The 21 cm line of hydrogen is strongly absorbed by interstellar dust.
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Q5: Reflection nebulae usually appear red.
Q6: Dark nebulae are opaque to all wavelengths
Q7: We often find dark nebulae associated with
Q8: The dark dust clouds can be explored
Q9: Interstellar matter is distributed very evenly through
Q11: H II regions are usually found isolated
Q12: Hydrogen is the major gas in the
Q13: Most molecules found in the giant molecular
Q14: The "Local Bubble" was probably created by
Q15: Emission nebulae appear red because they are
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