(Astronomy Class Notes)
Chapter 1 – Celestial Bodies
Celestial Bodies
Celestial bodies are natural objects located outside Earth’s atmosphere that
make up the universe. These include planets, moons, stars, nebulae, galaxies,
comets, asteroids, and meteoroids.
● Planets are large, spherical bodies that
orbit a star, like the Earth around the
Sun.
● Moons, or natural satellites, orbit
planets and vary in size and features.
● Stars are massive, luminous spheres of
gas powered by nuclear fusion.
● Nebulae are large clouds of gas and
dust, often sites for star formation.
● Galaxies are immense systems of
stars, planets, and interstellar matter held together by gravity.
● Comets are icy bodies that develop tails when they approach the Sun.
● Asteroids are rocky fragments that orbit the Sun, mostly found in the
asteroid belt between Mars and Jupiter.
● Meteoroids are smaller rocks or particles; if they enter Earth’s atmosphere
and burn up, they become meteors.
Planets and Moons
Planets are divided into two main groups:
1. Inner Planets (Terrestrial): Mercury,
Venus, Earth, and Mars. They are rocky,
smaller, and closer to the Sun.
2. Outer Planets (Gas Giants): Jupiter,
Saturn, Uranus, and Neptune. These are
, larger, made of gas, and have ring systems.
Moons are formed through various processes, such as being captured by a
planet’s gravity or forming from the debris of collisions.
● Europa: A moon of Jupiter with a possible subsurface ocean beneath its
icy crust.
● Titan: Saturn’s largest moon, known for its thick atmosphere and methane
lakes.
Moons vary widely in size and activity, with some, like Io (Jupiter), showing
volcanic activity, while others, like our Moon, are geologically inactive.
Stars and Nebulae
Stars undergo a life cycle depending on their mass:
1. Protostar: Formed from collapsing gas and dust in a nebula.
2. Main Sequence: Stars like the Sun, where nuclear fusion of hydrogen
occurs.
3. Red Giant or Supergiant: Expanded phase as the star exhausts
hydrogen.
4. End States: Low-mass stars become
white dwarfs; massive stars end in
supernovae, leaving behind neutron
stars or black holes.
Nebulae are classified by their appearance
and composition:
● Emission Nebulae: Glow due to
ionized gas (e.g., Orion Nebula).
● Reflection Nebulae: Reflect starlight.
● Planetary Nebulae: Shells of gas ejected by dying stars.
Galaxies and Comets
Galaxies are categorized by shape:
1. Spiral Galaxies: Disk-shaped with spiral arms (e.g., the Milky Way).
, 2. Elliptical Galaxies: Oval-shaped, often older with less active star
formation.
3. Irregular Galaxies: Lack a distinct shape, often formed by collisions.
Comets are icy objects that develop a visible tail when near the Sun due to
sublimation of ice into gas.
● Structure:
○ Nucleus: The solid core made of ice and dust.
○ Coma: The surrounding gas cloud.
○ Tail: Formed by solar wind pushing gas and dust away from the
nucleus.
● Famous comets include Halley’s Comet, visible from Earth every 76
years.
Chapter 2 – Astrophysics
Introduction to Astrophysics
Astrophysics is a branch of astronomy that studies the physical properties and
processes of celestial objects and the
universe. It combines principles from
physics, such as mechanics,
thermodynamics, and
electromagnetism, to explain
phenomena in space.
Key concepts in astrophysics include:
1. Gravity: The fundamental force
that governs the motion of
celestial bodies. It keeps
planets in orbit, forms stars and
galaxies, and influences the
large-scale structure of the universe.
