Stellar Evolution
6 marks · NormalA student studies two stars: Alpha (surface temperature 3400 K) and Beta (surface temperature 8500 K). Both are located at the same distance from Earth and have the same luminosity.
State the colour you would expect to see from each star based on their surface temperature.
Classify each star using the star classification table.
If both stars appear equally bright from Earth, describe what conclusions can be made about their absolute magnitude and their position in the Hertzsprung–Russell (HR) diagram.
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(a) Alpha: red; Beta: white or yellow-white.
(b) Alpha: class M (red); Beta: class A (white) or F (yellow-white), depending on precise temperature.
(c) Since both are equally bright and at same distance, they have the same absolute magnitude and luminosity; they will appear at different positions on the HR diagram due to their temperatures, likely both on the main sequence but at different places (Alpha to the right, Beta to the left).
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Marking points
B1 (a) · 1 mark · States Alpha is red (due to lower temperature).
B2 (a) · 1 mark · States Beta is white or yellow-white (due to higher temperature).
B3 (b) · 1 mark · Classifies Alpha as class M.
B4 (b) · 1 mark · Classifies Beta as class A or F.
B5 (c) · 1 mark · States absolute magnitude is the same for both stars (due to same distance and brightness).
B6 (c) · 1 mark · Mentions different positions on HR diagram due to temperature (B hotter, A cooler but same luminosity).
Worked solution
a) Alpha, with a surface temperature of 3400 K, would appear red. Beta, at 8500 K, would appear white or yellow-white.
b) 3400 K falls into class M (red), and 8500 K into class A (white) or possibly class F (yellow-white) by the classification table.
c) Since both stars are at the same distance and have the same luminosity, their absolute brightness (absolute magnitude) is the same. On the HR diagram, both would likely fall on the main sequence but at different positions; Alpha is cooler and to the right, Beta hotter and to the left.