Relegated to a blue-white existence well beyond the Lyrae-ian framework, Eta (η) Lyrae beams its fourth magnitude photons at us from about seven degrees due east of Vega, the jewel in the constellation’s crown. Fortunately it isn’t relegated to total stellar isolation, thanks to a surrounding trio of fainter double stars with cooperative separations. In fact, those stars all inhabit the same field of view with Eta (η), providing the observer with a rare four-for-the-price-of-one visual bonanza.
Eta (η) Lyrae was a popular item with the early double-star detectives of the late 18th and early 19th century. All of them were quick to capture Eta (η) in a telescope, with the result that each of their names became attached to it in the form of the four cryptic catalog designations listed at the end of the title line below.
Eta (η) Lyrae (Aladfar) (20 Lyrae) (Mayer 61) (H IV 2) (SHJ 291) (Σ 2487)
HIP: 94481 SAO: 68010
RA: 19h 13.8m Dec: +39° 09’
Magnitudes AB: 4.38, 8.58 AC: 4.38, 11.42
Separations AB: 28.30” AC: 162.10”
Position Angles AB: 79° (WDS 2013) AC: 151° (WDS 2013)
Distance: 1388 Light Years (Simbad)
Spectral Classification: “A” is B2.5, “B” is A0
NOTE: The catalog designations above, from Mayer 61 to (Σ 2487), refer to the AB pair.
Before we take a look at the historical record, which will include an attempt at decoding William Herschel’s H IV 2 observation of Eta (η), let’s take a peek:
Because of its remote location 1388 light years from us, Eta (η) lacks the wanderlust which is typical of stars located within fifty light years or so of planet earth. That’s borne out by the Simbad and WDS proper motion data for both the primary (-001 -001, which is .001”/year west and .001”/year south) and the secondary (+003 +007, or .003”/year east and .007”/year north). In other words, you would need to hang around for about a thousand years before noticing a barely perceptible difference – and I can think of a lot of things I would rather do over the next thousand years.
But at least Christian Mayer got us started down that road with the inclusion of Eta (η) in his 1779 double star catalog, Tabula Nova Stellarum Duplicium. Eight more stars were added in a 1781 revision, resulting in Eta (η) Lyrae being designated as Mayer 61, which the resourceful astronomer measured with a separation of 28.3” and a position angle of 81°.
Sir William Herschel turned his telescope to Eta (η) Lyrae on August 29th, 1779, and recorded this observation in his 1782 catalog (twelfth title from the top):
The separation of 25.4” he measured was slightly less than Mayer’s figure, while his position angle of 31° 51’ south preceding works out to a puzzling 238° 9’. Neil English and I have wrestled with Sir William’s Latin on the first line of the observation with little luck – whatever it means, it’s as much a puzzle as the position angle, which was also noticed by Sirs John Herschel and James South on June 16th, 1823 (source for their 1824 catalog, last title on the page):
The stellar Herschel-South duo made two separate measures of Eta (η) Lyrae on the same night, one with a 3.75 inch Dolland refractor (see p. 11 of the 1824 catalog) and the other with a five inch Tulley refractor (the “chef d’oeuvre of that eminent artist”, p.12 of the catalog), which are referred to in their report in terms of their lengths, five-feet and seven-feet, respectively. Those two instruments are described in detail on pp. 4-14 of their catalog.
When averaged together, the result is a separation of 29.34” and a position angle of 5° 58’ north following, which equates to a present day figure of 84° 2’. If you look at the individual measures obtained with the two telescopes, you’ll see a noticeable difference between them in separation and position angle. Also included in their report is an 1819 position angle measure of Eta (η) Lyrae by F.G.W. Struve, 5° 30’ north following, which is our present day 84° 30’. Struve measured the position angle again in 1830 at 85°, and that time included a separation of 27.90”. Additional measures up through 1900 are shown at the right, in an excerpt from Thomas Lewis’s book on Struve’s double stars.
It doesn’t happen frequently, but occasionally a position angle gets measured erroneously from the fainter of two stars, instead of from the brighter of the pair. But that doesn’t explain William Herschel’s position angle for Eta (η) Lyrae of 238° 9’, since adding 180° to the Herschel-South-Struve PA of 84° from 1819 and 1823 results in a figure of 264°. John Herschel and James South came to the conclusion they and Sir William were measuring different stars, pointing out Struve’s 1819 position angle was essentially in agreement with theirs. Yet when I look at William Herschel’s 1782 catalog entry, his description of the two stars as being “considerably unequal”, as well as his reference to “three other stars in view” (probably referring to the two eighth magnitude stars and one tenth magnitude star arranged in a slight curving arc to the south of Eta (η) Lyrae, the middle one of which is SHJ 289), I don’t get the impression Sir William was looking at a different star. His separation of 25.4” also supports that impression.
We’ll chalk the puzzling position angle up to what may have been the result of an unusually stimulating and vibrant stellar ether spread invisibly over William Herschel and his telescope on that August night in 1779. We’ll never know otherwise, so that’s a good place to leave this mystery.
The Surrounding Double Star Trio
The brightest of the threesome of doubles surrounding Eta (η) Lyrae, SHJ 289, was cataloged by John Herschel and James South on June 16th, 1823, the same night they measured Eta. Once again, as they did with Eta (η), they measured SHJ 289 once with each of their two refractors, resulting in an averaged separation of 40.4” and an averaged position angle of 32° 18’ north following, which translates into a present day figure of 57° 42’. Those numbers compare favorably with the 2013 data in the WDS shown below. (Here’s the sketch above).
SHJ 289 (H V 42) No HIP Number SAO: 68003
RA: 19h 13.5m Dec: +39° 02’
Magnitudes: 8.01, 8.71
Position Angle: 56° (WDS 2013)
No reliable distance known
Spectral Classification: Ap (“p” refers to “unspecified peculiarity” – see Sky and Telescope here)
William Herschel measured this pair on September 25th, 1781 (about two years after his measure of Eta (η) Lyrae), and cataloged it as H V 42:
He came up with a separation of 38.8” and a position angle of 26° 18’ north following, or as we would say today, 63° 42’, the last figure differing notably from the Herschel-South measure. It’s surprising that these two stars carry the SHJ designation instead of William Herschel’s H V 42 designation, since Sir William provided the first measures for it. Normally that’s the criteria for determining what prefix is attached to star, but as in this case, there are exceptions to the rule.
Simbad doesn’t list a parallax for either of the SHJ 289 components, but Stelladoppie reports a distance of 11,248 light years, which is too distant to be accurate if based on parallax. The most recent proper motion data from the URAT1 survey lends support to the idea that the two stars are rather distant from us, with motions reported for A of -007.3 +001.8 (.0073”/yr west, .0018”/yr north) and for B of -005.6 +001.2 (.0056”/yr west, .0012”/yr north). Translated into everyday language, the inherent motion of the two stars barely qualifies as a crawl.
Located 5.75′ due west of Eta (η) Lyrae is another pair of stars known as HLM 19 (here’s the sketch above once again).
HLM 19 No HIP or SAO
RA: 19h 13.2m Dec: +39° 08’
Magnitudes: 11.30, 11.87
Position Angle: 330° (WDS 2013)
No distance or spectral classification
The three letter prefix for these two stars belongs to Edwin Holmes, who discovered them sometime around 1900. According to this article by T.E. Espin, some of the position angles in the stars discovered by Holmes were in error, so he and M.A. Ellison made more precise measures in the early 1920’s. The first date of measure for HLM 19 in the WDS is 1922 (11.8”, 332°), which Espin made, and can be seen at the bottom of the first page of the article in the link above.
Holmes is also known for his 1892 discovery of the comet known as 17P/Holmes, which put on a spectacular display in the autumn of 2007. Holmes’ account of his discovery can be read here, and there’s also a detailed biography of him here.
Located 7’ northeast of Eta η Lyrae is the last of the trio of surrounding double stars, SEI 584 (here’s our earlier sketch once more):
SEI 584 No HIP or SAO Numbers
RA: 19h 14.3m Dec: 39° 11’
Magnitudes: 10.74, 11.60
Position Angle: 118° (WDS 2010)
No distance or spectral class
No distance or spectral class is available for this pair of stars, but as was the case with SHJ 289, the URAT1 proper motion data indicates they’re located a respectable distance from us.
Translated, the proper motion numbers are telling us A is moving east at a rate of .0061” per year and south .0074” per year, while B is moving east at the rate of .0015” per year and south at the rate of .0159” per year, which again is a relative crawl compared to stars located within fifty light years or so from us. Judging by the motion of A relative to B, it doesn’t appear the two stars are gravitationally linked.
That’s it for this trip through Lyra — not sure yet where the next stopping point is, so stay tuned.
Until then, clear and stable skies! 😎