Galaxy

• A galaxy is a massive system of millions to billions of stars, along with gas and dust, held together by gravity.
• Galaxies are the basic building blocks of the universe.
• Smallest galaxies have ~1 lakh stars, while the largest have up to 3000 billion stars.

Types of Galaxies

Broadly classified into:

1. Regular Galaxies
2. Irregular Galaxies
   • About 1/10th of all galaxies
   • Contain very old stars

Regular Galaxies

1. Spiral Galaxies

• Disc-shaped with a central bulge and spiral arms
• Non-uniform distribution of stars
• Old stars concentrated at the centre
• Young, bright stars are found in spiral arms
• Rich in interstellar gas and dust
• Active star formation occurs
• The Milky Way is a spiral galaxy

2. Elliptical Galaxies

• Smaller and less bright than spiral galaxies
• Contain mostly old stars
• Very little gas and dust
• No new star formation
• Among the brightest galaxies in the universe

Dark Matter

• Outer arms of spiral galaxies (including the Milky Way) rotate faster than expected
• This indicates the presence of extra unseen mass → Dark Matter
• Accounts for ~85% of the total matter in the universe
• Does not interact with light
• Detected only through its gravitational effects
• Dark matter and dark energy ≈ 95% of the universe

Why “Dark” Matter?

• Does not emit, absorb, or reflect electromagnetic radiation
• Invisible across the electromagnetic spectrum
• Interacts only through gravity

Our Galaxy – The Milky Way

• Hosts our Solar System
• A disc-shaped spiral galaxy with a central bulge
• Diameter: 1,50,000–2,00,000 light-years
• Thickness:
  • Nucleus: ~10,000 light-years
  • Disc: 500–2,000 light-years

Key Features of the Milky Way

• Contains 100–400 billion stars
• Supermassive black hole – Sagittarius A* at the centre
• The Solar System lies in the Orion Arm
• Distance of Solar System from centre: ~26,000 light-years
• Red dwarfs are the most common stars
• Sun-like stars are rare

Motion of the Sun

• Completes one revolution around the Milky Way in ~220 million years
• Orbital speed: ~285 km/s

Neighbouring Galaxy

• Andromeda Galaxy
• Closest large galaxy to the Milky Way
• Distance: ~2 million light-years