Lesson 1: Earth's Landforms

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A landform is a physical feature on Earth's surface. Each landform has specific characteristics and each landform forms in a different way.

Land Features

• A mountain is a landform that rises high above the ground.
• A hill is lower and rounder than a mountain.
• A valley is low land between hills or mountains.
• A canyon is a deep valley with high, steep sides.
• A cliff is a high, steep section of rock or soil.
• A plain is a wide, flat area.
• A plateau is flat land that is higher than the land around it.
• A desert is an area with very little precipitation.
• A beach is the land along the edge of a body of water.
• A dune is a mound or ridge of sand.

Water Features

• An ocean is a large body of salt water.
• A coast is where a body of water meets land.
• A tributary is a small river or stream.
• A river is a natural body of moving water.
• A waterfall is a natural stream of water falling from a high place.
• A lake is a body of water surrounded by land.
• An estuary is where river water and ocean water meet.
• A delta is the mass of land that forms at the mouth of a river.
• An inlet is a narrow body of water off of a larger body of water.

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Features on the Ocean Floor

There are several landforms at the bottom of the ocean, some that look like mountains and others look like valleys. An ocean basin is a large underwtaer area between continents. Along the coast of a continent, the ocean floor is called the continental shelf. At this point, the water is shallow but as you go further from the coast, the slope gets sharp. This is called Continental slope.

A submarine canyon is a steep sided valley in a continental slope. These are frequently associated with the mouth of a large river. At the end of a continental slope is another downward slope called a continental rise. Over 40% of the ocean floor is flat. These flat areas are called Abyssal plains. Trenches are the deepest parts of the ocean floor. They are usually long and narrow. A seamount is an underwater mountain that rises from the ocean floor but stops before it reaches the surface of the ocean. Mid-ocean ridges are underwater mountain ranges. An indentation called a rift valley occurs along the top of these mountains.

Mapping Earth's Features

Mapping the features on the earth surface requires several peopple with different expertise. Surveyors are people who measure the land. As the first step in making a map, a surveyor measures the elevation in a specific location. Elevation indicates how high the ground is above sea level (height above sea level). Mapmakers then use the surveyor's measurements to show changes in elevation on a map. The various elevations can be shown on a map using different shades of colors. This gives a 3D appearance on a 2D paper. Maps that use variation in shading to indicate elevation are called relief maps.

A map that uses lines to represent elevation is called a topographical map. each line is called a contour line and it represents a different elevation. Some lines might have numbers on them, these numbers represent the elevation, either in meters or feet. Contour lines that are close together mean that section is steep, as compared to contour lines that are farther apart.

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The Earth's Layers

The air around you is the Earth's atmosphere. The atmosphere includes all of the gases around Earth. All of Earth's liquid and solid water, including oceans, lakes, rivers, glaciers, and ice caps, makes up its hydrosphere. The hydrosphere covers approximately 70% of the Earth's surface.

Like an egg, Earth has several layers. The continents and ocean floor make up Earth’s outermost layer, called the crust. The crust is Earth’s thinnest and coolest layer. The layer below the crust is the mantle. Part of the mantle is solid rock. Part is nearly melted rock that is soft and flows. It is a lot like putty. At the center of Earth is the core. The core is the deepest and hottest layer of Earth. The outer core is melted rock. The inner core is solid rock. The biosphere means the parts of Earth where living things are found. Organisms have been found from the atmosphere to the ocean floor.

Lesson 2: Soil

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Soil is a mixture of minerals, weathered rocks, and other things. It has bits of decayed plants and animals called humus. So we can define soil as consisting of organic and inorganic material. Organic material are those coming from or having to do with living things, such as decaying plant and animal material. Inorganic materials do not come from living things, such as minerals and weathered rocks.

The making of soil starts with weathering. Weathering causes rocks to break down into smaller and smaller pieces. The tiny bits of weathered rock build up into layers.

Soil Layers / Horizons

Generally, soil can be divided into three horizons; A, B and C.

The A horizon contains most of the nutrients and humus. The nutrients in the humus are used by plants to grow. In addition, the humus absorbs water and keeps it for a longer period of time and releases it to plants. The soil in this layer is called topsoil.

The horizon B is called subsoil. This layer contians less humus and more particles of rocks.

The C layer is mostly made of larger pieces of rocks. This layer sits on solid unweathered rock called bedrock.

Different places have different depths of each soil layer, and some areas may even lack one of the layers totally.

Soil in different places has different properties and each type of soil has different amount of nutrients and can support different plant and animal life. The soil in a forest has a thin layer of humus and topsoil. Frequent rains carries minerals deep into the ground. Crops with shallow roots do not grow well in such environments because their roots cannot reach the minerals. Desert soil is sandy and does not contain much humus. Because of the sarce rains in the desert, plants here have special adaptations to survive. Desert soil, however, is rich in minerals because they are not washed away by rain water. Plants may grow in deserts if they are supplied with water artificially (irrigation). The grasslands are good for crops such as wheat, rye and corn. The soil is rich in humus, which provides nutrients for the plants. Animals feed on the grasses and also drop their waste on the ground, which adds to the organic matter.

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Soil is a resource that can be wasted through processes like erosion. Plants roots help to keep the soil aggregated and prevents erosion. Plants obtain nutrients from the soil as they grow, and they replace them when they die. If farmers remove all crops from the land, they are removing the nutrients from the land and over time the land becomes less able to support new crops.

Pollution

Pollution is the addition of harmful materials to soil, air or water. Soil can be polluted by chemicals such as those used to kill insects and weeds. Garbage that cannot decay, such as plastic contianers, can also pollute the soil if they are dumped.

Soil Conservation

Conservation generally refers to practices that protect and preserve natural resources. There are several ways of conserving soil including:

• Fertilizing: fertilization is the addition of nutrients to the soil to replace those taken away by the crops.
• Crop rotation: Farmers can plant different crops on the same land in different years/seasons. They can select crops that add nutrients back into the soil. For examples crops in the beans/peas family are able to replace nitrogen back in the soil. It is advisable to plant a pea/bean crop as part of the rotation.

Strip farming: a narrow line of planted crop is called a strip. In some cases, farmers can plant a strip of grass between rows of other crops. The grass helps to hold the soil together to reduce erosion.

Contour plowing: When the farm has a gentle slope, water can run downhill and carry away topsoil. To prevent this, farmers plant their crops in rows across the slope. Farmers can also plow furrows to slow down the water.

Terracing: farmers create terraces around a steep hill, to make what looks like steps. This prevents water from flowing fast down the hill and carrying away rich topsoil. It also creates flat shelves where plants can be grown easily.

Wind breaks: farmers plant tall trees along the adge of the farm to slow the speed of wind. This way the wind is less likely to blow the topsoil away.

Conservation education: farmers and the general population should be educated on wys to conserve soil and other natural resources and avoid pollution.

Laws: The government can enact laws making certain detrimental practices illegal.

Lesson 3: Fossils and Energy

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Fossils are the remains of ancient organisms preserved in soil or rocks. When an organism dies and are covered in soil, sand or other sediments, the sediments harden over and around the organism's remains. Almost all fossils are found in sedimentary rocks. Scientists can study fossils to understand the characteristics of organisms that lived many years ago. These characteristics may also be used to determine the environmental conditions that were present many years ago.

Fossil fuel is a material that formed from the decay of ancient organisms and is used as a source of energy. For example, decayed parts of ocean organisms were buried deep under the ocean. There, a combination of the weight of rock, heat, and the action of bacteria turned the decayed materials into oil and natural gas. Oil and natural gas also are fossil fuels.

Scientists can estimate the age of fossils by observing the layers of the sediments. Fossils found on the top layers are more recent, were deposited more recently, than fossils found deeper in the sediments. This is called relative age. The word relative, means it has been estimated in comparison to something else. The Absolute age of a fossil is the actual age in years. It can be estimated by estimating the age of the rock layer where the fossil was found. The age of the rocks is estimated by the amounts of various elements. The concentration/amount of certain elements reduces at a contant way. For example, lets say it takes 1 million years for half of element A to change into Element B. After 1 million years, the rock contains equal amounts of element A and B. This time point is called an element's half-life. As mentioned, during the first half life, you will observe equal amounts of Element A and B. During the second half-life, more Elemebt A will be converted to Element B, so by the 2 million year mark, there will be 75% of element B and 25% of element A. at the 3 million year mark, there will be 87.5% of element B and 12.5% of element A. Notice that at every subsequent half-life, the concentration of element A is halfed. Different elements different half-lives.

Using the relative age of fossils and rocks, scientists estimated that the age of the earth is around 4.6 billion years. during those 4.6 billion years, there has been several eras. An era is a unit of time measured in millions of years. Geological periods divide eras into shorter periods of time where specific major geological events occured.

Fossil fuels can be burned to release energy. For example, gasoline is used to make cars move. Natural gas is used to heat homes in parts of the world that experience cold seasons. Many homes use electricity to run devices and power appliances etc. Electricity is generated in power plants from generators and the electricity is then 'transported' through wires to the places where it is used.

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Lesson 4: Air and Water

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Approximately 70% of the Earth's surface is covered with water. However, because most of this water is in oceans, it is salty. In fact, 90% of the earth's water is salty. About 2.5% of the Earth's water is frozen at the North and South poles. About 0.1% of the water is in the air as water vapor. Only 0.6% of the Earth's water is fresh liquid water. Many living things utilize the fresh water to survive. Most of the fresh water is obtained from running water, standing water and groundwater.

Running Water
Many cities are built close to sources of running water such as rivers or streams. running water is a source of fresh water for homes, businesses and farms.

Standing Water
Standing water includes bodies of water such as lakes and reservoirs. A reservoir is an artificial lake built to store water. Reservoirs are usually made by building a dam across a river. The water is stored behind the dam and is released when needed.

Groundwater
Water seeps into the ground through aquifers. An Aquifer is an underground layer of rock or soil that has pores and is capable of absorbing water. When the seeping water reaches a layer of rock that does not have pores, it settles there builds up and can be used by humans by drilling or digging into the ground. Once ground water has been harvested, more water must seep from the surface so as to replace the ground water again.

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Some countries/regions use water for irrigation. Irrigation is supplying water to agricultural farms by artificial means.

The water used in most homes is supplied by a water company. The water usually undergoes some treatment processes then pumped through pipes into homes and businesses. Some homes may use well water. A well is used to obtain groundwater by drilling and installing a pipe deep into the ground and pumping water to the surface. This water may undergo treatment and then used for human consumption. Otherwise, well water may also be provided to farm animals.

Drinking water should be free from contamination. You should be on the look out for possible contamination in water, such as a change in color (fresh water should be clear), visible dirt in the water, any smell in the water (fresh water should not have any smell - odorless).

Cleaning Water

The water that runs in most homes is usually treated in a water treatment plant to remove contaminants. Water is first obtained from a fresh water source such as a reservoir or a lake and then it is run through several tanks, with each tank having a different function. The steps may vary with there the water was obtained from.

The first step is to remove dirt through a process called coagulation. This involves the use of sticky particles to attract the dirt.

The next step involves sedimentationwhere clumps of dirt and the sticky particles sink to the bottom of the tank.

The water then is passed through a series of filters made with layers of sand, gravel, and charcoal. These remove more particles from the water. Afte this tank, chlorine is added to the water to kill harmful bacteria. This step is called disinfection. The clean water is stored in tanks until it is pumped to homes and businesses.

Conserving and Protecting Water

Water conservation refers to all efforts made to reduce the waste of water. This may be targeting a small scale level at the individual level, like showering for a shorter time. Conservation can also be aimed at a larger scale, like a lake or a watershed.

Water protection involves the use of government regulations to limit the uses of water. For example, a government may restrict the use of water to domestic uses only and regulate the use of water for irrigation such as watering the lawn.

Air

The Earth's atmosphere holds the gases that living things need to stay alive. You cannot see air (its invisible), you cannot smell air (its odorless) and if air is not moving, you cannot feel it. Air is made up of Oxygen, carbon dioxide and nitrogen. Plants take in carbon dioxide and with the help of sunlight, they are able to make the food they need to survive. Some bacteria in the soil can convert nitrogen into forms that plants can absorb and use to grow.

Air pollution occurs when toxic gases, dust or chemicals are released into the atmosphere from vehicles, factories, mines and other sources. Smog is a type of air pollution caused by oarticles and gases from burning fossil fuels in factories/industries.

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Some gases and chemicals have been shown to affect the ozone layer. The ozone layer is a layer of ozone gas (similar to oxygen) found about 30 kilometers above the earth's surface. It protects the earth from dangerous energy from the sun. Some pollutants from air conditioners, aerosol cans have been shown to destroy the ozone layer. The destruction of the ozone layer results in dangerous radiations from the sun reaching the earth's surface and these may be associated with the occurrence of skin cancer.

Lesson 1: Earth and Sun

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Each planet in the solar system is drawn toward the sun by gravity. Gravity is a force of attraction between two objects. The strngth of the force depends on the mass of the two objects, and the distance between the two objects. The strength of the force decreases as the mass decreases and as the objects are positioned further apart.

The Earth is larger than the Moon so the expectation is that the strength of gravity will be higher on the Earth than on the Moon. In fact, the Moon's gravity is only about one sixth of the Earth's gravity.

Two objects do not have to touch each other to produce a force of gravity between them. The pull of gravity between Earth and the Sun acts across about 150 million kilometers of space. Gravity also acts across roughly 6 billion kilometers of space between the Sun and Pluto. However, because Pluto is located farther away from the Sun, the strength of gravity bwteen the Sun and Pluto is much lower than the gravity between the Sun and the Earth.

The force of gravity between the planets and between the planets and the Sun results in a path that the planets take around the Sun. These paths are called orbits.

As the Earth orbits around the Sun, it is pulled toward the Sun because of gravity but at the same time, inertia makes the planet move away from the Sun. Inertia is defined as the tendency of a moving object to continue moving in a straight line. The combination of gravity and inertia results in planets moving in a nearly cirvular orbit around the Sun called an ellipse.

When Earth is closest to the Sun, it is about 147,000,000 kilometers away. When Earth is furthest from the Sun, it is about 152,000,000 kilometers away.

The Earth moves aroudn the sun at a staggering 30km per second. The Earth's orbit is around 942,000,000 km long, so it takes the Earth about 365 1/4 days for the earth to make a complete trip around the Sun. This is called a Revolution and it is equal to one year.

An imaginary line drawn from the Earth's North to the South pole is called the Earth's axis. The Earth axis is tilted at an angle of about 23^o. As the Earth revolves around the Sun, sunlight strikes different parts of the Earth at different angles. The changes in the angle result in seasons. When the Northern Hemisphere is tilted away from the Sun, the ground does not receive much heat energy and temperatures are low. In the Northern Hemisphere, this is the beginning of winter. At the same time, summer begins in the Southern Hemisphere. The Southern Hemisphere is tilted toward the Sun, the ground receives more heat energy and temperatures are warmer. In spring and in fall/autumn, both hemispheres receive equal warmth from sunlight, which makes temperatures mild.

The solar energy that reaches the Earth is called insolation. Insolation does not warm all places on the Earth equally. Because of the shape of the Earth, (sphere) and with an imaginary line (the equator) dividing the earth into two equal hemispheres, the sun strikes with the most vertical angle at or near the equator.

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The atmosphere forms layers of gases around the earth. The layer of gsases cloest to the earth is called the trophosphere. This layer is thickest at the equator and thinnest at the poles. Above the trophosphere are the stratosphere, mesosphere, thermosphere, and exosphere.

Gases are more dense closer to the earth surface and become less dense as you move toward the exosphere. Tha gas particles press on the Earth's surface and on everything they surround. The force put on a given area by the weight of the air above it is called Air pressure or Atmospheric pressure. You do not feel this weight because atmospheric pressure pushes in all directions and these pushes balance each other.

Lesson 2: Earth and Moon

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The Moon has no atmosphere. Because there is no atmosphere, there are no winds and there is no weather on the Moon. There is no air to block radiation from the Sun or for astronauts to breathe. As a result, the temperatures on the moon can be as high as 123^oC or as low as -233^oC. Astronauts going to the moon need to wear spacesuits to protect them from the changes in temperature and from radiation. They also need containers of oxygen to breathe.

The cheese like craters on the moon are as a result of rocks traveling through space and hitting the Moon.

As the Moon revolves around Earth, different amounts of light reflect from the Moon’s surface and the Moon appears to change shape. A phase of the Moon is the appearance and shape of the Moon as you see it at a particular time. The phase depends on the location of the Moon in relation to Earth and the Sun.

Eclipse

An eclipse occurs when one object moves in front of another object in space.

A solar eclipse occurs when the Moon passes directly between the Sun and Earth. When this happens, the Moon casts a shadow on Earth. People on Earth see darkness move across the Sun.

A lunar eclipse occurs when the Moon moves into Earth’s shadow and is no longer reached by direct sunlight. This happens when Earth is between the Sun and the Moon.

If you are on Earth in the umbra during a solar eclipse, darkness covers the entire face of the Sun. This is called a total solar eclipse. If you are on Earth in the penumbra during a solar eclipse, darkness covers only part of the Sun. This is called a partial solar eclipse.

In lunar eclipses, Earth’s shadow causes the umbra and penumbra. Lunar eclipses may also be total or partial depending on whether or not the Moon is in the umbra or penumbra.

Tides

The pull of gravity between the Moon and Earth and between the Sun and Earth causes a bulge in the surface of Earth. On the part of Earth’s surface that is rocky, we do not notice this pull. However, the pull can be seen in the oceans and other large bodies of water. This pull causes the tide, or the rise and fall of the ocean’s surface.

When the Sun and the Moon line up and pull in the same direction. This causes higher high tides and lower low tides, called spring tides. The tides with the smallest range between high and low tides occur between these two spring tides. These more moderate tides are called neap tides. They take place when the Sun and the Moon pull in different directions and their pulls partly cancel each other.

Lesson 3: The Solar System

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Until 1610, when the telescope was developed by the Astronomer Galileo Galilei, people used to observe the sky using only their eyes. As you learnt in the previous topic that the air in the atmosphere has different densities. As light from stars travels through the air, the changes in density make the faint light of the stars appear fuzzy. Looking through a telescope makes dim objects seem brighter and also makes objects appear larger so more details can be observed.

Genius Minute

The invention of the telescope played an important role in advancing our understanding of Earth's place in the cosmos. While there is evidence that the principles of telescopes were known in the late 16th century, the first telescopes were created in the Netherlands in 1608. Galileo Galilei (1564-1642) was part of a small group of astronomers who turned telescopes towards the heavens. After hearing about the "Danish perspective glass" in 1609, Galileo constructed his own telescope. He subsequently demonstrated the telescope in Venice.

Shortly after Galileo's first telescopic observations of the heavens, he began recording and drawing his observations. He wanted to get his findings out. His observations and interpretations of stars, the moon, Jupiter, the sun and the phases of the planet Venus, were critical in refining our understanding of the universe.

Galileo Galilei's telescope

The structure of the moon as observed and drawn by Galileo Galilei in 1610. The drawing already shows the topography of the moon showing the craters on the surface.

Lesson 3: The Solar System - Cont'd

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Telescopes can be placed in space to avoid the interference caused by variations in air density in the atmosphere. In 1990, the Hubble Space Telescope was placed into orbit around the Earth. This and other space telescopes gather more than visible light from objects in space, they can detect heat given off by stars and other space objects.

Radio telescopes are located on Earth and record data from radio waves given off by objects in space. Several dishes focus the waves so the radio waves data can be recorded and computers covert the data into images. Radio waves are not affected by air densities so they can move through the atmosphere without interference.

A solar system is made up of a star and the objects that orbit around it. In our solar system, there are eight planets orbiting the Sun. A planet is a large object that orbits a star. Form neares to the sun, the planets in our solar system include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The planets travel around the sun in elliptical or nearly circular orbits.

The inner planets are closer to the Sun than the asteroid belt and have surfaces made of rock. These planets are Mercury, Venus, Earth, and Mars. The outer planets are beyond the asteroid belt and have surfaces made of gases. These planets are Jupiter, Saturn, Uranus, and Neptune. Pluto was once known as the ninth planet. Pluto’s elongated orbit and small size were different from the other planets. Because of this, scientists debated whether Pluto should be classified as a planet. In Aug 2006, the International Astronomical Union officially reclassified Pluto as a dwarf planet. Other dwarf planets include Ceres and 2003 UB313, which is larger than Pluto but farther from the Sun.

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Planet's unique features:
Jupiter: has the great red spot (aka red eye), which is a huge storm that has been blowing for over 400 years. It is believed that combination of sulfur and phosphorus are in Jupiter's atmosphere gives this storm its red color.
Saturn rings: First observed by Galileo in 1610. They are made of ice and rocks ranging in size from pea size to rocks larger than a house. Jupiter, Uranus and Neptune also have faint rings that are more difficult to observe.
Venus: The surface of Venus shows evidence of violent volcanic activity in the past. Venus has shield and composite volcanoes similar to those found on Earth. Long rivers of lava have also been observed on Venus.
Mars rocks! The dark boulders on the surface of Mars are volcanic rock fragments that have been found on Mars. These rocks look similar to rocks found near lava flows on Earth.

A moon is a natural object (natural satellite) that orbits a planet. Different planets have different numbers and sizes of moons. Generally, the outer planets have more moons. The Earth has only one moon while Jupiter has at least 63 moons. Saturn has 47 moons, Uranus has 27 and Neptune has 13.

An artificial satellite is an object that is put in space by man to orbit around the earth or other planets. These may be to monitor weather or conduct various forms of communication.

Moons vary in size. Ganymede is the largest moon in the solar system. In fact Ganymede is larger than Pluto and Mercury. The Earth's moon is also larger than Pluto and is clearly visible without a telescope.

When small objects in space collide with larger objects, a crater is formed. Craters are bowl-shaped holes on the larger object.

A comet is a mixture of frozen gases, ice, dust, and rock that moves in an elliptical orbit around the Sun.

An asteroid is a rock that revolves around the Sun. Most of the thousands of asteroids in the solar system are located between Mars and Jupiter in the asteroid belt.

An object that crosses paths with Earth and enters the atmosphere is called a meteor. Most meteors burn up before they reach the ground. When a meteor lands on the ground, it is called a meteorite.

Chelyabinsk meteor - 2013

Lesson 4: Stars and the Universe

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A star is an object that produces its own heat and light energy. Stars go through stages from beginning to ending depending on how much hydrogen tha star contains. The star's cycle ends when it stops giving off energy.

All stars form out of a nebula. A nebula is a cloud of gases and dust. Gravity pulls the mass of nebula, which contains a lot of hydrogen atoms and as the atoms move closer they collide with each other producing heat. The temperature increases and when the temperature reaches 10 million degrees Celcius, the hydrogen atoms combine to form Ehlium. This process produces huge amounts of heat and light. This marks the beginning of the formation of a star.

The sun is a star, like other stars, it uses hydrogen as the source of energy. As the heat in the sun increases, it forces the hydrogen at the endge of the sun to expand into space, as the hydrogen moves further away from the center of the sun, it cools slightly and turns red. This stage of the star is called a red giant.

Eventually, all the helium is gone and the star begins to cool off and shrink becoming a white dwarf. A white dwarf is a small dense star that sines with a cooler white light. This is the end of the cycle for medium sized stars.

Stars that start with larger amounts of hydrogen (larger stars) end their cycle differently. After they become red giants the atoms at the core become so hot that they combine to form iron atoms. Eventually the iron gets so hot and explodes into a supernova. Supernovas shine brightly for days or weeks then they fade away.

If a star is very massive, it may end its life as a black hole. A black hole is an object that is so dense and has such powerful gravity that nothing can escape from it,not even light.

The sun is a medium sized star with a temperature of around 6000 degrees celcius. Giant stars are about 100 times larger than the sun and super giant stars are 1000 times larger. Neutron stars are the smalles stars.

Stars that form patterns are called constellations. Constellations were often named after animals, characters from stories, or familiar objects. Some constellations have been extensively useful to both ancient and modern travelers. For example, if you can see either the Big Dipper or the Little Dipper in the night sky, you can follow the line that their stars make to find Polaris, the North Star. If you travel in the direction of Polaris, you will be moving north. If you ever become lost in the woods or at sea, look for Polaris (North star) in the night sky. It will help guide you to safety.

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The ancient Greeks divided the sky into 12 sections. They named some constellations after characters from Greek myths, such as Orion, a hunter, and Hercules, a hero.

Light Years

After the sun, the next closest star is called Proxima Centauri and is about 40,000,000,000,000 km away. This distance is so huge and becomes difficult to remember and comprehend. We can use the unit light year, which is equal to the distance that light travels in a year, and is equal to 9.5 billion kilometers. Proxima Centauri is 4.2 light years away from the earth.

Galaxies

A galaxy is a huge very distant collection of stars. Each galaxy holds billions of stars. The universe is full of galaxies and each galaxy differs in size and shape. Galaxies may be spiral, elliptical or irregular shaped.

Our solar system is part of the galaxy called the Milky Way. The stars you see in the sky are part of the Milky Way galaxy.

Clusters and Binary Stars

Some stars form clusters that may contain more than 100,000 stars. Clobular clusters are shaoed like a sphere. When two stars are close to each other, or somehow overlap and are seen as though they were only one star, they are called binary stars. the prefix -bi stands for 'two'. A star that seems to be blinking might actually be a binary star where one of the stars, the dimmer one, blocks the light from the brighter star.

The Big Bang Theory

The Big Bang Theory hypothesizes that the universe started with a big bang a single point and has been expanding ever since. Scientific evidence indicates that the big bang happened 13.7 billion years ago.