Stars & The Universe
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Notes
The Sun as a Star
- The Sun is a **medium-sized star** at the centre of the Solar System.
- It consists mostly of **hydrogen** and **helium**.
- It radiates most energy in the **infrared**, **visible** and **ultraviolet** regions.
- In the core, **nuclear fusion** converts hydrogen into helium, releasing huge energy.
- All stable stars are powered by nuclear fusion.
Scale of the Universe
- The **Universe** is a large collection of billions of **galaxies**.
- A **galaxy** is a large collection of billions of **stars**.
- The **Milky Way** is our galaxy; the Sun is one of its billions of stars.
- Other stars in the Milky Way are much further from Earth than the Sun.
- A **light-year** is the distance light travels in one year (≈ 9.5 × 10¹⁵ m).
- The Milky Way's diameter is about **100 000 light-years**.
Star Formation
- Stars form from a **nebula** (cloud of gas and dust).
- Gravity pulls particles together to form a **protostar**; density and temperature increase.
- When the core is hot enough, **nuclear fusion** begins → **stable star**.
- In a stable star, inward **gravitational forces** balance outward **thermal pressure**.
Life Cycle of Low-Mass Stars (≤ 8 solar masses)
- After stable phase: **red giant** → **planetary nebula** → **white dwarf**.
- Red giant: hydrogen depleted, core shrinks and heats, helium fuses to carbon, outer layers expand.
- Planetary nebula: outer layers ejected; core collapses.
- White dwarf: hot, dense core that cools and dims over time.
Life Cycle of High-Mass Stars (> 8 solar masses)
- After stable phase: **red supergiant** → **supernova** → **neutron star** or **black hole**.
- Red supergiant: fuses heavier elements up to iron; core collapses and expands repeatedly.
- Supernova: core collapses rapidly, triggering a huge explosion; heavy elements (heavier than iron) are formed.
- Neutron star: dense remnant at the centre; if massive enough, collapses to a **black hole**.
- Nebulae from supernovae may form new stars and planets.
Galactic Redshift
- **Redshift** is the increase in wavelength of light from a source moving away (Doppler effect).
- Light from distant galaxies is **redshifted** compared to hydrogen on Earth → galaxies are moving away.
- Greater redshift means the galaxy is **further away** and **moving faster**.
- This shows the **Universe is expanding**.
The Big Bang Theory
- The Universe began about **14 billion years ago** from a hot, dense single point.
- Evidence: **galactic redshift** (galaxies receding → expansion) and **cosmic microwave background radiation (CMBR)**.
- CMBR is uniform microwave radiation from all directions, left over from the early Universe.
- CMBR was originally high-energy gamma radiation; it has been **redshifted** as the Universe expanded.
Age of the Universe (Extended)
- **Hubble's law**: recession speed v is proportional to distance d: v = H₀ d.
- **Hubble constant** H₀ = 2.2 × 10⁻¹⁸ per second (current accepted value).
- Age of Universe ≈ 1 / H₀ ≈ **13.7 billion years**.
- Distance to galaxies measured using **redshift** and **brightness of supernovae** (standard candles).
Life cycle of low-mass and high-mass stars.
Comparison of hydrogen spectra showing redshift.
Practice questions
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1.Which of the following is the correct definition of a galaxy?
Easy- AA large collection of billions of stars
- BA large collection of billions of planets
- CA large collection of billions of galaxies
- DA large collection of billions of moons
2.What is the approximate diameter of the Milky Way galaxy in light-years?
Easy- A100 000 light-years
- B1 million light-years
- C10 000 light-years
- D1 billion light-years
3.The Sun consists mostly of which two elements?
Easy- AHydrogen and helium
- BCarbon and oxygen
- CNitrogen and oxygen
- DHydrogen and carbon
4.What is the name of the force that pulls the outer layers of a star inwards during its stable phase?
Medium- AGravitational force
- BThermal pressure
- CNuclear force
- DElectromagnetic force
5.What is the correct order of stages in the life cycle of a star similar in mass to the Sun after the stable star phase?
Medium- ARed giant, planetary nebula, white dwarf
- BRed supergiant, supernova, neutron star
- CPlanetary nebula, white dwarf, red giant
- DWhite dwarf, red giant, planetary nebula
6.What does the observed redshift in the light from distant galaxies indicate?
Medium- AGalaxies are moving away from Earth
- BGalaxies are moving towards Earth
- CGalaxies are stationary
- DGalaxies are shrinking
7.The Hubble constant H0 is approximately 2.2 × 10⁻¹⁸ per second. A galaxy is at a distance of 1.0 × 10²³ m. What is its recessional velocity?
Hard- A2.2 × 10⁵ m/s
- B2.2 × 10⁻⁴¹ m/s
- C4.5 × 10⁴⁰ m/s
- D2.2 × 10⁻⁵ m/s
8.Which of the following is evidence for the Big Bang theory?
Hard- AGalactic redshift and cosmic microwave background radiation
- BThe Sun's energy from nuclear fusion
- CThe existence of black holes
- DThe formation of planetary nebulae
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