“The Sky This Week” appears every Tuesday. It is written by Ian Clarke, Director of the Hatter Planetarium at Gettysburg College. The planetarium offers regular educational presentations about the stars and the skies; there’s something for early elementary through adults. Field trip requests are welcome. NOTE: field trip request form for 2022-23 is now live. Shows have concluded for Fall 2022, but we will post the schedule for the next semester around the new year. Images created with Stellarium.
We’ve been looking at notable features of the winter skies, and this week I’d like to move on to a new star—Sirius, the brightest star in our sky. Look southeast about 9:00 p.m., and you will probably spot it. If you need a little help, follow the line of Orion’s belt down toward the horizon. (Note that Mars and Jupiter are both as bright or brighter than Sirius, but they are planets, not stars.) Sirius is bright mostly because it’s close to us, astronomically speaking. At 8.7 light-years away, it is the closest star visible to the unaided eye from our latitude. Sirius is a binary (double) star system, consisting of a main sequence star twice the mass of our sun and a much smaller white dwarf companion too faint and close to be seen with the unaided eye.
All the stars in our galactic neighborhood are moving, and therefore Sirius has not always been our brightest star. Estimating past or future brightness of stars requires very accurate data on the 3D motions of numerous stars, data that have only become available in the last 25 years or so. That data shows that Sirius has been our brightest star for the last 90,000 years and will continue to be for another 210,000 years before Vega takes over. Interestingly, the estimated brightest star of the last 5 million years is in the same constellation as Sirius, Canis Major (the larger of Orion’s hunting dogs). Epsilon Canis Majoris (see illustration) would have been about as bright as our planet Venus at its brightest 4.7 million years ago, around the time hominids split from other apes.
Ian Clarke is the director of the Hatter Planetarium at Gettysburg College. In addition he has taught introductory astronomy labs and first-year writing there for over 30 years (not necessarily all at the same time). He was educated at Biglerville High School, the University of Virginia, and the University of Iowa. He lives in Gettysburg.