What You Need to Know
- The Sun is a star powered by nuclear fusion (H → He) in its core
- Stars evolve through different stages depending on their mass
- Low-mass stars end as white dwarfs; high-mass stars become neutron stars or black holes
- Distant galaxies show redshift, proving the Universe is expanding
- The Doppler effect explains redshift: wavelength increases when objects move away
- The Big Bang theory is supported by: galactic redshift, Hubble's Law, and cosmic microwave background radiation (CMBR)
- Hubble's Law tells us: recession velocity is proportional to distance (v = H₀d)
- We can estimate the Universe's age using Hubble's constant (about 13.7 billion years)
The Sun as a Star
The Sun is a medium-sized star at the centre of our Solar System. It consists mostly of hydrogen and helium gas and radiates most of its energy as infrared, visible, and ultraviolet radiation.
What makes the Sun shine? Nuclear fusion. Deep in the Sun's core, hydrogen nuclei are forced together under extreme pressure and temperature. When they collide and fuse, they transform into helium and release an enormous amount of energy — the energy that warms our planet and lights our sky.
Nuclear Fusion in Stars
A nuclear reaction where hydrogen nuclei (or lighter nuclei) merge at extremely high temperatures and pressures to form heavier nuclei (like helium), releasing huge amounts of energy in the process. This is the fuel that keeps all stable stars shining.
Key insight: Every stable star is powered by nuclear fusion. The pressure from the weight of all that gas pressing down keeps the core hot enough for fusion to occur. When fusion stops (millions or billions of years later), the star's life changes dramatically.
Why Nuclear Fusion Matters
You don't need to memorize the exact nuclear equation for the exam — but you must understand that hydrogen is the fuel, and it converts to helium, releasing energy. This is the single most important fact about stable stars.
The Scale of the Universe
When we talk about distances to distant stars and galaxies, we use a special unit: the light-year.
Light-Year
The distance that light travels through the vacuum of space in exactly one year. One light-year = 9.5 × 10¹⁵ metres (or 9.5 × 10¹² km).
Why use this unit? Because the distances are so enormous that using kilometres or metres produces unwieldy numbers. For example, the nearest star to us (Proxima Centauri) is 4.2 light-years away — meaning light from that star takes 4.2 years to reach us.
The Hierarchy of the Universe
It helps to think of the Universe as nested layers:
Universe (billions of galaxies)
↓
Milky Way Galaxy (our galaxy, ~100 billion stars)
↓
Our Solar System (Sun + 8 planets)
↓
The Sun (a medium-sized star)
The Milky Way is our galaxy. It contains roughly 100 to 200 billion stars, each one potentially surrounded by planets. Our Sun is just one ordinary star in this vast collection. The nearest star to us (besides the Sun) is about 4.2 light-years away — a distance that would take a spacecraft thousands of years to travel.
Example: Converting Light-Years to Kilometres
Question:
Part (a) — Time for light to travel: