Final answer:
The spectrum of a cloud of gas with no background is an absorption spectrum, which appears as dark lines in the spectrum. By analyzing these lines, scientists can determine the composition of the gas cloud.
Step-by-step explanation:
The spectrum of a cloud of gas with no background, or no emission behind the cloud, would be an absorption spectrum. An absorption spectrum occurs when the cloud of gas absorbs certain wavelengths of light, resulting in dark lines in the spectrum. These dark lines represent the specific wavelengths that have been absorbed by the gas cloud.
For example, when starlight passes through a cloud of gas with no emission behind it, the gas absorbs specific wavelengths, leaving dark absorption lines in the spectrum. By analyzing these absorption lines, scientists can determine the composition of the gas cloud.
Keywords: spectrum, cloud of gas, no background, absorption spectrum, dark lines, specific wavelengths, absorption lines, composition