Stars & the Universe

Stars and the Universe - IGCSE Physics (CIE)

6.2.1 The Sun as a Star

The Sun - Our Closest Star

Medium-sized star consisting mostly of hydrogen (≈70%) and helium (≈28%)
Radiates most energy in infrared, visible light, and ultraviolet regions of the electromagnetic spectrum
Surface temperature: ≈5,500°C
Core temperature: ≈15 million °C
Powered by nuclear fusion reactions that convert hydrogen into helium
This fusion process releases enormous amounts of energy according to E=mc²
The balance between gravitational collapse and outward pressure from fusion maintains stellar stability

6.2.2 Stars and Galaxies

Galaxies and Cosmic Distances

Galaxies are massive collections of billions of stars held together by gravity
The Sun is one star in the Milky Way galaxy
Other stars in the Milky Way are much farther from Earth than the Sun
Light-year (ly): distance light travels in one year in vacuum

1 light-year = 9.5 × 10¹⁵ metres

Stellar Life Cycle

Life Cycle of Stars

Less Massive Stars (like our Sun)

Interstellar Cloud

Gas & Dust

→

Protostar

Collapsing Cloud

→

Main Sequence

Stable Star

→

Red Giant

Expanding

→

Planetary Nebula + White Dwarf

Final Stage

More Massive Stars

Interstellar Cloud

Gas & Dust

→

Protostar

Collapsing Cloud

→

Main Sequence

Stable Star

→

Red Supergiant

Massive Expansion

→

Supernova

Explosion

→

Neutron Star or Black Hole

Remnant

Detailed Life Cycle Stages

Formation: Stars form from interstellar clouds of gas and dust containing hydrogen
Protostar: Cloud collapses under gravity, temperature increases due to compression
Stable Star: Achieved when gravitational attraction inward equals outward pressure from high core temperature
Fuel Depletion: All stars eventually exhaust their hydrogen fuel
Expansion Phase:
- Less massive stars → Red Giants
- More massive stars → Red Supergiants
End Stages:
- Red Giant → Planetary Nebula with White Dwarf at center
- Red Supergiant → Supernova explosion → Nebula with heavier elements → Neutron Star or Black Hole
Recycling: Supernova nebulae can form new stars with planets

6.2.3 The Universe

The Cosmic Scale

The Milky Way is one of billions of galaxies in the Universe
Diameter of Milky Way: ≈100,000 light-years
Redshift: Increase in observed wavelength of EM radiation from receding stars/galaxies
Light from distant galaxies appears redshifted compared to Earth-based sources
Redshift provides evidence for Universe expansion and supports the Big Bang Theory

Evidence for the Big Bang Theory

Key Evidence

Cosmic Microwave Background Radiation (CMBR):
- Microwave radiation observed uniformly throughout space
- Produced shortly after Universe formation
- Originally high-energy radiation, expanded into microwave region as Universe expanded
- Temperature: ≈2.7 K (-270.45°C)
Redshift Measurements:
- Speed (v) of galaxy recession found from wavelength change due to redshift
- Distance (d) of far galaxies determined using supernova brightness

Hubble's Law and the Age of the Universe

Hubble's Law: v = H₀ × d

Hubble Constant (H₀):

Ratio of galaxy recession speed to its distance from Earth
Current estimate: H₀ = 2.2 × 10⁻¹⁸ per second
Units: km/s/Mpc (kilometers per second per megaparsec)

Estimated Age of Universe: t ≈ 1/H₀

Significance: The relationship v = H₀ × d suggests all matter in the Universe was present at a single point, supporting the Big Bang Theory.

Key Relationships

Greater redshift = faster recession = greater distance
Hubble constant gives expansion rate of Universe
Inverse of Hubble constant (1/H₀) estimates Universe age
Current age estimate: ≈13.8 billion years

Key Exam Concepts

Describe stellar life cycles for different mass stars
Explain redshift as evidence for expanding Universe
Apply Hubble's Law and understand its implications
Understand the evidence supporting the Big Bang Theory
Remember key values:
- 1 light-year = 9.5 × 10¹⁵ m
- Milky Way diameter ≈ 100,000 light-years
- Hubble constant H₀ = 2.2 × 10⁻¹⁸ s⁻¹
- Universe age ≈ 13.8 billion years