A molecular fingerprint in radio waves
Sagittarius B2 is not visible as a reservoir of liquid alcohol. It is a vast region of gas and dust, and its ethanol molecules are separated by enormous distances. The eye could not pick them out; radio telescopes detect the faint signals they produce instead.
Molecules can rotate only in particular ways, each requiring a specific amount of energy. When a molecule changes its rotational state, it absorbs or emits radio waves at a characteristic frequency. Ethanol has many such frequencies, forming a pattern that astronomers can compare with laboratory measurements. Finding several matching lines is much stronger evidence than spotting a single signal, especially because a busy cloud can contain lines from many different molecules.
From faint signals to a staggering quantity
A spectrum—the pattern of signal strength across frequencies—can reveal which molecules are present and how their gas is moving. The lines may shift slightly because the cloud is moving toward or away from us. Their strengths also help researchers estimate how much ethanol is there, though that requires interpreting the cloud’s temperature and physical conditions as well as the signal itself.
The enormous beer-equivalent figure is therefore a way to make an astronomical abundance imaginable, not a measure of a liquid stash. Even a huge total of ethanol molecules can be spread thinly across an immense cloud, where the near-vacuum conditions keep them from forming a drinkable pool. What telescopes map is a chemical ingredient dispersed through space, molecule by molecule, through its radio fingerprint.
