Space Information Booklet PDF

Space Information Booklet Section 1 OUR UNIVERSE 1.1 OUR SOLAR SYSTEM Our solar system is the one we know the most about. The Sun is at its centre. Our solar system includes everything that orbits or goes around the Sun. Planets,...

Space Information Booklet Section 1 OUR UNIVERSE 1.1 OUR SOLAR SYSTEM Our solar system is the one we know the most about. The Sun is at its centre. Our solar system includes everything that orbits or goes around the Sun. Planets, moons, asteroids; comets and dust are all part of the solar system. Our solar system lies near the edge of the Milky Way galaxy. The Milky Way is shaped like a whirlpool. All the stars in the galaxy, including our Sun, orbit around the centre of the Milky Way. With the discovery of Pluto in The Solar System 1930, astronomers considered the Solar System to have nine planets. Pluto was classified as the ninth planet and it remained so for 75 years but in 2006, International Astronomical Union classified Pluto as a ‘dwarf planet’ due to its smaller mass. In view of this, our solar system now consists of eight planets. Mercury, Venus, Earth and Mars are known as the inner planets, as they are nearest to the sun. They form a group of rocky planets. The outer planets are Jupiter, Saturn, Uranus and Neptune. 1.1.1 The Sun The sun is a bright star, and it is the largest object in our solar system. The Sun At sunset or sunrise when it is safe to look at the sun, we can see the sun’s photosphere, the level in the sun from which visible photons escape most easily. Dark sunspots come and go on the sun but only rarely are they large enough to be visible to the unaided eye. The solar atmosphere consists of three layers of hot, low-density gas: the photosphere, chromosphere, and corona. The granulation of the photosphere is produced by convection currents of hot gas rising from below. Larger super-granules appear to be caused by larger convection currents deeper in the sun. The chromosphere is most easily visible during total solar eclipses, when it flashes into view for a few seconds. It is a thin, hot layer of gas just above the photosphere, and its pink color is caused by the Balmer emission lines in its spectrum. Filtergrams of the chromosphere reveal spicules, flame like structures extending upward into the lower corona. The corona is the sun’s outermost atmospheric layer and can be imaged using a coronagraph. It is composed of a very-low-density; very hot gas extending many solar radii from the visible sun. It’s high temperature — over 2 million K — is believed to be maintained by the magnetic field extending up through the photosphere — the magnetic carpet— and by magnetic waves coming from below the photosphere. Parts of the corona give rise to the solar wind, a breeze of low density ionized gas streaming away from the sun. Astronomers can study the motion, density, and temperature of gases inside the sun by analyzing the way the solar surface oscillates. Known Layers of Sun as helioseismology, this field of study requires large amounts of data 01 and extensive computer analysis. Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet Nuclear reactors on Earth generate energy through nuclear fission, during which large nuclei such as uranium break into smaller fragments called fission. The sun generates its energy through nuclear fusion, during which hydrogen nuclei fuse to produce helium nuclei. In nuclear fission or nuclear fusion, the energy comes from the strong force. 1.1.2 Mercury Mercury is the closest planet to the sun. It is very hot. It is only 57,900,000 Km away from the sun. Each day lasts 59 earth days and a year lasts 88 earth days. Mercury is a world of extremes. Because it is so close to the Sun, a visitor could easily cook to death. However because Mercury spins so slowly it gets very cold in the night time, which means a visitor could also freeze to death. The average temperature on the lit side of the planet is 350°C (660°F). Mercury is so close to the Sun, and so small, that it has only a very small atmosphere. Its atmosphere has been blown away by the Sun’s solar winds. It means that there is almost no air on Mercury. Mercury has no moons. Due to its smaller size Mercury has lesser gravity. Therefore, if someone weighs 70 pounds on Earth, he would weigh only about 27 pounds on Mercury. 1.1.3 Venus Venus is the second planet from the sun. It is the planet closest to the earth. It is almost the same size as the Earth. Apart from the moon, Venus is by far the brightest object in the night sky. The temperature of the surface is 480°C. Its thick cloud cover makes it impossible to see the surface, and traps much of the Sun's heat. This extra heat gives Venus the hottest average temperature of all the planets. There were once oceans on Venus but these have long since boiled away due to the great heat on the surface. In its early days, Venus was just like the Earth. It had oceans and may even have held life, but as it has so much carbon dioxide now, that life is not possible any more. In many ways Venus is similar to the Earth. It has active volcanoes, Venus quakes, mountains and valleys. The major difference is that Venus's atmosphere makes the planet far too hot for life. Aside from the fact that Venus rotates very slowly so that a day on Venus actually lasts longer than a year, Venus also rotates in the opposite direction as almost all the other planets. Due to this reason, the Sun instead of rising in the east and setting in the west, would appear to rise in the West, and set in the East on Venus. Venus 1.1.4 Earth Our planet ‘Earth’ is an oasis of life in an otherwise desolate universe. The Earth's temperature, weather, atmosphere and many other factors are just right to keep us alive. Earth is the 5th largest planet in our solar system. It is located at third position from the sun. Earth’s mass is 6.0 x 1024 Kg. Its atmosphere mainly contains 78.084% Nitrogen and 20.946% Oxygen. It takes 24 hours to rotate on its axis and 365 days to complete its one orbit around the sun. LUNA, the earth’s moon is our closest neighbour. Its average distance from earth is 384,000 km (239,000 miles). The moon takes 27 days to travel around the earth. This is the only planet we know which has life. Earth 02 Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet 1.1.4.1 The Earth’s Moon The Moon is Earth’s only natural satellite. It is 2,160 miles (3,476 kilo meters) across, which is a little more than one quarter of Earth’s diameter. The Moon orbits Earth once every 27.3 days. The Moon has no atmosphere and no liquid water at its surface, so it has no wind or weather at all. On the lunar surface, there is no protection from the Sun’s rays, and no ability to retain heat like the greenhouse effect on Earth. Temperature on the moon range from about 253°F (123°C) to –387°F (–233°C). The Moon’s surface is covered with rocks, mountains, craters, and vast low plains called Maria (“seas”). The Earth’s Moon On an average, the Moon is about 238,000 miles (384,000 kilo meters) away from Earth. This value was measured quite accurately by the ancient Greek astronomer Hipparchus, who lived in the second century B.C.E. Today, laser rangefinders have been used to measure a very precise value. Galileo Galilei, the first astronomer to study the universe with a telescope, observed that the Moon’s surface was not smooth, but rather covered with mountains and craters. The broad, dark patches on the Moon looked to him like seas on Earth, so he named them Maria, or “seas” in Latin. Moonlight is reflected sunlight. This was discovered long ago by the ancient Greek astronomer Parmenides, who lived and worked around 500 B.C.E. Depending on the location of the Moon in its orbit around Earth, different parts of the Moon will reflect sunlight onto Earth. Since Earth and the Moon are so close together, and since the Moon has such a shiny surface, large amounts of sunlight come to Earth after bouncing off the Moon. Although the Moon is very massive having a mass of 73.5 billion billion metric tons—it is so far away from Earth (238,000 miles or 384,000 kilo meters) that it has very little gravitational pull on objects at or near Earth’s surface. It produces about 1/300,000th the gravitational acceleration that Earth produces at its own surface which is far too weak to be felt by any person. A snapshot of the moon taken from CCD camera at Sonmiani 1.1.5 Mars Mars excites scientists because its mild temperament is more like the Earth's than that of any of the other planets. Evidence suggests that Mars once had rivers, streams, lakes, and even an ocean. But later on, Mars' atmosphere slowly depleted into outer space, the surface water began to permanently evaporate. Today the only water on Mars is either frozen in the polar caps, or is underground. Mars is 249 million km away from the sun. Mars’ day is calculated at 24 hours and 37.5 minutes. It tilts at an axis of 23.98 degrees. Summer and spring last for 381 days and autumn and winter last for 306 days, which makes it cold most of the time. It takes 687 Earth days for Mars to orbit the sun. Mars has two Mars moons. Their names are Deimos and Phobos. Its atmosphere is made up of mainly Carbon Dioxide. 03 Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet 1.1.6 Jupiter Jupiter is the fifth planet from the sun and is about 770 million km from it. It takes almost twelve years for Jupiter to orbit the sun. It spins in less than 10 hours. For that reason its middle has been stretched out. Rather than round it is short and fat. This rapid spinning causes powerful winds that push the clouds into colourful bands, streaks and swirls that circle the planet. The temperature on the surface is very cold about 150°C below freezing and that’s in the daytime! The centre of Jupiter is very hot. Jupiter has sixty three moons, and more are discovered with every space probe sent there. It is likely that there are more not yet identified. As of 2009, Jupiter has 67 satellites known and classified as moons. This Jupiter giant planet has no solid surface. Under its atmosphere is a large liquid ocean of hydrogen and water. There is nothing in between the ocean and atmosphere. The atmosphere slowly gets thicker and thicker until it becomes part of the ocean. In other words Jupiter's ocean has no surface on which you could float a boat. The sky slowly becomes the ocean. 1.1.7 Saturn Saturn is a favourite object for many observers. It is the sixth planet from the Sun. Saturn is 95 times heavier than earth. The main system has a total diameter of over 270,000 km. In many ways Saturn is similar to Jupiter, but it is much smaller. Under the clouds of methane and helium, the sky gradually turns into liquid until it becomes a giant ocean of liquid chemicals. It takes a whole 29 years and 167 days for Saturn to orbit the sun. The length of one-day is just 10 hours and 14 minutes. Saturn Saturn has 61 confirmed moons by now. 1.1.8 Uranus Uranus is almost identical to the planet Neptune. Unlike all the other planets and most of the moons in our Solar System, Uranus spins on its side. It is believed that long ago a very large object smashed into this planet. The crash was so powerful that it completely changed the direction of Uranus’ spin. Uranus may have an ocean of water beneath its clouds. It has a large rocky core, and because of the tremendous pressure it could possibly contain trillions of large diamonds. The atmosphere is thought Uranus to be 10,000 km deep (6,200 miles). It has 27 moons. 1.1.9 Neptune It is the last planet in our solar system. Neptune has a giant storm much like the storm on Jupiter. This storm is often called The Great Dark Spot. We do not know how long this storm has been active because it is so far away that we could not get a good view of Neptune. Neptune has six rings which circle the planet. The distance from the sun to the blue planet is 4,500,000,000 km (2,800,000,000 miles). One day (24 hrs) lasts for 16 hours 3 minutes and one year lasts 164 earth years. Neptune has 13 moons that we know of. Because Neptune is so far away, it is difficult to see any of these worlds. There are probably many more moons orbiting Neptune 04 this blue planet which we have not yet discovered. Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet 1.1.10 The Kuiper Belt and Beyond The Kuiper Belt (also called the Kuiper-Edgeworth Belt) is a doughnut-shaped region that extends between about three to eight billion miles (5 to 12 billion km) out from the Sun (its inner edge is about at the orbit of Neptune, while its outer edge is about twice that diameter). Kuiper Belt Objects (KBOs) are, as their name implies, objects that originate from or orbit in the Kuiper Belt. Pluto is the only one KBO which was known for more than 60 years. Many KBOs have been discovered since 1992, however, the current estimate is that there are millions, if not billions, of KBOs. KBOs are basically comets without tails: icy dirt-balls that have collected together over billions of years. If they get large enough, such as Pluto did, they evolve as other massive planet like bodies do, forming dense cores that have a different physical composition than the mantle or crust above it. Most short-period comets, those with relatively short orbital times of a few years to a few centuries are thought to originate from the Kuiper Belt. Plutinos are Kuiper Belt Objects that are smaller than Pluto, have many physical characteristics similar to Pluto, and orbit around the Sun in much the same way that Pluto does. The discovery of Plutinos led to the recognition that the Kuiper Belt is heavily populated, and that Pluto itself is a Kuiper Belt Object. Pluto is about 1,400 miles (2,300 kilometres) across, less than one-fifth the diameter of Earth and smaller than the seven largest moons in the solar system. Pluto is composed mostly of ice and rock, with a surface temperature between –350 and –380 degrees Fahrenheit (–210 to –230 degrees Celsius); the bright areas observed on Pluto are most likely solid nitrogen, methane, and carbon dioxide. The dark spots may hold hydrocarbon compounds made by the chemical splitting and freezing of methane. Pluto’s day is about six Earth days long, and its year is 248 Earth years long. Pluto travels in a highly elliptical orbit around the Sun compared to the terrestrial planets and gas giants. For 20 years out of its 248-Earth-year orbital period, it is actually closer to the Sun than Neptune. When Pluto is closer to the Sun, its thin atmosphere exists in a gaseous state, and is comprised primarily of nitrogen, carbon monoxide, and methane. For most of its very distant orbit, though, there is no standing atmosphere because it all freezes out and drops to the surface. Pluto has no rings and three known moons. The largest one, Charon, is large enough to be considered a dwarf planet in its own right. Rabinowitz (1960) discovered a new solar system body beyond the orbit of Pluto and larger than Pluto. The discovery of 2003UB 313, proved that Pluto was not the largest Kuiper Belt Object in the solar system. Further observations showed that 2003UB 313 even had its own moon. The discovery of 2003UB 313 hastened the need for planetary astronomers to define the term “planet” in a scientific way. Since it was larger and more distant than Pluto, it would have to be called the tenth planet, unless Pluto was not to be considered a planet any longer. After substantial debate, the objects were officially reclassified in August 2006 by the International Astronomical Union (IAU). That is why there are only eight planets in our solar system today, and why Pluto is not one of them. 05 Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet 1.2 ASTERIODS An asteroid is a large rock in outer space. Due to their smaller size, asteroids do not have enough gravity to pull themselves into the shape of a ball. It is believed that asteroids are left over materials from the formation of the Solar System. 26 very large asteroids have been discovered. There are still millions of smaller ones that we have yet to see because they are too tiny, only a mile or so across. If all the materials of all the asteroids were squashed up into one planet, it would be smaller than our moon. Asteriods Astronomers group asteroids into different categories based on the way they reflect sunlight. The asteroid belt is divided into an inner belt and an outer belt. The inner belt which is made up of asteroids that are within 250 million miles of the Sun contains asteroids that are made of metals. The outer belt, which includes asteroids 250 million miles beyond the Sun, consists of rocky asteroids. These asteroids appear darker than the asteroids of the inner belt, and are rich in carbon. 1.3 METEOROIDS AND METEORITES The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the solar system falls into Earth’s atmosphere creating temporary incandescence resulting from atmospheric friction. This typically occurs at heights of 80 to 110 kilometres above the Earth’s surface. The term is also used loosely with the word meteoroid referring to the particle itself without relation to the phenomena it produces when entering the Earth’s atmosphere. Even smaller particles are called micrometeoroids or cosmic dust grains, which include any interstellar material that should happen to enter our solar system. A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized. 1.4 COMETS A comet is a small body which scientists sometimes call a planetesimal. Comets orbit the Sun just as the planets do. That is why astronomers see the same comet return over and over again. But comets have huge oval-shaped orbits. They swing far out to the edge of the solar system. It takes Earth 365 days to go around the Sun whereas; the orbits of some comets are so big that it takes them hundreds of years to go around the Sun once. For example Halley's Comet appears about every 76 years. When a comet nears the Sun, some of the ice in the comet turns into gas. The gas and loose dust freed from the ice create a long, luminous tail that streams behind the comet. Comets come from places in the outer solar system called the Kuiper Belt and the Oort Cloud. The Kuiper Belt and the Oort Cloud are made up of chunks of ice and rock. Comets that orbit the Sun in less than 200 years come from the Kuiper Belt. The Kuiper Belt is just beyond the planet Neptune. Comets that take longer than 200 years to go around the Sun come from the Oort cloud. The Oort Cloud is far out at the edge of the solar system, beyond Pluto. Comets 06 Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet 1.5 STARS The most fascinating body in the sky at night is a star. A star is a big ball of hot, glowing gases. The gases are mostly hydrogen and helium. Stars give off heat, light, and other kinds of energy. At night stars look tiny because they are millions of miles away from our earth. The closest star to our Earth is the Sun. The Sun is a star at the center of our solar system. A star can get birth and can encounter death as well. The life of a star depends on its mass. The higher the mass the lower its life time because high mass stars burn their fuel more quickly as compared to low mass stars. Our Sun is believed to be about 4.6 billion years old and scientists believe that half of its life is passed away. There are stars older or younger as well as bigger than our Sun. Stars are born from swirling clouds of gas and dust. Gravity pulls the gas and dust together. The gas and dust form a spinning ball. As it spins, it gets hotter. The gas and dust get tightly packed and birth of new star takes place. Likewise when a star burns up its fuel it comes to death. Now question arises what makes a star glow and give off energy? As stated, mostly stars are made up of gases i.e., hydrogen and helium. A star has several layers. The center of a star is called its core. A star shines because of its core. The core is so hot and tightly packed that atoms crunch together. Hydrogen atoms crunch together and become helium atoms. This is called nuclear fusion. Nuclear fusion gives off enough energy to make stars shine. Planets differ from stars in a variety of ways. A star is a collection of hot glowing gases that are held together by gravity which give off enormous quantity of energy as light and heat caused by the thermonuclear fusion taking place in its core. A planet on the other hand is a body that orbits around a star and it can be rocky, gaseous or a combination of the two having its own gravitational field holding it together. The sun is a star and Earth is a planet. Stars 1.5.1 Constellations From ancient times, people living in different parts of the world defined their own ways of categorizing groups of stars termed as constellations. Their old scriptures also reflect their cultural influence on their depiction. Constellations are, thus, the ideas and picture that human mind have imposed in the sky in an effort to connect our lives on earth with the working of the heavens. An asterism is a group of stars in the sky that, when viewed from Earth, create an outline of some recognizable shape or pattern. Two well-known asterisms are the Big Dipper, which many astronomers use to point out the location of the North Star, and the Summer Triangle, which is marked by three of the most Orion Constellation depiction in an old Scripture prominent stars in the Northern Hemisphere’s summer night sky. A constellation is similar to an asterism, but it is usually much more complicated, containing more stars or larger areas of the sky. A few asterisms are constellations: the asterism called the Southern Cross, for example, is the constellation Crux (the Cross). Modern constellations are mostly named after mythological themes, such as gods, legendary heroes, creatures, or structures. Although most constellations resemble the figures after which they are named, others are not as recognizable. The constellations encompass the entire celestial sphere and provide a visual reference frame. Astronomers can plot the stars and other objects in the universe using constellations, charting the Orion Constellation 07 apparent movement that is caused by Earth’s own rotation and orbit. shown in star charts Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet The current, internationally agreed upon map of the sky contains 88 constellations. Some well-known constellations include Aquila (the Eagle), Cygnus (the Swan), Lyra (the Harp), Hercules and Perseus (two mythological heroes), Orion the Hunter and Ophiucus the Knowledge- seeker (two other mythological characters), Ursa Major and Ursa Minor (the Big Bear and Little Bear), and the constellations of the zodiac. 1.5.2 Colors of Stars Stars come in different colours. They can be deep red, orange, yellow, white, or even blue. The colour of a star depends on how hot the star is. The coolest stars are reddish and the hottest stars are bluish. It is hard to imagine how hot a star can be. The temperature at the surface of red stars is about 3000° Celsius. Yellow stars have surface temperatures about 6000° Celsius. Our Sun is a yellow star. White stars are about 10,000° Celsius. Astronomers study the light of stars. Patterns in the light can tell astronomers what the stars are made of and how hot they are. 1.5.3 Distance between Stars There are many different methods used to find distance between stars, but historically the most accurate has been parallax. This is the same effect that makes distant trees appear to move more slowly than nearby ones when you drive past them in a car. Basically, as the Earth orbits the Sun, nearby stars appear to move a teeny bit as a reflection of our motion, while stars farther away move less. Since we know how big the orbit of the Earth is, we can use trigonometry to calculate the stars' distances. 1.6 BLACK HOLE A Black Hole is an extremely dense celestial body that has been theorized to exist in the universe. The gravitational field of a black hole is so strong, that nothing including electromagnetic radiation can escape from its vicinity. It is surrounded by a spherical boundary called the horizon, through which light can enter but not escape, it therefore appears totally black. The black hole concept was developed by a German astronomer in Black Holes in the Galaxies 1961 on the basis of Einstein’s General Theory of Relativity. According to General Relativity, Gravitation severely modifies space and time near a black hole. In 1994, astronomers used the Hubble Telescope to uncover the first convincing evidence that black holes exist. 1.7 ORIGIN OF THE UNIVERSE Cosmology is the study of the origin, structure and the history of the universe as a whole. Astronomers and physicists who do research in cosmology are called cosmologists. The darkness of the night sky leads to the conclusion that the universe is not infinitely old. If the universe were infinite in extent, infinite in age, and static, then every spot on the sky would glow as brightly as the surface of a star. This problem, commonly labelled Olbers’s paradox, implies that the universe had a beginning. 08 Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet 1.7.1 Hubble’s Law In 1929, Edwin Hubble discovered that the galaxies are moving away from each other. He presented his finding in the form of a law which is known Edwin Hubble as Hubble’s law. Tracing the expansion of the universe backward in time bring us to imagine an initial high-density, high-temperature state commonly called the big bang. A rough estimate of the age of the universe based on the presently observed expansion rate is called the Hubble time. The cosmic microwave background radiation is blackbody radiation with a temperature of about 2.73 Kelvin, spread nearly uniformly over the entire sky. This radiation is the light from the big bang. The background radiation is clear evidence that the universe began with a big bang. 1.7.2 The Big Bang Theory During the earliest moments of the universe, matter and antimatter particles continually flashed in and out of existence. A slight excess of ordinary matter remained after most of the matter and antimatter particles annihilated each other. During the first three minutes of the big bang, nuclear fusion converted some of the hydrogen into helium but was unable to make many other heavy atoms because no stable nuclei exist with weights of 5 or 8. Now, hydrogen and helium are common in the universe, but heavier atoms are rare. For a period of hundreds of millions of years called the Dark Age, the universe expanded in darkness until the first stars came into existence. Astronomers have observed signs of re-ionization of the universe caused by that first generation of stars. The chemical composition of the oldest stars is about 75 percent hydrogen and 25 percent helium, which is what models of the big bang nuclear processes would predict. This is a further evidence supporting the big bang theory. 1.7.3 The Inflation Theory The inflationary theory, a modification to the big bang theory, proposes that the universe briefly expanded dramatically, just a tiny fraction of a second after the big bang. The energy to drive inflation would have been released when the four forces of nature changed their respective properties as the universe cooled in its earliest moments. This “separation” of forces is predicted by grand unified theories (GUTs) that explain the forces of nature as being aspects of a single force, unified in particle interactions with very high energies. A major role in the development of GUT is played by Dr. Abdus Salam, the founder of Pakistan's space program and the founding director of Pakistan Space and Upper Atmosphere Research Commission (SUPARCO). He was convinced that weak nuclear forces are not really different from electromagnetic forces, and that the two could inter-convert. Salam presented a theory that shows the unification of two fundamental forces of nature, strong and weak nuclear forces and electromagnetic forces. In 1961, Glashow extended electroweak unification models due to Schwinger by including a short range neutral current, the Z0. The resulting symmetry structure that Glashow proposed, SU (2) x U (1), forms the basis of the accepted theory of the electroweak interactions. For this discovery, Glashow along with Steven Weinberg and Abdus Salam was awarded the 1979 Nobel Prize in Physics. 09 Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet 1.7.4 Properties of Universe The universe is isotropic and homogeneous. In other words, in its major features, the universe looks the same in all directions and in all locations. Isotropy and homogeneity lead to the cosmological principle, the idea that there are no special places in the universe. Except for minor local differences, every place is the same, and the view from every place is the same. There are three models about the fate of the Universe. Closed universe models are finite in size, but their space-time is curved back on itself so they have no edge or center. Open universe models have curved space-time, but it is not curved back on itself. Such universes are infinite. Flat universe models have uncurved space-time and are infinite. Modern observations show that the universe is probably flat. 1.7.5 Dark Matter Matter component that does not radiate in the electromagnetic spectrum and, therefore, is not detected by means of telescopes is known as dark matter. In the 1930s, astronomer Fritz Zwicky (1898–1974) noticed that, in the Coma cluster of galaxies, many of the individual galaxies were moving around so fast that there had to be a tremendous amount of gravitational pull toward the centre of the cluster; otherwise, the Dark Matter galaxies would literally hurl themselves out of the cluster. The amount of matter that needed to exist in the cluster to produce that much gravity far exceeded the amount of matter observed in all the galaxies in the cluster put together. This extra matter became known as “dark matter.” After decades of further study, dark matter has now been confirmed as an important constituent of matter around galaxies, in clusters of galaxies, and throughout the universe as a whole. According to the latest measurements, about 80 percent of the matter in the universe is dark matter. 1.7.6 Dark Energy When Albert Einstein and others were working on the nature of the universe in the early twentieth century, Einstein introduced a mathematical term into his equations to keep a balance between cosmic expansion and gravitational attraction. This term became known as the “cosmological constant,” and seemed to represent an unseen energy that emanated from space itself. After Edwin Hubble and other astronomers showed that the universe was indeed expanding, the cosmological constant no longer appeared to be necessary, and so it was not seriously considered again for decades. Then, starting in the 1990s, a series of discoveries suggested that the “dark energy” represented by the cosmological constant does indeed exist. Current measurements indicate that the density of this dark energy throughout the universe is much greater than the density of matter—both luminous matter and dark matter combined. 10 Pakistan Space & Upper Atmosphere Research Commission  Space Information Booklet Statistical observations of the large-scale structure of the universe confirm that it is flat and contains 4 percent baryonic matter, 23 percent dark matter, and 73 percent dark energy. The mass equivalent of dark energy added to dark matter and baryonic matter makes the observed density of the universe equal to the critical density, thereby confirming the prediction made by inflation theory that the universe is flat. 1.8 GALAXIES A galaxy is made up of millions or billions of stars. Big clouds of gas and dust swirl in space between the stars. Galaxies give off different kinds of rays e.g. radio waves, heat or infrared rays, X-rays and gamma rays. All these rays are types of light. Though some of them can be seen with our naked eye but the others like X-rays and gamma rays could only be seen through electronic instruments. Astronomers and scientists study the light to learn about the galaxies. They have instruments that can detect each of these types. The different types of light provide clues about what galaxies are made of and how do they form. Powerful telescopes like Hubble Space Telescope have been launched in the space to take pictures of galaxies. The location in space of about 2 million Spiral Galaxy Lenticular Galaxy Elliptical Galaxy All the stars visible to the unaided eye from earth belong to Earth’s galaxy, the Milky Way. Galaxies come in different shapes. Some galaxies look like giant whirlpools or pinwheels. They have long arms made of gas and dust clouds and stars. These are called spiral galaxies because the arms spiral into the center. The Milky Way is a spiral galaxy. The stars and clouds of gas and dust in a spiral galaxy move slowly in a circle. They go around the center of the galaxy. New stars form in the clouds of a spiral galaxy. Another shape of galaxy is called Lenticular. These galaxies are spiral galaxies but without spiral structure. Mostly old stars are found in these galaxies. Some galaxies are oval or round in shape. These are called elliptical galaxies. Elliptical galaxies tend to have older stars. Few new stars form in elliptical galaxies. Some galaxies do not have any particular shape. These are called irregular galaxies. When galaxies come close to each other, their shapes can change. Sometimes galaxies collide with one another. Irregular galaxies may be galaxies whose original shapes were distorted by collisions. 11 Pakistan Space & Upper Atmosphere Research Commission