The universe is a huge collection of all the heavenly bodies including galaxies, stars, constellations, planets, satellites, asteroids, comets, etc. along with matter, heat, and light around us. The branch of science dealing with the study of the universe and its components is called astronomy. Among millions of galaxies, the sun and the solar system are located in the Milkyway galaxy.
Units used to measure distance in the universe
Bigger units like light year, astronomical unit, parsec, etc. are used for measuring the cosmic distances.
Light Year:
It is the distance travelled by the light in one year. Its value is 9.5 × 1015 m (9.5 × 1012 km).
1 year = 365 days
= 365×24×60×60 seconds
The speed of light in vacuum = 3×108 m/s, so the distance travelled by light in 1 year
1 light-year = 365×24×60×60×3×108
=9.5×1015 m
= 9.5 × 1012 km (approximately)
Astronomical Unit:
It is defined as the mean distance between the sun and the earth.
1 Astronomical Unit (AU) = 1.5×1011 m = 1.5×108km (approximately)
Parsec
It is defined as the distance between the sun and the point in the space, which subtends an angle of one second of arc with the mean radius of the earth’s orbit. Its value is approximately equal to 3.26 light-years or 2.06×105AU.
1 Parsec(pc) = 3.26 ly = 2.06×105AU
The Solar System
The solar system is the gravitationally bound system of the Sun and the objects that revolve around it like planets, satellites, asteroids, comets, etc. The solar system is located in an outer spiral arm of the Milky Way galaxy.
The Sun
The sun is a medium-sized star – a hot glowing ball of hydrogen and helium- at the centre of our solar system. Its gravity holds the solar system together. It is about a 4.6 billion years old yellow dwarf star. It consists of three layers; Photosphere- the central hottest layer, Chromosphere- the middle layer(it can be seen during total solar eclipse only), Corona- the outermost layer. A stream of charged particles (plasma) is continuously released from the corona which is called solar wind.
Age: 4.6 billion years
Average Diameter: 1.3924×106km(approximately)
Mass: 2×1030kg(approximately)
Distance from the earth: 1.5×1011m
Composition: 73.46% – Hydrogen; 24.85% – Helium; Remaining 1.69% – other elements like oxygen, carbon, nitrogen, neon,etc.
Temperature: surface-5700 °C; core-1.5×107°C
Planets
Planets are those celestial bodies that revolve around the sun in their own elliptical paths. The criteria required for a celestial body to be considered as a planet are:
- It must be in orbit around the sun.
- It must be have sufficient mass to be spherical.
- It must have cleared the neighbourhood around its orbit.
There are eight planets in the solar system; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The pluto has been removed from the list of planets because it does not meet the third criteria. The eight planets can be classified into two types; inner planets and outer planets
Inner Planets
Inner planets lie in between the sun and the asteroid belt. Mercury, Venus, Earth, and Mars are the inner planets.
Mercury
Characteristics: smallest planet, closest to the sun, fastest revolving planet
Time for revolution: about 88 earth days
Time for rotation: about 59 earth days
Radius: 2440 km
Distance from the sun: 5.8×107km(0.4AU)
Number of satellites: 0
Atmosphere: It has no atmosphere.
Venus
Characteristics: Hottest and the brightest planet; also known as morning and evening star.
Time for revolution: 224 days
Time for rotation: 116 days 18 hours
Number of satellites: 0
Atmosphere: Thick toxic atmosphere filled with CO2.
Earth
Characteristics: Known as the living planet because it has suitable temperature, atmosphere and sufficient amount of water for the survival of living beings.
Time for revolution: 365 days
Time for rotation: 24 hours
Surface temperature: 15°C
Radius: 6371 km
Distance from the sun: 1.5×1011m
Number of satellites: 1(Moon)
Atmosphere: Nitrogen, oxygen, carbon dioxide, and water vapour.
Mars
Characteristics: Known as the red planet because it is very rich in limonite(iron ore) which is red in colour.
Time for revolution: 687 days
Time for rotation: 1 day 37 min
Surface temperature: 60°C
Radius:3389.5 km
Distance from the sun: 22.56×1010m
Number of satellites: 2 ( Phobos and Deimos)
Atmosphere: 95% CO2 and little amount of oxygen
Outer Planets
Outer planets lie beyond the asteroid belt. Jupiter, Saturn, Uranus, and Neptune are the outer planets.
Jupiter
Characteristics: Largest planet, fastest spinning planet
Time for revolution: 12 years
Time for rotation: 55 minutes
Number of satellites: 67 (Ganymede, Europa, Callisto, etc)
Composition: Hydrogen and helium
Saturn
Characteristics: Second largest planet, planet having least density
Time for revolution: 29.5 years
Time for rotation: 10 hours and thirty minutes
Number of satellites: 62 (Titan, Dione, Rhea, etc)
Uranus
Characteristics: Second farthest planet, the first planet discovered with a telescope
Time for revolution: 84 years
Time for rotation: 17 hours 14 minutes
Number of satellites: 27 (Miranda, Ariel, Titania,etc)
Neptune
Characteristics: Farthest and coldest planet
Time for revolution: 164 years
Time for rotation: 16 hours
Number of satellites: 14 ( Triton, Nereid,etc)
Satellite
Satellites are those heavenly bodies that revolve around a planet.
Natural satellite: The celestial body that orbits a planet is called a natural satellite. Moon is the natural satellite of the earth. The time taken by the moon to completer one revolution around the earth is called the sidereal month. Its duration is 27.3 days. The time taken by the moon to complete one complete rotation is called synodic month. Its duration is 29.5 days.
Artificial satellite: Artificial satellites are human-built objects orbiting the earth and other planets.
Purpose of launching artificial satellites
- For forecasting weather
- For communication
- For the study of space and various parts of the earth making different space stations
- To carry out various scientific experiments in space
Asteroids
Asteroids are the small, rocky bodies that orbit the sun and lie between the orbit of Mars and Jupiter. The asteroid belt separates inner and outer planets. Examples: Vesta, Hermes, Eros, Ceres, Juno, etc.
Comet
Comet is a frozen mass made of ice, dust, and gases that orbits the sun and produces a tail when approaches the sun. A comet has three parts; nucleus, coma, and tail. The nucleus is the central part which is composed of rock particles trapped inside the frozen mass. The outer layer of the nucleus of a comet begins to evaporate when it approaches to the sun and forms a gaseous atmosphere which is called coma. When the comet approaches the sun, the high-energy solar radiations push the vapour and dust to form a long tail pointing away from the sun due to the solar wind.
The tail of a comet disappears when it goes away from the sun, why?
When the comet moves farther away from the sun, it experiences less solar radiation and solar wind due to which the evaporation of frozen mass slows down and eventually stops and the comet appears only as a nucleus. Thus, the tail of a comet disappears when it goes away from the sun.
Meteoroids, meteors and meteorites
A meteoroid is a small rocky or metallic body in outer space that orbits the sun.
When the meteoroid enters the earth’s surface at high speed and produces light, it is called a meteor. A meteor is also called a fireball or a shooting star.
When a meteor doesn’t burn completely in the atmosphere and reaches the surface of the earth, it is called a meteorite.
Why do meteors disappear before reaching the earth?
During the falling of a meteor, friction is produced with the atmosphere because of its high speed and it gets brunt; changes to vapour. So,
meteors disappear before reaching the earth.
Types of meteorites
On the basis of the composition of materials, meteorites are classified into three types; stony meteorite, irony meteorite and stony irony meteorite
Stony meteorite: These meteorites contain more amount of silica and less quantity of iron and magnesium.
Irony meteorite: These meteorites contain more amount of iron and a negligible quantity of silica.
Stony irony meteorite: These meteorites contain both silicon and iron in nearly equal amount.
Stars
Stars are the self-luminous celestial bodies. The thermonuclear reaction inside the star is responsible for the emission of heat and light energy in a star. The stars show a large number of colours including red, orange, yellow, white and blue. The temperature of the stars is in the order of
Blue>White>Yellow>Orange>Red
Why do the stars end in a black hole after a long time?
After a long time, the core of a star runs out of hydrogen and the star changes into a dense and high gravity mass. Even light cannot escape out of the mass due to its high gravity and thus it gets transformed into a black hole.
Constellations
A constellation is a group of stars that do not change their position relative to each other and create a pattern in the sky. 88 constellations are known among which 12 are used as a sign of the zodiac. Examples: Ursa Major, Ursa Minor, Orion, Aries, etc.
Superstitions about the signs of the zodiac
- Future telling by looking at zodiac sign
- Person born on new moon day are said to have bad character
- Wearing a ring according to zodiac
- Practice of choosing auspicious day, time, and direction based on their sign of zodiac to start a new business
- Getting married by matching planets and the signs of zodiac
Galaxies
An extremely large gravitationally bounded system of billions of stars, gases, and interstellar substances. All the stars of a galaxy revolve around a central imaginary point called the galactic centre.
On the basis of shapes, galaxies are divided into three types; spiral, elliptical, and irregular galaxy.
Spiral galaxy:
Spiral galaxy is seen as spinning spindle with winding arms all around its centre. Andromeda and Milky Way are the spiral galaxies.
Elliptical galaxy:
Elliptical galaxies are shaped as a sphere or a flat ellipse, the centre is bulged and bright light is emitted from the centre. Examples: Maffei-1, IC1101,etc
Irregular galaxy:
Irregular galaxies dod not have a distinct regular shape. The large and small Magellanic clouds are the examples of irregular galaxies.
Big bang theory
Big bang theory explains the formation and expansion of the universe. According to this theory, around 13.7 billion years ago the entire universe was condensed at a single point of infinite denseness and heat. This highly dense and extremely hot mass exploded and scattered all over. Thus galaxies, stars, planets, etc were formed.
Differences between
| Light Year | Cosmic Year |
| 1. It is the distance travelled by light in 1 year. | 1. It is the time taken by the sun to complete one revolution around the galactic centre. |
| 2. Its value is 9.5× 1015m. | 2. Its value is 2.5×108years. |
| Comet | Meteor |
| 1. Comet is a cosmic object made of ice, dust, and gases that orbits the sun. | 1. A meteor is a meteoroid that has entered the earth’s atmosphere. |
| 2. It stays in space. | 2. It falls into the earth’s atmosphere. |
| 3. It produces a tail when it approaches the sun. | 3. It doesn’t produce a tail but produces a streak of light after entering the earth’s atmosphere. |
| Stars | Planets |
| 1. They revolve around the galactic centre. | 1. They revolve around the sun. |
| 2. They are luminous heavenly bodies. | 2. They are non-luminous heavenly bodies. |
| 3. Thermonuclear reactions take place. | 3. Thermonuclear reactions do not take place. |
| Constellation | Galaxy |
| 1. Constellation is a group of stars that do not change their relative position with each other. | 1. Galaxy is an extremely large group of billions of stars, gases and interstellar substances. |
| 2. It is an imaginary pattern in the sky. | 2. It is a real concept. |
| 3. There are 88 known constellations. Examples: Ursa Major, Orion, Cassiopeia, Lyra, etc. | 3. There are millions of galaxies. Examples: Milky Way, Andromeda, etc. |
Reference:
- Image: The Earth – Sapling on High School Pitcher’s Mound Celebrates Earth Day