One remarkable aspect of looking up at the bright stars that outline our constellations is that those stars are often at vastly different distances. The super-luminous giant stars and the hottest blue dwarf stars can be seen much further away than the much more common cooler dwarf stars like our Sun. Because of this, when we look up at the night sky with the unaided eye, many of the stars we are seeing are of the super-luminous variety, the “whales among the fishes”.
Once you know the distances to these stars, it is easy to calculate when the light you are seeing tonight left each one of them. It is enjoyable to contemplate what was going on in Earth history when each star’s light began its long journey across interstellar space, the tiniest fraction of which is reaching your eyes as you look up on a clear night.
This article represents the first in a series that will feature the distances to the major stars of a prominent constellation. We begin with Scorpius, which is currently crossing the celestial meridian at the end of evening twilight.
Below you will find a chart showing the constellation Scorpius and the bright stars that define its outline. The official IAU-approved star names are listed, where available, or the Bayer designation. There’s a printer-friendly PDF version of this chart at the bottom of this article, where the white text boxes have room for you to write in the nominal year when the light we are currently receiving left the photosphere of each star. The base chart was generated using SkySafari 6 Pro.
Next you’ll find a table that contains all the relevant information. There are three tabs: Parallax, Distance, and Time. The first three columns of each tab show the star name, the Bayer designation, and the spectral type and luminosity class listed in SIMBAD.
On the Parallax tab, the parallax value in millarcseconds (mas) is listed in column D, along with the mean error of the parallax in column E, and the year the parallax was published in column F. All are from SIMBAD. I will update these values as new results become available, but please post a comment here if you find anything that is not current, or is incorrect.
On the Distance tab, the parallax and error for each star is used to calculate the range of distance to the star (in light years) in columns D through F, with the nominal value given in column E being our current “best guess” for the distance to the star.
On the Time tab, the range of distance from the Distance tab are used to calculate the range of years when the light we are seeing at this point in time would have left the star. The earliest year (given the uncertainty in parallax) is shown in column D, the most likely year in column E, and the latest year (given the uncertainty in parallax) in column F.
Here’s a printer-friendly PDF version of the Scorpius chart where you can enter the nominal year from column E of the Time tab for each star. The year values on the Time tab will update automatically to reference the current date. I’ve used the “Replace Color” adjustment feature in Adobe Photoshop to lighten the background color so your printer will save on ink when printing out the full-page chart.