Which of the following statements about interstellar dust is FALSE A. dust makes distance stars look redder than they should B. dust scatters blue light from stars C. dust gives HII regions their red color D. dust makes distant stars look fainter than they should
The correct answer and explanation is:
Correct Answer: C. Dust gives HII regions their red color
Explanation (300 words):
Interstellar dust plays a significant role in altering the appearance of celestial objects as seen from Earth. Let’s break down each option to identify the false statement:
A. Dust makes distant stars look redder than they should — ✅ TRUE
This phenomenon is called interstellar reddening. As starlight passes through interstellar dust, the shorter blue wavelengths are scattered more than the longer red wavelengths. This makes the stars appear redder than their actual color.
B. Dust scatters blue light from stars — ✅ TRUE
Dust grains in space are very effective at scattering short-wavelength blue light. This scattering removes blue light from the direct line of sight and redirects it, contributing to the reddening effect and making stars appear dimmer and redder.
C. Dust gives HII regions their red color — ❌ FALSE
This is the incorrect statement. The red color of HII regions comes from ionized hydrogen gas, not from dust. When ultraviolet light from hot, young stars ionizes nearby hydrogen atoms, the electrons recombine and transition between energy levels. One prominent emission line from this process is the H-alpha line at 656.3 nanometers — a red wavelength. Therefore, the red glow of HII regions is due to emission from hydrogen gas, not dust.
D. Dust makes distant stars look fainter than they should — ✅ TRUE
This is called interstellar extinction. As starlight passes through dust, some of it is absorbed and scattered out of our line of sight, making the star appear dimmer. This has to be accounted for when calculating stellar distances and luminosities.
Conclusion:
While dust affects the brightness and color of stars through scattering and absorption, it does not cause the red color of HII regions. That red hue is caused by emission from ionized hydrogen, making C the false statement.