Galaxies

• Gigantic systems of stars, gas, dust, and dark matter bound together by gravity.
• Contain billions of stars and their solar systems.
• Galaxies are not stationary — they rotate and move through space.
• Classified into three main types:
  
  • Spiral galaxies: Flat, disk-shaped with spiral arms (e.g., Milky Way).
  • Elliptical galaxies: Round/oval shapes, mostly older stars, less gas.
  • Irregular galaxies: No clear shape, often formed by collisions.
• The Milky Way is our home galaxy, about 100,000 light-years across.

Examples:

• Milky Way Galaxy – spiral galaxy that contains our Solar System.
• Andromeda Galaxy (M31) – nearest major galaxy to the Milky Way, about 2.5 million light-years away.

Stars

• Massive balls of hot gases (mainly hydrogen and helium).
• Shine by nuclear fusion: hydrogen nuclei fuse into helium, releasing energy.
• Stars differ in temperature, color, mass, and brightness.
• Colors indicate surface temperature:
  
  • Blue stars – hottest (above 10,000 K).
  • Yellow stars – medium (like the Sun, ~5800 K).
  • Red stars – coolest (~3000 K).
• Life cycle: nebula → protostar → main sequence → red giant/supergiant → white dwarf / neutron star / black hole.

Examples:

• The Sun – a medium-sized yellow star providing energy for life on Earth.
• Sirius – the brightest star in Earth’s night sky, located in the constellation Canis Major.

Planets

• Large spherical objects orbiting stars due to gravity.
• Do not produce light, only reflect starlight.
• Two types in our Solar System:
  
  • Terrestrial (rocky) planets: small, dense, solid surfaces (Mercury, Venus, Earth, Mars).
  • Gas giants/ice giants: very large, thick atmospheres, many moons (Jupiter, Saturn, Uranus, Neptune).
• Planets vary in temperature, mass, number of moons, and distance from the Sun.

Examples:

• Earth – the only known planet with life, has water and atmosphere.
• Jupiter – the largest planet, a gas giant with over 90 known moons.

Moons

• Natural satellites that orbit planets.
• Stabilize the rotation of their planets and can affect tides (like Earth’s moon).
• Can be rocky or icy, some with atmospheres or subsurface oceans.
• Numbers vary: some planets have none (Mercury, Venus), while others have dozens (Jupiter, Saturn).

Examples:

• Earth’s Moon – stabilizes Earth’s tilt and causes tides.
• Europa (moon of Jupiter) – icy moon with a possible subsurface ocean that may support life.

Meteors

• Small pieces of rock or dust that enter Earth’s atmosphere.
• Friction with air makes them burn brightly – this is called a “shooting star.”
• If large enough, part of the rock can survive and hit Earth’s surface – then it’s called a meteorite.
• Meteor showers occur when Earth passes through debris left by a comet.

Examples:

• Perseids Meteor Shower – occurs every August, from debris of Comet Swift-Tuttle.
• Chelyabinsk Meteor (2013) – exploded over Russia, causing damage and injuries.

Asteroids

• Rocky objects orbiting the Sun, mostly between Mars and Jupiter in the asteroid belt.
• Much smaller than planets, irregular in shape.
• Thought to be leftover building blocks from the formation of the Solar System.
• Some asteroids cross Earth’s orbit, creating impact risks.

Examples:

• Ceres – the largest asteroid (dwarf planet) in the asteroid belt.
• Vesta – one of the brightest and largest asteroids, visited by NASA’s Dawn mission.

Relative Sizes of Stellar Bodies

Moons

• Moons are natural satellites that orbit planets.
• They are usually the smallest of the main stellar bodies considered.
• Size range:
  
  • Small irregular moons can be less than 10 km across (e.g., Deimos of Mars ≈ 12 km).
  • Larger moons like Earth’s Moon (3,474 km) or Jupiter’s Ganymede (5,268 km) rival small planets in size.
• Moons are generally smaller than planets but can be geologically active (like Io) or icy (like Europa).

Planets

• Planets are larger bodies that orbit stars.
• They are significantly bigger than moons, with diameters ranging from a few thousand kilometers to over 100,000 km.
• Examples of size variation:
  
  • Mercury: 4,879 km (smallest planet).
  • Jupiter: 139,820 km (largest planet, ~11 times Earth’s diameter).
• Gas giants (Jupiter, Saturn) are much larger than rocky planets (Mercury, Venus, Earth, Mars).

Stars

• Stars are massive balls of plasma undergoing nuclear fusion.
• The Sun is a medium-sized star with a diameter of about 1.39 million km (~109 times Earth’s diameter).
• Relative sizes:
  
  • Small stars (red dwarfs): smaller than the Sun, but still many times larger than Earth.
  • Supergiants (like Betelgeuse): can be more than 1,000 times the Sun’s diameter.
• This means stars can range from tens of thousands to billions of km across.

Solar Systems

• A solar system includes a star and all objects bound to it by gravity (planets, moons, asteroids, comets, etc.).
• Solar systems are vastly larger than individual stars or planets because they cover the entire region of orbiting bodies.
• Our Solar System:
  
  • Extends well beyond Pluto (~6 billion km from the Sun).
  • The Oort Cloud extends trillions of km, marking the far edge of the Sun’s gravitational influence.

Galaxies

• Galaxies are vast collections of stars, planets, gas, and dust held together by gravity.
• They contain billions of stars and solar systems.
• Size range:
  
  • Dwarf galaxies: a few thousand light-years across.
  • Large galaxies (like the Milky Way): about 100,000 light-years across.
  • Supergiant galaxies: several million light-years across.

Universe

• The Universe is the largest scale known, containing billions of galaxies.
• The observable Universe is about 93 billion light-years in diameter.
• Beyond this, the Universe may be much larger, but it is beyond what we can currently observe.
• The scale difference between the smallest moons and the Universe is almost unimaginable, spanning over 20 orders of magnitude in size.