylor Redd Discovery: Other galaxies exist Hooker 100-inch Reflector 1923: Other galaxies exist In the early 1900s, astronomers were debating the makeup of spiral nebulae — cloudy, spiral-shaped objects found throughout the night sky.

Were they gas clouds located within our Milky Way galaxy, or were they vast groups of stars located far beyond our galaxy? Get to the root of it Understanding the size of the universe interactive In 1919, American astronomer Edwin Hubble tackled the question. His keen astronomical knowledge was combined with a powerful tool – the Hooker telescope with its 100-inch mirror, on top of Mount Wilson in California. Hubble used the telescope’s resolution and light-gathering power to take a series of photographs of the great nebula in Andromeda. For the first time, the images revealed faint stars in the nebula. Hubble now knew the Andromeda nebula was a collection of stars, but how far away was it? To find out, he used a known method for calculating distance based on very bright variable stars . A galaxy far, far away Enlarge image In 1923, Hubble found dozens of these variable stars in Andromeda, and determined their distance. He calculated that Andromeda must be at least 10 times farther away than the farthest stars in the Milky Way. The Andromeda nebula was really the Andromeda galaxy.

This discovery implied that the other, even fainter, spirals were probably also galaxies even farther away. Hubble published his work in 1929 and changed forever our view of the universe. Astronomers no longer thought our galaxy was the entire universe. Now they knew that the universe was composed of many, many galaxies. Seven decades later, a telescope named in Hubble’s honor helped discover that the observable universe was truly vast, and contained nearly 100 billion galaxies. Famous Astronomers | List of Great Scientists in Astronomy Famous Astronomers | List of Great Scientists in Astronomy By Nola Taylor Redd, Space.com Contributor | June 11, 2012 10:07am ET MORE Throughout human history, scientists have struggled to understand what they see in the universe. Famous astronomers — many of them great scientists who mastered many fields — explained the heavens with varying degrees of accuracy. Over the centuries, a geocentric view of the universe — with Earth at the center of everything — gave way to the proper understanding we have today of an expanding universe in which our galaxy is but one of billions.

On this list are some of the most famous scientists from the early days of astronomy through the modern era, and a summary of some of their achievements. Any list of famous astronomers has to include a varied collection of great scientists from the Greeks to the modern era, big thinkers who tackled many fields as well as modern astronomers who made significant discoveries and helped popularize astronomy. Credit: Karl Tate, SPACE.com The Ptolemaic geocentric model of the universe, devised by the Greek scientist Claudius Ptolemy, had everything revolving around Earth. Credit: Bartolomeu Velho, Public Domain When most people believed the world was flat, the notable Greek mathematician, astronomer and geographer Eratosthenes (276 BCE- 195 BCE) used the sun to measure the size of the round Earth. His measurement of 24,660 miles (39,690 kilometers) was only 211 miles (340 km) off the true measurement.

The ancient Greek astronomer and mathematician Claudius Ptolemy (AD 90- 168) set up a model of the solar system in which the sun, stars, and other planets revolved around Earth. Known as the Ptolemaic system, it remained in place for hundreds of years, though it turned out to be flat wrong. Persian astronomer Abd al-Rahman al-Sufi (903-986), known as Azophi to Westerners, made the first known observation of a group of stars outside of the Milky Way, the Andromeda galaxy. Nicolaus Copernicus Credit: Public Domain In 16th century Poland, astronomer Nicolaus Copernicus (1473-1543) proposed a model of the solar system that involved the Earth revolving around the sun. The model wasn't completely correct, as astronomers of the time struggled with the backwards path Mars sometimes took, but it eventually changed the way many scientists viewed the solar system.

Using detailed measurements of the path of planets kept by Danish astronomer Tycho Brahe , Johannes Kepler (1571-1630) determined that planets traveled around the sun not in circles but in ellipses. In so doing, he calculated three laws involving the motions of planets that astronomers still use in calculations today. Galileo Galilei is credited with discovering the first four moons of Jupiter. Credit: NASA Born in Italy, Galileo Galilei (1564-1642) is often credited with the creation of the optical telescope, though in truth he improved on existing models. The astronomer (also mathematician, physicist and philosopher) turned the new observational tool toward the heavens, where he discovered the four primary moons of Jupiter (now known as the Galilean moons), as well as the rings of Saturn. Though a model of the Earth circling the sun was first proposed by Copernicus, it took some time before it became widely accepted. Galileo is most widely known for defending the idea several years after Kepler had already calculated the path of planets, and Galileo wound up under house arrest at the end of his lifetime because of it.

Dutch scientist Christiaan Huygens (1629-1695) proposed the earliest theory about the nature of light, a phenomena that puzzled scientists for hundreds of years. His improvements on the telescope allowed him to make the first observations of Saturn's rings and to discover its moon, Titan. Building on the work of those who had gone before him, English astronomer Sir Isaac Newton (1643-1727) is most famous for his work on forces, specifically gravity. He calculated three laws describing the motion of forces between objects, known today as Newton's laws. Famed physicist Albert Einstein at the blackboard. Credit: NASA In the early 20th century, German physicist Albert Einstein (1879-1955) became of of the most famous scientists ever after proposing a new way of looking at the universe that went beyond current understanding. Einstein suggested that the laws of physics are the same throughout the universe, that the speed of light in a vacuum is constant, and that space and time are linked in an entity known as space-time, which is distorted by gravity.

At the same time Einstein was expanding man's view of the universe, American astronomer Edwin Hubble (1899-1953) calculated that a small blob in the sky existed outside of the Milky Way. Prior to his observations, the discussion over the size of the universe was divided as to whether or not only a single galaxy existed. Hubble went on to determine that the universe itself was expanding, a calculation which later came to be known as Hubble's law. Hubble's observations of the various galaxies allowed him to create a standard system of classification still used today. American astronomer Carl Sagan (1934-1996) may not have been a great scientists in comparison to some on this list, but he is one of the most famous astronomers. Sagan not only made important scientific studies in the fields of planetary science, he also managed to popularized astronomy more than any other individual. His charismatic teaching and boundless energy impacted people around the world as he broke down complicated subjects in a way that interested television viewers even as he educated them. Professor Stephen Hawking speaks about "Why We Should Go into Space" for the NASA Lecture Series, April 21, 2008.

Credit: NASA/Paul Alers Stephen Hawking (born 1942) has made many significant insights into the field of cosmology. He proposed that, as the universe has a beginning, it will likely also end. He also suggested that it has no boundary or border. Despite being viewed as one of the most brilliant minds since Einstein, many of Hawking's books and lectures are steered toward the general public as he seeks to educate people about the universe they live in. Other astronomers that achieved significant discoveries and are often mentioned among the greats: Italian astronomer Giovanni Cassini (1625-1712) measured how long it took the planets Jupiter and Mars to rotate, as well as discovering four moons of Saturn and the gap in the planet's rings. When NASA launched a satellite to orbit Saturn and its moons in 1997, it was fittingly dubbed Cassini.

Edmond Halley (1656-1742) was the British scientist who reviewed historical comet sightings and proposed that the comet which appeared in 1456, 1531, 1607, and 1682 were all the same, and would return in 1758. Although he died before its return, he was proven correct, and the comet was named in his honor. French astronomer Charles Messier (1730-1817) composed a database of objects known at the time as "nebulae," which included 103 objects at its final publication, though additional objects were added based on his personal notes. Many of these objects are often listed with their catalog name, such as the Andromeda Galaxy, known as M31. Messier also discovered 13 comets over the course of his lifetime. British astronomer William Herschel (1738-1822) cataloged over 2,500 deep sky objects. He also discovered Uranus and its two brightest moons, two of Saturn's moons, and the Martian ice caps.

William trained his sister, Caroline Herschel (1750-1848), in astronomy, and she became the first woman to discover a comet, identifying several over the course of her lifetime. Henrietta Swann Leavitt (1868-1921) was one of several women working as a human "computer" at Harvard college, identifying images of variable stars on photographic plates. She discovered that the brightness of a special flashing star known as a Cepheid variable was related to how often it pulsed. This relationship allowed astronomers to calculate the distances of stars and galaxies, the size of the Milky Way, and the expansion of the universe. [This entry was corrected June 12, 2012.] American astronomer Harlow Shapley (1885-1972) calculated the size of the Milky Way galaxy and general location of its center. He argued that the objects known as "nebula" lay within the galaxy, rather than outside of it, and in 1920 participated in the "Great Debate". He also incorrectly disagreed with Edwin Hubble's observations that the universe boasted galaxies other than the Milky Way.

Frank Drake with a young observer at the Lick Observatory. Credit: SETI Institute Frank Drake (born 1930) is one of the pioneers in the search for extraterrestrial intelligence. He was one of the founders of the Search for Extraterrestrial Intelligence ( SETI ) and the deviser of the Drake equation, a mathematical equation used to estimate the number of extraterrestrial civilizations in the Milky Way galaxy able to be detected. American astronomer William K. Hartmann (born 1939) put forth the most widely accepted theory on the formation of the moon in 1975. He proposed that, after a collision with a large body scooped, debris from the Earth coalesced into the moon. —Nola Taylor Redd A Science Odyssey: People and Discoveries: Hubble identifies a new galaxy Hubble identifies a new galaxy 1924 Photo: Andromeda nebula plate with Hubble's handwritten circle around the variable star he discovered In 1919 American astronomer Edwin Hubble began work at the Mount Wilson Observatory in California, which boasted the most powerful telescope to date. He chose to investigate the nebulae, which were believed to be clouds of glowing gas.

But with this new, larger telescope, Hubble was able to resolve images of the nebulae that showed individual stars. Around 1924, Hubble was looking at the Andromeda nebula and found to his amazement that one of the stars he'd observed wasn't just any star, but a cepheid variable. In 1912 Henrietta Leavitt had shown that cepheid variables could be used as a virtual "yardstick to the universe" -- knowing the brightness of the star and the time period it took to vary from bright to dim would tell you its distance from Earth. Using this formula Hubble was able to find out how far away from Earth his newly discovered cepheid -- and Andromeda -- was. He calculated that the distance was about 800,000 light years -- more than eight times the distance to the furthest stars in the Milky Way! This proved that the "nebula" were actually separate systems outside and independent of our galaxy. It was the first evidence of galaxies outside our own, and Hubble soon went on to discover about 23 more galaxies. HubbleSite - Reference Desk - FAQs Reference Desk Frequently Asked Questions What is a galaxy?

A galaxy is an enormous collection of gas, dust and billions of stars held together by gravity. One galaxy can have hundreds of billions of stars and be as large as 200,000 light years across. What is the name of our galaxy? The name of our galaxy is the Milky Way. Our Sun and all of the stars that you see at night belong to the Milky Way. When you go outside on a dark night and look up, you will see a milky, misty-looking band stretching across the sky. When you look at this band, you are looking into the densest parts of the Milky Way, the "disk" and the "bulge." Where is the Earth in the Milky Way galaxy? Our solar system is at the edge of a spiral arm called the Orion Arm, and is about two-thirds of the way from the center of our galaxy to the edge of the starlight. The Earth is the third planet from the sun in our solar system of nine planets. What is the closest galaxy like our own, and how far away is it? The closest spiral galaxy is Andromeda, a galaxy much like our own Milky Way. It is 2.2 million light years away from us.

Andromeda is approaching our galaxy at a rate of 670,000 miles per hour (1,078,260 kph). Five billion years from now it will probably collide with our Milky Way galaxy. What are the parts of a galaxy? A galaxy contains stars, gas and dust. In a spiral galaxy like the Milky Way, the stars, gas, and dust are organized into a "bulge," a "disk" containing "spiral arms," and a "halo." Elliptical galaxies have a bulge-like central region and a halo, but do not have a disk. Bulge The bulge is a round structure made primarily of old stars, gas, and dust. The outer parts of the bulge are difficult to distinguish from the halo. The bulge of the Milky Way is roughly 10,000 light years across. Disk The disk is a flattened region that surrounds the bulge in a spiral galaxy. The disk is shaped like a pancake. The Milky Way's disk is 100,000 light years across and 1,000 light years thick. It contains mostly young stars, gas and dust, which are concentrated in spiral arms.

Some old stars are also present. Spiral Arms The spiral arms are curved extensions that begin at the bulge of a spiral galaxy, giving it a "pinwheel" appearance. Spiral arms contain a lot of gas and dust as well as young blue stars. Spiral arms are found only in spiral galaxies. Halo The halo primarily contains individual old stars and clusters of old stars ("globular clusters"). The halo also contains "dark matter," which is material that we cannot see but whose gravitational force can be measured. The Milky Way's halo may be over 130,000 light years across. Stars, gas, and dust Stars come in a variety of types. Blue stars, which are very hot, tend to have shorter lifetimes than red stars, which are cooler. Regions of galaxies where stars are currently forming are therefore bluer than regions where there has been no recent star formation. Spiral galaxies seem to have a lot of gas and dust, while elliptical galaxies have very little gas or dust. How are galaxies classified? Astronomer Edwin Hubble classified galaxies into four major types: spiral, barred spiral, elliptical and irregular.

Most of the nearby, bright galaxies are spirals, barred spirals or ellipticals. Spiral galaxies have a bulge at the center and a flattened disk containing spiral arms. Spiral galaxies have a variety of shapes and are classified according to the size of the bulge and the tightness and appearance of the arms. The spiral arms, which wrap around the bulge, contain numerous young blue stars and lots of gas and dust. Stars in the bulge tend to be older and redder. Yellow stars like our Sun are found throughout the disk of a spiral galaxy. The disks of spiral galaxies rotate somewhat like a hurricane or a whirlpool. Barred spiral galaxies are spiral galaxies that have a bar-shaped collection of stars running across the center of the galaxy. Elliptical galaxies do not have a disk or arms. Instead, they are characterized by a smooth, oval-shaped appearance. Ellipticals contain old stars, and possess little gas or dust. They are classified by the shape of the ball, which can range from round to oval (baseball-shaped to football-shaped). In contrast to the disks of spirals, the stars in ellipticals do not all revolve around the center in an organized way.

The stars move on randomly oriented orbits within the galaxy, like a swarm of bees. Irregular galaxies are galaxies that are neither spiral nor elliptical. They tend to be smaller objects that are without definite shape, and tend to have very hot newer stars mixed in with lots of gas and dust. Galaxy names are identified by a group of letters and numbers. What do they stand for? The letters indicate the catalog listing of the galaxies. Galaxies are listed in several different catalogs. The most common catalog is NGC, which stands for New General Catalog. Other catalogs include M (Messier), ESO (European Southern Observatory), IRAS (Infrared Astronomical Satellite), Mrk (Markarian), and UGC (Uppsala General Catalog). The numbers following the letters, such as Mrk 917 or NGC1433, indicate a galaxy’s entry in the catalog and are often related to the galaxy’s relative position in the sky. Sometimes a galaxy appears in more than one catalog and can have more than one name. Why do astronomers study galaxies in ultraviolet light? Galaxies emit all kinds of electromagnetic radiation, from x-rays to radio waves.

From this "light," astronomers get a clear picture of what these galaxies look like. But very distant galaxies pose a special problem. Light from these galaxies travels great distances (billions of light-years) to reach Earth. During its journey, the light is "stretched" due to the expansion of space. As a result, much of the light from the most distant galaxies is no longer visible, but has been shifted to the infrared where present instruments are less sensitive. The only light now in the visible region of the spectrum comes from regions where hot, young stars reside. These stars emit mostly ultraviolet light. But because this light has been stretched, it appears as visible light by the time it reaches Earth. Studying these distant galaxies is like trying to put together a puzzle with some of the pieces missing. So, astronomers are studying nearby galaxies in ultraviolet light to compare their shapes with those of their distant relatives. What are colliding galaxies? When two galaxies are close enough, their gravitational attraction draws them to each other. That attraction increases as the galaxies move closer. The Milky Way and Andromeda are examples of two galaxies that will probably eventually collide (no sooner than 5 billion years in the future).

In a direct encounter between two massive spiral galaxies such as these, huge cold clouds of molecules (gas) will be compressed, and millions of new stars will burst into life like a string of lights. As the galaxies first swing by each other, their once orderly disks will become jumbled with dust, gas, and brilliant blue star clusters. Then the galaxies may do a slow, graceful U-turn and plunge into each other. This second encounter will trigger another burst of star formation, which will drive the remaining gas and dust from the combined system. As the stars settle into randomly oriented orbits, the resulting system may take on an elliptical appearance. How do astronomers measure the distances to galaxies? Astronomers measure the distance to a galaxy in the same way we estimate the distance to an oncoming car by the brightness of its headlights. We know from experience how much light a car's headlights emits, so we can determine how far away the car is. To measure the distance to a galaxy, we try to find stars in that galaxy whose absolute light output we can measure. We can then determine how far away the galaxy is by observing the brightness of the stars. Such stars can help us measure the distance to galaxies 300 million light years away.

If a galaxy is too far away for us to distinguish individual stars, astronomers can use supernovae in the same manner, because the light output of supernovae at their peak brightness is a known fact. Supernovae can be used to measure the distance to galaxies as far as 10 billion light years away. Who is Edwin P. Hubble and what has he to do with galaxies? Edwin P. Hubble revolutionized cosmology by proving that the clouds of light astronomers saw in the night sky were actually other galaxies beyond our Milky Way. His greatest discovery was in 1929, when he identified the relationship between a galaxy's distance and the speed with which it is moving. The farther a galaxy is from Earth, the faster it is moving away from us. This is known as Hubble's Law. He also constructed a method of classifying the different shapes of galaxies. Edwin Powell Hubble was born in Marshfield, Missouri. In 1910, he received his undergraduate degree from the University of Chicago and studied law under a Rhodes Scholarship at Oxford University. His true love, however, was astronomy, and he returned to the University of Chicago to earn a Ph.D.

in that subject and work at the Yerkes Observatory. He served in the infantry during World War I. He once said that he "chucked the law for astronomy," knowing that even if he was second-rate or third-rate, it was astronomy that mattered. HubbleSite and STScI are not responsible for content found outside of hubblesite.org and stsci.edu Return to question list for Galaxies

Question: Who is credited with the discovery of galaxies outside of our own?
Answer: Edwin Hubble