CIE iGCSE Co-ordinated Sciences-P6.2.3 Galaxies and the Universe- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-P6.2.3 Galaxies and the Universe – Study Notes
CIE iGCSE Co-ordinated Sciences-P6.2.3 Galaxies and the Universe – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Galaxies and the Sun’s Position
(a) Galaxies Contain Billions of Stars
- A galaxy is a huge system of stars, gas, dust, and dark matter held together by gravity.
- Each galaxy contains many billions of stars, often arranged in spiral, elliptical, or irregular shapes.
- Our universe contains an estimated hundreds of billions of galaxies.
- Galaxies are separated by vast regions of mostly empty space.
(b) The Sun in the Milky Way
- The Sun is one of the billions of stars in our galaxy, called the Milky Way.
- The Milky Way is a spiral galaxy, with arms extending out from its centre.
- The Sun lies on one of these spiral arms, about two-thirds of the way from the centre.
- Our Solar System orbits the centre of the Milky Way, taking about 225 million years to complete one orbit.
(c) Relative Distances of Stars
- Although all stars in the night sky are part of the Milky Way, they are at vastly different distances from Earth.
- The Sun is only about \( \dfrac{1}{500{,}000{,}000} \) of the distance to the centre of the Milky Way.
- Other stars in the galaxy are much further away than the Sun.
- This is why the Sun appears so much brighter than other stars it is our nearest star.
Example
Explain why the Sun appears much brighter than other stars in the Milky Way, even though many of them are far larger and more luminous.
▶️Answer/Explanation
Step (1): The Sun is the closest star to Earth, at a distance of only 150 million km (1 AU).
Step (2): Other stars in the Milky Way are many light years away, so their light spreads out over vast distances before reaching Earth.
Step (3): Even though some stars are much larger and more luminous than the Sun, their great distance makes them appear faint.
Final Answer: The Sun looks brighter because it is extremely close to Earth compared to other stars, not because it is the largest star.
The Milky Way and the Universe
(a) The Milky Way in the Universe
- The Milky Way is our home galaxy, a huge spiral system of stars, gas, dust, and dark matter.
- It contains an estimated 100–400 billion stars, including our Sun.
- The Milky Way is only one of many billions of galaxies that make up the Universe.
- The Universe itself is vast, with galaxies grouped into clusters and superclusters separated by enormous voids.
(b) Size of the Milky Way
- The Milky Way is roughly 100,000 light-years in diameter.
- 1 light-year is the distance light travels in one year: \( 1 \, \text{ly} \approx 9.46 \times 10^{12} \, \text{km} \).
- This means the Milky Way’s diameter is about \( 9.46 \times 10^{17} \, \text{km} \).
- Even at the speed of light, it would take 100,000 years to cross from one side of the galaxy to the other.
- The Sun is located about 26,000 light-years from the galactic centre, on one of the spiral arms.
(c) The Vastness of the Universe
- The Universe contains billions of galaxies, each with billions of stars.
- Our nearest large galaxy, Andromeda, is about 2.5 million light-years away.
- Modern telescopes (e.g., Hubble, James Webb) have revealed galaxies billions of light-years away.
- This demonstrates the truly incomprehensible scale of the observable Universe.
Example
Estimate how many kilometres wide the Milky Way is if its diameter is approximately 100,000 light-years.
▶️Answer/Explanation
Step (1): 1 light-year \( = 9.46 \times 10^{12} \, \text{km} \).
Step (2): Multiply by the galaxy’s diameter:
\( 100,000 \times 9.46 \times 10^{12} = 9.46 \times 10^{17} \, \text{km} \).
Step (3): This is an enormous distance — far beyond practical human travel.
Final Answer: The Milky Way is about \( 9.46 \times 10^{17} \, \text{km} \) in diameter.
The Big Bang Theory
The Big Bang Theory is the leading scientific explanation for the origin and evolution of the Universe. It is strongly supported by astronomical evidence.
(a) The Universe Expanded from a Single Point
- The theory proposes that about 13.8 billion years ago, all matter, energy, space, and time were concentrated in an extremely small, hot, and dense point known as a singularity.
- This singularity suddenly expanded — this event is called the Big Bang.
- Space itself expanded, carrying matter and energy with it, leading to the formation of the first fundamental particles.
- As the Universe cooled, protons, neutrons, and electrons formed, eventually leading to atoms such as hydrogen and helium.
- Galaxies, stars, and planets developed much later from these building blocks.
(b) The Universe is Still Expanding
- Edwin Hubble (1929) observed that galaxies are moving away from us, and the further away a galaxy is, the faster it is receding. This is known as Hubble’s Law.
- This observation provides strong evidence that the Universe is still expanding, just as predicted by the Big Bang Theory.
- The expansion is not an explosion into space; rather, space itself is stretching, increasing the distances between galaxies.
- Cosmic Microwave Background Radiation (CMBR), discovered in 1965, is leftover heat from the early Universe and is considered “fossil evidence” of the Big Bang.
- Modern data from telescopes and satellites (e.g., Planck satellite) confirm this ongoing expansion.
(c) The Age of the Universe
- By studying the rate of expansion (Hubble constant) and the oldest observed stars, scientists estimate the Universe’s age.
- Current evidence suggests the Universe is approximately 13.8 billion years old.
- This value is consistent with measurements of CMBR and models of cosmic evolution.
- Thus, the Big Bang Theory provides a timeline that matches both astronomical data and physical theory.
- It explains not only the origin but also the evolution of galaxies, stars, and cosmic structures we observe today.
Example
What are two key pieces of evidence that support the Big Bang Theory?
▶️Answer/Explanation
Step (1): Hubble’s observations showed that galaxies are moving away from each other — evidence that the Universe is still expanding.
Step (2): The discovery of Cosmic Microwave Background Radiation (CMBR) provided direct evidence of leftover heat from the early Universe.
Final Answer: The expansion of galaxies and the existence of CMBR both support the Big Bang Theory.