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The Universe

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If You've ever looked up in the sky and questioned you are on the way of becoming an astronomer. Learning about the universe is one of the most adventurous task you can do. Ever wondered how did all of this begin? or how would all of this end? What exactly is our sun made of up? And what was the shining object I saw last night in the sky or why do we launch satellites? Here you will find answers to these questions. 
Introduction:
The Universe is all of space-time and everything that exists therein, including all the planets, satellites, stars, galaxies, the contents of intergalactic space, the smallest subatomic particles and all matter and energy. In short, everything you've seen or learned about including you yourself are the part of this Universe. The vastness of the Universe is beyond human imagination.Edwin  Hubble  for the first time stated that the universe is expanding and it will continue to do so in the future as well. Scientists believe that about 13.82 billion years ago the most powerful explosion called "the Big Bang" happened. This explosion set Universe into motion which continues even today. Then, Let's begin with the first question: "What is the Big Bang" theory?
The Big Bang Theory
At present the big bang theory is one of the most leading explanations about the birth of Universe. It states that the Universe that we see today started with a single singularity, then inflated over the next 13.82 billion years to the cosmos that we know today. A super dense and extremely hot body having neutrons and protons exploded with the big bang which led matter within it to fly all over the space in all directions. These matter started to cool down since they started to take up  space.  The tiny particles grouped together and formed atoms. These atoms further grouped together because of gravity to form stars and galaxies. The fusion inside the stars went on. The first stars created bigger atoms and group of atoms called molecule. This led to more stars being born. The galaxies even crushed together and grouped again forming more objects like planets, satellites, comet, meteorite as well as the most mysterious object "The black hole".Though the technology today doesn't allow us to look back at the birth of Universe, much part of this theory comes from explanations and mathematical calculations. Astronomers however can see the 'echo' of expansion through a phenomenon known as "Cosmic Microwave Background".
Units to measure the distance between heavenly bodies:
 All of us are familiar with the road distance from Biratnagar to Kathmandu. It's a little over 300 km. Multiply that by 10 you get half of the Earth's radius. Now, multiply it with 100 you get 3*10 km, which is almost moon-earth distance. 500 times of that distance you reach Sun. So, that's really a large unit, isn't it? In astronomy you may even have to measure more large distances for which the units such as Meter or Kilometre  are not at all suitable. Therefore, there are a set of Units that are designed to measure the vastness of the Universe and they are:

1. Astronomical Unit(AU): The mean distance between the Earth and the Sun is called one astronomical unit. It is equal to 1.5*10 km.

2. One light Year: The distance traveled by light in one year is called one light year. The diameter of our solar system is 10.5 light years. We know, distance traveled by light is calculated as speed of light times time in a year. The speed of light is approximately 3*10^8 m/s. There are approximately 31,536,000 seconds in one year. Multiplying both we get that one light years is:9.46*1012km. 

3. 1 parsec: The distance having value 3.26 light years is called one par sec.
SOLAR SYSTEM 
The solar system is the family of Our Sun, Planets, comets, asteroids, meteorites and many other celestial objects including the Kuiper belt and Oort cloud. All members of the solar system revolve around the Sun in elliptical orbits. This motion is caused due to gravitation among the sun and all other planets. Beginning from Mercury to Kuiper belt all lie inside the solar system. Let's know about some of the major objects of our solar system.
The Sun:
The sun is a medium sized, G2 and main sequence star. It is the main source of energy of the entire solar system. It appears larger than other stars since it's closer than the other stars. The sun has the mass of about 1.989*10^30 kg and radius around 700,000 km. It's almost at it's half way through it's main sequence lifetime of 10 billion years. The relative density of the sun is 1.41. The solar radiation is coming out from it's surface due to thermonuclear fusion reaction.  The Sun emits solar wind; it is the stream of charged gaseous ions emitting from the surface of the Sun. It has a speed of 500 km/s.  The sun does not have a solid surface. It's just a gaseous ball of hot gases mostly made up of Hydrogen and Helium. Some of the mass is lost during nuclear fusion reaction that gets converted into energy by Einstein's mass energy equation. It's mean distance from Earth is 1 AU. It's surface temperature is 5800 K and central temperature is 1.5*10^7 K. There are a lot more phenomena that happen in the Sun such as Coronal Mass ejection, Solar flares, Irridances, Sunspots and solar wind. These events help us understand the sun better. 
The Solar atmosphere consists of:
a. The Photosphere,
b. The Chromosphere,
c. The Corona( The outermost layer).

Importance of the Sun:
The Sun is the most important factor for all objects in the solar system. It's the key life supporting element for Earth. It helps heavenly bodies maintain  their position in space.The Sun helps in photosynthesis of plants. Solar(photo cells) can be operated with the help of solar energy. It provides necessary gravitational force to all the planets of the solar system. It's the ultimate source of heat and light on Earth. All other energy sources are subordinate to it.  

Planets

Planets are the heavenly bodies that revolve around the Sun in elliptical orbits. They are much smaller than the stars and do not have their own heat and light. They shine because of the reflected light from the sun and appear larger since they are closer compared to other stars. They are named planets which means"wanderers" in Greek language. There are eight planets in our solar system namely: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Their elliptical path lies almost at the same plane with sun at one of it's two foci. According to new definitions of planets by IAU set in 2006, a planet is a celestial body that:

1. is in orbit around the Sun,

2. has sufficient mass to assume hydrostatic equilibrium,

3. has "cleared neighborhood" around its orbit.

The Pluto was not considered a planet anymore due to these reasons:

1. It's mass is very small compared to that of the other planets,

2. It's orbit doesn't lie in the same plane as that of the other planets,

3. It's orbit coincides with that of Neptune.

The planets of our solar system are divided as inner planets or terrestrial planets and Outer planets or the Jovian planets. The inner planets are those planets inside the asteroid belt. They include Mercury, Venus, Earth and Mars. They have a solid surface and support no ring system. The planets Mars is however expected to have rings in few hundred years since it's Moon Phobos is approaching the Roche Limit. The inner planets have few or no moons and are comparatively smaller in size. Three of them(except Mercury) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features such as rift valleys and volcanoes. Unlike the inner planets, the Jovian planets are very massive, support ring systems, mostly gaseous as well as have large number of moons. The Jupiter is often referred to as "the star never formed" due to it's massive size and composition. 

Satellite:

A satellite is a heavenly body which revolves around the planet in it's own orbit. There are two types of satellites namely: Natural and artificial satellite.

1. Natural satellite:

A satellite which is made by nature and revolves continuously around the planet is called natural satellite. Our Earth has one such satellites: Our Moon. The no. of satellites of other planets are: 

a. Mercury-0     e. Jupiter-67 

b. Venus-0        f. Saturn-62

c. Earth-1           g. Uranus- 27

d. Mars-2            h. Neptune-14

The Moon

Moon is the natural satellite of the Earth. The diameter of the Moon is 3456 km. Gravitational force of the earth holds Moon in it's orbit. The Moon has no air and water. It's surface is covered with hard and loose dust, craters and mountains.It is at a distance of 382400 km from Earth. It doesn't have it's own heat and light rather reflects the light of the sun and appears to be glowing. The moon completes its rotation around the Earth in 27.33 days. This time is called 'sidereal period'. the relative density of Moon is 3.3. The moon rotates around it's own axis in every 27 days. As a result it doesn't appear to be moving at all. It is called synchronous rotation.

Differences between Sidereal and Synodic Months period:

2. Artificial satellite:

Those satellites that are made by humans and launched into space are called artificial satellites. For eg. SPUTNIK, INSAT, METSAT, etc. They revolve around the planet from a fixed distance. They are sent into space for the following purposes:

1. For the communication of radio. television, and telephone signals across the world.

2. for weather forecasting.

3. for spying or military purpose.

4. for study of solar rays and cosmic rays as well as study various regions of the Earth and Space.

There are other particles that keep floating in space. When viewed from earth they look beautiful at the same time they may even cause potential damage to the planet. Therefore, it's necessary to learn more about these objects, their composition and most importantly their orbits. While Jupiter saves us from most of the comets it is also responsible for pertubing the orbit of asteroids towards Earth. This raises another question in astronomy:"Jupiter a friend or Foe?" That's a question to be answered in a long run but first let's know the composition of Asteroids, Comets and Meteors.

Asteroids-

The rock and metal objects present between Mars and Jupiter and revolve around he sun are called asteroids. There are as many as 100000 asteroids in our solar system. The biggest asteroid is Ceres and the smaller ones could be the size of pebble. The other asteroids are: Juno, Vesta, Eros, etc. The other asteroids like Aten, Amor, Apollo, etc. are located outside the asteroid belt. They move at a speed of 25 m/s and intersect with Earth's orbit which increases chances of collision with Earth. The CENOS in United States is one of those organization tracking movements of these minor planets. If you would love to hunt few of those asteroids( discover asteroids) you could be the part of Asteroid search campaigns. 

Meteors (falling stars or shooting stars)

The pieces of stone or Metal which enter the Earth's surface are meteors. These are flashes of light in the sky which disappear shortly after lighting up. They could be the pieces of asteroids that have collided. When an heavenly body enters Earth's atmosphere with high speed, friction is developed between body and air.  Due to this friction, heat is produced in it, and a burning object is seen in atmosphere as a streak of light. Sometimes the big meteor do not completely vaporize and reach the earth's surface. This object is called meteorite. A large number of meteors fall near poles of the Earth known as the meteor shower. An extremely bright meteor can be termed as the 'fire ball'. Depending upon the composition of materials, meteors are of three types: 

1. Stony meteorite,

2. Iron meteorite

3. stony iron meteorite.

( A meteorite fall in Arizona created a huge crater which shows large meteors could be a risk for us.) There are a lot of meteor showers that happen every month. Recently, there was etta-aqaridds meteor shower and we'll soon observe Arietids meteor shower. These meteor showers are named based on which constellations they appear.

Comets

Comets are mostly located in the Kuiper belt and oort cloud region of our solar system. It appears broom like tailed structure revolving around the sun. It is made of of ice, gas and dust. The head is made up of ice. The head of comet is called coma and the central part is called the nucleus. As a comet approaches the sun it develops a long tail that is visible to us. They do not have their own light rather reflect the light of Sun. Some comets are visible after a long period of time and loses it's tail as it moves away from the sun. Some of the most famous comets are shoemaker-levy, Halley's comet, etc. 

Galaxy

Space contains a lot of stars that are grouped together as galaxies. The galaxy can be of different types: spiral, elliptical, irregular galaxy. It is a vast system of stars, dust particles, and gases isolated from similar structure in space.  These galaxies revolve around common center known as the galactic center. There are about 10^11 galaxies in the universe. our galaxy 'The Milky Way Galaxy' falls under spiral galaxy and is about 1.33*10^11 times heavier than Sun.

Constellations

In contemporary astronomy, IAU recognizes 88 constellations in the sky. Each of these constellation are the region of the sky bordered by the arcs of right ascension and declination. Together they cover the entire celestial sphere. Unlike this, it is also defined as the group of stars resembling a particular shape. The brightest star in the constellation is named as Alpha star of that constellation. The twelve of those constellations which lie on the solar path are called zodiacs. They lie at 9 degrees on either sides of the ecliptic. There are various superstitions related to zodiacs which must be discarded. They are just used in order to find time at night, forecast weather and identify other stars. Navigators find their locations in sea based on various zodiacs. Some of the most famous constellations include: Orion( The hunter), Ursa major(Big bear, Saptarishi), Ursa minor(Little bear), Andromeda(Princess of Eithiopia), Hercules(son of zeus), etc.

Stars

All the twinkling objects in the night sky are mostly stars. They have their own source of light and nuclear fusion keeps going inside a star. It is made up of hydrogen and helium mostly. There are various sequence of stars such as the main sequence, pre-main sequence and post-main sequence. The color of star changes in order of VIBGYOR. The size and life of star depends on amount of energy produced and mass of stars. Though very big they appear smaller due to prolonged distance from Earth.