1. Nebula

• A vast cloud of hydrogen, helium, and dust.
• Birthplace of stars.
• Collapse begins due to gravitational instability (often triggered by shock waves from supernovae).  

2. Protostar

• Early stage of star formation.
• Nuclear fusion has NOT started.
• Emits energy due to gravitational contraction, not fusion.
• Surrounded by dust → visible mainly in infrared, not optical light.

3. T Tauri Star

• An intermediate stage between a protostar and main sequence star.
• Still contracting under gravity.
• Commonly asked in UPSC as a young stellar object.

4. Main Sequence Star

• Hydrogen → Helium fusion occurs in the core.
• Produces stable energy due to a balance between:
  • Outward radiation pressure
  • Inward gravitational force
• About 90% of all stars, including the Sun, are main-sequence stars.

5. Red Dwarf

• Smallest and faintest main-sequence stars.
• Low luminosity, surface temperature ~ 4000°C.
• Make up ~75% of Milky Way stars.
• Example: Proxima Centauri (nearest star to the Sun).

6. Red Giant / Red Supergiant

• Occurs when core hydrogen is exhausted.
• Hydrogen fusion continues in a shell around the core.
• Red Giant → Low/medium mass stars
• Red Supergiant → Massive stars
• Helium fusion produces carbon and heavier elements.

7. Degenerate Matter

• Forms when fusion slows, and gravity dominates.
• Electrons are forced closer to nuclei.
• Found in white dwarfs and neutron stars.
• Allows extreme densities.

8. Planetary Nebula

• Outer layers ejected by a dying low-mass star.
• No planets involved (name is misleading).
• Short-lived (tens of thousands of years).

9. White Dwarf

• Final stage of stars like the Sun.
• Made of degenerate matter.
• Extremely dense: one spoonful weighs several tonnes.
• No fusion; shines due to residual heat.

10. Nova

• Occurs in a binary system.
• A white dwarf pulls hydrogen from a companion star.
• Causes a surface nuclear explosion, but the star survives.
• Recurrent phenomenon.

11. Black Dwarf

• A fully cooled white dwarf.
• Theoretical only — none exist yet.
• Cooling time > age of the universe (13.8 billion years).

12. Brown Dwarf

• Between planet and star.
• Insufficient mass for hydrogen fusion.
• Often called “failed stars”.

SUPERNOVA

Definition

• Explosive death of a star.
• Can shine as brightly as 100 million suns.
• A major source of cosmic rays.

Types of Supernovae

Type Ia Supernova

• Occurs in binary systems.
• White dwarf accretes mass → runaway carbon fusion.
• Completely destroys the star.
• Used as standard candles to measure the expansion of the universe.

Type II Supernova

• Core collapse of a massive star (iron core).
• Leaves behind a neutron star or black hole.

Importance of Supernovae

• Create elements heavier than iron (gold, uranium).
• Spread elements into space → stellar recycling.
• Trigger formation of new stars.

NEUTRON STAR

• Formed after a Type II supernova.
• Protons + electrons → neutrons.
• Extremely dense: 3 solar masses in ~20 km.
• Beyond a limit → collapses into a black hole.

Chandrasekhar Limit

• The maximum mass for a white dwarf ≈ 4 solar masses.
• Beyond this → neutron star or black hole.

BLACK HOLES

• End stage of very massive stars.
• Gravity is so strong that not even light escapes.
• Density → effectively infinite.
• Distorts spacetime and can absorb nearby matter.

 CONSTELLATIONS

• A group of stars forming a recognisable pattern.
• Examples:
  • Ursa Major (Saptarshi / Great Bear)
  • Orion
  • Cassiopeia
  • Leo Major

Key Facts

• Stars appear to revolve around the Pole Star.
• Some northern constellations are not visible from the southern hemisphere.
• Orion is visible in winter evenings.
• Sirius (the brightest star) lies near Orion.

POLE STAR (POLARIS)

• Lies along Earth’s rotational axis.
• Appears stationary in the sky.
• Visible only from the Northern Hemisphere.
• Actually, a triple star system.