The Universe
Explore the origins of the universe, the life cycle of stars, types of galaxies, and the evidence for an expanding cosmos.
The Big Bang Theory
The Big Bang theory is the leading scientific explanation for the origin of the universe. It proposes that approximately 13.8 billion years ago, all matter and energy were concentrated in an incredibly hot, dense singularity. This singularity rapidly expanded (and continues to expand), creating space, time, and all the matter in the universe.
Key Evidence for the Big Bang:
- Cosmic Microwave Background Radiation (CMB): A faint glow of microwave radiation detected uniformly in all directions — the “afterglow” of the Big Bang, discovered in 1965 by Penzias and Wilson.
- Redshift of galaxies: Light from distant galaxies is shifted toward the red end of the spectrum, indicating they are moving away from us (see below).
- Abundance of light elements: The observed ratios of hydrogen (~75%) and helium (~25%) in the universe match Big Bang predictions.
Important: The Big Bang was not an explosion “into” space. Rather, space itself expanded (and continues to expand) from that initial state. Every point in the universe was once at the singularity.
Life Cycle of Stars
Stars are born, live, and die over millions to billions of years. Their fate depends on their initial mass. All stars begin as a nebula (a cloud of gas and dust) and undergo nuclear fusion in their cores.
Stellar Evolution Pathways
Low-Mass Stars (like our Sun):
Nebula → Protostar → Main Sequence → Red Giant → Planetary Nebula → White Dwarf (slowly cools and fades)
High-Mass Stars:
Nebula → Protostar → Main Sequence → Red Supergiant → Supernova → Neutron Star or Black Hole
The Hertzsprung-Russell (HR) Diagram
The HR diagram plots stars according to their luminosity (brightness) against their surface temperature (or colour). It reveals distinct groupings:
Main Sequence: A diagonal band where most stars (including our Sun) spend the majority of their lives, fusing hydrogen into helium.
Red Giants/Supergiants: Upper right — cool but very luminous (large surface area).
White Dwarfs: Lower left — hot but dim (very small surface area).
Types of Galaxies
A galaxy is a massive collection of stars, gas, dust, and dark matter bound together by gravity. There are billions of galaxies in the observable universe, classified into three main types:
Spiral
Flat disc with spiral arms radiating from a central bulge. Rich in gas and dust. Example: Milky Way, Andromeda.
Elliptical
Oval-shaped, ranging from nearly spherical to elongated. Mostly old stars, little gas and dust. Largest galaxies are elliptical.
Irregular
No defined shape. Often chaotic with active star formation. Example: Large and Small Magellanic Clouds (visible from Australia).
Our Milky Way is a barred spiral galaxy approximately 100,000 light-years across, containing 100–400 billion stars. Our solar system is located about 26,000 light-years from the galactic centre in the Orion Arm.
Evidence for an Expanding Universe
Redshift
When a galaxy moves away from us, its light waves are stretched to longer wavelengths (shifted toward the red end of the spectrum).
Edwin Hubble (1929) observed that almost all galaxies show redshift, and more distant galaxies show greater redshift — they are moving away faster.
Blueshift
When a galaxy moves toward us, its light waves are compressed to shorter wavelengths (shifted toward the blue end).
Very few nearby galaxies (like Andromeda) show blueshift because they are gravitationally bound to us and approaching.
Hubble's Law
v = H₀ × d
The recessional velocity (v) of a galaxy is proportional to its distance (d) from us. H₀ is the Hubble constant. This means the universe is expanding uniformly in all directions.
Analogy: Imagine dots drawn on a balloon. As the balloon inflates, all dots move away from each other — and dots that are farther apart move away faster. This is how the expansion of the universe works: space itself is stretching.
Key Vocabulary
| Term | Definition |
|---|---|
| Nebula | A cloud of gas and dust in space; the birthplace of stars. |
| Nuclear fusion | The process by which hydrogen nuclei combine to form helium, releasing enormous energy (powers stars). |
| Supernova | A massive explosion that occurs when a high-mass star exhausts its fuel and collapses. |
| Light-year | The distance light travels in one year (~9.46 × 10¹² km); a unit of astronomical distance. |
| Redshift | An increase in the wavelength of light from an object moving away from the observer. |
| Luminosity | The total amount of energy a star emits per unit time (intrinsic brightness). |
Worked Examples
Predicting the fate of a star based on its mass.
Question: A star has approximately 15 times the mass of our Sun. Describe its life cycle.
Step 1: This is a high-mass star (much greater than ~8 solar masses).
Step 2: It will burn through its hydrogen fuel much faster than the Sun.
Step 3: Life cycle: Nebula → Protostar → Main Sequence (blue/white, very luminous) → Red Supergiant → Supernova
Step 4: After the supernova, the remaining core will likely form a neutron star or, if massive enough, a black hole.
Explaining how redshift supports the Big Bang theory.
Step 1: Observations show that light from distant galaxies is redshifted — shifted to longer wavelengths.
Step 2: This indicates these galaxies are moving away from us (Doppler effect).
Step 3: More distant galaxies have greater redshift, meaning they move away faster (Hubble's Law).
Step 4: If all galaxies are moving apart, then running time backward means they were all once closer together — consistent with expansion from a single point.
Answer: Redshift provides strong evidence that the universe is expanding, which supports the Big Bang theory — all matter originated from a single, extremely dense state.
Classifying a galaxy from a description.
Description: A galaxy with a bright central bulge, rotating arms of gas, dust, and young stars, and an overall flat disc shape.
Step 1: Central bulge + spiral arms = characteristic of a spiral galaxy.
Step 2: The presence of gas, dust, and young stars indicates active star formation.
Step 3: Flat disc shape is typical of spiral galaxies (unlike the rounded shape of elliptical galaxies).
Answer: This is a spiral galaxy, similar to our Milky Way.
Knowledge Check
Select the correct answer for each question. Click “Check Answer” to see feedback.
Question 1
What is the approximate age of the universe according to the Big Bang theory?
Question 2
What will our Sun eventually become at the end of its life cycle?
Question 3
Redshift in the light from a distant galaxy indicates that the galaxy is:
Question 4
On the HR diagram, where would you find the hottest and most luminous stars?
Question 5
What type of galaxy is the Milky Way?
Key Concepts Summary
- •The Big Bang (~13.8 billion years ago) explains the origin and expansion of the universe.
- •Star fate depends on mass: low-mass → white dwarf; high-mass → supernova → neutron star or black hole.
- •The HR diagram classifies stars by temperature and luminosity into main sequence, giants, supergiants, and white dwarfs.
- •Three galaxy types: spiral (like our Milky Way), elliptical, and irregular.
- •Redshift and Hubble's Law provide key evidence that the universe is expanding.