Every now and then I stumble across a star that pulls me into a whirling vortex with all the force of a one million solar mass black hole. I suspect I have Mr. Spock to blame for this one, since as it turns out the creators of Star Trek placed his home planet, Vulcan, in motion around 40 Eridani. But that star has also been tagged with numerous other names, such as Keid, Omicron-2 Eridani, d Eridani, H II 80, and STF 518. And to steal a phrase from Mr. Spock, there are things about this star that “just aren’t logical, Jim.” Which is something of an under-statement.
So where do we begin . . . . . . .
I think we’ll do what Cap’n Kirk would do if he wanted to fly the Enterprise to Vulcan – determine where it is. Of course we could just ask Spock, except that he’s already on Vulcan cooking dinner for us.
Now that we have a rough idea where we’re going, we can zoom in several light years:
Time to fasten your seat belt and pull it tight. We’re about to warp our way through inter-stellar space as fast as this ship will go. Since we only have a short trip of 16.4 light years, we should be there in time for dinner at Spock’s abode. Hope you have a taste for Vulcan shellfish and Romulan ale.
Omicron-2 (Keid) (40 Eri) (H II 80) (Σ 518)
HIP: 19849 SAO: 131063
RA: 04h 15.3m Dec: -07° 39’
. Magnitudes Separation Position Angle WDS Dates
A, BC: 4.51, 9.70 82.40” 102° 2011
AC: 4.51, 11.47 77.30” 97° 2011
AD: 4.51, 12.17 481.40” 38° 1998
AE: 4.51, 12.50 569.90” 24° 1998
BC: 10.02, 11.47 8.20” 331° 2011
BD: 10.02, 12.20 452.60” 28° 1998
BE: 10.02, 12.50 559.50” 16° 1998
Distance: 16.4 Light Years
Spectral Classifications: “A” is K0, “B” is G9, “C” is M5
I first stumbled across 40 Eridani quite innocently at the end of December (2013) on a dark and murky night, which was appropriate since it seemed to be enshrouded in mystery at first glance. The four inch refractor I was using was nowhere near enough aperture for the murky atmosphere, which only made the mysterious even more mysterious. To shorten what became a long story, hardly anything in my sketch matched the three star charting software programs I consulted, nor did it match well with the photos I ferreted out of various internet sites.
It was obvious I needed to go back for a more detailed look. But true to winter-time form, the weather refused to cooperate except on bright moonlit nights that overwhelmed my primary quarry, the twelfth magnitude “D” and “E” components. In the meantime, Eridanus was creeping lower in the southwestern sky and nearer to a cluster of ravenous star-starved coastal pines. And then, just as I was about to give up hope for this season, I found myself looking at a clear sky as the sun set on the last day of February. As dark descended, I could see Sirius and Rigel sparkling like airport beacons on steroids, but I decided to defy the poor seeing with my 9.25 inch SCT. I needed an extra dose of aperture to pull the troublesome twelfth magnitude twins, “D” and “E”, out of the oceanic murk.
Finally, as darkness dropped its black curtain over the entire sky, I gritted my teeth and aimed the SCT at 40 Eridani. Not expecting much from the shuddering seeing, I started with a 26mm Plössl (94x) in hopes it would be enough to unearth “D” and “E” from the murk. It worked, but just barely:
An unexpected bonus was the delicate split of the BC pair, which I was able to improve with a 12mm Radian (204x). The actual view was quite a bit mushier than the sketch, though, and it took a determined effort to cobble the trembling pair into a series of mental snapshots resembling a stable image – sort of a mind over matter counterpart to photographic stacking of several images.
Now that we’ve covered the visual aspect of 40 Eridani, it’s time to slip into its whirling vortex of details. Make sure that seat belt is still snug.
First, if you look at the WDS or Stelledoppie entry for 40 Eridani, you’ll see the first date of observation is 1783, but no mention is made as to who made that observation. Fortunately, that date points directly to Sir William Herschel, who was apparently the only person on the planet obsessed with double stars that year. And the reason for pointing out that point is that Herschel’s is a very interesting observation (source, sixth title down).
This is actually Sir William’s observations of both BC and AB, but he treats BC as double (his catalog number is H II 80) and ignores the AB connection. It’s somewhat confusing on first reading, but the first three lines refer to BC, and the last two to “A” and “B”.
Starting with the first line, in the statement “About 1 1/3 min s. following d Eridani”, his ”d” refers to 40 Eridani (meaning the primary, “A”). On the third line, Herschel gives the position angle for the BC pair – his “56° 42’ n. preceding” translates to 326° 42’ in our current system for measuring position angles. On the fourth line, in the statement “Distance of L. from d Eridani”, the “L.” refers to “B”, the “Larger” star of the BC pair. He then provides the distance of “B” from d Eridani, which he puts at 1′ 21″ 47”’ (81.78”), as well as the position angle – his “Position of L. 17° 53’ s. following d Eridani” translates to 107° 53’.
The second amazing thing we discover as we delve deeper into the whirling vortex is that F.G.W Struve observed 40 Eridani BC in 1825 and was unable to measure the distance separating the two stars because they were too close – a rarity for an early Struve observation. But remember, Herschel described “C” as “hardly visible” at 227x and “very obscure” at 460x. It wasn’t until 1851 that the BC pair was finally measured, with the meticulous Reverend Dawes only able to make an estimate of 3”, while Otto Struve, using a larger telescope, contributed a more exact 3.91” that year, followed by 4.14” in 1855. But his position angles of 155.8° and 154°, respectively, sound a clangorous note with the 2011 WDS figure of 331°. (All of that information comes from S.W. Burnham‘s 1906 catalog and can be seen by clicking on the thumbnail images at the right or by going to this link for a slightly different version).
And it turns out there’s a very good reason for the dissonant clang. “C” is in a 252 year orbit around “B” that varies from twenty-one astronomical units to forty-nine AU (an astronomical unit, or AU, is the distance between the earth and Sun). Fortunately, as fate would have it, “B” and “C” are about as far apart as they can get at the moment – 2013 marked their furthest point of separation, so they’re beginning to slowly close now. Their orbital data can be seen here. (On p. 381 at the immediate right, “A and a” is AD, “A and b” is AE).
But as we descend further into the vortex, we also find “B” is in orbit around “A.” That orbit is much larger, since it takes “B” about 7200 years to make a full circuit around “A”, and the separation between the two stars is much larger, at least 400 astronomical units according to James Kaler’s information on 40 Eridani.
So what we have here is a real rarity: a genuine, gravity bound triple star. And the rarity is even more rare than that – because all three stars are dwarf stars of varying flavors.
The primary is one of the very few class K (orange) dwarfs visible to the naked eye. In fact, “A” is only four-tenths as luminous as the Sun, which means if it was a few dozen light years further away, it would fade from naked eye view. “B” is a genuine white dwarf in the true sense of the meaning of dwarf. It has passed through its final stage of stellar evolution, having shed its outer envelope of gases completely, and is now estimated to be about one and a half times the radius of the earth with an average density of about a quarter of a ton per cubic centimeter. Despite its diminutive radii, it burns at an astronomical 16,700 degrees Kelvin.
Last, but hardly least, “C” is a red dwarf with a mass equivalent to sixteen hundredths of the Sun’s mass and glows at a cool temperature of 3500 Kelvin. Despite that cool temperature, it shines almost as brightly as does “B”, which has a mass equivalent to half of the Sun’s. But “C” has another characteristic that sets it apart from most stars in the sky – it’s a flare star, meaning its magnetic field shorts out at unpredictable intervals, causing it to suddenly become brighter (a short discussion is included in Kaler’s discussion of 40 Eridani).
That takes us to the last curled up corner of our whirling vortex, the one that caused me so much consternation when comparing my four inch refractor sketch with star plotting software and photos. The Simbad chart below provides the first clue to why nothing matched my sketch:
This chart is a correct image view of proper motion – in other words, it matches what you would see visually without a telescope – whereas my sketch is a refractor (mirror-image) view, meaning it reverses east and west. So the long red arrows (there are three if you look closely) on the chart are pointing to visual southwest. Those arrows illustrate the proper motion of the “A”, “B”, and “C” components of 40 Eridani.
There’s one very important thing to point out about this chart. In the upper left corner you’ll see the radius of view shown in arcminutes. Normally, Simbad defaults to a 10 arcminute view, which is what most of the Simbad charts I’ve included in the past have been. But in the case of 40 Eridani, a ten arcminute view was nowhere near large enough to capture all the motion shown on the chart. I had to set the chart for a 50 arcminute radius (five times larger than normal) in order to capture all of the motion of the three stars.
Which gets us to the reason my four inch refractor sketch didn’t match the photos and star charting software I compared it with. The three orbitally linked components of 40 Eridani (“A”, “B”, and “C”) are, relatively speaking, racing southwest across the sky. In fact, the 40 Eridani system ranks twelfth in rate of proper motion among the stars in the Hipparcos Catalogue. If you’re wondering what those other stars are, you’ll find a list of them here.
Looking at the specific numbers for all five of the stars associated with 40 Eridani in the WDS, we find Simbad lists these values for their proper motion:
. Simbad Data What the Data Means in Arcseconds per Year
“A”: -2240 -3420 2.240” west, 3.420” south
“B”: -2228 -3377 2.228” west, 3.377” south
“C”: -2239 -3419 2.239” west, 3.419” south
“D”: +0039 -0042 0.039” east, 0.042” south
“E”: +0009 -0001 0.009” east, 0.001” south
When you combine the southern and western components of motion for “A”, “B”, and “C”, you get these values for total arcseconds of motion per year: for “A”, 4.083”; and for “B” and “C”, 4.073” each (those values come from this source: “A” is here, “B” is here, and “C” is here). Also, the proper motions of both “D” and “E” clearly point to both of them having no physical relation whatever to the A-B-C trio.
That brings us to the final segment of this 40 Eridani Odyssey.
When I was lost in space, mired in a mystified muddle because the background stars in my first sketch didn’t match well with other sources, attempting to locate “D” and “E” gave me no end of tortuous trouble. Once I discovered how quickly 40 Eridani “A”, “B”, and “C” were moving relative to other background stars, it was obvious that a history of the measures of AD and AE would provide a window into the past. Brian Mason at the U.S. Naval Observatory, home of the Washington Double Star Catalog (WDS), was kind enough to provide me with that data, from which I put together three charts.
First, though, I needed some current measurements of AD and AE (the WDS data measures are from 1998), so I turned to Steve Smith, who had also clued me in on the 40 Eridani connection with Vulcan. After taking a photograph of the 40 Eridani system, Steve employed his expertise with AutoCad to provide this work of art:
Now we’ll add the historical observational data provided by Brian Mason to plot the configurations of AD and AE from 1850 to 2014. Because both “D” and “E” are twelfth magnitude stars, meaning they aren’t visible except through a telescope, we’ll return to the same mirror-image refractor/SCT view used in my sketch at the beginning of this post. So once again (unlike the correct image Simbad views), west will be on the left side of the view and east on the right:
If you were looking at 40 Eridani “A” in 1850, you would have seen 12.17 magnitude “D” south and very slightly west of “A”. Otto Struve’s measurement of the position angle for that year was 197.0° and the separation was 128.3”. Just thirty-six years later, you would find “D” was almost due east of “A” – Asaph Hall’s 1886 measures put the position angle at 94.05° and the separation at 40.46”. And so on until 2014, when we see “D” parked northeast of “A” at about 37° and a distant 541”.
Looking at 12.50 magnitude “E” in relation to “A” in 1850, you would find it sitting almost due west. Otto Struve’s measurements for that year were 278.7° and 99.4”. A mere fourteen years later, you would have to look northwest of “A” to find “E” – August Winnecke’s 1864 measures were 312.5° and 89.4”. And by 2014, we find “E” situated at 25° and 627” from “A”.
Now the amazing thing is, if you combine the left and right halves of the sketch, you’ll realize that in 1850 the alignments of AD and AE are nothing at all like what we see in 2014.
BUT – keep in mind, it’s not “D” and “E” that are responsible for the changes in positions you see . It’s the trio of “A”, “B”, and “C” that are moving in relation to “D” and “E”. (To keep things simple, only “A” is shown on the charts).
So what would it look like if we reversed the eyepiece plots above in order to show what is actually occurring? Let’s put “D” and “E” at the center of our eyepiece view and cut “A” loose to move freely:
So there you have a very comprehensive view of a very interesting triple star system, one which would do perfectly fine on its own without the additional dimension of interest added to it by considering 40 Eridani as the location of Mister Spock’s home planet, Vulcan.
Which reminds me, we’re almost there, and dinner will be served shortly. Pull up whatever passes for a seating device and prepare yourself for a voluptuous Vulcan dinner. By the way, I recommend eating for several minutes before sipping the Romulan ale – it can play havoc with the head when the stomach is empty!
Desert is coming up shortly – a tour of a group of John Herschel and F.G.W Struve pairs within shuttle-hopping distance of 40 Eridani.
Clear Skies! 😎
P.S: And thanks to Steve Smith, Brian Mason, and Chris Thuemen for their contributions to this overly long dissertation.
Filed under: Eridanus