Astronomers analyze the spectrum of a distant star and find three prominent dark…
PHYSICS · Multi-part · A Level · Physics · Particles and radiation · Particles and radiation
Problem
Astronomers analyze the spectrum of a distant star and find three prominent dark lines at the following wavelengths measured on Earth: 656 nm, 486 nm, and 434 nm. A laboratory reference table of atomic emission lines lists the following prominent transitions:
• Hydrogen: 656 nm, 486 nm, 434 nm
• Helium: 588 nm, 668 nm, 706 nm
• Calcium: 393 nm, 397 nm, 422 nm
Answer the questions below about the star’s spectrum.
(a) Identify which element is most likely responsible for the observed stellar absorption lines and justify your choice using evidence from the provided data.
(b) Explain why the star’s spectrum shows these lines in absorption rather than emission, even though the same wavelengths appear as bright lines in a laboratory gas-discharge tube of that element.
(c) If the outer layers of the star were to cool significantly while the deeper layers remained at the same temperature, predict qualitatively how the depth (darkness) of the identified absorption lines would change and explain the physical reasons for your prediction.
(d) Discuss how the presence or absence of additional absorption lines from other elements can provide information about the star’s chemical composition and physical conditions. Include at least two distinct pieces of information that astronomers can infer.