What is Astronomy
Astronomy is a branch of science that studies the physics and chemistry of celestial objects such as planets, stars, comets, nebulae and galaxies. It helps us to understand the formation of stars and their composition, space, solar system, gravitational waves, supernovas and other phenomena
Distances
Astronomical Unit (A.U): The distance between the earth and the sun is 1.496 X 10^8 Km.
Parsec: The distance between the sun and an object whose parallax angle is one arcsecond is called a parsec. It is approximately 3.26 lightyears. 1 parsec = 1.3 X10^13 Km
Lightyear: The distance traveled by light in one year in a vacuum is called lightyear. It is 9.46 Trillion km.
The nearest star to our solar system is Proxima Centauri which is at 1.31 parsecs. Astronomers use Megaparsecs to measure the distances between galaxies. one megaparsec is equal to 1 million parsecs.
Solar System
Our solar system is part of the milky way galaxy with eight planets with the sun as the center starting with mercury, venus, earth, mars, Jupiter, Saturn, Uranus and Neptune. Even though initially we had nine planets but now pluto is considered a dwarf planet. Venus is the hottest planet in the solar system while Uranus is the coldest planet even though it's not the farthest planet from the sun but the core temperature is 5000°C when compared to Neptune which has 7000°C.
All the planets revolve around the sun in their orbits. The planet close to the sun will revolve fast compared to other planets due to the high gravitational pull of the sun.
The time taken by the earth to rotate about its axis such that the position of the sun is the same in the sky is called solar day. It is the time taken from noon to noon when the distance of the sun is at maximum. The sidereal day is the time taken by the earth to rotate about its axis such that the distant stars appear in the same position in the sky.
The distant objects appear to move slower than the closer objects when moving at a constant speed which is termed a parallax error.
Celestial objects
meteoroid: A “space rock”—a relatively small object traveling through space, between the size of a grain of dust and a small asteroid.
meteor: A meteoroid that enters Earth’s atmosphere and burns up.
meteorite: A meteoroid, especially one that has hit Earth’s surface.
asteroid: A rocky object that orbits the sun and has an average size between a meteoroid and a planet.
comet: An object made mostly of ice and dust, often with a gas halo and tail, that sometimes orbits the sun.
Constellations
A group of stars that forms a pattern that looks like animals, mythological subjects or inanimate objects in a celestial sphere is called a constellation. There are 88 official constellations known to date.
These constellations act as a compass in the olden days to move from one direction to another direction by looking at the night sky.
Milky way galaxy
Our solar system is part of the milky way galaxy which has more than 200 billion stars. The center of the galaxy is very dense with start dust and matter and has a black hole known as Sagittarius A*. The diameter of the galactic disc is about 30 Kpc while its thickness is about 1 Kpc. The sun is located approximately 8 Kpc from the center of the galaxy. The stars near the center of the galaxy move slower compared to stars on the outer edges of spiral arms. Andromeda is the nearest galaxy which is at a distance of 2.5 million light-years away milky way.
A group of galaxies forms into clusters and these clusters form into superclusters that constitute the universe.