A galaxy is an enormous collection of a few million to trillions of stars, gas, and dust held together by gravity. Galaxies can be several thousand to hundreds of thousands of light-years across.
The name of our galaxy is the Milky Way. All of the stars that you see at night and our Sun belong to the Milky Way. When you go outside in the country 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.
Our solar system is in 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. Earth is the third planet from the Sun in our solar system of eight planets.
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. Five billion years from now it may even collide with our Milky Way galaxy.
By studying other galaxies, astronomers learn more about the Milky Way, the galaxy that contains our solar system. Answers to such questions as "Do all galaxies have the same shape?," "Are all galaxies the same size?," "Do they all have the same number of stars?," and "How and when did galaxies form?" help astronomers learn about the history of the universe. Galaxies are visible to vast distances, and trace the structure of the visible universe with their collections of billions of stars, gas, and dust.
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-shape and a halo, but do not have a disk.
Bulge — A round structure made primarily of old stars, gas, and dust. The bulge of the Milky Way is roughly 10,000 light-years across. The outer parts of the bulge are difficult to distinguish from the halo.
Disk — A flattened region that surrounds the bulge in a spiral galaxy. The disk is shaped like a pancake. The disk of the Milky Way is 100,000 light-years across and 2,000 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 — Curved extensions beginning 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). It may be over 130,000 light-years across. The halo also contains dark matter, which is material that we cannot see but whose gravitational force can be measured.
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.
Edwin Hubble classified galaxies into four major types: spiral, barred spiral, elliptical, and irregular (see also Question 8 and Question 10). Most galaxies are spirals, barred spirals, or ellipticals.
A spiral galaxy consists of a flattened disk containing spiral (pinwheel-shaped) arms, a bulge at its center, and a halo. Spiral galaxies have a variety of shapes, and they 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 many 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. These galaxies rotate somewhat like a hurricane or a whirlpool.
A barred spiral galaxy is a spiral galaxy that has a bar-shaped collection of stars running across its center.
An elliptical galaxy does not have a disk or arms; rather, it is characterized by a smooth, ball-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). The smallest elliptical galaxies (called dwarf ellipticals) are probably the most common type of galaxy in the nearby universe. In contrast to spirals, the stars in ellipticals do not revolve around the center in an organized way. The stars move on randomly-oriented orbits within the galaxy like a swarm of bees.
An irregular galaxy is neither spiral nor elliptical. Irregular galaxies tend to be smaller objects without definite shape, and they typically have very hot newer stars mixed in with lots of gas and dust. These galaxies often have active regions of star formation. Sometimes their irregular shape is the result of interactions or collisions between galaxies. Observations such as the Hubble Deep Fields show that irregular galaxies were more common in the distant (early) universe.
Today we classify galaxies mainly into two major groups following Hubble's examples, as shown in the diagram below. Elliptical galaxies range from round shapes (E0) to oval shapes (E7).
Spiral galaxies have a pinwheel shape and are classified according to their bulge, as well as how tightly their arms are wrapped around the bulge. They range from Sa, which has a large bulge and tight, smooth arms, to Sc, which has a small bulge and loose, lumpy arms.
Barred spiral galaxies, classified as SB, are pinwheel-shaped and have a distinct bar of stars, dust, and gas across their bulge. They range from an SBa, which has a bar across its large bulge and tight, smooth arms, to an SBc, which has a bar across its small bulge and loose, lumpy arms.
Irregular galaxies have no definite shape but still contain new stars, gas, and dust. The chart below the diagram summarizes the properties of the main classes of galaxies.
Edwin P. Hubble revolutionized cosmology by proving that galaxies are indeed island universes beyond our Milky Way galaxy. 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 with the Hubble Tuning Fork (see previous question).
Edwin Powell Hubble was born in Kentucky, where he grew up observing the habits of birds and animals. In 1910 he received his undergraduate degree from the University of Chicago and studied law under a Rhodes Scholarship at Oxford University. Later he changed his mind and completed a Ph.D. in astronomy at Chicago's Yerkes Observatory in 1917. He had several other interests, and for a while he thought of becoming a professional boxer. He also enjoyed basketball, and even answered a call in World War I to serve in the infantry.
Hubble 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.
Scientists classify galaxies in different catalogs. The most common catalog is NGC, which stands for New General Catalog. Other catalogs include M (Messier), ESO (European Southern Observatory), IR (Infrared Astronomical Satellite), Mrk (Markarian), and UGC (Uppsala General Catalog). Sometimes a galaxy appears in more than one catalog and can have more than one name.
The numbers following the letters, such as Mrk917 (Sc) or NGC1433 (SB), indicate the galaxy's entry in the catalog and are often related to the galaxy's relative position in the sky.
When two or more galaxies are close enough to each other, gravitational forces will pull the galaxies toward each other. This gravitational attraction increases as the galaxies travel toward each other. The galaxies may pass by each other or collide. Two galaxies that are interacting or colliding may be referred to as a pair, or one galaxy may be referred to as a companion of the other.
The Hubble images in the chart below show how different colliding galaxies can look. The appearance of an interacting system of galaxies depends on many factors, including the stage of the interaction, the number of galaxies involved in the interaction, their masses and types, how close they are, and how they approach each other.
The Antennae galaxies (upper left in the chart) are an example of two spirals that are in the process of colliding. We will not see the end result during our lifetimes because this process takes hundreds of millions of years. Sometimes smaller galaxies plunge into larger galaxies. This type of collision produces a ripple effect, like a rock thrown into a pond. The Cartwheel galaxy (upper right in chart) is an example of this type of collision. The outer ring of blue stars in this galaxy indicates a ripple of star formation resulting from the collision.
Our Milky Way and Andromeda are two spiral galaxies that may eventually collide (about 5 billion years in the future).
When galaxies collide, they experience a gravitational pull toward each other. This gravitational pull distorts the shapes of the galaxies, and can pull material from one galaxy to the other. In many cases, the pull of gravity may result in the galaxies merging. The individual stars within the galaxies do not collide since they are so far apart, relative to their size.
Clouds of gas within the galaxies do collide, however. As a result, large amounts of gas become concentrated in one or more areas of the system. As the collision compresses the gas, the gas becomes dense. The clouds of gas collapse under gravitational forces and form large numbers of new stars. This rapid, short-lived episode of star formation activity is referred to as a starburst. Intense starbursts can use up nearly all of the available gas.
Interacting galaxies do not always merge together to form a single object. Scientists think that some galaxies (such as M51 and its companion, NGC 5195) will simply pass by each other without merging. Galaxies can also pass through each other without merging. The Cartwheel galaxy is believed to be the aftermath of such a pass-through. Some galaxies do merge. The Antennae are thought to represent a merger between two gas-rich spiral galaxies. When the merger is complete, the Antennae may end up looking like a single elliptical galaxy.
When we study astronomical objects, we are actually looking back in time. Light from the Sun takes eight minutes to reach Earth. The light we see today from the next nearest star was emitted about four years ago. Light from the nearest galaxy like our own, Andromeda, takes over 2 million years to reach us. That is, we see Andromeda as it appeared more than 2 million years ago. Observations of distant galaxies show us what the universe looked like at an earlier time in the history of the universe. By studying the properties of galaxies at different epochs, we can map the evolution of the universe.
They observe many properties of each galaxy, including size, shape, brightness, color, amount of star formation, and distance from Earth. This information helps astronomers to determine how these structures may have formed and evolved.
In astronomical terms, a deep field is a long-exposure observation taken to view very faint objects. Light from these objects is collected over a large period of time, so the detectors have a chance to gather as much light as possible. Objects can be very far away and appear faint to us due to the vast distances over which the light must travel; and/or objects can lie close to us and be faint because they don't give off much light. So deep doesn't necessarily mean far. However, in the case of all of the Hubble deep field surveys, deep does mean far away since the images were taken in areas that we know have few nearby stars.
The Hubble Deep Field project was inspired by some of the first deep images to return from the Wide Field and Planetary Camera 2 (WFPC2), after it was installed during the 1993 Hubble Space Telescope servicing mission. These images showed that the early universe contained galaxies in a bewildering variety of shapes and sizes. Some had the familiar elliptical and spiral shapes seen among galaxies today, but there were many peculiar shapes as well. Such images of the early universe are likely to be one of the enduring legacies of the Hubble Space Telescope. Few astronomers had expected to see this activity presented in such amazing detail.
Impressed by the results of earlier observations such as the Hubble Medium Deep Survey, astronomers at the Space Telescope Science Institute (STScI), and STScI's Director, Robert Williams, realized that they could provide a service to the entire astronomical community by taking the deepest optical picture of the universe. The research was done by aiming Hubble at a single piece of the northern sky for 10 days (150 consecutive orbits) in December, 1995. Images from the Hubble Deep Field project were made available to astronomers around the world shortly after completion of the observation.
A few thousand previously unseen galaxies are visible in the original deepest ever view of the universe, called the Hubble Deep Field (later named the HDF-North). In addition to the classical spiral and elliptical forms, the variety of other galaxy shapes and colors seen in the image are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang.
Hubble took a second deep look in the Southern Hemisphere in October of 1998 — the HDF-South — to see if a similar result would be obtained.
In 2004, the Hubble Ultra Deep Field (HUDF) was captured in visible light using the Advanced Camera for Surveys and NICMOS. The addition of Wide Field Camera 3 resulted in the 2009 HUDF image take in infrared light. Finally, in 2012, ten years of Hubble data centered on the 2004 HUDF were combined to form the eXtreme Deep Field (XDF) image.
The HDF-N covers a very small fraction of the sky. It would have taken 27 million fields and well over 500,000 years to use Hubble's Wide Field and Planetary Camera 2 to survey the entire sky to the depths of the HDF. Instead, astronomers rely on several thin looking-through-a-soda-straw views across the cosmos to infer the history of star and galaxy formation.
Taking a second Deep Field helped astronomers to confirm that the HDF-N is representative, and that it is not unusual in some way. The two HDFs are, in fact, consistent with the common assumption that the universe should look largely the same in any direction.
Each of the Hubble Deep Fields represents a carefully selected random spot on the sky. To allow the Hubble Space Telescope to peer deeply into the sky, astronomers selected a special region of Hubble's orbit where Hubble could view the sky without being blocked by Earth or experiencing interference from the Sun or Moon. The field also had to be far away from the plane of our own galaxy to avoid being cluttered with objects within the Milky Way. Finally, the field needed to have nearby guide stars, which are used to keep Hubble pointed at the field. These criteria led to the selection of a spot of sky near the handle of the Big Dipper, in the Northern Hemisphere, and a spot of the sky in the constellation Tucana, in the Southern Hemisphere.
When produced, the HDFs contained the faintest galaxies we'd ever been able to see over a large range of distances. Since seemingly empty spots were chosen, most of the galaxies in the Deep Fields lie billions of light-years away.
The images show some galaxies in their early stages of formation, appearing in peculiar shapes never previously seen by astronomers. The variety of new galaxy shapes and colors seen in the HDFs, along with the classical elliptical and spiral shapes, are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang. The HDFs are important because they can help answer such questions as:
If this is typical evolution for spiral galaxies, then predictions can be made for what they should have looked like at half their present age — their size, color and abundance. This information, combined with actual distances derived from ground-based spectroscopic observations, will provide a new test for theories of spiral galaxies.
The other major class of galaxies seen in the nearby universe is the elliptical type — football-shaped aggregates of stars that appear to be very old and stopped forming stars long ago. There is currently debate about when such galaxies formed and whether they formed through collisions of other types of galaxies or through collapse of a pristine cloud of primordial gas in the very early universe.
The Hubble Deep Field images, along with other deep Hubble images, provide a snapshot through time, which can be used to search for distant elliptical galaxies or primeval galaxies that might later evolve into elliptical galaxies.
They use the colors of the galaxies. Different types of galaxies tend to be different colors. For example, elliptical galaxies have reddish colors because they are mostly composed of old red stars. Astronomers study the colors of nearby elliptical, spiral, and irregular galaxies and compare these colors to those of the galaxies in the Hubble Deep Fields. Comparing the colors allows them to classify the galaxies.
Because of the way astronomers' instruments work, they can be reasonably sure that they have detected all galaxies with a certain range of brightnesses in the Hubble Deep Fields. Astronomers may be able to identify fainter objects, but they cannot be sure that they have detected all of the fainter objects that exist.
When studying populations of objects, astronomers need to make sure that the sample they choose is representative. The very faintest objects do not form a representative sample since astronomers do not know if they have detected all of the faintest objects. Therefore, they limit their sample to objects in a certain brightness range. The sample is then said to be statistically complete to that brightness level. For the HDF-N, the statistically complete sample consists of 1,067 galaxies.
In 2003, the Hubble Space Telescope looked even farther back in time and found as many as 10,000 galaxies, using the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-object Spectrometer (NICMOS). The million-second-long exposure (11.3 days) captured galaxies that are too faint to be seen by ground-based telescopes, or even by Hubble in its previous faraway looks, called the Hubble Deep Fields.
The ACS image, known as the Hubble Ultra Deep Field (HUDF), shows a wide range of galaxies of various sizes, shapes, colors and ages. Some were formed just a short time after the universe was created, about 13 billion years ago. The infrared NICMOS image complements the visible-light ACS image and reveals some of the most-distant galaxies ever seen.
The ACS picture required a series of exposures taken over the course of 400 Hubble orbits around Earth. This is such a big chunk of the telescope's annual observing time that Space Telescope Science Institute Director, Steven Beckwith, used his own Director's Discretionary Time to provide the needed resources. Just like the previous HDFs, the new data are expected to galvanize the astronomical community and lead to dozens of research papers that will offer new insights into the birth and evolution of galaxies.
The Hubble Deep Fields represented the faintest, deepest visible light images ever taken using the technology available at that time — the Wide Field and Planetary Camera 2. The Hubble Ultra Deep Field was produced using an instrument with newer technology — the Advanced Camera for Surveys. The differences in the HDF and HUDF are due to the quality of the instruments being used. Concerning the variety of galaxies visible in the fields, it has been said that the HDF captured images of galaxies when they were youngsters but the HUDF captured images of galaxies as toddlers.
"Q&A: Galaxies" is a series of questions and answers about galaxies written for teachers and students. The questions are ones that students might ask while studying galaxies. Teachers can use this Q&A to gain additional knowledge about galaxies, or use it in the classroom as outlined below.
• An engagement activity. Use selected questions to start a discussion.
• An inquiry tool. Use selected questions and answers to help students generate questions. Propose a question, such as "Why do we study galaxies?"(see question 5 in Q&A: Galaxies). Have students read the answer to the question and write down questions they would like answered as a result of reading the material.
• A source of information. Students can use the questions and answers as part of their research on galaxies.
• A form of review. Use the questions as a review at the end of a unit on galaxies.
• A follow-up. Have students read the questions and answers to gain additional information about galaxies following a related activity.
• A starting point for a debate. "Will the Andromeda galaxy merge with the Milky Way galaxy when they collide in 5 billion years?" Questions that address this idea include "What is the closest galaxy like our own, and how far away is it?" (see question 4 in Q&A: Galaxies) and "What happens when galaxies collide?" (see question 12 in Q&A: Galaxies).