MSU researchers have made precise mass measurements of four such nuclei, 68-selenium, 70-selenium, 71-bromine and an excited state of 70-bromine. The results may make it easier to understand X-ray bursts, the most common stellar explosions in the galaxy. No one likes to say exactly how much they weigh. Rare atomic nuclei are similarly coy, obviously not because of their own volition, but rather because they are exceedingly difficult to produce and, while they exist, very short-lived and difficult to corral and accurately measure. Now, MSU researchers have made precise mass measurements of four such nuclei, 68-selenium, 70-selenium, 71-bromine and an excited state of 70-bromine (yes, a nucleus weighs measurably more when it is excited because of Einstein's famous E=mc˛ declaration). The results may make it easier to understand X-ray bursts, the most common stellar explosions in the galaxy.