CIE IGCSE Physics (0625) Stars Study Notes - New Syllabus
CIE IGCSE Physics (0625) Stars Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Stars
Key Concepts:
- Galaxies and Astronomical Distances
- Life Cycle of a Star
Galaxies and Astronomical Distances
Galaxies and Astronomical Distances
(a) Galaxies contain many billions of stars
A galaxy is a vast collection of stars, dust, gas, and dark matter held together by gravity. Each galaxy can contain hundreds of billions of stars, many of which may also have planets and other celestial bodies orbiting them.
Examples of galaxies include the Milky Way, the Andromeda Galaxy, and the Triangulum Galaxy.
(b) The Sun is a star in the Milky Way galaxy
Our Sun is just one of the stars in the Milky Way Galaxy. The Milky Way is a spiral galaxy, and the Sun is located in one of its outer spiral arms called the Orion Arm, about 27,000 light-years from the galactic center.
(c) Other stars in the Milky Way are much farther from the Earth than the Sun
The Sun is approximately 150 million km (1 AU) from Earth, making it the nearest star to us. All other stars in the Milky Way are much farther away, often measured in light-years. For example, the closest star system, Alpha Centauri, is about 4.37 light-years away.
Due to their great distance, these stars appear as tiny points of light in the night sky, whereas the Sun appears large and bright.
(d) Astronomical distances are measured in light-years
1 light-year is defined as the distance light travels in a vacuum in one year.
Speed of light, \( c = 3.0 \times 10^8 \, \text{m/s} \)
Time in one year: \( 365.25 \times 24 \times 60 \times 60 = 31,557,600 \, \text{seconds} \)
So, \( 1 \, \text{light-year} = c \times t = 3.0 \times 10^8 \times 31,557,600 \approx 9.46 \times 10^{15} \, \text{m} \)
This is approximately \( 9.5 \times 10^{15} \, \text{km} \)
Example:
Which of the following statements is correct?
A. The Sun is the only star in the Milky Way galaxy.
B. All stars in the Milky Way are closer to Earth than the Sun.
C. A light-year is the time light takes to travel around the Earth.
D. The Sun is a star located within the Milky Way galaxy, and other stars in the Milky Way are much farther from Earth than the Sun is.
▶️ Answer/Explanation
Correct Answer: D
Option D correctly states that the Sun is one of the many stars in the Milky Way, and other stars in the galaxy are much farther away from Earth than the Sun.
Options A and B are false because there are billions of stars in the Milky Way and they are farther away than the Sun. Option C misdefines a light-year, which is actually a distance, not a time around Earth.
Life Cycle of a Star
Life Cycle of a Star
1. Formation from a Nebula (Interstellar Cloud)
Stars begin their lives in vast clouds of gas and dust known as nebulae. These clouds are mostly made of hydrogen, the simplest and most abundant element in the universe.
2. Protostar Stage
Gravity pulls the particles in the cloud closer together, causing the cloud to collapse. As it collapses, the temperature and pressure in the center increase. This dense, hot object is called a protostar.
3. Main Sequence Star (Stable Phase)
When the core of the protostar becomes hot and dense enough, nuclear fusion of hydrogen into helium begins. This releases enormous amounts of energy. The outward pressure from the energy balances the inward pull of gravity. The star is now stable — this is the main sequence phase, and it lasts for most of the star’s lifetime.
4. Exhaustion of Hydrogen Fuel
Over time, the hydrogen in the core is used up. Without fuel for nuclear fusion, the balance is disturbed, and the star begins to change.
5. Expansion into Giant Phase
What happens next depends on the mass of the star:
- Low- and medium-mass stars (like the Sun) expand into red giants.
- High-mass stars become red supergiants.
The outer layers expand as helium starts fusing into heavier elements like carbon.
6. Final Stages for Low-Mass Stars
A red giant sheds its outer layers into space, creating a glowing cloud called a planetary nebula. The hot core left behind becomes a white dwarf, which slowly cools over time.
7. Final Stages for High-Mass Stars
A red supergiant undergoes a massive collapse and explodes in a supernova. This is an extremely energetic event that spreads elements into space, forming a new nebula enriched with heavier elements (like iron and gold).
8. Stellar Remnants after a Supernova
Depending on the mass of the remaining core:
- If it’s moderately massive, it becomes a dense neutron star.
- If it’s extremely massive, it collapses into a black hole.
9. Recycling of Stellar Material
The nebula formed by a supernova contains hydrogen and newly formed heavier elements. These materials can combine again to form new stars and planetary systems, continuing the cycle of stellar evolution.
Example:
Below shows some stages in the life cycle of a star similar in mass to the Sun.
Nebula → Protostar → Main Sequence Star → Red Giant → ? → White Dwarf
Which stage is missing from the sequence above?
- A. Supernova
- B. Black Hole
- C. Planetary Nebula
- D. Red Supergiant
▶️ Answer/Explanation
Correct answer: C. Planetary Nebula
A star with a mass similar to the Sun does not go supernova. Instead, it becomes a red giant and then sheds its outer layers, forming a planetary nebula. The core left behind cools and becomes a white dwarf.
Other options:
- A. Supernova – occurs in more massive stars, not in Sun-like stars.
- B. Black Hole – forms from very massive stars after a supernova.
- D. Red Supergiant – is a stage for stars much more massive than the Sun.
Hence, the correct stage between red giant and white dwarf is planetary nebula.