My Science School

In the middle of the 20th century the USA and the Soviet Union were struggling to be the most powerful country in the world. Both countries wanted to be the first to send spacecraft and people into space, and so the Space Race began.

The first man-made object to travel into space was the Soviet satellite Sputnik 1. It was launched on 4 October 1957.

A month later, on 3 November 1957, the Soviet Union sent a dog into space. She was called Laika, became the first living creature to orbit the Earth.

In April 1959, the US introduced its first group of astronauts, known as the Mercury 7. They were an elite group of pilots who did special training to travel to space.

But the Soviet Union sent a human to space first! On 12 April 1961, Russian cosmonaut Yuri Gagarin orbited the Earth.

In September 1962, US President John F. Kennedy set the goal of landing a man on the Moon by the end of the decade.

But the Soviets were still ahead, and in June 1963, Valentina Tereshkova became the first woman to travel to space.

In a further triumph, on 18 March 1965 the Soviet cosmonaut Alexei Leonov became the first person to walk in space!

However, the United States were first to the Moon. The Apollo 11 mission launched on 16 July 1969 and successfully landed on the Moon four days later.

On 20 July 1969, Neil Armstrong and Buzz Aldrin became the first people to walk on the Moon. The Space Race was over.

 

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Scientists think the Moon was formed when the Solar System was very young and an object about the size of Mars collided with the young Earth. They think the Moon is debris from the collision, pulled together in Earth’s orbit by gravity.

The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, in the Hadean eon; about 20 to 100 million years after the Solar System coalesced. The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks, published in a 2016 report, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies.

The giant-impact hypothesis is currently the favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes:

• Earth's spin and the Moon's orbit have similar orientations.


• Moon samples indicate that the Moon's surface was once molten.


• The Moon has a relatively small iron core.


• The Moon has a lower density than Earth.


• There is evidence in other star systems of similar collisions, resulting in debris disks.


• Giant collisions are consistent with the leading theories of the formation of the Solar System.


• The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin.

 

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The Moon is our closet neighbour and the only place in the Solar System, other than Earth, that humans have set foot on. The Moon is desert-like, with plains, mountains and valleys, and a black sky. It is covered with craters, because there is no atmosphere to protect it from space rocks.

Like the Earth, the Moon has layers. The innermost layer is the lunar core. It only accounts for about 20% of the diameter of the Moon. Scientists think that the lunar core is made of metallic iron, with small amounts of sulfur and nickel. Astronomers know that the core of the Moon is probably at least partly molten.

Outside the core is the largest region of the Moon, called the mantle. The lunar mantle extends up to a distance of only 50 km below the surface of the Moon. Scientists believe that the mantle of the Moon is largely composed of the minerals olivine, orthopyroxene and clinopyroxene. It’s also believed to be more iron-rich than the Earth’s mantle.

The outermost layer of the Moon is called the crust, which extends down to a depth of 50 km. This is the layer of the Moon that scientists have gathered the most information about. The crust of the Moon is composed mostly of oxygen, silicon, magnesium, iron, calcium, and aluminum. There are also trace elements like titanium, uranium, thorium, potassium and hydrogen.

Moon landing

A total of twelve men have landed on the Moon. This was accomplished with two US pilot-astronauts flying a Lunar Module on each of six NASA missions across a 41-month period starting 20 July 1969 UTC, with Neil Armstrong and Buzz Aldrin on Apollo 11, and ending on 14 December 1972 UTC with Gene Cernan and Jack Schmitt on Apollo 17. Cernan was the last to step off the lunar surface.

All Apollo lunar missions had a third crew member who remained on board the Command Module. The last three missions included a drivable lunar rover, the Lunar Roving Vehicle, for increased mobility.

Moon exploration

The physical exploration of the Moon began when Luna 2, a space probe launched by the Soviet Union, made an impact on the surface of the Moon on September 14, 1959. Prior to that the only available means of exploration had been observation from Earth. The invention of the optical telescope brought about the first leap in the quality of lunar observations. Galileo Galilei is generally credited as the first person to use a telescope for astronomical purposes; having made his own telescope in 1609, the mountains and craters on the lunar surface were among his first observations using it.

NASA's Apollo program was the first, and to date only, mission to successfully land humans on the Moon, which it did six times. The first landing took place in 1969, when astronauts placed scientific instruments and returned lunar samples to Earth.

People last visited the Moon in 1972, but the footprints they left will last for millions of years because there is no wind to blow them away. This means future Moon explorers will be able to see them.

Solar eclipse

Sometimes when the Moon passes between the Earth and the Sun, the Moon briefly blocks out light from the Sun, causing an eclipse to be seen on Earth.

An eclipse is a natural phenomenon. However, in some ancient and modern cultures, solar eclipses were attributed to supernatural causes or regarded as bad omens. A total solar eclipse can be frightening to people who are unaware of its astronomical explanation, as the Sun seems to disappear during the day and the sky darkens in a matter of minutes.

Since looking directly at the Sun can lead to permanent eye damage or blindness, special eye protection or indirect viewing techniques are used when viewing a solar eclipse. It is technically safe to view only the total phase of a total solar eclipse with the unaided eye and without protection; however, this is a dangerous practice, as most people are not trained to recognize the phases of an eclipse, which can span over two hours while the total phase can only last a maximum of 7.5 minutes for any one location. People referred to as eclipse chasers or umbraphiles will travel to remote locations to observe or witness predicted central solar eclipses.

Mining the Moon

In the future there could be a Moon base, where people could live. Some scientists are even interested in mining the Moon for resource they could turn into rocket fuel.

If human beings are to explore the Moon and, potentially, live there one day, we’ll need to learn how to deal with these challenging environmental conditions. We’ll need habitats, air, food and energy, as well as fuel to power rockets back to Earth and possibly other destinations. That means we’ll need resources to meet these requirements. We can either bring them with us from Earth – an expensive proposition – or we’ll need to take advantage of resources on the Moon itself. And that’s where the idea of “in-situ resource utilization,” or ISRU, comes in.

Underpinning efforts to use lunar materials is the desire to establish either temporary or even permanent human settlements on the Moon – and there are numerous benefits to doing so. For example, lunar bases or colonies could provide invaluable training and preparation for missions to farther flung destinations, including Mars. Developing and utilizing lunar resources will likely lead to a vast number of innovative and exotic technologies that could be useful on Earth, as has been the case with the International Space Station.

 

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From the biggest whale in the ocean to a tiny mouse, all life on Earth has one thing in common – it is all made from the same stuff.

Stardust

Nearly everything that makes up our bodies, and everything else on Earth, was created when dying stars exploded. These explosions send raw materials like carbon and oxygen hurtling across space, and these raw materials are what we are made of. That means that you are made of stardust! When the universe started, there was just hydrogen and a little helium and very little of anything else. Helium is not in our bodies. Hydrogen is, but that's not the bulk of our weight. Stars are like nuclear reactors. They take a fuel and convert it to something else. Hydrogen is formed into helium, and helium is built into carbon, nitrogen and oxygen, iron and sulfur—everything we're made of. When stars get to the end of their lives, they swell up and fall together again, throwing off their outer layers. If a star is heavy enough, it will explode in a supernova.

So most of the material that we're made of comes out of dying stars, or stars that died in explosions. And those stellar explosions continue. We have stuff in us as old as the universe, and then some stuff that landed here maybe only a hundred years ago. And all of that mixes in our bodies.

 

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Although there may be life elsewhere in our Solar System, we haven’t discovered it yet. The only place we know has life for sure is Earth. Our home planet is at just the right distance from our Sun for liquid water to exist, and has all the other key ingredients to make life possible.

Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased (they have died), or because they never had such functions and are classified as inanimate. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. The criteria can at times be ambiguous and may or may not define viruses, viroids, or potential synthetic life as "living". Biology is the science concerned with the study of life.

There is currently no consensus regarding the definition of life. One popular definition is that organisms are open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve. However, several other definitions have been proposed, and there are some borderline cases of life, such as viruses or viroids.

Recipe for life

In the mixing bowl are the key ingredients needed for life as we know it:

You will need:

Raw materials, such as oxygen, nitrogen, and carbon

Liquid water

Energy

Raw material

The raw materials needed for life are found all over Earth – for example in soil. However soil needs water and energy from the Sun before life can appear. Life as we know it contains specific combinations of elements including carbon, hydrogen, nitrogen, and oxygen that combine to form proteins and nucleic acids which can replicate genetic code. All the basic elements are formed in stars and distributed throughout space as a result of giant explosions called supernovas. Since these essential chemicals are quite common in other places in the Universe we can expect that the development of life somewhere else is also possible.

Water

Liquid water is essential for life. It allows crucial changes to take place between raw materials. Liquid water is essential because biochemical reactions take place in water. Water is also an excellent solvent that easily dissolves and carries nutrients and other compounds in and out of cells. Life forms are usually made primarily of water. In fact, our human bodies are more than 60% water.

Energy

Life on Earth would not be possible without a constant source of energy, such as the Sun. Organisms require energy to assimilate or put together the chemicals that form an individual. Energy is also required for the organism to grow, reproduce, and respond to the environment. Energy sources may include other organisms, light, or inorganic compounds. The most common source of energy on the Earth is photosynthesis, which transforms sunlight into food. This process will not work very well for the outer Solar System, because not much light reaches such great distances. However, we can look to extremophiles here on Earth for help in figuring out where and what to search for. Extremophiles live in extreme conditions and typically get their energy from a source other than the sun.

 

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Earth and Venus are about the same size, and are made up of similar rocky materials. They are also neighboring planets. However, Venus and Earth are also very different. Venus has an atmosphere that is about 100 times thicker than Earth's and has surface temperatures that are extremely hot. Venus does not have life or a water ocean like Earth does. Venus also rotates backwards compared to Earth and the other planets.

But that’s where the similarities end! Venus is a deadly world. It‘s boiling hot, covered in volcanoes, and cloaked in an atmosphere of deadly poisonous gases

Volcanoes: Venus is covered in volcanoes. There is evidence that some may still be erupting. On Earth, volcanoes are mainly of two types: shield volcanoes and composite or stratovolcanoes. The shield volcanoes, for example those in Hawaii, eject magma from the depths of the Earth in zones called hot spots. The lava from these volcanoes is relatively fluid and permits the escape of gases. Composite volcanoes, such as Mount Saint Helens and Mount Pinatubo, are associated with tectonic plates. In this type of volcano, the oceanic crust of one plate is sliding underneath the other in a subduction zone, together with an inflow of seawater, producing gummier lava that restricts the exit of the gases, and for that reason, composite volcanoes tend to erupt more violently.

Barren surface: There are no rivers or lakes on the surface of Venus. The only rain it gets is acid rain that would burn trough your skin.

Toxic clouds: Venus is covered in clouds of sulphuric acid. The atmosphere is so thick it would crush you in seconds.

Atmosphere: Earth’s atmosphere protects it from dangerous space radiation, and contains gases like oxygen that we need to breathe. Venus’s atmospheric pressure is greater than that of any other planet – more than 90 times that of Earth’s. This pressure is equivalent to being almost one kilometre below the surface of Earth’s oceans. The atmosphere is also very dense and mostly carbon dioxide, with tiny amounts of water vapour and nitrogen. It has lots of sulphur dioxide on the surface. This creates a Greenhouse Effect and makes Venus the hottest planet in the solar system. Its surface temperature is 461 degrees Celsius across the entire planet, while Mercury (the closest planet to the Sun) heats up to 426 Celsius only on the side facing the Sun.

Life: Earth is home to an amazing variety of plants and animals.

Water: About 71 per cent of Earth’s surface is covered by water. It is a vital ingredient for life.

Temperature: The surface temperature on the planet Earth goes only to about 100 degrees Fahrenheit, which makes it possible for life to thrive on this planet. On the other hand, the surface temperature on the planet Venus is nine times hotter than that on planet Earth. As such, it is extremely impossible for any form of life to survive and thrive on Venus’ surface.

 

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Mercury has a rocky surface, but inside is a very large metallic core, part of which is molten (liquid).

Crust: Mercury has a thick crust that is composed mostly of silicate rocks. Mercury may have small ice caps at its north and south poles; this ice stays frozen inside deep craters that are shaded from sunlight. 



Mantle: Beneath the crust it is a mantle (also made of silicate rocks) that is hundreds of kilometers thick. 



Core: At the center of Mercury is a partly-molten iron core about 2,300 miles (7,500 km) in diameter (almost half of the diameter of Mercury). This core accounts for about 80% of Mercury's mass. This core generates a magnetic field (which is how we know that Mercury has an iron core). 



Density: Mercury has a density of 5,430 kg/m3, slightly less than that of Earth. Mercury is the second-densest planet in the solar system (after Earth) because of its large iron core. 

People have been observing Mercury for a very long time, but nobody knows who discovered it. Sometimes it can be seen from Earth around sunset and sunrise. Sunrise and sunset on Mercury are spectacles to behold. Two and one half times larger in the sky than seen on Earth, the sun appears to rise and set twice during a Mercurian day. It rises, then arcs across the sky, stops, moves back toward the rising horizon, stops again, and finally restarts its journey toward the setting horizon. These aerial maneuvers occur because Mercury rotates three times for every two orbits around the sun and because Mercury's orbit is very elliptical.

Visible at night: Mercury is not the only planet that can be seen with the naked eye. Mercury can generally be observed with a naked eye as it has the sun as a bright backdrop. Mercury is best observed with the naked eye during times right before and after the sun has set, which gives enough light pollution to contrast the shadow of Mercury. A general time to try and view Mercury with your naked eye is 90 minutes before sunrise or after sunset.

 

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 Mercury is the closest planet to the Sun and the least explored of the four inner rocky planets. Its surface is covered in greyish-brown dust and looks similar to our Moon, with lots of craters where it has been hit by space rocks. Scientists think there is no possibility of life here.

Mercury is smallest of the eight planets in our Solar System – it is only slightly bigger than the Earth’s Moon.

Mercury is a world of extreme temperatures. By day it is scorching hot, but at night it is very cold.

Mercury is not the hottest planet in the solar system. With no atmosphere to trap heat, surface temperatures on Mercury can swing from 800 degrees Fahrenheit during the day to -290 degrees Fahrenheit at night. Mercury may even have reservoirs of ice sitting deep inside permanently shadowed craters at its poles. By contrast, the surface of hazy Venus sits at a sweltering 880 degrees Fahrenheit year-round, making it the hottest planet in our solar system.

Lack of an atmosphere also means Mercury’s surface is pockmarked by numerous impact craters, since incoming meteors don’t encounter any friction that would cause them to burn up. Seen via telescopes and spacecraft, Mercury looks like a battered world covered in overlapping basins, soaring cliffs, and occasional smooth plains.

Bright lines called crater rays also crisscross the surface where impacts crushed the rock and kicked up reflective debris. One of the most notable features on Mercury is Caloris Basin, an impact crater about 960 miles wide that formed early in the planet’s history. Mercury has no rings, no moons, and a relatively weak magnetic field.

 

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 As well as heat and light, the sun blasts out special particles called solar wind. When these get trapped by Earth’s magnetic field near the poles they can create spectacular light shows, called auroras. Even though auroras are best seen at night, they are actually caused by the sun. The sun sends us more than heat and light; it sends lots of other energy and small particles our way. The protective magnetic field around Earth shields us from most of the energy and particles, and we don’t even notice them.

But the sun doesn’t send the same amount of energy all the time. There is a constant streaming solar wind and there are also solar storms. During one kind of solar storm called a coronal mass ejection, the sun burps out a huge bubble of electrified gas that can travel through space at high speeds.

When a solar storm comes toward us, some of the energy and small particles can travel down the magnetic field lines at the north and south poles into Earth’s atmosphere.

There, the particles interact with gases in our atmosphere resulting in beautiful displays of light in the sky. Oxygen gives off green and red light. Nitrogen glows blue and purple.

 

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The sun is so ginormous that all of the planets of the solar system could fit inside it hundreds of times over. The sun is nearly a perfect sphere. Its equatorial diameter and its polar diameter differ by only 6.2 miles (10 km). The mean radius of the sun is 432,450 miles (696,000 kilometers), which makes its diameter about 864,938 miles (1.392 million km). You could line up 109 Earths across the face of the sun. The sun's circumference is about 2,713,406 miles (4,366,813 km).

It may be the biggest thing in this neighbourhood, but the sun is just average compared to other stars. Betelgeuse, a red giant, is about 700 times bigger than the sun and about 14,000 times brighter.

 

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Located at the centre of the solar system is the sun. It is a star, like the ones you see in the night sky. A burning ball of gas, made of mostly hydrogen and helium, it provides us with the heat we need to survive. The sun is doing massive that its gravity – the force that pulls things together - keeps the planets in orbit around it. Like other stars, our sun is basically a large ball of gas that is 91% hydrogen and 8.9% helium. The sun’s mass is around 70.6% hydrogen and 27.4% helium.

While a majority of our sun may be gas it does have six distinct regions: the core, the radiative zone, and the convective zone in the interior, the visible surface, called the photosphere; the chromospheres; and the outermost region, the corona.

The sun is held together due to gravitational attraction that produces an intense temperature and pressure at the core. The core’s temperature is about 27 million degrees F/15 million degrees C.

This is hot enough to continue the constant state of thermonuclear fusion, a process where atoms combine to create larger atoms and in that process they release huge amounts of energy.

Our star: The Sun is 150 million kilometers (93 million miles) away from the Earth (this distance varies slightly throughout the year, because the Earth’s orbit is an ellipse and not a perfect circle). The Sun is an average star – there are other stars which are much hotter or much cooler, and intrinsically much brighter or fainter. However, since it is by far the closest star to the Earth, it looks bigger and brighter in our sky than any other star. Energy is constantly being generated deep within the sun. It can take up to 100,000 years for energy to reach the surface, but then it only take 8 minutes to reach the earth!

Solar flare: Huge eruptions from the surface of the sun are called solar prominences. They form loops because of the Sun’s invisible magnetic field. Flares are closely associated with the ejection of plasmas and particles through the Sun's corona into outer space; flares also copiously emit radio waves. If the ejection is in the direction of the Earth, particles associated with this disturbance can penetrate into the upper atmosphere (the ionosphere) and cause bright auroras, and may even disrupt long range radio communication. It usually takes days for the solar plasma ejecta to reach Earth. Flares also occur on other stars, where the term stellar flare applies. High-energy particles, which may be relativistic, can arrive almost simultaneously with the electromagnetic radiations.

Sunspots: Dark patches that appear on the surface of the sun are called sunspots. They are cooler areas that usually last for a few weeks. Individual sunspots or groups of sunspots may last anywhere from a few days to a few months, but eventually decay. Sunspots expand and contract as they move across the surface of the Sun, with diameters ranging from 16 km (10 mi) to 160,000 km (100,000 mi). Larger sunspots can be visible from Earth without the aid of a telescope. They may travel at relative speeds, or proper motions, of a few hundred meters per second when they first emerge.

 

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The solar system is located in the Milky Way, a huge spiral galaxy containing billions of stars. They are grouped in “arms” that spiral outwards. All of the stars are travelling around a point at the centre. The Milky Way does not sit still, but is constantly rotating. As such, the arms are moving through space. The sun and the solar system travel with them. The solar system travels at an average speed of 515,000 mph (828,000 km/h). Even at this rapid speed, the solar system would take about 230 million years to travel all the way around the Milky Way. Scientists think there is a super massive black hole located there that sucks in anything that gets too close to it.

The Night sky: The Milky Way used to be visible on every clear, moonless night, everywhere in the world. Today, however, most people live in places where it's impossible to see the Milky Way because of widespread light pollution caused by lights left on all night long. Seeing the Milky Way requires a special effort for most people, but its well worth the trouble.

To see the Milky Way, you'll need to travel far from any city, to a wilderness area. Even in rural farming country, there are still a lot of bright lighting fixtures that wipe out the night sky. 

Black hole: Tucked inside the very center of the galaxy is a monstrous black hole, billions of times as massive as the sun. This supermassive black hole may have started off smaller, but the ample supply of dust and gas allowed it to gorge itself and grow into a giant. The greedy glutton also consumes whatever stars it can get a grip on. Although black holes cannot be directly viewed, scientists can see their gravitational effects as they change and distort the paths of the material around it, or as they fire off jets. Most galaxies are thought to have a black hole in their heart.

 

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The solar system is made up of our star called the sun, and everything that travels, or orbits, around it. This includes eight planets and their moons, dwarf planets, asteroids, comets, and smaller bits of rock and dust. The solar system is one of many solar systems that exist in the Universe.

Beyond our own solar system, there are more planets than stars in night sky. So far, we have discovered thousands of planetary systems orbiting other stars in the Milky Way, with more planets being found all the time. Most of the hundreds of billions of stars in our galaxy are thought to have planets of their own, and the Milky Way is but one of perhaps 100 billion galaxies in the universe.

While our planet is in some ways a mere speck in the vast cosmos, we have a lot of company out there. It seems that we live in a universe packed with planets — a web of countless stars accompanied by families of objects, perhaps some with life of their own.

Gas planets: The four outer planets – Jupiter, Saturn, Uranus, and Neptune –are the largest planets in the Solar System. They are mostly made of gas and spacecraft are unable to land on them. Jupiter is the largest planet in our solar system. It has a radius almost 11 times the size of Earth. It has 50 known moons and 17 waiting to be confirmed, according to NASA.  Saturn is about nine times Earth's radius and is characterized by large rings; how they formed is unknown. It has 53 known moons and nine more awaiting confirmation, according to NASA. Uranus has a radius about four times that of Earth's. The planet has 27 moons, and its atmosphere is made up of hydrogen, helium and methane, according to NASA. Neptune also has a radius about four times that of Earth's. It has 13 confirmed moons and an additional one awaiting confirmation, according to NASA.

Asteroids:  Asteroids are lumps of rock and metal left over from when the solar system first formed. Most can be found in the asteroid belt, which is located between the planets Mars and Jupiter. There are millions of asteroids in our Solar System. Scientists estimate the asteroid belt has between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Most of the undiscovered asteroids are likely the smaller ones (less than 100 km across) which are more difficult to detect. Some astronomers estimate there could be 150 million asteroids in the entire Solar System.

Rocky planets: Closest to the sun are the four rocky planets-Mercury, Venus, Earth and Mars. They all began their existence in the same way, but over time became very different worlds. They are the closest four planets to the Sun. They are made of rocks and metals. They have a solid surface and a core which is mainly made of iron. They are much smaller than the gas planets and rotate more slowly.

Super-sized: The solar system is so big that if the sun were the size of a basketball, the earth would be the size of a sesame seed – and it would be located more than 25 m (80 ft) away! The Earth is the largest of the four inner, rocky planets in our Solar System, at more than 12,000 km in diameter. But even at this size, Earth is dwarfed by all four of the gas giant planets, which range in size from Neptune (at nearly 4 times the size and 17 times the mass of Earth) all the way up to our Solar System's giant, Jupiter, with more than 11 times the Earth's diameter and over 300 times its mass.

Dwarf Planets: Dwarf planets, such as Pluto, also travel around the sun; these worlds are smaller than the other planets. Scientists think there may be dozens of undiscovered dwarf planets hiding in the solar system. It is possible that there are another 40 known objects in the solar system that could be rightly classified as dwarf planets. Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.

 

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A star is a body of luminous gas, like the sun. But as stars are much farther away from the earth than the sun, they appear to be only small points of twinkling light. With the naked eye it is possible to see about 2,000 stars at any one time or place but with the most powerful telescope over 1,000 million stars are visible. Although light travels at 186,000 miles a second, the light from the stars takes many years to reach the earth.

     Stars are not fixed in space, but are travelling in different directions at different speeds.  Seen from the earth, these movements appear to be so small that groups of stars, or constellations, seem to have a permanent relationship. The star patterns we see in the sky are almost the same as those seen by our ancestors hundreds, or even thousands of years ago.

    The sizes of stars vary tremendously, from less than the diameter of the sun to thousands of times its size. Most stars appear white when looked at with the naked eye, but some are bluish-white, yellow, orange and red. The varied colours are due to differences in surface temperature. The brilliant, white stars are the hottest with surface temperatures of several hundred thousand degrees. The less brilliant, orange and res stars have surface temperatures of about 2,000 degrees.

      There are exceptions, however. Te red giant, betelgeux, in the constellation (or group) of Orion, appears to be brilliant because of its size. Its diameter is 250 million miles, which is greater than the diameter of the earth’s orbit round the sun.

     Shooting stars which are sometimes seen moving across the night sky for a few seconds are really meteors. These small particles flare up as they strike the earth’s atmosphere and usually burn out.

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The Milky Way, or Galaxy, is the whole concourse of stars and other bodies which can be seen stretched across the heavens. It includes our own sun and its planets, as well as all stars visible to the naked eye. But the name is commonly restricted to the luminous band or belt where most classes of stars are concentrated.

       The spiral arms of the Milky Way are rich in hot, bright stars, interstellar clouds of gas (mainly hydrogen) and dust. The first evidence of spiral arms was obtained in 1951 by the American astronomer W.W. Morgan, who identified three.

     Our own system of sun and planets appears to be situated towards the inner edge of one of the arms, which is about 1,300 light-years away. The Andromeda nebula, a vast mixture of gaseous and solid matter, is visible as a small luminous patch in our sky. But it is comparable in size to the Milky Way and seems remarkably similar to our own galaxy.

    The Palomar telescope, 200 inches in diameter, situated on Mount Palomar in California, has perhaps 1,000 million galaxies within the scope of its vision.

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