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Introduction Astronomy Basics The Sky, The Stars, and You Ways of Looking Inside Astronomy Sky Stories	Astronomy Basics Have you ever looked at the night sky and wondered about your place in the universe? This section will help you get oriented in the cosmic "big picture." Find out where we are and where we're headed. Learn about Earth and our solar system, plus some of the secrets astronomers have uncovered about the stars, galaxies and universe. Take a flight through the universe. This woodcut shows the astronomer's quest to look beyond Earth's horizons and discover a deeper reality.  |  Anonymous. From L'Atmosphere Météorologie Populaire (Paris, 1888), © Camille Flammarion. Naming the Stars The word astronomy comes from two Greek words: "astron," which means "star," and "nemein," which means "to name." Our Place in Space We live on a tiny blue planet, one of nine worlds that orbit an average-size star called the Sun. Our Sun and its family of planets exist within a vast group of stars called a galaxy. Our galaxy is called the Milky Way, and it contains nearly 200 billion stars-that's about the same number as there are grains of sand on a beach. That might sound like a lot, but it's only the beginning. The Milky Way is one of about 50 galaxies that form a loose cluster called the Local Group. Some of our neighbours in the Local Group are the Large and Small Magellanic Clouds, the Andromeda galaxy and the Triangulum galaxy. The Andromeda galaxy-the largest member of the Local Group of galaxies-is about 2.25 million light-years away.  |  Chris Cook © 2002 The Local Group belongs to a cluster of more than 2 500 galaxies, known as the Virgo Cluster. Altogether, these galaxies and galaxy groups form the Local Supercluster. And things still get bigger!  More... The Big Picture As far as astronomers can tell, the universe contains about 100 billion galaxies. These galaxies are grouped into clusters, which in turn are grouped into even bigger groups called superclusters. Superclusters are arranged in long filaments that reach through space. These filaments are like tendrils of light that embrace large bubbles of empty space called galactic voids. It's as if the universe were a vast, frothy sea, with galaxies forming the surface of the bubbles. More than 1.6 million galaxies are shown in this large-scale view of the nearby universe. The closest ones are blue and the farthest ones are red.  |  © 2MASS (2 Millimeter All Sky Survey) / CALTECH / MIT. Smaller than a pinprick At the other end of the scale, everything in the universe - people, planets, stars and galaxies - is made of atoms. Atoms are unimaginably small-about 10,000 billion could fit in the dot at the end of this sentence. An atom is a tiny nucleus surrounded by clouds of negatively charged electrons. The nucleus, in turn, consists of even smaller protons and neutrons. Neutrons, like their name implies, are electrically neutral, while protons carry a positive charge. Finally, the smallest particles that we've found are quarks, which are what protons and neutrons are made of. A helium atom has two protons, two neutrons and two electrons.  |  © Planétarium de Montréal. The speed of light Light moves through space at about 300 000 km per second. If you flew to the Sun in a Boeing 747, it would take you 20 years to get there, but light travels the same distance in only eight minutes. Light-years Cosmic distances are so vast that astronomers measure them in light-years. A light-year is the distance a beam of light travels in a year-about 10 trillion kilometres. The closest star to us (apart from the Sun) is Proxima Centauri, which is 4.2 light-years away. Its light takes 4.2 years to reach the Earth. A beam of light is fast enough to circle the Earth 7½ times in one second.  |  © Planétarium de Montréal. The Solar System Our solar system is the group of planets, moons and other space debris circling our Sun. There are also at least 120 moons circling the planets, and millions of asteroids in a belt between Mars and Jupiter. At the outer edge of the Solar system, far beyond the orbit of distant Pluto, is a halo of icy comets called the Oort cloud. The Sun is at the centre of our solar system. The four inner planets are Mercury, Venus, Earth and Mars. The five outer planets are Jupiter, Saturn, Uranus, Neptune and Pluto.  |  © Planétarium de Montréal. Some asteroids come close to Earth's orbit, although astronomers don't know of any that pose an immediate threat to the Earth. By studying asteroids, astronomers hope to find a way to counteract any that come too close for comfort in the future.  |  © NASA / JPL. The Sun The Sun has the mass of 333 000 Earths; its massive gravity is what holds the solar system together. As stars go, the Sun is medium-sized. It generates almost all of the light in the solar system. We see the planets and moons because they reflect sunlight. More about the Sun Sky Woman For Indigenous Australians, the Sun is a Sky Woman who has her camp in the East. Each morning she wakes up and lights a bark torch that carries across the sky during the day. Read the full story. Pisim (Cree) or Keesis (Anishinabe) is translated as the Sun, in English. The Planets There are nine planets in our solar system. The four inner planets-Mercury, Venus, Earth and Mars-are small, dense and rocky and are called the "terrestrial" planets. Four of the outer planets-Jupiter, Saturn, Uranus and Neptune-are mostly gas and are called the "gas giants." The ninth planet, Pluto, does not fit into either category: it is a small, icy world, more similar to comets than the other planets. Our planet, Earth, is third from the Sun. Earth takes about 365 days to circle the Sun- or one year. The length of a year on each planet depends on its orbit around the Sun; the closer a planet is to the Sun, the faster the planet moves. Mercury, the innermost planet, races around the Sun in only 88 Earth days, while Pluto, the outermost planet, takes 248 Earth years to complete one orbit. More about the planets: Mercury Venus Earth Mars Saturn Uranus Neptune Pluto The Moon, Sun and Morning Star are Sky Beings in the Blackfoot culture of the North American Plains. Find out how they helped the Blackfoot's ancestors. More... The Sun The Sun is a star, just like the stars we see at night. The Sun is much closer to us than the other stars, so it appears much bigger and brighter. The Sun's visible disk is called the photosphere. Surrounding the photosphere is a thin layer called the chromosphere, which is only seen during a total solar eclipse. The outer atmosphere of the Sun is called the corona. Eclipses of the sun inspire creative works and cultural rituals. One rock-climbing performance group from Canada performed a piece during the solar eclipse of May 1994. More... The Sun.  |  © NASA / Extreme UV Imaging Consortium. The basics: Period of revolution (around centre of galaxy): 220 000 000 years Period of rotation 27 days on average Diameter: 1 392 000 km Surface gravity: 27.9 times greater than Earth's Composition: 75% hydrogen, 25% helium, with traces of heavier elements Temperature: 5 500 degrees Celsius at surface; 15 000 000 degrees Celsius at core Appearance in Earth's sky: The Sun appears as a blindingly bright disk of light. In fact, the Sun is bright enough to cause serious eye damage, so it's important never to look at it directly. When the Sun is in the sky, the atmosphere of the Earth scatters the blue component of the Sun's light, turning the sky blue. This blue sky is bright enough that we cannot see the other stars in the sky. (The stars are still there, however, and can be seen in the daytime during a solar eclipse.) Telescopic appearance: WARNING! NEVER LOOK DIRECTLY AT THE SUN! The Sun is so bright that looking at it through a telescope for even an instant can cause permanent blindness. The best way to observe the Sun is by using a pinhole camera or another indirect viewing method. The most obvious features of the solar disk are sunspots. These are cooler regions of the Sun's surface and are sometimes visible as dark spots. The Sun rotates relatively slowly, once every 27 days. In the Anishinabe culture of Central North America, the Sun is Grandfather Sun, a being of the sky world who has seven daughters. More... Mercury Mercury is the densest planet (5.4 times the density of water) and the closest planet to the Sun. Mercury was last visited by the Mariner 10 spacecraft in 1974. Mercury.  |  © NASA / JPL. The basics: Order (outwards from the Sun): 1 Average distance from the Sun: 57 900 000 km (0.39 astronomical units) Period of revolution (length of year): 87.95 days Period of rotation (length of day): 58.6 days Diameter: 4 879 km Surface gravity: 0.38 times greater than Earth's Atmosphere: Virtually none Satellites: None Composition: Basaltic rocks and dust; nickel-iron core Surface temperature: Average 227 degrees Celsius during the day, -173 degrees Celsius at night Appearance of surface: Rocky and covered with meteorite craters. Appearance similar to Earth's Moon. Appearance in Earth's sky: Mercury is visible to the unaided eye, and appears as a bright star-like object. In the northern hemisphere, Mercury stays close to the glow of dusk and dawn, either very low in the west after sunset, or very low in the east before sunrise. In the southern hemisphere, Mercury is high enough above the horizon that it can be viewed in total darkness several times a year. Telescopic appearance: Mercury is hard to observe because it is usually close to the horizon, where it's obscured by atmospheric turbulence. Because it is closer to the Sun than the Earth, Mercury shows phases, like the Moon. It is unlikely you will see any surface details on Mercury. Venus Venus is the hottest planet in the solar system Venus is the hottest planet in the solar system because the constant cloud cover traps heat like a greenhouse. In 1975, the Soviet Venera probes landed on Venus, sending back TV pictures of the surface. In 1992, NASA's Magellan went into a Venus orbit and produced a complete radar picture of the surface. Venus.  |  © NASA / JPL. The surface of Venus.  |  © NASA / JPL. Venus spins in the opposite direction of every other planet. This means that, on Venus, the Sun rises in the west and sets in the east. The basics: Order (outwards from the Sun): 2 Average distance from the Sun: 108 200 000 km (0.72 astronomical units) Period of revolution (length of year): 224.70 days Period of rotation (length of day): 243.02 days Diameter: 12 104 km Surface gravity: 0.91 times greater than Earth's Atmosphere: Carbon dioxide, hydrogen sulphide, sulphuric acid, 90 times thicker than Earth's atmosphere. Dense clouds completely cover the planet. Constant lightning. Satellites: None Composition: Basaltic and granitic (hard volcanic) rocks Surface temperature: 460 degrees Celsius both day and night, due to greenhouse effect caused by heavy cloud cover Appearance of surface: Landscapes of rocky rubble or flat rock outcrops. The rocks are partially molten and flow slowly. Light level similar to an overcast day on Earth. Appearance in Earth's sky: A brilliant star-like object seen as a "morning star" or "evening star." Always appears close to the Sun in the sky. Venus outshines every other object in the sky except the Sun and Moon. It is often misidentified as an aircraft or an unidentified flying object (UFO) because of its striking brilliance. Telescopic appearance: Thick clouds make it impossible to see the surface of Venus, but the phases (similar to Mercury's and the Moon's) are easily seen with any telescope. Even in a thin crescent phase, Venus is large enough that the phase can be seen with binoculars. Morning Star In the far north of Australia, Barnumbir (the Morning Star - the planet Venus) lives on the Island of the Dead. Because she was so bright, her people asked her to join them in their fishing boats in the morning, so they could see better. But Barnumbir was afraid of drowning and refused to go. Read the full story. Kisikaw Achak (Cree) or Geezhigo Anung (Anishinabe), the Day Star, is the planet Venus when seen during the day. Kisikaw Achak (Cree) or Geezhigo Anung (Anishinabe) is really a planet orbiting the Sun. Its position, therefore, cannot be fixed on a star map. Earth Earth is unique in the solar system. It is the only planet that has both liquid water and an oxygen-nitrogen atmosphere, two features essential to life as we know it. It is also the only planet known to harbour living creatures. There are over six billion people on Earth now, and countless animals and plants. Earth.  |  © NASA  As the Earth orbits the Sun it also rotates on its axis. When we're facing the Sun it's daytime, and when we're facing away it's night. Because the Earth rotates on its axis as it orbits the Sun, parts of the Earth that face the Sun experience daytime while parts that face away experience night. Though the Earth rotates once every 23 hours 56 minutes, the period from one sunrise to the next is actually 24 hours. That's because the Earth changes position from one day to the next as it moves around the Sun. As a result, it must rotate for an additional 4 minutes to bring the Sun back to the same position in the sky. Because the Earth rotates on its axis as it orbits the Sun, parts of the Earth that face the Sun experience daytime while parts that face away experience night. Though the Earth rotates once every 23 hours 56 minutes, the period from one sunrise to the next is actually 24 hours. That's because the Earth changes position from one day to the next as it moves around the Sun. As a result, it must rotate for an additional 4 minutes to bring the Sun back to the same position in the sky.  |  © Planétarium de Montréal. The basics: Order (outwards from the Sun): 3 Average distance from the Sun: 149 600 000 km (1.00 astronomical unit) Period of revolution (length of year): 365.25 days Period of rotation (length of day): 23 hours 56 minutes Diameter: 12 756 km Atmosphere: 78% nitrogen, 21% oxygen, traces of carbon dioxide, water vapour. Satellites: 1 (the Moon), plus numerous man-made satellites Composition: Basaltic rock continents with an iron core. Liquid water covers about two-thirds of the Earth's surface Surface temperature: Range of -89 degrees Celsius to 55 degrees Celsius, depending on season and location. Appearance of surface: Mostly covered with liquid water. Rocky continents with a wide variety of geologic formations. Green plants cover large areas of the land. Effects of human population (farming, mining, nighttime lighting of cities) visible from orbit. The Moon The Moon is our closest neighbour in space. It orbits the Earth at an average distance of 384,000 km and takes 27.3 days to complete one revolution. As it orbits the Earth the Moon is illuminated from different angles by the Sun, which produces a quarter lunar phase roughly each week. Since the Moon is only one quarter the Earth's diameter, it has much less mass and only 1/6th the Earth's gravity. Because the Moon takes the same amount of time to rotate on its axis as it does to revolve around the Earth, one side of the Moon always faces us. Long ago, the Moon used to rotate more quickly but the earth's gravity gradually slowed it down. Scientists believe the Moon formed 4.6 billion years ago when a Mars-sized asteroid tore a huge chunk out of the Earth. This chunk later solidified in orbit around the Earth and became the Moon. Lunar samples, gathered by the Apollo astronauts, show that the Moon consists of exactly the same minerals as parts of the Earth. Mother Earth in the Anishinabe culture of Central North America came from Grandmother Moon and Grandfather Sun. More... Mars In August 1996, scientists announced the discovery of a 1.9 kg (4.2 Ib) rock, determined to have originated on Mars and showing evidence of what appears to be the fossil remains of microscopic bacteria that lived on Mars 3.6 billion years ago. If confirmed, this discovery will be one of the most significant in astronomy. The Mars Global Surveyor camera has sent back photos showing recent flow patterns, areas which look like dry riverbeds, that may have been caused by water. Mars.  |  © NASA and the Hubble Heritage Team (STScI/AURA). The basics: Order (outwards from the Sun): 4 Average distance from the Sun: 227 900 000 km (1.524 astronomical units) Period of revolution (length of year): 686.95 days Period of rotation (length of day): 24 hours 37 minutes Diameter: 6 794 km Surface gravity: 0.38 times greater than Earth's Atmosphere: Thin carbon dioxide Satellites: 2 small moons (Phobos and Deimos) Composition: Basaltic rock Surface temperature: As low as -140 degrees Celsius, but occasionally as high as 27 degrees Celsius Appearance of surface: Cold desert world covered with rocky, dusty terrain. Ice caps of frozen carbon dioxide at the north and south poles. Craters, valleys and dormant volcanoes, much larger than those found on Earth, dot the surface. Appearance in Earth's sky: Mars changes its appearance dramatically over the course of its year. At its closest approach to Earth it can be brighter than any of the stars, and quite obviously reddish-coloured. When farthest away, it is a dim red point in the sky. Telescopic appearance: Mars is the only planet whose surface is easily visible from Earth. Mars is the only planet whose surface is easily visible from Earth. When Mars is at its closest point to Earth (every two years or so), a telescope can show a reddish disk, as well as the white polar ice caps and some of the dark surface markings. At other times, Mars appears so small that little detail can be seen even with a large telescope. Jupiter Jupiter.  |  © NASA. See Jupiter in orbit around the Sun. In July 1994, the fragments of Comet Shoemaker-Levy collided with Jupiter, producing the largest explosions ever observed in our solar system. The fragments left behind dark patches in the clouds of Jupiter, allowing astronomers to observe the way Jupiter's atmosphere works. Jupiter rotates on its axis every 9 hours and 55 minutes. Watch a video of Jupiter. Can you see the Great Red Spot? In December 1995, the Galileo spacecraft went into orbit around Jupiter and began returning close-up images of Jupiter's atmosphere and moons. The basics: Order (outwards from the Sun): 5 Average distance from the Sun: 778 300 000 km (5.20 astronomical units) Period of revolution (length of year): 11.86 years Period of rotation (length of day): 9 hours 55 minutes at the equator Diameter: 142 980 km at the equator Surface gravity: 2.54 times greater than Earth's Composition: Scientists do not agree on whether Jupiter has a solid rocky core or if it consists only of gases that condense in the middle to form liquids. The interior ocean is made primarily of hydrogen and helium, with a few other gases Atmosphere: Hydrogen, helium, some methane and ammonia Satellites: 61 moons and one faint ring Surface temperature: 10 000 degrees Celsius in the interior ocean; -140 degrees Celsius at the cloud tops; 25 000 degrees Celsius at the centre. Therefore, Jupiter radiates more heat than it receives Appearance of surface: The visible cloud tops of Jupiter are divided into multicoloured cloud belts and zones, stretched out by Jupiter's rapid rotation. The Great Red Spot, easily visible with small telescopes, is the largest example of the many violent storms in Jupiter's atmosphere. There is a vast liquid hydrogen ocean under the visible cloud tops. Appearance in Earth's sky: Jupiter is easily visible for about 10 months of the year, appearing as a very bright, star-like object. Telescopic appearance: Jupiter offers a wealth of detail to telescope users. The cloud bands, the Great Red Spot, and the four Galilean moons are all easily visible. One of Jupiter's moons is Europa. It has inspired Australian writer Dorothy Porter: "Ever since I saw the first photos of Europa that were sent back to Earth by the unmanned Voyager probe I have been intrigued by this strange moon of Jupiter's. Europa provides amazingly fertile ground for a poet's imagination." Dorothy Porter and the Poetry of Europa Sawanachak (Cree) or Shawan Anung (Anishinabe) is translated into English as the Southern Star. This is the planet Jupiter as seen in the south. Sawanachak (Cree) or Shawan Anung (Anishinabe) is really a planet orbiting the Sun. Its position, therefore, cannot be fixed on a star map. Saturn Saturn is best known for its beautiful system of rings. The ring system is only about 10 kilometres thick and is made up of particles of ice and dust. Most of the rings are less than one metre across, but a few are as large as 100 kilometres across. During 1995, Saturn's rings were visible almost edge-on, making them appear to vanish as viewed from Earth. Saturn.  |  © NASA. The basics: Order (outwards from the Sun): 6 Average distance from the Sun: 1 429 000 000 km (9.56 astronomical units) Period of revolution (length of year): 29.42 years Period of rotation (length of day): 10 hours 40 minutes at the equator Diameter: 120 540 km at the equator; rings are 275 000 km across Surface gravity: 1.08 times greater than Earth's Composition: Scientists are not sure if Saturn is made completely of gases or if it has a solid rocky core. The entire planet has a density less than water- if you could put it in a big enough bathtub, it would float Atmosphere: Hydrogen, helium, some methane and ammonia Satellites: 31 moons and a ring system of 23 major rings Surface temperature: -190 degrees Celsius at the cloud tops. Like Jupiter, Saturn radiates more heat than it receives from the Sun Appearance of surface: Saturn has cloud structures similar to Jupiter, but not as pronounced. There is probably a liquid hydrogen ocean under the visible cloud tops. Appearance in Earth's sky: Saturn can be seen in the sky as a bright star-like object. It moves very slowly against the background stars, and several weeks may be required to see it change its position relative to the stars. Telescopic appearance: A good pair of binoculars will reveal that Saturn is not quite round,due to its ring system. Almost any telescope that magnifies over 30 times will show the rings of Saturn and perhaps a faint cloud band or two on the disk of the planet. The view of Saturn through a telescope is one of the most beautiful sights in astronomy. Uranus Uranus was discovered by amateur astronomer William Herschel in 1781 using a six-inch telescope. The planet is tilted on its side relative to its orbit around the Sun. The north pole of Uranus sometimes points almost directly towards the Sun, and sometimes almost directly away. This means that the "daytime" and "nighttime" on Uranus can be over 40 years long! "Daytime" and "nighttime" on Uranus can be over 40 years long! Uranus was visited by the Voyager 2 robotic spacecraft in January 1986. Voyager 2 increased our knowledge of Uranus tremendously by providing close-up pictures of the planet and its moons and ring system. Uranus.  |  © STScI / NASA. The basics: Order (outwards from the Sun): 7 Average distance from the Sun: 2 875 000 000 km (19.22 astronomical units) Period of revolution (length of year): 83.75 years Period of rotation (length of day): 17 hours 14 minutes at the equator Diameter: 51 120 km at the equator Surface gravity: 0.91 times greater than Earth's Composition: Possibly has an inner rocky core, overlaid with ice (frozen methane, water and ammonia) covered with an ocean of liquid hydrogen Atmosphere: Hydrogen, helium, methane Satellites: 22 moons, and 9 dark rings Surface temperature: -195 degrees Celsius at the cloud tops Appearance of surface: Probably a liquid surface under the visible cloud tops. Appearance in Earth's sky: Uranus is technically visible to the unaided eye, but it is near the limit of detection. It appears as an extremely faint greenish star. Telescopic appearance: A large telescope reveals that Uranus is not a star but a tiny blue-green disk. The planet is so far away that cloud features are not visible in small telescopes. Neptune Neptune's existence was predicted by both English mathematician John Adams and French mathematician U.J.J. Leverrier, based on irregularities in Uranus' orbit. The German astronomer J.G. Galle began searching for a new planet and found it very near the predicted position. In August 1989, the robotic spacecraft Voyager 2 flew past Neptune, radioing back close-up pictures of the planet and its family of moons. Voyager also discovered a large storm system, similar to Jupiter's Great Red Spot, which is called The Great Dark Spot. Due to the elliptical shape of Pluto's orbit, Neptune is the furthest planet from the Sun for about 20 years out of every 248 years. This was the case from about 1979 to 1999. Neptune.  |  © NASA / JPL. The basics: Order (outwards from the Sun): 8 or 9 Average distance from the Sun: 4 504 400 000 km (30.11 astronomical units) Period of revolution (length of year): 163.73 years Period of rotation (length of day): 16 hours 3 minutes at equator Diameter: 49 530 km at the equator Surface gravity: 1.19 times greater than Earth's Composition: Hydrogen and helium ice Atmosphere: Hydrogen, helium, methane Satellites: 11 moons and 5 incomplete ring arcs Surface temperature: -205 degrees Celsius at the cloud tops Appearance of surface: Perhaps a rocky core surrounded by massive layers of ice. May be covered by a deep ocean of liquid hydrogen under the visible cloud tops. Appearance in Earth's sky: Neptune is so far away it cannot be seen without the aid of a telescope. Neptune revolves so slowly that it spends years in basically the same area of the sky. Telescopic appearance: Even in large telescopes, Neptune appears as a small pale blue speck, with no detail visible. Pluto Pluto's orbit is very different from that of the other planets. Pluto orbits in a highly oval path called an ellipse, which is angled to the other planets' orbits by about 17.1 degrees. Pluto's distance from the Sun varies because of this oval path, so that sometimes it is closer to the Sun than Neptune. In 1976, James Christie discovered Pluto's moon, Charon. Charon is about half as big as Pluto, which makes Pluto and Charon more like a double planet than a planet and moon. Pluto and its moon Charon, as revealed by the Hubble telescope.  |  © STScI / NASA / ESO. The basics: Order (outwards from the Sun): 9 or 8 Average distance from the Sun: 5 915 800 000 km (39.55 astronomical units) Period of revolution (length of year): 248.03 years Period of rotation (length of day): 6 days 9 hours 17 minutes Diameter: 2 300 km Surface gravity: 0.05 times greater than Earth's Composition: Probably rock covered with methane ice Atmosphere: A thin atmosphere of nitrogen, methane, and carbon monoxide that is produced only when Pluto is closest to the Sun Satellites: 1 moon (Charon, diameter = 1 200 to 1 300 km) Surface temperature: -215 degrees C Appearance of surface: Scientists suppose that the rocky surface of Pluto is covered by ice frozen as hard as steel. There are also polar ice caps of frozen methane, which partially melt when Pluto is nearest the Sun, releasing the gas that forms Pluto's atmosphere. Appearance in Earth's sky: Pluto is invisible without a moderate-sized telescope. Telescopic appearance: Even with the largest telescopes on Earth, Pluto appears as a faint star-like point. No details can be seen at all. The Hubble Space Telescope has been able to resolve some rough details on Pluto's surface, and also detect its moon, Charon. Stars Many cultures have stories to explain the stars and their role in our lives . Astronomers study the stars using telescopes on Earth and in space. Thanks to their work, we've learned quite a lot about these distant gems that reign far above our world. The birth of stars For ages, people have gazed at the night sky and wondered about the stars, and for ages, the stars have remained distant and serene. Today, thanks to astronomers, we understand what the stars are made of and where they come from. More... The Hercules Cluster of galaxies is 650 million light-years from Earth. In this image, the spiral galaxies appear bluish because they are home to hot, newborn stars. The elliptical galaxies are yellowish because they are full of cool, old stars.  |  © US Gov public domain. [Credit:] NASA. The life of stars From the moment a star is born, its fate is sealed. Depending on its mass, it could end its life quietly or as a blazing supernova. Throughout their lives, stars evolve: they grow and degenerate; they can be violent and explosive; and they are more central to our existence than we ever imagined. More... To some ancient people, the stars looked like tiny pinholes in the sky, with the fires of creation shining through them. The Woman who married Morning Star The Blackfoot people of Canada tell a story about a young girl who married the Morning Star (also called Venus). She was happy living with him above the Earth until one day she dug up a special turnip and looked through a hole in the sky, which was the North Star. When she saw all her family down below, she realized how much she missed them. Read the full story Groups of stars can make up constellations. Their shapes in the sky have inspired the stories of indigenous peoples for generations. Learn about the six lost boys, the fisher and other stories. More... A Star is Born Stars are born within vast, cool clouds of dust and gas in the spiral arms of galaxies like our own. These clouds include molecular hydrogen and helium, plus traces of other elements, such as oxygen, silicon and carbon. Over time, a lot of this gas and dust comes together, probably due to shock waves from exploding stars or passing galaxies. New stars light up clouds of gas and dust in the Trifid Nebula.  |  © US Gov public Domain. [Credit] HST/STSI. The gas and dust form into globules, and small disturbances make these globules start to rotate. As the mass of the globules slowly increases, so does the force of gravity, causing the globules to condense into what are called protostars. As more material spirals inward, the newly formed protostars are surrounded by disk-shaped clouds. Under increasing pressure and temperature, these infant stars begin to glow, radiating light, heat and radio waves. At this point, they're still not true stars. Their subtle glow is obscured by the surrounding clouds, but we can detect their energy using infrared and radio telescopes. A cluster of hot young stars in 30 Doradus illuminates interstellar gas in the Large Megellanic Cloud.  |  © US Gov public Domain. [Credit] NASA. As the protostar's core continues to collapse, its temperature increases. Once the core temperature reaches about 4 million degrees Celsius, which takes between 100 000 and 10 million years, nuclear fusion starts,-and a new star is born. It's a Star's Life When a star begins to shine, it produces a tremendous amount of energy, thanks to the nuclear fusion of hydrogen into helium that happens in its core. Eventually, the star runs out of hydrogen fuel. What happens next depends on how much mass the star had to begin with. Stars in globular cluster NGC6354 formed more that 12 billion years ago: the universe must be even older.  |  © CFHT. White dwarf When a star with about half the Sun's mass runs out of hydrogen fuel, it contracts into a white dwarf. A white dwarf is a tiny compact star, so dense that just one teaspoon of its matter would weigh about a tonne on Earth. Although no longer burning, a white dwarf is still incredibly hot. It slowly cools until all that's left is a burnt out black core. After about 100 billion years, the star's life is over. Red giant A star with up to 2½ times the mass of our Sun goes through a red giant stage before it becomes a white dwarf. The star's core gets so hot that its atmosphere balloons outwards. If you could put a red giant where our Sun is, it would extend as far as the orbit of Venus, or even Earth. Eventually, a red giant collapses and becomes a white dwarf, then slowly cools to a black core. As a red giant collapses, it expells the outer layers of its atmosphere. The star's core remains as a white dwarf near the centre of the huge bubble.© WIYN /NOAO / NSF. Our own Sun will turn into a red giant when it reaches the end of its life, but that won't happen for about four billion years. Supernova Supergiant stars are up to 100 times more massive than our Sun. When a supergiant runs out of fuel, it collapses and blows apart in a supernova explosion. The remaining core continues to collapse and becomes either a neutron star or a black hole, depending on how much mass was left after the explosion. We come from the stars We are made of stardust.  |  National Research Council of Canada. The atoms that make up our bodies were originally created in the thermonuclear fires at the heart of supermassive stars that exploded billions of years ago. In a very real sense, we are all made of stardust! AAT 66. The light echo of supernova 1987A. | © Anglo-Australian Observatory, photograph by David Malin. Galaxies Galaxies are the largest single structures in nature, and certainly among the most beautiful. These vast, starry islands have slowly been sculpted over billions of years by the force of gravity. It's believed that there are about 100 billion galaxies in the universe, and that most of them have a black hole at their centre. Stars in globular cluster NGC6354 formed more that 12 billion years ago: the universe must be even older. The origin of galaxies The first galaxies began forming about one billion years after the Big Bang , very early in cosmic history. Until recently, astronomers were hard-pressed to explain how such enormous structures could have emerged so quickly. The answer, it turns out, is intimately linked to the Big Bang itself. More... Grouping galaxies Like snowflakes, no two galaxies are exactly alike. However, all galaxies have certain features in common, which means we can group them into categories. Though there are many ways to do this, most people still use the system developed by Edwin Hubble in 1925. Hubble grouped galaxies according to two distinct shapes: elliptical and spiral. More... Spiral galaxy NGC 268.  |  © CFHT. Our home-the Milky Way On a clear night in the country, you can look into the sky and see a soft band of light stretching overhead. That glowing band is the starry disk of our galaxy-the Milky Way-seen edge-on from within. Yet this entire cosmic panorama, as vast as it might seem, represents a very small part of our galaxy. More... Priepriggie The Australian tribal hero Priepriggie was as famous for his songs and dances as for his hunting. When he was carried into the sky by flying foxes, his rhythms and songs made the stars twinkle and dance, and arrange themselves in a wide, glittering ribbon-the Milky Way. Read the full story The Spirit Road Cheepahi Meskanaw (Cree) is translated into English as the Spirit Road. This is the path marked across the sky by the Milky Way galaxy when it is turned westward. According to traditional beliefs, the spirit of a person who dies on Earth ascends into the star world, then dances along this path to the place of eternal happiness in the West, beyond the setting Sun. In the Anishinabe language, the Milky Way is called Binessiwi Mekuna, the Bird's Path. In autumn, when it points South, the birds follow it. In spring, it turns North and the birds follow it back again. Galactic Origins Most astronomers believe that the universe began about 15 billion years ago in a fiery explosion called the Big Bang . In one instant, all the space, time, matter and energy in the universe came into being as a tiny speck, then started to expand rapidly. As the universe got bigger and cooler, the force of gravity started to take hold. At the moment of the Big Bang A fraction of a second after the Big Bang, the infant universe went through a sudden growth spurt, called inflation. In far less than a billionth of a second, the universe grew faster than at any other time in its history. Inflation only lasted an instant, and then the universe slowed down to the rate of expansion we see today. After one second As the universe continued to expand and cool, protons, neutrons and electrons condensed out of the background energy, especially in areas where the energy was slightly "denser." After 100 seconds Protons and neutrons combined to form helium nuclei. Hydrogen nuclei, which are simply protons, already existed. In less than two minutes, all the sub-atomic elements that exist today had been formed. After 300 000 years By this time, the universe had cooled enough for electrons to combine with atomic nuclei, thereby forming atoms. These atoms slowly gravitated into vast strand-shaped clouds, from which the galaxies would soon emerge. After one billion years Clouds of hydrogen and helium began "clumping" under the force of gravity. As the clouds grew denser, early galaxies-called protogalaxies-began to take shape. These protogalaxies grew bigger and bigger. Some of them started spinning and flattening out into disk-like shapes. Others stayed more or less spherical as they grew. Eventually, these primal galaxies gathered enough mass for stars to ignite within them, and for the first time, the universe took on the appearance we see today. These images show the evolution of the universe from top to bottom. Frame 1: Temperature fluctuations in the oldest light in the universe. Frame 2: Matter condensing under the force of gravity. Frame 3: The first stars form, just 200 million years after the Big Bang. Frame 4: The first galaxy chains start to form, 1 billion years after the big bang. Frame 5: The modern era, nearly 14 billion years after the Big bang. This remarkable image, called the Hubble Deep Field, represents a "core sample" of the cosmos. It reveals about 1500 galaxies, seen in an area of sky no larger than a grain of sand held at arm's length! Many of these were among the first to emerge after the Big Bang.  |  © STScI / NASA. Galaxy Shapes Galaxies come in many shapes and sizes. No two are exactly alike, but they do tend to follow certain general forms. The Hubble "tuning fork" arranges galaxies according to shape. However, the galaxies do not evolve from one shape to the next. Their shapes are the product of the initial conditions under which they formed.  |  © STScI. Spiral galaxies Spiral galaxies are flat with bright and dark arms winding around a central core. Some have bright bars across their centres or a bulge in the middle. The spiral shape comes from a wave of star formation that proceeds along the disk-like a wave of sports fans standing up in the bleachers. Our Milky Way galaxy is spiral shaped. Elliptical galaxies Elliptical galaxies are fat balls of old, cool stars. Most formed when spiral galaxies collided and lost their star-forming gas as they merged. Elliptical galaxies take either an ellipsoidal (like a U.S. football) or spherical (like a beach ball) shape. Irregular galaxies Some galaxies are neither elliptical nor spiral-these galaxies are called irregular. Irregular galaxies are small and take a wide range of forms. They are often distorted by the violence of star birth or stretched into odd shapes by the gravitational pull of neighbouring galaxies. Space collisions When galaxies collide, they pass through one another like ghosts moving through a wall. Their stars are spaced so far apart that they rarely touch. However, the pull of gravity distorts the galaxies, spraying their stars across space. "Polar Ring" galaxies temporarily form when two galaxies collide.  |  © US Gov public Domain. [Credit] Space Telescope Science Institute. The Milky Way Our galaxy is often called the Milky Way, but this hazy glow also has many other names. The Blackfoot people of the North American Plains call it the Wolf Trail, after the wolves that taught them how to hunt together. Some Indigenous Australians see the Milky Way as a river in the sky, with the big stars as fish, and the small ones as lily bulbs. The name Milky Way comes from the ancient Greeks, who thought the galaxy looked like the milk of the goddess Hera flowing across the sky. The term "galaxy" comes from the Greek word "galaktos," which means milk. It should come as no surprise that the term "galaxy" comes from the Greek word "galaktos," which means milk. Watch a video about the Milky Way. What shape is our galaxy in? Astronomers now believe that the Milky Way is a barred spiral galaxy (more about galaxy shapes) about 100 000 light-years in diameter. It's home to nearly 200 billion stars and their planets. The Milky Way has three parts: a disk, in which our solar system resides, a central bulge in the middle, and a spherical halo that surrounds the whole galaxy. If you could see the Milky Way from 10 million light-years away, it would closely resemble the spiral galaxy M83.  |  © Bill Schoering / NOAO / AURA / NSF. Stellar nurseries The Milky Way's disk contains large clouds of interstellar dust and gas, which clump together into pinkish, star-forming regions. These, in turn, replenish the spiral arms with fresh groups of hot, young blue stars, known as open clusters. The disk's bluish color comes from the young stars in the spiral arms. The Milky Way has hundreds of star-forming regions, and about 1 500 known open clusters. Young stars become visible as their radiation blows dust and gas away from the centre of the Rosetta Nebula.  |  © CFHT. Seeing through the dust We're inside our galaxy, so it's hard for us to get a good look at it. Also, all the interstellar dust and gas obscures our view along the plane of the galaxy, which makes it hard to see its exact shape. But modern instruments using radio waves have penetrated the haze, revealing many astounding features. For one thing, the Milky Way is not the quiet galaxy we once imagined. Intense X-rays and plumes of gamma radiation blaze from its core, where stars whip around a central black hole at over 900 km/s. That's fast enough to circle the Earth in 45 seconds. Also, the Milky Way is consuming several of its smaller neighbour galaxies. In fact, we're in mid-collision with one of them. The Sagittarius dwarf, a spherical galaxy, is currently embedded in the Milky Way's disk, just 80 000 light-years from Earth, on the far side of the central bulge. Two nearby irregular galaxies, the Large and Small Magellanic clouds, are being gravitationally consumed as the Milky Way tears streamers of dust and gas away from them. River in the sky Some Indigenous Australians think of the Milky Way as a river in the sky. The big stars in it are fish and the small stars are lily bulbs. For the Blackfoot people of the North American Plains, the Milky Way is the Wolf Trail. More... Indigenous peoples of central Canada have different names for the Milky Way. The Anishinabe call it Binessiwi Mekuna, the Bird's Path. More... The Universe What holds the galaxies together? Why is the universe still expanding from the Big Bang, and will it ever collapse again in a big crunch? Cosmologists-scientists who study the large-scale structure of the universe-ask these kinds of questions. Here are some answers they've come up with and some areas where they're still in the dark. The mysteries of gravity Gravity is everywhere. It keeps our feet planted firmly on the ground, it holds the planets in orbit around the Sun and it keeps the stars and galaxies from flying apart. Basically, gravity holds the universe together. It's also the most mysterious of nature's forces. More... Artist's conception of a black hole.  |  © NASA / Dana Berry (STScI). The expanding Universe In 1929, astronomer Edwin Hubble discovered that other galaxies are racing away from our galaxy, the Milky Way. In fact, the farther away the galaxy is, the faster it's receding. More... Dark Matter and the fate of the Universe Only 10 per cent of the universe radiates light, the other 90 per cent is invisible to us. This "dark matter" is vital to our understanding of the fate of the universe. Without enough dark matter to put on the "gravitational brakes," cosmic expansion will continue thinning out the universe forever. More... Our quest to understand the cosmos is inspired by our curiousity about who we are and where we come from. For indigenous peoples, nature is a source of spiritual guidance. Stories about creation enhance peoples' understanding of human existence in the cosmos. More... Understanding Gravity Gravity is what holds the universe together. It keeps the galaxies from flying apart, the planets in orbit around the Sun, and our feet firmly on the ground. But we didn't always know what gravity is. To some extent, it's still a mystery. Gravity holds the Moon in orbit around the Earth.  |  © NASA / U.S. Geological Survey. The world according to Newton In 1687, Sir Isaac Newton proved that gravity is the mutual force of attraction between all masses in the universe and that it extends infinitely across space but gets weaker with distance. For us, the most massive object around is the planet we live on. The Earth's mass is about 6 trillion trillion kilograms-that's "6" followed by 24 zeros. That much mass puts out a lot of gravitational force, and all that gravity pulls down towards the centre of the Earth. That's why no matter where you live on the Earth, you can't fall off the "bottom"-gravity always pulls you towards the Earth's centre. Gravity also pulls the Moon toward the Earth. But because of its orbital motion, the Moon doesn't hit the Earth, it basically falls around it. Likewise, Newton concluded that the Sun's gravity pulls the Earth toward the center of the solar system and keeps all the planets in orbit. Einstein's general theory of relativity About two centuries later, in 1915, Albert Einstein predicted how gravity would act near massive objects like stars, which have very strong gravity. He suggested that we think of space as a flat rubber sheet that is stretched tight. A large ball placed on this sheet will make a dent. When a smaller object is placed nearby, it rolls into the dent and "gravitates" toward the larger object. The more massive the object is, the deeper the dent, and the greater the gravitational attraction. A black hole is an extremely massive object embedded in the sheet. It makes such a deep dent that even light can't escape. See how gravity warps space. Your weight on the Moon Gravity actually determines what the "weight" of an object is. Every object, including you, has a certain mass. If you move that object to a planet with a different gravity, it will "weigh" a different amount because a different amount of gravity will be pulling on it. If you went to the Moon, you'd weigh less because the Moon only has about one-sixth the gravity of Earth. So if you weighed 60 kilograms on Earth, you'd only weigh 10 kilograms on the Moon. But on Jupiter, which is more massive than the Earth, you would weigh much more-130 kilograms. And in space, far from any celestial objects, you'd weigh practically nothing. Jupiter's gravity is much stronger than Earth's. If you weighed 60 kilograms on Earth, you'd weigh 130 kilograms on Jupiter.  |  © NASA / CICLOPS / University of Arizona. The Expanding Universe See the expansion of the universe. This remarkable image, called the Hubble Deep Field, represents a "core sample" of the cosmos. It reveals about 1500 galaxies, seen in an area of sky no larger than a grain of sand held at arm's length! Many of these were among the first to emerge after the Big Bang.  |  © STScI / NASA. In 1924, the American astronomer Edwin Hubble began studying spiral nebulae. Back then, astronomers thought spiral nebulae were cloud-like eddies within the Milky Way. They didn't know there were other galaxies in the universe, and the Milky Way was thought to be a simple stream of stars in an otherwise vacant cosmos. Hubble found that there were stars in the "Andromeda nebula." By carefully measuring their brightness he concluded that the nebula was really a huge island of stars far beyond the Milky Way. The nature of spiral nebulae was finally resolved and the Andromeda nebula was re-classified as a galaxy. By 1929, observations of other spiral nebulae led Hubble to conclude that the Milky Way was surrounded by galaxies, and that these galaxies were all moving away from us. In fact, the farther away the galaxies were, the faster they were receding. Photographic Images of Different Nebulae by Australian Photographer of the Night Skies, David Malin. View David Malin images. Nebulae inspired Canadian composer Allan Gordon Bell to write the musical work Nebulae in 1995. Learn more about this work and hear Canadian musician Colleen Athaparia perform the third movement Helix. Growth rate Knowing the rate of expansion is key to understanding the age, size and destiny of the universe. Astronomers are constantly seeking to refine the Hubble constant, a ratio of speed to distance that measures the expansion rate. Stretching universe According to Einstein's theory, the space between galaxies is what expands: the galaxies do not actually move through space. To picture this, imagine lampposts fixed to an expanding highway. As the road stretches, the space between lampposts increases. See it. Dark Matter In this image of galaxy cluster Abell 2218, the light from objects behind the cluster is spread in arc-like patterns by the cluster's gravitational field. This is similar to what happens when light passes through a glass ball. However, the mass needed to produce this distortion is much greater than the mass we can actually see. Images like these confirm the existence of dark matter.  |  W.Couch, University of New South Wales R. Ellis, Cambridge University. © NASA/STScI. We can detect dark matter by the effect that its gravity has on the visible universe. Galaxy clusters move as if they contain more mass than we can actually see. Computer studies show that galaxies are imbedded in vast halos that contain 10 times more mass than we can see-this mass is dark matter. So what is dark matter made of? One possibility is MACHOs (Massive Compact Halo Objects). MACHOs are slow-moving chunks of cosmic material, perhaps burnt out stars, or Jupiter-sized objects, swarming in galactic halos. Or it could come from WIMPs (Weakly Interacting Massive Particles), which are fast-moving, exotic particles, left over from an earlier cosmic epoch. So far, only a few MACHOs have been detected, but no exotic WIMPs. However, neutrinos now seem to be a promising dark matter candidate. Neutrinos are subatomic particles with no electrical charge and very little mass. They are so numerous that even with one-fifty thousandth the mass of an electron, they could account for as much matter as the entire visible universe. Canada's Sudbury Neutrino Observatory (SNO) is located 2000 meters underground in a nickel mine near Sudbury, Ontario. SNO detects neutrinos from the Sun and other astrophysical objects.  |  © E.O. Lawrence Berkley National Lab. Why does it matter to us? Dark matter does matter. In fact, it could tell us whether the universe will end in a big crunch or keep expanding forever. Dark matter does matter. In fact, it could tell us whether the universe will end in a big crunch or keep expanding forever. Depending how much mass exists in the universe, there are three possible scenarios: If the universe has more than enough mass, gravity will eventually stop the expansion of the universe, and everything will fall back together in a "big crunch." This is called a "closed universe." If the universe has just enough mass, it will keep expanding forever, but at a slower and slower rate-it will never stop. This is called a "flat universe." See what a flat or closed universe would look like. If there is less than enough mass, the universe will expand forever, but at an accelerating rate. This is called an "open universe." So far, it looks like number 3 is what will happen. Even with the mass from MACHOs, WIMPs and neutrinos, there isn't enough mass to stop the universe from expanding forever.