My Science School

          Although viruses are the simplest living things, they need to live and reproduce themselves inside a larger organism, so they are unlikely to have been the first living things on Earth. The earliest evidence of life that has been found is tiny fossils of primitive bacteria in rocks about 3800 million years old. Later, blue-green algae evolved. They could use energy from the Sun and in so doing gave off oxygen. Modern plants and animals share these simple organisms as ancestors.

          The earliest evidence for life on Earth arises among the oldest rocks still preserved on the planet. Earth is about 4.5 billion years old, but the oldest rocks still in existence date back to just 4 billion years ago. Not long after that rock record begins, tantalizing evidence of life emerges: A set of filament-like fossils from Australia, reported in the journal Astrobiology in 2013, may be the remains of a microbial mat that might have been extracting energy from sunlight some 3.5 billion years ago. Another contender for world's oldest life is a set of rocks in Greenland that may hold the fossils of 3.7-billion-yer-old colonies of cyanobacteria, which form layered structures called stromatolites.

          Some scientists have claimed to see evidence of life in 3.8-billion-year-old rocks from Akilia Island, Greenland. The researchers first reported in 1996 in the journal Nature that isotopes (forms of an element with different numbers of neutrons) in those rocks might indicate ancient metabolic activity by some mystery microbe. Those findings have been hotly debated ever since — as, in fact, have all claims of early life.

          Most recently, scientists reported in the journal Nature that they had discovered microfossils in Canada that might be between 3.77 billion and 4.29 billion years old, a claim that would push the origins of life to very shortly after Earth first formed oceans. The filament-like fossils contained chemical signals that could herald life, but it's hard to prove that they do, researchers not involved in the study told Live Science. It's also hard to prove that fossils found in ancient rocks are necessarily ancient themselves; fluids have penetrated cracks in the rock and might have allowed newer microbes in to older rock. The researchers used samarium-neodymium dating to arrive at the 4.29 billion maximum age for the fossils. This method, which uses the decay of one rare-earth element into another, may measure the age of the magma that formed the rocks rather than the rocks themselves, an issue that has also dogged claims of the Earth’s oldest rocks.   

          Still, the fact that suggestive evidence of life arises right as the rock record begins raises a question, said University of California, Los Angeles, geochemist Elizabeth Bell in a SETI Talk in February 2016: Is the timing a coincidence, or were there earlier forms of life whose remnants disappeared with the planet's most ancient rocks?

          The period that occurred before the rock record begins is known as the Hadean. It was an extreme time, when asteroids and meteorites pummeled the planet. Bell and her colleagues said they might have evidence that life arose during this very unpleasant time. In 2015, the research team reported discovering graphite, a form of carbon, in 4.1-billion-year-old crystals of zircon. The ratio of isotopes in the graphite suggested a biological origin, Bell and her colleagues wrote in the journal Proceedings of the National Academy of Sciences.

          "There is some skepticism, which is warranted," Bell told Live Science. Meteorites or chemical processes might have caused the odd carbon ratios, she said, so the isotopes alone aren't proof of life. Since the publication of the 2015 paper, Bell said, the researchers have found several more of the rare-carbon inclusions, which the scientists hope to analyze soon.

          From what is known of this period, there would have been liquid water on the planet, Bell told Live Science in an interview. There might have been granite, continental-like crust, though that's controversial, she said. Any life that could have existed would have been a prokaryote (a single-celled organism without membrane-bound nuclei or cell organelles), Bell added. If there was continental crust on Earth at the time, she said, prokaryotes might have had mineral sources of nutrients like phosphorus.

          A different approach to the hunt for Earth's early life suggests that oceanic hydrothermal vents may have hosted the first living things. In a paper published in July 2016 in the journal Nature Microbiology, researchers analyzed prokaryotes to find the proteins and genes common to all of these organisms, presumably the final remnants of the Last Universal Common Ancestor (LUCA) — the first shared relative from which all life today descends.

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          The Earth began to be formed over 4.5 billion years ago, but for millions of years nothing could live here. Gradually, the Earth’s crust and the atmosphere formed. The simplest forms of life, bacteria and algae, probably began to grow less than four billion years ago. Human beings did not appear until about two million years ago.

          Microbial life forms have been discovered on Earth that can survive and even thrive at extremes of high and low temperature and pressure, and in conditions of acidity, salinity, alkalinity, and concentrations of heavy metals that would have been regarded as lethal just a few years ago. These discoveries include the wide diversity of life near sea–floor hydrother­mal vent systems, where some organisms live essentially on chemical energy in the absence of sunlight. Similar environments may be present elsewhere in the solar system.

          Under­standing the processes that lead to life, however, is complicated by the actions of biology itself. Earth’s atmosphere today bears little resemblance to the atmosphere of the early Earth, in which life developed; it has been nearly reconstituted by the bacteria, vegetation, and other life forms that have acted upon it over the eons. Fortunately, the solar system has preserved for us an array of natural laboratories in which we can study life’s raw ingredients — volatiles and organics — as well as their delivery mechanisms and the prebiotic chemical processes that lead to life. We can also find on Earth direct evidence of the interactions of life with its environments, and the dramatic changes that life has undergone as the planet evolved. This can tell us much about the adaptability of life and the prospects that it might survive upheavals on other planets.

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          One interesting aspect of volcanic eruptions is that surrounding areas are covered rapidly in molten rock or ash, sometimes preserving the animals, plants and structures underneath. Archaeologists have been able to study life in Roman times, for example, by examining the remains of Pompeii, in Italy, buried when Vesuvius erupted in AD 79.

          I guess the main good effect that volcanoes have on the environment is to provide nutrients to the surrounding soil. Volcanic ash often contains minerals that are beneficial to plants, and if it is very fine ash it is able to break down quickly and get mixed into the soil.

          Perhaps the best place to look for more information about this would be to look up references about some of the countries where lots of people live in close proximity to volcanoes and make use of the rich soils on volcanic flanks. These would include Indonesia, The Philippines, Japan, Italy, etc.

          I suppose another benefit might be the fact that volcanic slopes are often rather inaccessible, especially if they are steep. Thus they can provide refuges for rare plants and animals from the ravages of humans and livestock.

          Finally, on a very fundamental scale, volcanic gases are the source of all the water (and most of the atmosphere) that we have today. The process of adding to the water and atmosphere is pretty slow, but if it hadn’t been going on for the past 4.5 billion years or so we’d be pretty miserable.

          Volcanoes have done wonderful things for the Earth. They helped cool off the earth removing heat from its interior. Volcanic emissions have produced the atmosphere and the water of the oceans. Volcanoes make islands and add to the continents.

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          Like earthquakes, volcanoes mainly occur along fault lines. Molten rock, gases and ash are forced out through a gap in the Earth’s crust to release the pressure beneath. Over thousands of years, cooled rock sometimes builds up around the fissure in the ground to form the familiar conical shape of a volcano.

          Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Since it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually, some of the magma pushes through vents and fissures to the Earth's surface. Magma that has erupted is called lava.

          Some volcanic eruptions are explosive and others are not. The explosivity of an eruption depends on the composition of the magma. If magma is thin and runny, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano. A good example is the eruptions at Hawaii’s volcanoes. Lava flows rarely kill people because they move slowly enough for people to get out of their way. If magma is thick and sticky, gases cannot escape easily. Pressure builds up until the gases escape violently and explode. A good example is the eruption of Washington’s Mount St. Helens. In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra. Tephra can range in size from tiny particles of ash to house-size boulders.

          Explosive volcanic eruptions can be dangerous and deadly. They can blast out clouds of hot tephra from the side or top of a volcano. These fiery clouds race down mountainsides destroying almost everything in their path. Ash erupted into the sky falls back to Earth like powdery snow. If thick enough, blankets of ash can suffocate plants, animals, and humans. When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes.

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          In some areas, underground lakes, rivers and springs are heated by molten rocks below. The hot water bubbles to the surface in springs and forms pools, or it may shoot upwards under great pressure, forming a geyser.

          Geysers result from the heating of groundwater by shallow bodies of magma. They are generally associated with areas that have seen past volcanic activity. The spouting action is caused by the sudden release of pressure that has been confining near-boiling water in deep, narrow conduits beneath a geyser. As steam or gas bubbles begin to form in the conduit, hot water spills from the vent of the geyser, and the pressure is lowered on the water column below. Water at depth then exceeds its boiling point and flashes into steam, forcing more water from the conduit and lowering the pressure further. This chain reaction continues until the geyser exhausts its supply of boiling water.

          The boiling temperature of water increases with pressure; for example, at a depth of 30 metres (about 100 feet) below the surface, the boiling point is approximately 140 °C (285°F). Geothermal power from steam wells depends on the same volcanic heat sources and boiling temperature changes with depth that drive geyser displays.

          As water is ejected from geysers and is cooled, dissolved silica is precipitated in mounds on the surface. This material is known as sinter. Often geysers have been given fanciful names (such as Castle Geyser in Yellowstone National Park) inspired by the shapes of the colourful and contorted mounds of siliceous sinter at the vents.

         Geysers are rare. There are more than 300 of them in Yellowstone in the western United States —approximately half the world’s total—and about 200 on the Kamchatka Peninsula in the Russian Far East, about 40 in New Zealand, 16 in Iceland, and 50 scattered throughout the world in many other volcanic areas. Perhaps the most famous geyser is Old Faithful in Yellowstone. It spouts a column of boiling water and steam to a height of about 30 to 55 metres (100 to 180 feet) on a roughly 90-minute timetable.

          No structure can withstand very large earthquakes, but by using reinforced materials and foundations that allow for movement, architects have been able to design buildings able to survive even quite strong shocks.

          Many engineers and architects now believe it's possible to build an earthquake-proof building -- one that would ride the waves of the most fearsome temblor and remain as good as new once the shaking had stopped. The cost of such a building, however, would be staggering. Instead, construction experts strive for something slightly less ambitious -- earthquake-resistant buildings, which are designed to prevent total collapse and preserve life, as well as construction budgets.

          In recent years, the science of building earthquake-resistant structures has advanced tremendously, but it's not an entirely new subject. In fact, a few ancient buildings still stand today despite their location in active seismic zones. One of the most notable is the Hagia Sophia, a domed church (now museum) built in Istanbul, Turkey, in A.D. 537. About 20 years after it was completed, the massive dome collapsed after a quake shook the area. Engineers evaluated the situation and decided to rebuild the dome, but on a smaller scale. They also reinforced the whole church from the outside.

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          The earth’s crust is made up of 15 pieces or “plates”, which float on the molten rock below. The places where these plates meet are called faults. Along the lines of faults, the plates move and push against each other. Sometimes this causes a violent shock, with waves of tremors moving out and shaking the Earth’s surface.

          The Earth’s crust consists of seven large lithospheric plates and numerous smaller plates. These plates move towards each other (a convergent boundary), apart (a divergent boundary) or past each other (a transform boundary).

          Earthquakes are caused by a sudden release of stress along faults in the earth's crust. The continuous motion of tectonic plates causes a steady build-up of pressure in the rock strata on both sides of a fault until the stress is sufficiently great that it is released in a sudden, jerky movement. The resulting waves of seismic energy propagate through the ground and over its surface, causing the shaking we perceive as earthquakes.

         Induced quakes are caused by human activity, like tunnel construction, filling reservoirs and implementing geothermal or fracking projects.

          Volcanic quakes are associated with active volcanism. They are generally not as powerful as tectonic quakes and often occur relatively near the surface. Consequently, they are usually only felt in the vicinity of the hypocentre.

          Collapse quakes can be triggered by such phenomena as cave-ins, mostly in karst areas or close to mining facilities, as a result of subsidence.

          Take a look at recent seismic activity, and you might get the impression that Earth, perhaps a bit too overcaffeinated, has a bad case of the shakes. Earthquakes rattled Chile on and off during 2010-11, beginning with a magnitude-8.8 temblor (or earthquake) that struck just off the coast near Concepcion in February 2010. Then, in March 2011, a magnitude-9.0 quake rocked Japan, triggering a tsunami that killed an estimated 29,000 people and damaged nuclear reactors. And finally, in August 2011, a magnitude-5.8 quake centered near Mineral, Va., spooked residents up and down the Atlantic seaboard and damaged the Washington Monument.

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          The size of the shock waves from an earthquake is measured on the Richter scale. No earthquake has ever measured more than 9 on this scale. However, the size of the shock waves cannot tell us how much damage the earthquake has done. That depends on many factors, such as the kind of soil on which buildings are constructed, how they are built and so on. The effects of earth-quakes are measured on the Modified Mercalli scale. The highest point on this scale is 12, which describes the total destruction of all buildings but has luckily rarely been used.

          Today, an earthquake's size is typically reported simply by its magnitude, which is a measure of the size of the earthquake's source, where the ground began shaking. While there are many modern scales used to calculate the magnitude, the most common is the moment magnitude, which allows for more precise measurements of large earthquakes than the Richter scale.

          In the news, however, when an earthquake’s magnitude is given , the scale used to calculate the magnitude is not usually specified since the modern scales are all very similar. A network of geological monitoring stations, each with instruments that measure how much the ground shakes over time called seismographs allow scientists to calculate an earthquake's time, location and magnitude.

          Seismographs record a zigzag trace that shows how the ground shakes beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world.

          From these measurements, a quake's magnitude is usually reported as, for example, a magnitude-7.0 in the case of the earthquake that struck Haiti on Jan. 12. Based on their magnitude, quakes are assigned to a class, according to the U.S. Geological Survey. An increase in one number, say from 5.5 to 6.5, means that a quake's magnitude is 10 times as great. 

          After an earthquake strikes, its magnitude is continuously revised as time passes and more stations report their seismic readings. Several days can pass before a final number is agreed upon.

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          Beneath the land and water that cover the Earth's surface lie layers of rock and metal at very high temperatures. The deepest mines ever dug have not reached the bottom of the outer layer, called the crust. Under the crust, a layer called the mantle is thought to be made partly of solid and partly of molten rock. At the centre of the Earth, there is an outer core of molten metal and an inner core of solid metal, probably largely iron.

          The Earth's interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the Sun. The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter. Although this inner core is white hot, the pressure is so high the iron cannot melt. The iron isn't pure—scientists believe it contains sulfur and nickel, plus smaller amounts of other elements. Estimates of its temperature vary, but it is probably somewhere between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius).

          Above the inner core is the outer core, a shell of liquid iron. This layer is cooler but still very hot, perhaps 7,200 to 9,000 degrees Fahrenheit (4,000 to 5,000 degrees Celsius). It too is composed mostly of iron, plus substantial amounts of sulfur and nickel. It creates the Earth's magnetic field and is about 1,400 miles (2,300 kilometers) thick.

          The next layer is the mantle. Many people think of this as lava, but it's actually rock. The rock is so hot, however, that it flows under pressure, like road tar. This creates very slow-moving currents as hot rock rises from the depths and cooler rock descends. The mantle is about 1,800 miles (2,900 kilometers) thick and appears to be divided into two layers: the upper mantle and the lower mantle. The boundary between the two lies about 465 miles (750 kilometers) beneath the Earth’s surface.

          The crust is the outermost layer of the Earth. It is the familiar landscape on which we live: rocks, soil, and seabed. It ranges from about five miles (eight kilometers) thick beneath the oceans to an average of 25 miles (40 kilometers) thick beneath the continents. Currents within the mantle have broken the crust into blocks, called plates, which slowly move around, colliding to build mountains or drifting apart to form new seafloor.

          Continents are composed of relatively light blocks that float high on the mantle, like gigantic, slow-moving icebergs. Seafloor is made of a denser rock called basalt, which presses deeper into the mantle, producing basins that can fill with water.

          Except in the crust, the interior of the Earth cannot be studied by drilling holes to take samples. Instead, scientists map the interior by watching how seismic waves from earthquakes are bent, reflected, sped up, or delayed by the various layers.

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          Of course, the Moon does not really change shape — it just seems as though it does. The Moon orbits the Earth once every 27.3 days. It has no light of its own, but as it moves around, it is lit by the Sun. Only the part of the Moon that is both turned towards the Earth and lit by the Sun is visible on Earth. The amount of the Moon’s surface that can be seen changes as the Moon’s position changes.

          You see, the Moon doesn’t actually produce any light at all.  The Sun is a ball of burning gas that provides a constant source of light, zooming away from it in all directions.  Some of that light bounces off objects in space – moons, planets, asteroids, etc., meaning that the same light bounces into our eyes so we can see them.  When you look at the Moon you see the Sun’s light that has reflected off it, to you!  I mean, that works for everything: the screen you’re looking at now produces light but turn the device around (tablet, phone, laptop, etc.) – can you see the back of it?  Of course you can, but it isn’t producing light like the screen. So, if you can see it, where’s the light coming from?  That light, reflecting off the back of the device to your eyes, allows you to see it.

          Why the Moon changes shape, there’s the Moon, it looks like it's hanging up there in the sky, but in fact, it’s travelling around the Earth, but the Earth is spinning too. OK then, on a clear night the amount of Moon that you can see is dependent upon two things: where it is on its LUNAR CYCLE (takes about a month … or “moon-th”!) and where the Sun is in relation to the Moon. You see, if the face of the Moon you can see is also facing the Sun (even though it’s night time to us), then it’s fully lit by the Sun’s light. That’s a full Moon.

          To us, the Sun may have set, but remember, its light is still streaming out in all directions and any that falls on the Moon may be reflected in your eyes. Now, as the Moon undertakes its lunar cycle, different faces of the Moon are lit by the Sun and, depending on where you are, you may be able to see all of the lit side (Full Moon) or only part of the lit side (Crescent moon/Gibbous Moon, etc.). Golly, complicated! Well, some pictures really help to understand this.  So, let’s have a look at the different shapes (or phases) of the Moon.

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          The first artificial satellite, Sputnik 1, was launched by the USSR in October 1957. The same year, a dog called Laika was the first living creature to travel in space in Sputnik 2. It was the USSR again that put the first human in space in 1961, when Yuri Gagarin travelled in Vostok1. In 1969, US astronauts were the first to land on the Moon in Apollo 11.

          For centuries, scientists had been looking at the prospects of traveling into outer space. In the 1940s, experimental rocket launches into outer space were carried out time after time, but none could reach the desired altitudes. On October 4, 1957, the Soviet Union sent the first unmanned mission into space. They launched a satellite called Sputnik 1, which successfully remained in outer space for 3 months. On November 3, 1957, they subsequently launched another satellite known as the Sputnik 2, which carried a dog into orbit for 7 days. The Americans were envious of the success of the Russians, and the fact that there was a cold war between the two countries did not make things better. This led to the beginning of the “space race”.

          Space scientists were always looking for the possibility of sending human beings into outer space. After experimenting with animals, it was time for the first manned space mission. The first successful manned space mission was launched by Russia on April 12, 1961, carrying a young man known as Yuri Gagarin. The spacecraft was Vostok 1, and it orbited around the earth in 1 hour 48 minutes. One month later, the United States launched their first manned space mission with astronaut Alan Shepard, who managed to complete a suborbital flight. John Glen achieved his first orbital flight on February 20, 1962.

          With advancement in technology, it became easier and safer to launch manned missions. This led to an attempt to land on the moon, and it was achieved when Neil Armstrong and his crew in the Apollo 11 made a safe landing on the moon on July 20, 1969. Neil Armstrong proceeded to make the first moon walk. This great achievement catapulted America’s reputation in the space race.

           Not all space missions have been successful though. There were several tragedies involving space missions, and some of them had fatal results. On January 27, 1967, the Apollo 1, which was also known as Apollo/Saturn 204, caught fire during its simulation launch, killing all crew members. Russia’s attempt to land on the moon ended in tragedy too when the Soyuz 1 encountered technical problems soon after its launch. The sole crew member perished as he could not repair the fault.

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          Human beings have always tried to see pictures in the patterns the stars make. The names given to those pictures by European scholars in medieval times and earlier are often used by astronomers today. Most of them are in Latin as that was the language of scholarship in Europe for hundreds of years.

          Not necessarily. Each constellation is a collection of stars that are distributed in space in three dimensions – the stars are all different distances from Earth. The stars in a constellation appear to be in the same plane because we are viewing them from very, very, far away. Stars vary greatly in size, distance from Earth, and temperature. Dimmer stars may be smaller, farther away, or cooler than brighter stars. By the same token, the brightest stars are not necessarily the closest. Of the stars in Cygnus, the swan, the faintest star is the closest and the brightest star is the farthest!

          Most of the constellation names we know came from the ancient Middle Eastern, Greek, and Roman cultures. They identified clusters of stars as gods, goddesses, animals, and objects of their stories. It is important to understand that these were not the only cultures populating the night sky with characters important to their lives. Cultures all over the world and throughout time — Native American, Asian, and African — have made pictures with those same stars. In some cases the constellations may have had ceremonial or religious significance. In other cases, the star groupings helped to mark the passage of time between planting and harvesting. There are 48 “ancient” constellations and they are the brightest groupings of stars – those observed easily by the unaided eye. There actually are 50 “ancient” constellations; astronomers divided one of the constellations (Argo) into 3 parts.

          “Modern” constellations — like the Peacock, Telescope, and Giraffe — were identified by later astronomers of the 1500s, 1600s, and 1700s who used telescopes and who were able to observe the night sky in the southern hemisphere. These scientists “connected” the dimmer stars between the ancient constellations. There are 38 modern constellations.

          Most of the constellation names we know came from the ancient Middle Eastern, Greek, and Roman cultures. They identified clusters of stars as gods, goddesses, animals, and objects of their stories. It is important to understand that these were not the only cultures populating the night sky with characters important to their lives. Cultures all over the world and throughout time — Native American, Asian, and African — have made pictures with those same stars. In some cases the constellations may have had ceremonial or religious significance. In other cases, the star groupings helped to mark the passage of time between planting and harvesting. There are 48 “ancient” constellations and they are the brightest groupings of stars – those observed easily by the unaided eye. There actually are 50 “ancient” constellations; astronomers divided one of the constellations (Argo) into 3 parts.

          “Modern” constellations — like the Peacock, Telescope, and Giraffe — were identified by later astronomers of the 1500s, 1600s, and 1700s who used telescopes and who were able to observe the night sky in the southern hemisphere. These scientists “connected” the dimmer stars between the ancient constellations. There are 38 modern constellations.

          In 1930 the International Astronomical Union officially listed 88 modern and ancient constellations (one of the ancient constellations was divided into 3 parts) and drew a boundary around each. The boundary edges meet, dividing the imaginary sphere — the celestial sphere — surrounding Earth into 88 pieces. Astronomers consider any star within a constellation boundary to be part of that constellation, even if it is not part of the actual picture.

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          A Galaxy is an enormous group of stars held together by gravity. Our galaxy, the Milky Way, is in the shape of a spiral. Other galaxies are elliptical or irregular. There may be 100 billion galaxies in the universe. Many of them are grouped together in clusters, with huge areas of space in between.

          A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems, all held together by gravity. We live on a planet called Earth that is part of our solar system. But where is our solar system? It’s a small part of the Milky Way Galaxy. A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems. A galaxy is held together by gravity. Our galaxy, the Milky Way, also has a supermassive black hole in the middle.

          When you look up at stars in the night sky, you’re seeing other stars in the Milky Way. If it’s really dark, far away from lights from cities and houses, you can even see the dusty bands of the Milky Way stretch across the sky.

          There are many galaxies besides ours, though. There are so many, we can’t even count them all yet! The Hubble Space Telescope looked at a small patch of space for 12 days and found 10,000 galaxies, of all sizes, shapes, and colors. Some scientists think there could be as many as one hundred billion galaxies in the universe.

          Some galaxies are spiral-shaped like ours. They have curved arms that make it look like a pinwheel. Other galaxies are smooth and oval shaped. They’re called elliptical galaxies. And there are also galaxies that aren’t spirals or ovals. They have irregular shapes and look like blobs. The light that we see from each of these galaxies comes from the stars inside it.

          Sometimes galaxies get too close and smash into each other. Our Milky Way galaxy will someday bump into Andromeda, our closest galactic neighbor. But don’t worry. It won’t happen for about five billion years. But even if it happened tomorrow, you might not notice. Galaxies are so big and spread out at the ends that even though galaxies bump into each other, the planets and solar systems often don’t get close to colliding.

          The sun is our nearest star. It is 149.6 million km (92.9 million miles) away from Earth. Stars are massive nuclear reactors, generating energy in their cores. It is the heat and light from the Sun that makes life on Earth possible. The huge gravity pull of the Sun keeps the planets of our Solar System orbiting around it.

          The closest stars to Earth are three stars in the Alpha Centauri system. The two main stars are Alpha Centauri A and Alpha Centauri B, which form a binary pair. They are an average of 4.3 light-years from Earth. The third star is Proxima Centauri. It is about 4.22 light-years from Earth and is the closest star other than the sun.

          Alpha Centauri A and B orbit a common center of gravity every 80 years. The average distance between them is about 23 astronomical units (AU) — a little more than the distance between the sun and Uranus. Proxima Centauri is about one-fifth of a light-year or 13,000 AUs from the two other stars, a distance that makes some astronomers question whether it should be considered part of the same system.

          Proxima Centauri may be passing through the system and will leave the vicinity in several million years, or it may be gravitationally bound to the binary pair. If it's bound, it has an orbital period around the other two of about 500,000 years.

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          The universe is the name we give to all of space. Astronomers use huge telescopes, both on Earth and in space, to measure light, x-rays and radio waves from objects that are billions of light years away. Earth is one of the nine planets that orbit our Sun. It is part of the Milky Way Galaxy, one of billions of galaxies in the universe.

          Knowledge of the location of Earth has been shaped by 400 years of telescopic observations, and has expanded radically since the start of the 20th century. Initially, Earth was believed to be the center of the Universe, which consisted only of those planets visible with the naked eye and an outlying sphere of fixed stars. After the acceptance of the heliocentric model in the 17th century, observations by William Herschel and others showed that the Sun lay within a vast, disc-shaped galaxy of stars. By the 20th century, observations of spiral nebulae revealed that the Milky Way galaxy was one of billions in an expanding universe, grouped into clusters and superclusters. By the end of the 20th century, the overall structure of the visible universe was becoming clearer, with superclusters forming into a vast web of filaments and voids. Superclusters, filaments and voids are the largest coherent structures in the Universe that we can observe. At still larger scales (over 1000 mega parsecs) the Universe becomes homogeneous, meaning that all its parts have on average the same density, composition and structure.

          Since there is believed to be no "center" or "edge" of the Universe, there is no particular reference point with which to plot the overall location of the Earth in the universe. Because the observable universe is defined as that region of the Universe visible to terrestrial observers, Earth is, because of the constancy of the speed of light, the center of Earth's observable universe. Reference can be made to the Earth's position with respect to specific structures, which exist at various scales. It is still undetermined whether the Universe is infinite. There have been numerous hypotheses that the known universe may be only one such example within a higher multiverse; however, no direct evidence of any sort of multiverse has been observed, and some have argued that the hypothesis is not falsifiable.