SOLAR SYSTEM

• What is it?
  • The gravitational bound system of the Sun + objects that orbit it (directly or indirectly).
• What does it consists of?
  • The sun
  • Everything bound to it by gravity, such as:
    • the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune;
    • dwarf planets such as Pluto; 
    • dozens of moons; and 
    • millions of asteroids, comets, and meteoroids.
• Formation:
  • From the gravitational collapse of a giant interstellar molecular cloud, 4.6 billion years ago.

STARS

• What is it?
  • an astronomical object comprising a luminous spheroid of plasma held together by its gravity.
  • cosmic energy engines that produce heat, light, ultraviolet rays, x-rays, and other forms of radiation.
• Composition:
  • Largely gas and plasma (a superheated state of matter composed of subatomic particles
• They are the fundamental building blocks of galaxies.

MOON

• Earth’s only natural satellite
• Size:
  • ¼ the diameter of Earth;
  • 5th largest satellite in the solar system;
  • Largest satellite in the Solar System relative to its major planet;
  • Larger than any known dwarf planet.
• Formation:
  • 4.6 billion years ago
  • 30-50 million years after the formation of the solar system
• Rotation:
  • Synchronous with Earth, meaning the same side is always facing the Earth
• Revolution:
  • Revolves around the Earth in 27 days 7 hours 43 minutes 11.47 seconds
  • Rotates on its own axis in exactly the same time
  • Hence only one face of the Moon is seen from the Earth.
• Other facts:
  • Total forces of the Moon and the Sun are in the ratio of 9:4.
  • Study of the Moon is known as Selenology.

CONSTELLATION

• Group of stars that form a pattern that is recognisable in some shape or form.
• At present, there are 88 constellations.
• Some important ones:
  • Ursa Major:
    • Also known as Great Bear or Saptarishi
    • Consists of 7 bright stars in the shape of a bear
    • Visible during the summer season in early night
    • Appears to move from east to west
  • Orion:
    • Also known as Hunter or Mriga
    • Consists of 7 or 8 bright stars
    • Visible during the winter season in the late evening
    • One of the brightest stars is called Sinus
  • Leo Major:
    • Mainly consists of 9 or 10 stars
    • Shape similar to that of a lion
    • Visible during the summer in the early part of the night
  • Cassiopeia:
    • Mainly consists of 5 stars, arranged in the form of letter W or M depending on its position in the sky
    • Supposed to represent an ancient queen named Cassiopeia in Greek Mythology
    • Visible during the winter in the early part of the night

PLANETS

• A planet is a celestial body that
  • (a) is in orbit around the Sun,
  • (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  • (c) has cleared the neighbourhood around its orbit.
• Examples: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
• Dwarf Planet: a celestial body that
  • (a) is in orbit around the Sun,
  • (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
  • (c) has not cleared the neighbourhood around its orbit, and 
  • (d) is not a satellite.
• Why is Pluto a dwarf planet?
  • On account of its size; and
  • It resides within a zone of other similarly sized objects known as the transneptunian region.
• Some facts:
  • Planets with rings: Jupiter, Saturn, and Uranus have rings around them – belts of small debris
  • Smallest planet: Mercury
  • Largest planet: Jupiter
  • Inner planets: Mercury, Venus, Earth, and Mars
  • Outer planets: Jupiter, Saturn, Uranus and Neptune

Milky Way Galaxy

• What is a galaxy?
  • a large group of stars, gas, and dust bound together by gravity.
• Milky Way:
  • It is named so due to its appearance as a milky band of light in the sky. 
  • It is a large barred spiral galaxy.
• In ancient India, it was imagined to be a river of light flowing in the sky. Thus, it was named Akash Ganga.

Sun

• Composition:
  • made up of extremely hot gases, particularly by hydrogen (70%), Helium (26.5%), and others (3.5%) gases.
• Distance:
  • 150 million km away from earth. 
  • The sunlight takes 8 minutes to reach the earth’s surface.
• Energy:
  • Continuously emits in the form of visible light, infra-red, ultraviolet, X- rays, gamma rays, radio waves, and plasma gas.
• Solar Flares:
  • The sudden flash of brightness observed near the sun’s surface
  • It is a collection of magnetic energy including electrons, protons, and nuclei released from the surface of the sun.
  • They are concise particles and are harmful for satellite communication.
  • Solar flares can be accompanied by Coronal Mass Ejections (CMEs)
  • CMEs are bubbles of radiation and particles from the Sun’s corona
  • They explode into space when the Sun’s magnetic field lines suddenly reorganize
  • CMEs can trigger auroras on Earth
• Nuclear Fusion:
  • The phenomenon in which the hydrogen atoms which fuse together due to compression and creates helium.
  • It happens in the core of the sun.
  • It produces a huge amount of energy. 
  • It is radiated outward to the surface, atmosphere, and beyond.
• Layers of the Sun:
  • Core: where Nuclear Fusion takes place
  • The convection zone: next to the core. Here the temperature drops to 2-million-degree C.
  • Photosphere: temperature drops to 6,000°C.
  • Atmosphere: consists of the chromosphere and corona.
• Corona:
  • It is seen in a form of spectral lines emitted by iron, calcium, and Nickle ions. 
  • The ionization of these elements increases the temperature of the corona.
• Auroras:
  • When the charged particles released from solar flares get trapped by the earth’s magnetic field while entering the upper atmosphere, it results in auroral (light) display.
  • In northern hemisphere, it is called Aurora Borealis (the Northern Light)
  • In the southern hemisphere, it is called as Aurora Australis (the Southern Lights).
• Sun spots:
  • dark appearing areas present in the photosphere from where solar flares originate.
  • Appears and disappears after every 11 years
  • This period is called the Sunspot cycle, which is marked by the increase and decrease of sunspots – visible as dark blemishes on the sun’s surface or photosphere.
  • The greatest number of Sunspots in any given solar cycle is designed as “solar maximum” and the lowest number is the “solar minimum“.
  • They typically consist of a dark region called the ‘umbra’, which is surrounded by a lighter region called the ‘penumbra’.

• Solar Wind:
  • a stream of charged particles released from the upper atmosphere of the Sun, called the corona. 
  • This plasma mostly consists of electrons, protons, and alpha particles with kinetic energy between 0.5 and 10 keV.
• Solar Storm:
  • a disturbance on the Sun, which can emanate outward across the heliosphere, affecting the entire Solar System, including Earth and its magnetosphere
  • It is the cause of space weather in the short-term with long-term patterns comprising space climate.
• Geomagnetic storm:
  • a temporary disturbance of the Earth’s magnetosphere caused by a solar wind shock wave and/or cloud of a magnetic field that interacts with the Earth’s magnetic field.

Earth

• It is the 3rd planet from the sun and is 93 million miles away.
• A day on Earth is 24 hours long because of its rotation.
• Air composition:
  •  78% nitrogen, 21% oxygen, and 1% other stuff, which is good for living things to breathe.
• It’s the only planet with air we can breathe.
• Earth has 1 moon that orbits around it.
• The air protects us from falling rocks (meteoroids) from space.

Satellite

• Natural Satellite:
  • A celestial body that moves around the planets in the same way as the planets move around the sun.
• Human-made Satellite:
  • An artificial body designed by scientists to gather information about the universe or for communication. 
  • It is carried by rocket and placed in the orbit around the earth.

Asteroids

• What are they?
  • Minor planets especially those of the inner solar system.
  • They orbit our sun in a region of space between the orbits of Mars and Jupiter known as the Asteroid Belt.
  • They are solid, rocky and irregular bodies.
  • They do not have atmospheres (hence no life support) nor rings.
• Over 150 asteroids have moons.
• The first asteroid with a moon was discovered in 1993, known as asteroid Ida and its moon Dactyl.
• NASA has studied asteroids through missions like the Dawn mission, which orbited an asteroid for the first time in 2011.
• Ceres is the largest and first discovered asteroid and makes up a large part of the total mass of all asteroids.

Meteors and Meteorites

• Meteoroids:
  • Asteroids that crash and break into smaller pieces.
  • They orbit the sun.
  • They are small and rocky.
• Meteor:
  • When a meteoroid comes near Earth, it burns up in the atmosphere and becomes a meteor.
• Meteroites:
  • The meteoroids that don’t burn up and hit the ground.

Asteroid	Meteoroid
Asteroids are minor planets.	Meteorites are falling stars.
The orbital shape is elliptical and orbits the Sun	The orbital shape is elliptical and orbits the Sun but, get pulled into larger bodies.
Believed to be a leftover from the planet	Believed to be a smaller disintegrated element of comet or asteroid.
Do not have an atmosphere	Do not produce an atmosphere, but burn up when falling into a planet.
1 to more than 100 kilometers in diameter	Typically, less than 10 meters.

Meteor	Meteorite
Meteors are still up in the sky.	Meteorites are on the earth.
Meteoroids break down in the earth’s atmosphere which results in the flash of light known meteors.	Meteorites are the broken meteoroids that land on the earth.

Comets

• What are they?
  • Comets or ‘dirty snowballs’ are mostly made of dust, rocks, and ice and can range in their width from a few miles to tens of miles wide.
  • Example: C/2020 F3

• When they orbit closer to the sun, they heat up and release debris of dust and gases.
• Composition:
  • The solid portions consists mostly of water, ice, and embedded dust particles are inactive when far away from the sun.
• Activity:
  • When comets are close to the sun, they release gas and dust and form a glowing head and a long tail.
  • The more a comet orbits the sun, the more material it loses until it eventually disappears.
  • 
  • NASA estimates that there are millions of comets orbiting the sun and over 3,650 have been discovered.
• Predictable comets:
  • These are the short-period comets that take less than 200 years to orbit around the sun.
  • These can be found in the Kuiper belt, where many comets orbit the sun in the realm of Pluto.
  • One of the most famous short-period comets is called Halley’s Comet that reappears every 76 years. It will be sighted next in 2062.
• Less-predictable comets:
  • They can be found in the Oort cloud that is about 100,000 AU (Astronomical Unit which is the distance between the Earth and the Sun and is roughly 150 million km) from the sun or 100,000 times the distance between the Earth and the sun.
  • Comets in this cloud can take as long as 30 million years to complete one rotation around the sun.
• Visibility
  • Comets do not emit their own light, they are only visible because of reflection from the sun and the energy from the sun that is absorbed by the gas.
  • For a comet to be visible, it must either approach the sun closely and release a lot of gas and dust or approach the Earth closely.

Goldilocks Zone:

• The Goldilocks Zone is a habitable zone around a star where the temperature is suitable for liquid water to exist on a planet.
• Earth is in the Sun’s Goldilocks Zone.
• If Earth were where Pluto is, all its water would freeze, and if it were where Mercury is, all its water would boil off.
• Water is a necessary ingredient for life as we know it.
• Scientists search for alien life by looking for rocky exoplanets in the habitable zone of their star.
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