Unit 5: Observational Astronomy

Answer: D — The correct answer is D because larger telescopes have a larger aperture, which allows them to collect more light from the observed object. This is essential for studying faint or distant objects. Option A is incorrect because while larger telescopes can provide better resolution, it's not the primary reason for using them. Option B is incorrect because larger telescopes are often more expensive. Option C is incorrect because the time required for data analysis is not directly related to the size of the telescope.

Answer: A — The correct answer is A) The star is moving away from Earth because the redshift of the star's spectrum indicates that the light is being stretched due to the Doppler effect, which occurs when an object is moving away from the observer. Option B is incorrect because a blueshift would be observed if the star were moving towards Earth. Option C is incorrect because if the star were not moving relative to Earth, there would be no shift in the spectrum. Option D is incorrect because the size of the star is not related to the shift in the spectrum.

Answer: B — CORRECT (B): Angular resolution is fundamentally determined by the diffraction limit, described by the Rayleigh criterion: θ ≈ 1.22λ/D. A larger diameter (D) directly decreases the minimum resolvable angle, creating smaller diffraction disks that allow closely spaced objects to be distinguished. Telescope B's 2-meter mirror creates a diffraction disk approximately 1/4 the angular size of Telescope A's, enabling the binary stars to be resolved. WRONG (A): While larger mirrors do collect more light and improve sensitivity for faint objects, light-gathering power does not determine angular resolution—two unresolved stars will still appear as one bright spot. WRONG (C): Magnification alone cannot resolve unresolved objects; magnifying a blended image only creates a larger blended image. Resolution is a physical limit, not a magnification problem. WRONG (D): Mirror quality affects contrast and optical aberrations but does not change the fundamental diffraction limit imposed by the wavelength and aperture diameter.

Answer: B — The correct answer, B, is due to the concept of parallax, where the apparent position of a nearby star appears to shift against the background of more distant stars as the Earth orbits the Sun. This is a fundamental concept in astronomy for determining the distances to nearby stars. Option A is incorrect because while stars do move, this motion would not cause a noticeable shift over six months. Option C is incorrect because a star's expansion would not cause its apparent position to shift in this way. Option D is incorrect because gravitational lensing would cause distortions in the star's appearance, not a simple shift in position.

Answer: B — CORRECT (B): Hot ionized gas emits primarily in the ultraviolet range due to its high temperature, while cool dust emits strongly in the infrared range. Using the appropriate wavelength for each target allows the student to detect the strongest emissions from each component and gather optimal data. This demonstrates understanding of blackbody radiation and how temperature relates to peak emission wavelength (Wien's Law). WRONG (A) - Visible-light-only misconception: While visible light is useful, it represents only a narrow portion of the electromagnetic spectrum. Hot gas and cool dust emit most of their energy outside the visible range, making visible light inadequate for studying these specific phenomena. This reflects a misconception that visible light is universally the 'best' choice. WRONG (C) - Over-generalization error: While infrared does penetrate dust, this doesn't make it ideal for observing hot gas, which emits primarily at shorter ultraviolet wavelengths. Using only infrared for both observations would miss the characteristic emission from the hot gas and represent incomplete understanding of temperature-dependent radiation. WRONG (D) - Resolution confusion: Radio telescopes have the poorest angular resolution of standard astronomical instruments (though interferometry can improve this). They also don't match the emission peaks of either hot gas or cool dust as directly as UV and IR. This reflects confusion between different telescope capabilities and their appropriate applications.