Free Astronomy Magazine September-October 2021

33 SEPTEMBER-OCTOBER 2021 ASTRO PUBLISHING I n astronomy, high speeds are the norm. With just local examples, let’s take the orbital speed of the Earth, almost 30 km/s, or the speed with which the entire solar system moves around the Milky Way center, about 230 km/s. Astronomers can measure such rapid displacements with relative ease and they can do so with all the stars approaching or moving away from us (radial veloc- ity). But with what maximum preci- sion are astronomers able to meas- ure those speeds? The answer is to within less than 1 m/s. To get an idea of how small a displacement this is, measured in the spectra of objects tens or hundreds of light years away, we must consider that these are stars with diameters of hundreds of thousands or millions of kilometers. Further, the surfaces of these stars can be affected by local and global phenomena that develop at speeds well above a few m/s, phenomena that can be erro- neously interpreted as a translation in space of the entire star. For exam- ple, a star subject to a pulsation that causes it to periodically expand and contract at a speed of a few m/s ap- pears to approach and move away from us with a period equal to that A glimpse of the Kitt Peak National Observa- tory. In the foreground, the structure that houses the 3.5 meter diameter WIYN telescope, on which the new NEID spectrograph has been mounted. [NSF’s National Optical-Infrared Astron- omy Research Laboratory/KPNO/NSF/AURA]