"Deep-sky" objects refer to astronomical objects such as stars, clusters, nebulae and galaxies that are outside of our own solar system. They do not include the Sun, Moon, planets, dwarf planets, asteroids, comets, meteors, or satellites.

Most of the thumbnail images on the right side of this page are linked to a full-sized image of the object.

• Stars

  A star is a giant self-luminous ball of extremely hot hydrogen and helium gas, held together by its own gravity, that creates and radiates light and energy through the process of nuclear fusion.

  Sirius

  Our Sun is the nearest star. All of the other stars in the night sky are so far away that they appear only as points of light.

  Sirius is the brightest star in the night sky. It is located 9 light-years away. A light-year is a measure of distance, not time. A light-year is the distance that light travels in a year, about 6 trillion miles.

  Some stars are much larger than the Sun and some are smaller. Red giant stars are stars that start out in size similar to the Sun, but that have greatly expanded near the end of their lifetimes. Blue giants are large, extremely massive, hot young stars that will eventually die in supernovae explosions. White dwarves and red dwarves are stars that are smaller than the Sun.

  Star Colors

  Stars come in different colors and temperatures. They are usually classified according to their spectral characteristics. Types run from hot blue stars to cool red stars with seven major categories: O, B, A, F, G, K and M. These can be easily remembered with the mnemonic "Oh, Be A Fine Girl, Kiss Me." with the first letter of each word representing the spectral categories. O-type stars are hot and blue, M-type stars are cool and red.

  

  Here are the temperature ranges for each spectral class:

  • O = more than 30,000°K
  • B = 10,000 to 30,000°K
  • A = 7,500 to 10,000°K
  • F = 6,000 to 7,500°K
  • G = 5,000 to 6,000°K
  • K = 3,700 to 5,000°K
  • M = 2,400 to 3,700°K

  Each class has many subdivisions. These are designated with a number with 0 representing the hottest and 9 being the coolest. For example, an F1 star would be hotter than an F8 star.

  Roman numerals are then added to the spectral class to designate the luminosity of the star. This distinguishes small dwarf stars from large giant stars with the following designations:

  • Class I = Supergiants
  • Class II = Bright Giants
  • Class III = Regular Giants
  • Class IV = Sub-Giants
  • Class V = Dwarfs or Main-Sequence
  • Class VI = Sub-Dwarfs

  Our Sun is classified as a G2V star, with "G" meaning it is yellow in color, "2" meaning it is relatively hot for that class, and "V" meaning it is a main-sequence star.

  The Sun

  Although the Sun is classified as a yellow star, we see its light as white because our eyes have evolved under its light.

  Several additional classes have been added for extreme stars off the ends of the original categories, such as WN and WC stars on the blue end, and L, T, Y, C and S stars on the red end. C stars are carbon stars that appear extremely red.

  Spica and Rigel are examples of B-type spectral-class blue-giant stars. Antares and Betelgeuse are examples of M-type spectral-class red-giant stars.
  T Lyrae and V Aquilae are examples of C-type carbon stars.

  Star Brightness

  Stars are ranked in brightness by a scale of magnitude. This is a logarithmic scale where each magnitude is 2.512 times as bright as the next magnitude.

  Fainter stars have higher numbers and brighter stars have lower numbers. The brightest objects in the sky have negative magnitudes. For example, Sirius, the brightest star in the night sky, has a magnitude of -1.44. Venus can get as bright as magnitude -4.9. The Sun is the brightest object in the sky at magnitude -27. Polaris is a variable star whose brightness varies from a maximum magnitude of 1.86 to a minimum magnitude of 2.13.

  Magnitude measurements are usually made with a standard set of UBV filters in three wavelengths: U for ultraviolet, B for blue light, and V for visual. Magnitudes listed here are usually measured in the "V" band of wavelengths, which is similar, but not exactly equal to normal human vision. Sometimes magnitudes for some objects are only available in the "B" band.

  Magnitudes quoted in this book are also apparent magnitudes. That is a measurement of how bright an object is in the sky as it appears to us on Earth.

  Because objects appear dimmer when they are farther away, apparent magnitudes do not tell us how bright an object really is. For this, we use absolute magnitudes.

  Absolute magnitudes can be used to compare objects directly. The absolute magnitude of an object is calculated as if the object were located 10 parsecs (32.6 light-years) away from Earth.

  
  
• Double and Multiple Stars

  Stars that form in nebulae out of gigantic clouds of dust and gas will frequently form in pairs and groups of multiple stars. Sometimes these pairs (binaries) and groups are gravitationally bound and orbiting each other. These are true double stars and multiple star systems where the stars orbit around each other or a common center of gravity such as Almach (Gamma Andromedae).

  Theta Orionis

  Double stars that can be resolved with a telescope are called visual binaries. Double stars that are too close to be resolved visually, but that can be detected through spectroscopy are called spectroscopic binaries. Other doubles can be detected if their orbital plane happens to align with our viewing position so that we can detect one star moving in front of, or behind, another. These are called eclipsing binaries.

  Some stars, however, that appear to be double stars in the sky are just chance alignments with the stars at different distances along the same line of sight. These stars do not gravitationally interact with each other. These are called optical binaries.

  Scientists think that as many as 85 percent of all stars may be true binaries or members of multiple-star systems.

  Examples of multiple star systems are Theta Orionis, the Trapezium, in the heart of the Orion Nebula, and Epsilon Lyrae, the "Double Double."

  Algedi, Alpha Capricorni, is an example of an optical double that can be seen with the unaided eye by observers with good eyesight. The two components of Algedi appear to be separated by 6.33 arcminutes but are not gravitationally interacting and are at vastly different distances.

  
  
• Variable Stars

  Variable stars vary in brightness. There are different types of variable stars with several sub-groups in each.

  Delta Cephei

  Intrinsic variables change brightness because of changes in the star itself, such as pulsations where the star expands or contracts. There are pulsating variables such as Delta Cephei, eruptive variables such as R Coronae Borealis, and cataclysmic variables such as novae and supernovae.

  Extrinsic variables change brightness because of factors outside of the star itself, such as a dimmer binary companion that moves in front of the brighter star and blocks its light. Algol, Beta Persei, is the most famous example of an eclipsing binary, varying in brightness from magnitude 2.1 to 3.4 over a period of 2 days 20 hours and 49 minutes.

  
  
• Asterisms

  An asterism is a group of stars that form a pattern but not one that is officially recognized as a constellation.

  Brocchi's Cluster

  These patterns are usually in the mind of the observer, but some are actual groupings of stars that are close together in space. Others are just chance alignments of stars at different distances that happen to lie along the same line of sight as we look out into the night sky.

  Brocchi's Cluster in Vulpecula is an example of an asterism. The Summer Triangle and Great Square of Pegasus are other examples.

  
  
• Constellations

  Constellations are groupings or patterns of stars whose boundaries, but not figures, are officially recognized and defined by the International Astronomical Union (IAU).

  Orion

  The fanciful figures that make up the constellations and the mythology that surrounds them goes back, in some cases, thousands of years. Many different cultures had stories and myths associated with the patterns that they saw in the stars in the sky.

  Orion is an example of a distinctive constellation.

  
  
• Molecular Clouds

  A molecular cloud is a large dense area of cool gas and dust in space made mostly of molecular hydrogen and helium. These clouds are usually dark and mostly opaque.

  Taurus Molecular Cloud

  Giant molecular clouds can be hundreds of light-years in diameter such as the Orion Molecular Cloud (OMC) and the Taurus Molecular Cloud (TMC). They can contain thousands, or even millions of times the mass of the Sun.

  Molecular clouds are star forming regions. Once stars form, such as in the Orion Nebula, strong ultraviolet light from hot young stars ionizes the hydrogen gas, causing it to glow, forming emission nebulae. Over time, these stellar winds clear out the gas and dust in the area, and the stars that were hidden become visible.

  
  
• Open Star Clusters

  When stars form in a nebula in molecular clouds, they usually don't form alone. Groups of stars may form in clusters at the same time.

  The Pleiades

  These stars may stay together, gravitationally bound, in an open cluster for a while after they are born. The Pleiades, M45, is a beautiful example of an open cluster.

  Open clusters may contain only a few dozen stars, or may have hundreds or thousands of members. There are some 1,100 open clusters in our Milky Way Galaxy.

  Open clusters eventually disperse and the stars go their individual ways and end up orbiting the center of the galaxy by themselves. It is believed the Sun formed in an open cluster more than four billion years ago and is now on its own.

  
  
• Globular Star Clusters

  Globular clusters contain hundreds of thousands of stars that are tightly gravitationally bound in a sphere. These spheres are usually more than 100 light-years in diameter. Globular clusters usually orbit a galaxy in a halo around it.

  Omega Centauri

  The gravity that makes globular clusters very dense in stars in their cores also holds them together over long time periods. Indeed, most globular clusters don't contain any young O or B type stars, indicating their old age. Most of the Milky Way's globular clusters are believed to be at least 10 billion years old.

  The Milky Way contains more than 150 globular clusters. The giant elliptical galaxy M87 has more than 10,000 globular clusters.

  Omega Centauri, NGC 5139, is the largest and most spectacular globular cluster in the Milky Way. It is larger than the full Moon and contains more than 10 million stars.

  
  
• Stellar Associations

  Stars that originally formed together in clusters, but that are not necessarily gravitationally bound to one another anymore may still be located in the same general area. These groups are called stellar associations. They are sometimes located in the cloud of gas and dust out of which they originally formed.

  NGC 206

  The Orion Association contains most of the stars in the constellation of Orion.

  Stellar associations in other galaxies can also be seen and photographed, such as NGC 206, an OB association of type-O and type-B giant stars in M31, the Andromeda Galaxy.

  Other stars that formed together in clusters may no longer be gravitationally bound, but nevertheless still share a common motion through space. These are called stellar streams or moving groups. Most of the stars in the Big Dipper are part of the Ursa Major Moving Group or Stream.

  
  
• Emission Nebulae

  Emission nebulae are glowing clouds of plasma (high temperature gas) in space.

  Messier 8

  Ultraviolet radiation from nearby hot stars is absorbed by the atoms in the gas. This energy strips electrons from atoms and ionizes the gas. As atoms and electrons recombine and fall back to a lower energy state, they emit light through fluorescence.

  The light in emission nebulae is usually red because the most dominant emission line of hydrogen gas, the most abundant gas in these nebulae, is at the hydrogen-alpha wavelength at 656.28 nanometers, which is in the deep red portion of the visible spectrum.

  The Lagoon Nebula, Messier 8, and the North America Nebula, NGC 7000, are examples of red emission nebulae.

  
  
• Reflection Nebulae

  Reflection nebula are clouds of interstellar dust which scatter and reflect the light of nearby stars.

  NGC 1977

  These nebulae are mostly blue because shorter wavelengths scatter more efficiently. This same process happens in the Earth's atmosphere with sunlight. The long blue wavelengths are scattered, making the sky blue during the daytime, and the Sun red at sunrise and sunset.

  The blue color of these nebulae also reflect the predominant color of the stars that illuminate them. These stars are often hot young O and B type stars that are blue colored.

  There are approximately 500 reflection nebulae known. M45, the Pleiades, M20, the Trifid Nebula, and NGC 1977, the Running Man Nebula, are examples of blue reflection nebulae.

  The Antares Nebula is a rare red-orange reflection nebula illuminated by the red giant star Antares.

  
  
• Dark Nebulae

  Dark nebulae are clouds of cold dust in interstellar space that block most of the light from objects behind them. These dark nebulae are not visible because they are self luminous like emission or reflection nebulae. They are visible because they are silhouetted in front of other bright nebulae, or star clouds of the Milky Way.

  Barnard 73

  Dark nebulae are often involved in bright nebulae and molecular clouds and star forming regions. Dark nebulae can eventually be eroded away by the hot stellar winds from nearby young stars such as in the "Pillars of Creation" in M16, the Eagle Nebula.

  Bok globules are isolated small dark nebulae such as those found inside of M8, the Lagoon Nebula.

  The Great Rift in the Milky Way that runs from Deneb in Cygnus through Aquila, Ophiuchus, Scorpius and Centaurus is the most conspicuous example of a dark nebula. Other dark nebulae that can be observed without optical aid include Le Gentil 3, the Coalsack, and the Pipe Nebula. Dark nebulae best seen in binoculars include Barnard 168 and Barnard's "E" Nebula. A fine example of a telescopic dark nebulae is B72, the Snake Nebula.

  
  
• Planetary Nebulae

  Stars with the mass of 8 Suns or less evolve into a red giant after all of the hydrogen in the core has been turned into helium by nuclear fusion and the star expands and brightens. These stars then shed their outer layers and form a planetary nebula illuminated by the remaining stellar core, a white dwarf, which is about as heavy as the Sun, but with all of its mass packed into a size about the same as that of the Earth.

  The gas in the planetary nebula is ionized by the hot ultraviolet radiation from the white dwarf and it fluoresces and glows. The planetary nebula is visible for about 10,000 to 100,000 years. After that the white dwarf cools and fades.

  Messier 57

  Planetary nebula have nothing to do with planets. They derived this name from William Herschel because they were similar in color to the planet Uranus, which he had discovered in 1781.

  Many planetary nebulae appear blue-green in color from emission lines of doubly ionized oxygen at 495.9 nanometers and 500.7 nanometers, and the hydrogen-beta emission line at 486.1 nanometers.

  In about 5 billion years, our Sun will go through a red-giant phase and produce a planetary nebula.

  Messier 57, the Ring Nebula, Messier 27, the Dumbbell Nebula, and NGC 7009, the Saturn Nebula are three examples of very interesting planetary nebulae.

  
  
• Novae

  A nova is a violent explosion in binary star systems, usually caused when material from the larger star in the pair accumulates on the surface of the smaller star (often a white dwarf) until it becomes unstable which then starts runaway nuclear fusion resulting in an explosion. This can increase the brightness of the star by 10 to 15 magnitudes.

  Dwarf Nova in M27

  Unlike supernovae where the star is destroyed, the stars involved in novae survive and return to their normal brightness.

  There are several sub-species of novae including dwarf novae, recurrent novae and classical novae.

  This image at right of M27 shows a dwarf nova that brightened by about 9 magnitudes.

  
  
• Supernovae

  A supernova is an explosion of a star so violent that the star is basically destroyed, sometimes leaving behind only the core as a neutron star or black hole.

  Supernovae can increase the brightness of a star to an absolute magnitude of -18 or -19 where it can briefly outshine its host galaxy.

  There are two types of supernovae: Type I and Type II, with several sub-categories.

  Type I supernovae are similar to regular novae in that a binary star system is involved where a white dwarf accumulates material from a companion. When enough mass is gathered, the core reaches a critical density and uncontrolled fusion takes place which causes the supernova explosion.

  Stars up to a mass of about eight times that of the Sun usually expand into red giants at the end of their lives, eventually leaving behind a white dwarf. If that white dwarf is involved in a binary star system, it can eventually end up as a Type I supernova.

  When a star explodes as a Type I supernova, nothing is left behind except elements created in the blast and a supernova remnant.

  Type II supernovae occur in single massive stars at the end of their lives. Nuclear fuel is exhausted, the star's remaining iron core collapses and the star explodes.

  Stars with more than 8 stellar masses end their lives in a supernova explosion. If the star has less than 25 stellar masses, all that remains after the explosion is the incredibly dense core that is comprised almost completely of neutrons forming a neutron star. The neutron star is a little more massive than the Sun, but its mass is packed into a sphere about 10 miles in diameter.

  If the original star has a mass of 25 times or more that of the Sun, the core collapses into a black hole where nothing can escape its gravity, not even light. Most of the mass of the original star is blown away, but several stellar masses remain in the black hole.

  A stellar-mass black hole with 3 stellar masses has a diameter of about 6 miles (9 kilometers), and one with 10 stellar masses has a diameter of about 18 miles (30 kilometers).

  Supernovae are relatively rare, with only two or three occurring in an entire galaxy about once every 100 years. The last supernova recorded inside of the Milky Way Galaxy was the supernova of October 9, 1604, recorded by Johannes Kepler.

  Supernova SN 1987A occurred in the Large Magellanic Cloud, a companion satellite galaxy to the Milky Way, on February 23, 1987. It was visible to the unaided eye from the southern hemisphere.

  Supernova in M51

  The thumbnail image at right shows images of galaxy Messier 51 before and during Supernova 2011dh. This was a Type IIP supernova.

  Supernovae are important because they can be used as "standard candles" to determine distances across the vast reaches of the universe because they are so bright they can be detected in galaxies that are very far away.

  Observations of type Ia supernovae in distant galaxies led to the staggering discovery that the expansion of the universe after the big bang has actually been accelerating. This discovery was completely unexpected and led to theories of dark energy as the cause of the acceleration.

  These and other observations have led to our current best understanding of the composition of the mass and energy in the universe as a whole. It is now believed that the universe contains only about 4.9 percent of the ordinary matter that we are intimately familiar with, and the rest of the universe is comprised of about 26.8 percent dark matter, and 68.3 percent dark energy. At this point in time, we basically have a poor understanding of exactly what dark matter or dark energy are. This means that 95 percent of the universe is still a giant mystery for us to solve.

  
  
• Supernova Remnants

  A supernova remnant is the structure that remains in space after a supernova explosion.

  Veil Nebula

  It is the visible portion of a large bubble of material ejected from the star, and other material in the neighborhood of the star, that has been swept up after the supernova exploded. The boundary of the bubble is marked by a shock wave that heats up the material causing it to glow, which is what we see as the supernova remnant.

  The Veil Nebula, Simeis 147, and IC 443 are examples of a shell-type supernova remnants.

  M1, the Crab nebula, is an example of a core-collapse supernova remnant powered by a rapidly spinning neutron star called a pulsar.

  
  
• Black Holes

  A black hole is an object that is so dense its gravity prevents anything, even light, from escaping.

  Black holes are thought to form after the supernova explosion of large stars. Smaller stars, such as the Sun, will end their lives as incredibly dense white dwarfs or neutron stars. Stars that start with a mass of about 25 times that of the Sun and end up with a mass of three to four solar masses after they explode in a supernova will turn into a black hole.

  Stars are very massive. Nuclear fusion in their cores normally exerts a constant outward pressure that prevents the material in the star from collapsing due to its own gravity.

  Eventually, however, a massive star will run out of fuel for fusion. The star's gravity causes the core to collapse and the star explodes in a supernova where most of the star's material is blasted out into space. The remaining core has no fusion taking place and gravity causes it to continue to shrink until it forms a black hole.

  At this point the normal mathematics of physics fails us. Here we find a "singularity", a point described with zero volume, infinite density and where the curvature of space-time becomes infinite. We do not currently understand what happens inside of a singularity.

  The "event horizon" around a black hole is a boundary where gravity is so strong that light cannot escape.

  Once a black hole has formed, it can grow by absorbing matter that strays to close to its vicinity. This does not mean that a black hole will eventually suck everything into it. The gravity of a black hole is the same as that of a regular object, it is based on its mass. So a black hole that ended up with the mass of the Sun would have gravitational interaction with the surrounding area in a manner similar to the area around the Sun. If the Sun were somehow turned into a black hole (which won't happen), the planets in the solar system would keep orbiting around it exactly the same as they do now.

  Larger black holes can form from larger progenitor stars, and black holes in the center of galaxies can become extremely massive. A typical black hole at the center of a galaxy may have a million solar masses in an area only about four times as large as the Sun. A solar-mass black hole might have a diameter of only four miles! Some large black holes may contain billions of solar masses.

  Almost every galaxy is thought to have a massive black hole at its center, including our own galaxy, the Milky Way.

  Messier 87 Jet

  Material falling into the massive black hole at the center of a galaxy can form an active galactic nucleus that shoots out powerful beams of energy, such as the jet emanating from the core of galaxy M87, seen in the image at right.

  Because no normal radiation can escape a black hole, they cannot be observed directly. Scientists infer the existence of black holes by studying the properties of the matter and energy in the area around them, and the radiation emitted as material falls into them.

  Scientists have a lot of very good evidence that black holes do, in fact, exist. Cygnus X-1 was considered to be the first black hole discovered in our galaxy, in 1971. Today astronomers believe that there may be 10 million to 1 billion stellar-mass black holes in the Milky Way Galaxy.

  The massive black hole at the center of our Milky Way Galaxy is named Sagittarius A*, pronounced "Sagittarius A-star". It has a mass of about 4 million solar masses with a diameter of 27 million miles (44 million kilometers), about the size of the diameter of the orbit of Mercury.

  Primordial black holes may have also formed at the beginning of the universe when matter was extremely dense.

  
  
• Galaxies

  Just as stars usually form in open clusters and globular clusters, they also form into larger aggregations called galaxies. The Milky Way is the galaxy that our own Sun is located in.

  Messier 31

  Galaxies can contain hundreds of billions of stars, and it is estimated that there are hundreds of billions of galaxies in the visible universe.

  A galaxy is composed of visible dust, gas, stars, clusters, bright and dark nebulae, and invisible dark matter. The Milky Way is thought to contain ten times as much dark matter as visible matter.

  Galaxies are usually classified according to their shape with ellipticals, lenticulars, spirals, and irregular galaxies being the main types, with sub-categories of each.

  Some galaxies are small dwarfs like NGC 6822, Barnard's Galaxy, in Sagittarius, and some are enormous behemoths , such as Messier 87 in Virgo. Some galaxies interact with other galaxies causing distortions, such as the Antennae Galaxies in Corvus and M51, the Whirlpool Galaxy, in Canes Venatici.

  Galaxy Messier 31, the Great Galaxy in Andromeda, is 2.5 million light-years away and yet is bright enough to be seen with the unaided eye from a true dark-sky observing location.

  Some galaxies contain supermassive black holes at their centers such as M87. These black holes can cause Active Galactic Nuclei (AGN), and Quasars - Quasi-Stellar Objects (QSO).

  The true nature of galaxies was not really understood until the early part of the 20th century. In 1924 Edwin Hubble used photographs to identify Cepheid variables in M31 and M33. His studies proved that these "spiral nebulae" were too far away to be part of our own Milky Way Galaxy, and that they were, in fact, other completely separate galaxies. This discovery changed our understanding of the entire universe.

  Later, Hubble studied the red-shifted spectra of galaxies and discovered observational evidence for the Big Bang theory and the expansion of the universe.

  
  
• Quasars

  A quasar is a "quasi-stellar object" or QSO.

  Quasar 3C 273

  Although appearing to look like a star, a quasar is really the tremendously bright nucleus of an extremely distant galaxy.

  Quasars are not completely understood, but scientists believe that they are associated with a supermassive black hole in the center of their host galaxy that powers an active galactic nucleus.

  Quasars have high red-shifts, an effect of the expansion of the universe after the Big Bang. Their red-shifts indicate that they are extremely far away.

  Quasars are the most luminous objects known in the universe. A quasar can output hundreds to thousands of times the energy output of an entire galaxy.

  Most galaxies have black holes at their centers. As material falls into a black hole, it usually spirals around due to angular momentum. This in-falling material forms an accretion disk around the black hole. Since a black hole's gravity is too strong to let light escape, we only see energy released outside of the black hole in the accretion disk.

  Due to the magnetic properties involved, the accretion disk can produce twin jets of extremely powerful energy that beam out at tremendously high speeds in opposite directions. If one of these beams happens to be pointed near us, we see a quasar. If the beam is pointed directly at us, we see a "blazar."

  More than 200,000 quasars have been found, mostly by the Sloan Digital Sky Survey.

  Quasar 3C 273, in Virgo, is one of the brightest and closest quasars to us.

  
  
• Galaxy Groups and Galaxy Clusters

  Just as stars form in clusters, so do galaxies. Large galaxy clusters can contain thousands of galaxies. Small compact galaxy groups may contain only three or four members.

  Virgo Cluster

  Our own Milky Way Galaxy and M31 are two prominent members of what is called the Local Galaxy Group, which, in turn, is a member of the Coma-Virgo Supercluster of galaxies, at the heart of which are the giant elliptical galaxies M87, M86 and M84.

  Other galaxy clusters include Abell 426, the Perseus Galaxy Cluster, Abell 1656, the Coma Galaxy Cluster, and Abell 2151, the Hercules Galaxy Cluster.

  Galaxies also congregate in smaller associations called compact galaxy groups, such as Hickson 44, and Stephan's Quintet.

  
  
• The Universe

  Evidence indicates that the universe formed about 13.798 ± 0.037 billion years ago in the Big Bang. It has been expanding since then. This expansion has also been accelerating. The observable universe is now believed to be about 93 billion light-years in diameter because of this expansion.

  Some of the largest structures in the universe are comprised of galaxy clusters that have formed filaments and walls with giant empty voids in between. These enormous structures make up a cosmic web that fills the observable universe.

  The Great Wall, also called the Coma Wall, is an example of such a structure. It was officially cataloged as the "CfA2 Great Wall" during the Center for Astrophysics (CfA) Survey to map the large-scale structure of the universe. It is a sheet of galaxies that is about 500 million light-years long, 300 million light-years wide, and only 25 million light-years thick. It is located 200 million light-years away and includes the Hercules Supercluster, the Coma Supercluster and the Leo Galaxy Cluster.

  Another structure that is even larger is the Sloan Great Wall (SGW) of galaxies and galaxy clusters. It was discovered with the Sloan Digital Sky Survey. The SGW is more than 1.3 billion light-years long and is located about 1 billion light-years away.

  Even larger structures may have been detected recently such as the Hercules-Corona Borealis Great Wall and the Huge Large Quasar Group.