The effect is stronger at sunset because the light passes through more of the Earth's atmosphere when the sun is near the horizon. This means that a dust cloud close to a bright star can scatter light. The bright patch that is seen is called a reflection nebula . When light is absorbed the energy goes into the dust grain and heats it up. The temperature is perhaps 20 - 50 K, which is very cold by Earth standards, but enough to produce strong infrared emission.
Infrared emission is also useful because it is scattered and absorbed even less than red light.
One can, thus, penetrate deep into clouds by this method. This is particularly useful for the study of newly forming stars. The gas in a nebula is often the remnants of the gas cloud out of which the stars formed. Diffuse gas Much of the diffuse gas is made of neutral atoms. A few elements have absorption lines in the visible, though more often they are in the ultraviolet.