The otherwise visibly unremarkable star Xi Scorpii is in truth a set of five, two pairs traveling together and a fifth member orbiting one of those.
Many of the stars that we perceive as solitary points of light easily resolve into pairs and sets when viewed with a modest telescope. Some naked-eye interest in the night sky reveals the pair Alioth-Mizar in the middle of the Handle of the Big Dipper. Medieval astronomers knew the Beehive Cluster of 1000 stars as the Manger of seven stars, just as the Pleiades (also about 1000 stars) were the Seven Sisters to the ancient Greeks (among very many other traditions). The invention of the telescope and the advent of micrometrical recording expanded our understanding of space and time: some arrangements are accidental to our point of view; others move together temporarily locked into association by gravity. Amateur astronomers call them all “binary stars” and many are true pairs. The term also includes larger sets such as Xi Scorpii.
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| After a first view on 19 June 2022 at 0038, I returned that night at 2357 |
I found Xi Scorpii from a new book by Agnes Clarke, Discovering Double Stars: Double Stars for Light-Polluted Skies (Transtextual Books, 2022). I met Clarke on the Cloudy Nights discussion board. They are listed by their Struve numbers STF 1998 and STF 1999 in An Anthology of Visual Double Stars by Argyle, Swan, and James (Cambridge, 2019). Being self-published on demand through a contractor, Clarke’s book is frequently updated. The Cambridge volume provides more astronomical and astrophysical data for each listing.
Earler in the week, I was working with the Celestron AVX computerized “goto” mount and tripod carrying an Explore Scientific 102mm achromatic refractor. However, for this, I used a 130mm reflector from Astronomers Without Borders and the ES102mm on manual mounts.
I started at dusk, as soon as I could align the red-dot finders on Spica. I just kept checking stars for the familiar pattern. I refered to the Sky & Telescope Pocket Atlas to check my bearings relative to Yed Posteriori and Yed Priori in Ophiucus
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| Discovering Double Stars by Agnes Clarke. She uses 5x and 8x finder scopes for measuring. |
The easily visible pair has a period of only 44.5 years. They are about 2.7 billion km apart about the same as the distance from the Sun to Uranus. The fifth member of the set takes 1514.3 years to orbit them.
The other pair is designated Struve 1999 and was originally considered separate but in modern times was computed to be traveling with the other three. They orbit so slowly – hence at a great physical distance –that no discernable change has been noted. The entire family is about 80 to 90 light years from us. According to the SIMBAD database they are coming to us at 36.33 km/sec (Burnham gave 20 mi/sec). So, we have about 700,000 years to make plans.
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| From Argyle, Swan, and James. The stars are near maximum separation. |
The close pair AB are both subgiant F5 IV stars on the Hertzsprung Russell Diagram: OBAFGKM. They are on the main sequence, fusing hydrogen. Their temperatures are about 6700 kelvins (the Sun is 5,778 K) and they are about 2.4 times more luminous than the Sun. Metals are present.
The wider pair are K-types, dK0 and dK3, meaning that they are dwarfs, smaller and cooler than the Sun. K0 mass 0.88 suns and 5270 K while K3 are smaller 0.78 suns and 4830 K. They are classified as orange though they did not appear so to me in either my 130mm reflector or my 102mm refractor over two different nights.
(What's funny about this is that I just wrote an article about Scorpius X-1 the first discovered stellar x-ray source. So, it has a symmetry: ex-one, ex-eye.)
Previously on Necessary Facts
Parallax: The Race to Measure the Cosmos
Copernicus on the Revolution of Heavenly Bodies
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