- 2930 - GALAXIES - how many galaxies is the Universe? The universe is an immensely large place. Even distances between the nearest objects are staggering, and the distances across the Milky Way Galaxy and certainly between galaxies in the universe are astonishingly huge to living beings stuck on a planet
---------------- 2930 - GALAXIES - how many galaxies is the Universe/
- It is absolutely amazing to know that shortly after the Big Bang, the universe was a relatively small, nearly infinitely dense place. That was 13,800,000,000 years ago. Lots has happened since then. The expanding universe means the entirety of what we know is now incredibly large and is getting more immense every day.
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- A model of the Milky Way wherein the Sun is a grain of sand brings this home. On this scale, stars (sand grains) are 4 miles apart in the Milky Way’s disk and the disk is about 40,000 miles across.
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- The Big Bang theory tells us that once the universe was very small. We know the fastest that radiation or any information can travel is the speed of light, 186,000 miles per second.
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- We’re confident that the universe is 13.8 billion years old. We also know that one light-year is equal to approximately 6 trillion miles. In nearly 14 billion years, we might expect radiation to expand radially outward to something like 30 billion light-years across.
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- But, the Big Bang was not like an explosion that went off in a room. Following the Big Bang, space-time itself expanded radially outward at all points, meaning all of space expanded too, not just the stuff within it.
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- As the expansion of the universe began, just 1 centimeter of “empty space” interstitially became 2 centimeters over time, and so on. So the best ideas about the size of the universe allowing for its expansion over time point to a radius of slightly more than 46,000,000,000 light-years and therefore a diameter for the universe of approximately 93 billion light-years.
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- That diameter is the visible universe we can see from Earth. Inflation theory, suggests the portion of the universe we can see is by no means the entire cosmos. The cosmos, at least the part that we can observe, is about 93 billion light-years across.
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- That says that traveling at 186,000 miles per second, or 670,633,500 miles per hour, it would take you 93,000,000,000 years to get across it.
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- A thorough understanding of our neighborhood, our solar system, our area of the Milky Way, our galaxy, and so on is critical to comprehending how the universe works. And exploring the cosmic distance scale also unveils a slew of interesting objects astronomers use to determine distances to objects near and far.
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- The history of measuring the universe stretch back in time all the way to the Greek astronomer Aristarchus of Samos (ca. 310–230 B.C.), who had correct notions of parallax in mind with regard to distances of the Sun and Moon. Parallax is the technique of measuring the offset of nearer bodies to the distant background of stars and geometrically calculating a distance.
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- Little progress took place after Aristarchus until Polish astronomer Nicolas Copernicus (1473–1543) proposed the heliocentric model of the cosmos, and it was one of the last great visual astronomers, Danish nobleman Tycho Brahe (1546–1601), who made the first parallax measurements of comets and helped define a more modern distance scale to nearby objects.
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- The physical scale of just our solar system, only the Sun, its nine planets and debris, and our little island of life inside it can be better visualized by changing the scale. . To envision our immediate vicinity a little better imagine a scale solar system with the Sun on one end and 1 centimeter representing the distance between our star and Earth, called an astronomical unit (AU). That is, 1 AU = 1 centimeter.
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- You actually can draw this out on paper to help crystallize it in your mind. Tape several sheets of paper together and have at it. With the Sun at one end, Earth is 1 centimeter away, and Mercury and Venus are in there too at 0.4 centimeter and 0.7 centimeter, respectively.
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- Outward from Earth, we have Mars at 1.5 centimeters, the main-belt asteroids centered around 2.5 centimeters, Jupiter at 5 centimeters, Saturn at 9.5 centimeters, Uranus at 19 centimeters, and Neptune at 30 centimeters. Pluto can be placed at 40 centimeters.
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- The outer solar system is sparse, consisting of the Kuiper Belt region from 30 to 50 centimeters from the Sun, and you can even indicate some of the more interesting objects in the area of Pluto, Haumea at 40 centimeters, Makemake at 45 centimeters, and Eris at 60 centimeters.
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- Now you can finish by indicating the region of the scattered disk, a sparse body of energetically “spun up” icy asteroids, between 50 and 100 centimeters from the Sun. This gives you a complete scale model of the solar system in a region spanning 1 meter, or 3 feet, across.
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- Now appreciate that on this scale, the inner edge of the Oort Cloud, the vast halo of 2 trillion comets on the solar system’s perimeter, is 100 meters farther away than the edge of your diagram. The outer edge of the Oort Cloud, on this scale, is 1,000 meters away (0.6 mile, more than 10 football fields away).
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- Yet as human astronaut-explorers, we only have traveled as far away as the Moon, about 1/389 AU, or on our scale 1/389 centimeter, from Earth, which on this scale is about the size of a human red blood cell. That distance is imperceptibly close to our planet’s “dot” on our scale drawing.
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- The distances to the nearest stars are larger than our imagined scale of the Oort Cloud. And then come perhaps 400 billion stars scattered across the bright disk of our Milky Way Galaxy, 150,000 light-years across, and a hundred billion more galaxies spread across a vast cosmos.
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- The next time you’re out under the stars, look up and think carefully about the enormity of the universe. It is one of the great humbling feelings of humanity.
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- Edwin Hubble got the impression of this. He was one of the first astronomers that introduced this expanding Universe to us. On the evening of Oct. 4, 1923, near Los Angeles, he got into his car and began a motorized trek up to Mount Wilson. There, he arrived at the observatory that housed the 100-inch Hooker Telescope, at the time the largest telescope in the world.
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- Edwin Hubble was a fourth-year astronomer at Mount Wilson; he enjoyed using the Hooker Telescope because he was interested in, among other things, studying spiral “nebulae.” These mysterious gas clouds were scattered across the sky, and no one understood their nature. In the early days of the 1920s, Hubble had assigned himself the task of figuring out those mysterious nebulae.
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- He pointed the great telescope toward his favorite object, the nebula in Andromeda, M31. This spiral-shaped cloud is faintly visible to the naked eye under a clear, moonless sky. He then captured its image on a photographic plate.
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- Hubble was excited to find a suspected “nova“, an “exploding star“. The next night, he photographed M31 again, hoping to catch the nova and record it under better atmospheric stability. The second plate did indeed record the nova, but little did he know, he also had captured a plate that would become legendary in the history of science.
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- Astronomer Edwin Hubble made an exposure of the “Andromeda Nebula” with the Hooker Telescope at Mount Wilson Observatory near Los Angeles on Oct. 5, 1923. He was initially excited, believing he had recorded a nova, an exploding star.
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- The nova was not a nova at all, but a particular type of star that changed its brightness, a Cepheid variable. Checking earlier plates, he was able to confirm that, and he realized that the star’s faintness had incredible implications.
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- The star, and the nebula that encompassed it, must lie at a distance of a million light-years, three times larger than anyone at the time believed the size of the whole universe to be. Today, thanks to improved measurements, astronomers know the object is 2.5 million light-years away.
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- At Arizona’s Lowell Observatory, as early as 1912, Vesto M. Slipher had recorded the apparent velocities of spiral nebulae and, with the work now done by Hubble, it was clear the universe was expanding, the galaxies were flying apart from one another over time. The universe was not only far larger than anyone had previously believed, but it was growing as time went on.
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- By 1929, astronomers had put a cosmic picture of the past together. If you traced the histories of many of the galaxies backward in time, it meant that the cosmos began with a small, infinitely dense point at its origin.
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- This research was an extension of work originally done by Belgian astronomer Georges Lemaître. Astronomers understood this cosmic point of origin, later called the Big Bang, as the start of the universe, and, they calculated, it must have occurred billions of years ago. The Big Bang had commenced the expansion that was driving all the galaxies away from each other as time rolls on. The whole universe seemed to be flying apart.
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- In the 1930s, Hubble began to study and classify galaxies into their various so-called morphological types, the array of structures astronomers saw in photographs. He eventually assembled the types of galaxies he observed into a tuning fork-shaped diagram.
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- It contained spiral galaxies, barred spiral galaxies, spirals containing a linear “bar” of material passing through their centers, lenticular (lens-shaped) galaxies, and elliptical galaxies. He also identified irregular galaxies, clouds of stars and gas that lacked an organized shape.
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- Later on, astronomers would identify peculiar galaxies, systems that appeared to be wracked with explosive or disruptive events. They also identified a class of galaxies called dwarf spheroidals, which seemed to be numerous in the local universe.
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- By the 1950s, French astronomer Gérard de Vaucouleurs of the University of Texas had expanded Hubble’s classification scheme into a more complex system that took into account many observed properties of galaxies. De Vaucouleurs produced a pseudo-three-dimensional plot showing the galaxies’ relationships, nicknamed the “Cosmic Lemon” due to its shape.
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- De Vaucouleurs included details on bars in galaxies, descriptions of rings of matter visible in them, and an evaluation of how loosely or tightly the spiral arms of a galaxy were wound. He also included evaluative details about the nature of irregular and peculiar galaxies.
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- The last generation of extragalactic astronomy has moved into far more sophisticated analyses than cataloging. By using the Hubble Space Telescope, astronomers have estimated that some 100 billion galaxies must exist in the cosmos. And the number may be much greater than that. Probably some 2 trillion galaxies existed in the early universe, but it seems clear that galaxies near each other are drawn together by gravity and combine over cosmic time.
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- Despite the universal expansion, then, normal galaxies like the Milky Way are probably made of dozens or more protogalaxies that merged into larger systems. You can see these primitive blobs of matter, bluish protogalaxies, in the early universe within the Hubble Ultra Deep Field pictures.
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- As astronomers have studied greater numbers of galaxies over the past few decades, they’ve discovered many things, but one that is impossible to ignore is that the universe is incredibly large as the beginning of this Review tried to illustrate.
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- If you look at a galaxy in your telescope’s eyepiece tonight, the photons striking your eye have been traveling at the fastest speed there is, 186,000 miles per second (300,000 kilometers per second). Nonetheless, they have taken 2,500,000 years at that velocity to reach us from the Andromeda Galaxy.
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- And that galaxy is nearly on our cosmic doorstep. Of course, the knowledge of our own galaxy, in a primitive sense, goes back to antiquity. The name Milky Way comes from the Latin via lactea, which derives from the original idea, the Greek term, galaxías kýklos, “milky circle.”
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- The band of Milky Way visible in our sky, most prominently in the summer and winter evenings, is the unresolved light from billions of stars lying along the plane of our galaxy. Only in the past few decades have we come to understand that the Milky Way is one of the 100 billion galaxies in the universe, and that its disk stretches some 100,000 light-years across.
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- It contains some 400 billion stars, although we don’t know exactly how many because dwarf stars are faint and difficult to see over long distances. For decades, astronomers believed the Milky Way was a simple spiral galaxy. But studies in this century have shown the Milky Way is a barred spiral, and that our sun and solar system lie some 26,000 light-years from the center, in one of the galaxy’s arms.
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- The Milky Way consists of a bright disk, a slowly spinning platter of stars and gas that contains most of the stars we see. Our sun orbits the center of the galaxy once every 220 million years, meaning that we’ve rotated around the galactic center about 20 times since the formation of the solar system.
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- Far away, in the center of the galaxy, lies a supermassive blackhole containing around 4.3 million times more mass than the sun. In recent times, astronomers have discovered that supermassive black holes in the centers of galaxies are the norm. Nearly all galaxies, except for dwarfs, have them.
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- The galaxy’s disk is encapsulated by a halo of a small number of stars, along with huge spheres of ancient stars called globular star clusters, and a big envelope of dark matter. Astronomers don’t yet know what dark matter consists of, but they know it is there because of the gravitational influence it has on the visible matter they can observe.
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- The Milky Way belongs to a group of at least 54 galaxies called the “Local Group of galaxies“, a name Hubble gave to this local cloud of objects as he mapped the nearby cosmos.
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- The primary members of the Local Group are the Milky Way, the Andromeda Galaxy, and the Pinwheel Galaxy (M33). But each of these big three spirals has a cloud of attendant galaxies, too.
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- The Milky Way’s satellites include the Large and Small Magellanic Clouds, visible to the naked eye in the Southern Hemisphere, and many dwarf galaxies. The diameter of the Local Group is about 10 million light-years, some 100 times the diameter of the Milky Way.
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- Beyond our Local Group of galaxies is a larger assemblages of galaxies called clusters and very large ones called superclusters. Despite the overall expansion of the universe, meaning that most galaxies are moving away from each other as the cosmos grows, gravity keeps smaller numbers of galaxies bound to each other on their journeys.
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- Our Local Group is a member of the so-called Virgo Cluster of galaxies, named so because its richly populated center lies in the constellation Virgo in our sky.
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- The Virgo Cluster contains at least 1,500 galaxies and is centered some 54 million light-years from Earth. You can see some of the brightest galaxies near the core of the Virgo Cluster in amateur telescopes, in an array called Markarian’s Chain. This line of galaxies contains supermassive elliptical galaxies such as M84 and M86, and a variety of spiral galaxies.
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- Most of the Virgo Cluster galaxies contain supermassive blackholes in their centers. M87 is quite an example. Whereas the Milky Way’s central black hole weighs in at 4.3 million solar masses, the colossal black hole inside M87 contains an estimated mass of 5 billion to 7 billion suns, some 1,000 times more massive than ours.
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- M87 is one of the largest galaxies in our part of the universe. ia so-called “cD galaxy“, short for “centrally dominant“, and it has “eaten” many of the smaller galaxies that once surrounded it. That’s what massive galaxies do, they consume their neighborhood partners.
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- A cluster containing 1,500 galaxies is one thing, but much larger assemblages of galaxies also exist. The Virgo Cluster itself is a member of the so-called Virgo Supercluster, which holds thousands of galaxies on a scale an order of magnitude larger yet.
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- The Virgo Supercluster holds our Milky Way, the Local Group, the Virgo Cluster, and altogether some 100 galaxy groups and clusters. This amazingly large framework stretches some 110 million light-years across, and is one of about 10 million superclusters that make up the entire cosmos.
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- Despite the huge number of galaxies existing in the Virgo Supercluster, astronomers now know that most of the space in this volume is essentially empty. The diameters of these great voids range from dozens to hundreds of millions of light-years.
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- Filamentary chains of galaxies wind their way around the dark, empty spaces. On large scales, galaxies in clusters and superclusters are like soap bubbles, with galaxies coating the surfaces and voids lying in between.
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- By the 1980s, astronomers had identified the “Great Wall“, a sheet of galaxies measuring 500 million light-years across. More recently, the Sloan Digital Sky Survey uncovered the Sloan Great Wall, an assemblage of galaxies at least twice the size of the Great Wall, which covers a long dimension of some 1.4 billion light-years.
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- As astronomers discovered more and more distant galaxies, they found that some large mass seemed to be tugging on the local universe, pulling us in the direction of the southern constellations Triangulum Australe and Norma. Called the Great Attractor, this anomaly, some 200 million light-years away, puzzled astronomers.
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- Astronomers eventually discovered that an even larger mass in that direction was pulling us. This mammoth structure, called the Shapley Supercluster, is 650 million light-years away and contains the greatest concentration of galaxies in our local part of the cosmos.
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- Additional surprising discoveries have occurred, too. In 2014, astronomers identified a new supercluster based on the relative motions of galaxies analyzed in a more sophisticated way than ever before.
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- ‘Laniakea“, which is also sometimes called the Local Supercluster, contains some 100,000 galaxies, including the Local Group and the Milky Way. This massive cluster and all its members are traveling together through space, but not all of the galaxies within it are gravitationally bound. Some will splinter apart from the rest of the cluster as time rolls on.
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- The Laniakea Supercluster has four major components — the Virgo Supercluster, the Hydra-Centaurus Supercluster, the Pavo-Indus Supercluster, and the Southern Supercluster. Altogether, Laniakea contains around 500 galaxy clusters and groups. And surrounding Laniakea in the local universe are other galaxy super clusters, the Shapley Supercluster, the Hercules Supercluster, the Coma Supercluster, and the Perseus-Pisces Supercluster. Each of these structures holds hundreds of galaxy clusters and are linked by the fabric like web of cosmic structure.
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- Beginning in the 1980s, astronomers found evidence of structures even larger than superclusters. At first, objects now called Large Quasar Groups (LQG) baffled astronomers.
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- In 1982, Scottish astronomer Adrian Webster found what would become known as the Webster Large Quasar Group, a collection of five quasars, or actively feeding black holes, stretching over 330 million light-years.
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- Now, nearly two dozen LQGs are known. A structure known as the Huge LQG contains 73 quasars over a diameter of some 4 billion light-years. This massive structure, dismissed by some astronomers, may hold the title as the largest collection of related matter in the cosmos.
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- Truly, the universe is so big that it’s hard to comprehend. On one hand, the enormity of the universe makes us feel small. Our brief lives happen so quickly, and we wink out, mostly unaware of the incredibly large cosmos around us.
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- But the fact that we are sentient, that we can ponder the stars and galaxies far away from us, makes life in the universe a truly amazing thing. And we’re just starting to get to know the immense world of galaxies. The more I learn the smaller I feel.
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- December 8, 2020 GALAXIES - how many galaxies? 2928
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