The Milky Way appears as a white band of star light in the sky. It is made up of hundreds of billions of stars that form a great star system, a galaxy. Our Sun and nine planets are a part of the Milky Way Galaxy. It is of the gravitational pull of the stars that holds the system together. If you were above looking down on the Milky Way Galaxy, it would have the appearance of a pinwheel 100,000 light years across. The curving arm of the pinwheel are stars that together circle the center for the galaxy. Our Sun travels at a speed of 155 miles per second and takes more than 220 million years to travel once around the middle of the Milky Way.
Astronomers believe that there is a black hole in the center of our galaxy. A black hole can be defined as very dense remains of a massive star that has collapsed to pinpoint size. The black hole’s gravitational pull is so strong that light and matter entering the hole cannot escape.
Astronomers believe that about 15 billion years ago, an explosion called the Big Bang created space, time, energy, and matter, and signified the beginning of the Universe.
FORMATION OF THE SOLAR SYSTEM
The Earth is estimated to be about 4700 million years old. The Sun is obviously as old as the Earth. There are many theories attempting however to explain the formation of the Solar System. Modern theory says that the Earth and other planets were built up by accretion from a solar nebula. If this is the case, the process was a gradual one and the planets are expected to be approximately the same age. It is known that the Moon and the Earth are about the same age.
What do you really see when you look up into the sky on a dark clear night? Well, there is a lighted band that we call the Milky Way. Our galaxy appears to be flattened. Detailed studies indicate that it has a spiral shape. The Sun and its planets is situated near the main plane.
The Milky Way Galaxy contains about 100,000 million stars of which our Sun is not a particularly distinguished number. In size and luminosity, the Sun is an average star. It as important to us because the Earth moves around it.
The Sun is 30,000 light years form the center of the Galaxy. A light year is defined as the distance a ray of light travels in one year equivalent to 5.886 million million miles. The entire Galaxy is constantly in a state of rotation and it takes the Sun 225 million years to complete one journey around the center.
The Sun is a great distance from its nearest stellar neighbor. The closest star is a red dwarf, Proxima Centauri. Sirius is also one of our neighbors. The closest star that has some similarity to our Sun is Epsilon Eridani and Tau Ceti. These stars are eleven light years from earth.
The major body of the Solar System is the Sun. The Solar System is made up of nine planets and their satellites, comets, and countless minor bodies of meteors and meteorites.
The Sun is an ordinary star in the Galaxy. The Sun is not the largest star nor is it the smallest. The Sun is the center of our Solar System. Nine planets orbit around the Sun in their individual path. Mercury, Venus Earth, and Mars are the terrestrial planets. These rocky planets are small in comparison to the four giant jovian planets. The four jovian planets Jupiter, Saturn, Uranus, and Neptune are made mostly of gases. The outermost and smallest planet is Pluto. Seven of the nine planets have satellites (moons) circling around them.
Mercury is the closest planet to the Sun. Although Mercury revolves quickly around the Sun it rotates slowly on its axis. Consequently a day on Mercury is equivalent to a year.
Mercury is the second smallest planet. It is smaller than Saturn’s and Jupiter’s largest moon. Mercury has no moons.
Mercury gets its name from the Roman god, fleet messenger. It is hard to spot Mercury because it is only visible during the twilight hours on some days, and it is always close to the Sun’s bright glare. Some telescopic observations of Mercury reveal that it appears to change its shape from day to day much like our Moon.
The surface of Mercury is heavily cratered due to the bombardment of countless meteorites and asteroids.
Mercury is almost airless because of the closeness of the Sun. The temperature can rise above 750 degrees fahrenheit during the day, which is hot enough to melt lead. During the long nights the temperature drops to -300 degrees fahrenheit, which is colder than the South pole.
Next to our Moon Venus is the brightest object in the night sky. Venus is named after the Roman goddess of love and beauty. Although Venus is not a star, it is sometimes called the Morning or the Evening Star. Venus is the second planet from the Sun and rotates from East to West, the opposite of most planets and Moons. Venus has no moons. It is always shrouded in clouds.
Venus is also referred to as Earth’s sister planet although it is very different from Earth. Venus is covered by thick clouds of sulfuric acid. Below the cloud the atmosphere is carbon dioxide.
Venus is a scorching desert with temperatures of 900 degrees fahrenheit. It is the hottest planet in the Solar System. The atmosphere is credited mostly for this intense temperature. It traps the heat and does not allow it to escape, in a process that is known as the greenhouse effect.
Earth is the third planet from the Sun. It is the only planet with large amounts of liquid water on its surface and in the atmosphere. The Earth is filled with living things.
From space Earth looks like a perfect ball. It is 27 miles wider at the equator than at the poles. Planet Earth is larger than Mercury, Venus, Mars, and Pluto.
Earth spins like a giant top. One complete spin is called a day. It tilts on one side as it travels around the Sun. Part of the year the northern half experience summer because it tilts toward the Sun while the southern half tilting away from the sun experiences winter. As the Earth continues to orbit the southern half will tilt toward the Sun and experience summer while the northern half experiences winter.
Encircling the Earth is a protective blanket of atmosphere. The atmosphere is made up of nitrogen, oxygen, carbon dioxide, and trace gases. Most of the Earth’s weather occurs in the layer of atmosphere closest to it, the troposphere.
Earth has one natural satellite, the Moon. We can only see that part of the Moon lit by the sunlight. The Moon is barren with no water, air, clouds, and no living things.
The Moon is covered with thousands of craters caused by bombardments of meteorites and asteroids. There are also mountains, hills, valleys, and flatlands. The gravity is 1/6 of that of the Earth.
Mars is the fourth planet from the Sun. Due to the closeness to us it appears bright in the sky. Mars has a red color. The Romans named it after their god of war because the red reminds them of blood and war.
Mars has craters, mountains, and valleys. The valleys are deep and longer than the Grand Canyon. The surface on Mars is made up of an orange dusty soil. The orange color is due to the presence of iron oxide in the soil. It is windy and cold there. The atmosphere is also thin.
Astronomers believe that when Mars was a young planet some water flowed there. Because of the properties of its atmosphere, water cannot exist there in a liquid form. In some unexplored area there may still be some frozen water and possibly life. However, from samples gathered by the Viking missions there is no evidence that life exist on Mars.
Jupiter is the giant planet in the Solar System. The “rings” in Jupiter is speculative and it probably does not exist most times. Only Saturn has prominent rings. The gases that make up Jupiter are primarily hydrogen and helium.
An outstanding featured on Jupiter is the giant wind storm called the Great Red Spot. It was sighted 300 years ago and has not changed position.
Jupiter has at least sixteen moons. The outer moons are small. The four larger moons are Io, which has exploding volcanoes that spur out liquid sulfur, it also undergoes metamorphism with each explosion. Europa is covered by a smooth ocean of gas. Ganymede is the largest and is larger than Mercury. Callisto is primarily ice with some rock on top of a deep frozen ocean.
Saturn is the second largest planet and it too is a gas planet composed mostly of hydrogen and helium. It is the sixth planet from the Sun and was named after the Roman god of farming.
Saturn’s rings are flat and are made of thousands small rings within rings. The rings are made of ice. The size of objects in the rings vary from the size of a house to that of a fingernail. The rings contain bits of rock and dust. The rings are less than three miles thick but extend to a distance of 170 miles across. No one knows for certain just how the rings were formed. It is speculative that they contain material left over from the formation of the planet.
Saturn has more than twenty moons, more than any in the Solar System. Titan is the largest moon. Titan is larger than Mercury and Pluto together. Saturn has six medium size moons and fourteen smaller moons.
Titan is the only moon in the Solar System known to have atmosphere. The atmosphere is mostly nitrogen gas that covers the moon surface with a thick haze.
Uranus is planet number seven from the Sun. It is named after the Greek god of heaven and ruler of the world. If Uranus were hollow, about fifty planets Earth could fit inside. It is covered with pale blue-green clouds. The eleven rings around Uranus are made of chunks of an unknown black material.
Unlike other planets, Uranus lies on its side. Alternating poles when pointing toward the sun will have 42 years of dark.
Uranus has at least fifteen moons, five large and at least ten small. Uranus’s moon Miranda is quite unique in that it has huge canyons, deep grooves, ridges, and rope like markings on the surface.
You cannot see Neptune from Earth without the use of a telescope. Neptune is named after the Roman god of the sea. It is the eighth planet from the Sun.
The rings here are not visible except from a close probe. It has dark storms, giant hurricanes, and streaky white clouds of methane-ice. Frigid winds blow at a speed of 700 miles an hour. Atmospheric methane absorbs red light which is why the planet has a blue color. Haze high above the clouds causes a red rim.
As Voyager 11 went past Neptune it discovered the two bright outer rings. There are eight moons of which Triton is the largest. Triton is said to be colder than any object in the Solar System. Once a hot volcanic place, now Triton is a frozen imprint of that earlier life. The planet has the appearance of a rind of an orange.
Pluto is usually the most distant planet from the sun. Its odd tilted orbit pulls it on occasions closer to the sun than Neptune. It is named after the Roman god of the underworld.
Pluto is the coldest and smallest planet in the Solar System. It appears to be an ice ball of methane gas and water mixed with rock. It is surrounded with methane gas.
Comets appear to be shining patches of light in the night sky
Most are too faint to be seen without a telescope. They are named, after their discoverer.
A comet is a dirty snowball. It has an icy core of water and gas mixed with bits of rock and metal, and covered with black dust.
When a comet approaches the Sun its surface material begins to evaporate. This exchange enables the gas to carry away some of the gas and dust particles that spread out around the nucleus in a large cloud called a coma. The sunlight causes the coma to glow. An average comet can create a coma 60 thousand miles across with a small amount of material spread very thin.
As the comet approaches the Sun, the pressure of the Sun and solar winds causes the gases of the coma to be swept away from the direction of the Sun. it is possible for the glow of a straight tail to extend millions of miles. Sometimes a second tail appears, this dust tail is usually slightly curved and shorter then the gas tail.
Most comets have an oval shaped orbit. Short period comets take less than 200 years to travel around the sun. Halley’s Comet is an example of a short period comet. Many short period comets have orbits that extends as far as Pluto. The comet with the shortest known period is Encke’s comet, which returns every 3.3 years.
There are two types of comet tails. The gaseous tail is usually straight and the dust tail is curved.
The mass of a comet is small compared to planets or satellites.
The tail of comets are mainly tenuous gases of hydrogen compounds, ammonia, methane and cyanogen. Many comets can be seen with the naked eye. They appear as a blurred patch of light and not all of them have tails.
A comet’s brightness is dependent upon its distance from the sun. The closer they are to the sun the brighter they appear. Some may fade away when they get close to the sun because their nucleus break apart.
In the past 200 years the comet IRA Araki-Alcock, has come closer to the Earth (2.9 million miles) than any other known comet.
There is a cloud of 100 billion comets that surrounds the Solar System.
Asteroids are chunks of rock that circle the Sun. Sometimes they are referred to as minor planets. The diameters of asteroids are only a few miles across. They are far smaller than planets.
METEORS/ METEORITES/ ASTEROIDS
Meteoroids are members of the Solar Systems that are so small that they cannot be seen beyond Earth’s atmosphere. They are the most common bodies in the sky.
Due to the high velocity of a streaking meteoroid, sometimes they plunge into Earth’s atmosphere. The friction caused by the streaking causes them to glow red. Once a meteor enters the upper atmosphere, friction causes it to become heated and break up, leaving a luminous streak called a meteor or a shooting star. In reality they are not stars at all. Stars are Suns much beyond our Solar System.
Meteors originated as meteoroids, which are bits of rock and metal. There are two types of meteoroids, showers and sporadic. Meteor showers occur when Earth passes through an old comet’s orbit and collides with some of the particles remaining from the comet’s nucleus. The 1966 meteor storm was the greatest recorded in history.
The meteors that do fall to the Earth are called meteorites. Meteorites have the appearance of a rock or a hunk of metal.
Unless someone sees them fall they are difficult to find. The stony meteorites are mostly chondrites. Chondrites contain small glassy globs. The meteorite that was found in Antarctica is very much like that brought back from the moon by the Apollo astronaut. Meteoroids that are mostly iron and nickel do not look like ordinary rock and are attracted to magnets. A third group of meteoroids, stony iron, are about half stone and half nickel iron.
Large space rocks are asteroids. Most of these rocks are in the asteroid belt circling the sun between the orbits of Mars and Jupiter. There are about three thousand known asteroids. Ceres, about 600 miles in diameter is the largest known asteroid. Pallas and Vesta are next in size.