Question 1: Define the term 'Main Sequence' in the context of stellar evolution.

Answer: The Main Sequence is the continuous and distinctive band of stars that appear on the Hertzsprung-Russell diagram, where stars fuse hydrogen into helium in their cores, representing the longest stable phase of a star's life.

Question 2: Explain why stars in the Main Sequence have a relatively stable luminosity and size.

Answer: Stars in the Main Sequence maintain a balance between gravitational force pulling inward and nuclear fusion pressure pushing outward, resulting in stable size and luminosity.

Question 1: A star has a luminosity of 5 times that of the Sun. Using the Stefan-Boltzmann law, if its surface temperature is 6000 K, calculate its radius. Use the formula L = 4πR²σT⁴, where σ = 5.67 × 10⁻⁸ W/m²K⁴.

Answer: R = √(L / (4πσT⁴)) = approximately 1.55 times the Sun's radius (6.96 × 10⁸ m).

Question 2: Estimate the main sequence lifespan of a star twice as massive as the Sun, given that the Sun's lifespan is about 10 billion years. Assume the lifespan scales inversely with the mass to the power of 2.5.

Answer: Lifespan ≈ 10 billion years / (2)^{2.5} ≈ 10 billion / 5.66 ≈ 1.77 billion years.

Question 1: Describe a simple experimental setup to measure the surface temperature of a Main Sequence star using spectral analysis. List the variables involved and safety precautions.

Answer: Students would use a spectrometer to analyse the star's light spectrum, identifying absorption lines to determine surface temperature via Wien's law. Variables include the star's spectral data, calibration settings, and atmospheric conditions. Safety precautions involve protecting eyes from intense light sources and handling equipment carefully to avoid damage.

Question 1: A star in the Main Sequence exhibits a higher luminosity than the Sun but has a similar surface temperature. What can you infer about its size compared to the Sun?

Answer: The star must be larger in radius than the Sun, since higher luminosity at a similar temperature indicates a greater surface area.

Question 1: Explain how the balance between gravitational pull and nuclear fusion in the core of a Main Sequence star contributes to its stability. (6 marks)

Answer: In a Main Sequence star, gravitational force pulls inward while nuclear fusion produces outward pressure from energy released by hydrogen fusion. This equilibrium maintains the star's size and luminosity over a long period, preventing collapse or expansion.

Question 2: A star's position on the Hertzsprung-Russell diagram indicates it is in the Main Sequence. Describe two observational features that support this classification.

Answer: It has a surface temperature characteristic of Main Sequence stars and exhibits a luminosity consistent with hydrogen fusion in its core, placing it along the diagonal band on the HR diagram.

Question 1: Describe how knowledge of Main Sequence stars aids in the search for exoplanets and the assessment of their habitability.

Answer: Studying Main Sequence stars helps identify stable stars with consistent luminosity, which are ideal candidates for hosting habitable exoplanets. Understanding their properties allows astronomers to estimate planetary conditions and potential for life.