• Choose a post by category or constellation

  • Learn the Night Sky

  • Search strategies

    Use the Search box below to find doubles by popular name, RA, or telescope size. For example, a search on "15h" will find all doubles we've reported on that have an RA of 15 hours. A search for "60mm" will find all doubles where we used that size telescope.

Algol: A Demon Double

Algol, or “al Ghul” (as in ghoul), has been called the Demon Star, the head of Medusa, and the “unlucky” star, as well as Gorgona, or Gorgonea Prima, meaning ogre, or monster, or even the main monster, apparently.

All are names capable of sending a shiver up your spine, especially if you happen to catch its chameleon-like behavior on a Halloween night.   It’s a masked star with a reprehensible reputation because it’s periodic dimming and brightening had the ancients perplexed and petrified.   If you’re not familiar with it, Algol waxes and wanes every 2.867 days, ranging from a magnitude of 2.1 at its brightest to 3.4 when dimmest.   What’s most unsettling, though, is the waxing-waning takes place in just a few hours.  Imagine seeing that happen for the first time on a dimly moonlit night in Autumn with fog draped across the trees and a branch rattling breeze ruffling your hair.  Argh.

But Algol isn’t really as evil as all that.  Not only is there a logical reason for its dimming and un-dimming, but when approached with a minimum of mortification, it provides a lesson in stellar interaction that needs no more technical apparatus than the two eyes in your head.  Algol is actually an eclipsing variable, which means the eclipse is caused by an orbiting companion — and that means Algol is a double star.  And since double stars attract my attention like a magnet to a refrigerator door, it’s well past time it garnered some Star Splitter attention.

Not that we’re ever going to catch sight of that wee orbiting companion, mind you.

But because Algol had peaked, peeked, and piqued my attention (take your choice, all three words have possibilities), in order to quell my simmering curiosity, I decided to see if it had other companions visible in an average-sized telescope, and darned if I didn’t stumble onto this list:


coord_2000 discov# comp first last  pa   sep mag1 mag2
03082+4057 CSI 1 Aa1,2 2006 2010   43   0.00   2.12
03082+4057 LAB 2 Aa,Ab 1973 2010 304   0.10   2.12   4.60
03082+4057 BU 526 AB 1878 2009 156 58.50   2.00 12.70
03082+4057 BU 526 AC 1878 2009 146 67.70   2.00 12.50
03082+4057 BU 526 AD 1878 2009 193 81.70   2.00 10.50
03082+4057 BU 526 AE 1899 2009 186 85.50   2.00 12.50
03082+4057 DAL 46 AF 2000 2009  111 93.20   2.00 13.70
03082+4057 JNN 3 AH 2008 2009 206   5.10   2.00 17.10
03082+4057 BU 526 BC 1878 2002 103 13.70 12.70 12.50
03082+4057 BU 526 DE 1878 2000 120 11.10 10.50 12.50
03082+4057 DAL 46 FG 2000 2009 283   6.20 13.70 14.70


Merciful Mirfak!    A veritable cornucopia of companions!

Now before we get under way here, I need to backtrack just a few arc seconds.  That “wee orbiting companion” is actually larger than the primary, if you can believe it.  Yet it’s not quite that simple. Although the orbiting companion, a class K giant, has the primary out-sized – 3.5 solar radii to 2.9 solar radii for the primary – it’s the less massive of the two stars.  The companion weighs in at .81 solar masses, while the primary suffocates the scales at 3.7 solar masses.   In reality, the larger diameter orbiting companion is a dying star, losing its mass to the primary.  And as much as I would love to get a glimpse of this close-knit pair, it’s just a slight bit beyond our range.  The reason is the two stars are separated by a distance equivalent to five percent of that between the earth and sun – which mathematizes out to a mere 4,650,000 miles – hardly a separation at all on stellar scales.  And given their distance from earth, 93 light years, the odds are permanently stacked against our glimpsing their circular waltz.  Rats.

But on the presumption that compensation could be found in the long list above, I sojourned forth into the darkness of night with my six inch f/10 refractor to see what I could see.  And even though I had every intention of ignoring the ghouls, they were intent on making their presence known.  Once again, as so often this winter, the rare clear nights seem unable to appear without bringing a temperature inversion with them, usually accompanied by a brisk wind out of the east.  The inversion spreads a layer of warm air over cooler air, which means the stars waver between semi-focused blobs and non-focused blobs, and the brisk breeze animates the entire evening by contributing a constant ghoulish rattle and whoosh to the trees.  But at least I didn’t freeze my focus fingers off since the temperature hovered round fifty degrees.

Before we get under way here, let’s take a look at where we’re going first:

Algol is caught here shining at full blast on the west side of Perseus, near its southern tip. (Stellarium screen image with labels added, click for a larger view).

Algol is caught here shining at full blast on the west side of Perseus, near its southern tip. (Stellarium screen image with labels added, click for a larger view).

Next, let’s reduce the list above to those stars we have a reasonable chance of picking out of the Al-ghoulish glare:

Algol                  HIP: 14576   SAO: 38592
Distance: 93 Light Years
Spectral Classification: A is B8, B is K0, C is A7, F is B8, and G is B8

coord_2000 discov# comp first  last  pa   sep mag1 mag2
03082+4057 BU 526  AB 1878 2009 156 58.50  2.00 12.70
03082+4057 BU 526  AC 1878 2009 146 67.70  2.00 12.50
03082+4057 BU 526  AD 1878 2009 193 81.70  2.00 10.50
03082+4057 BU 526  AE 1899 2009 186 85.50  2.00 12.50
03082+4057 DAL 46  AF 2000 2009 111 93.20  2.00 13.70
03082+4057 BU 526  BC 1878 2002 103 13.70 12.70 12.50
03082+4057 BU 526  DE 1878 2000 120 11.10 10.50 12.50

And now, let’s go see what we can see of that list.    ➡

The first thing I had to puzzle out was a missing star.  If you clicked on the link at the top left of the very first list above, you might have seen the “Aa,Bb” companion listed with magnitudes of 2.12 and 4.60 at a distance of 39.47”.  That surprised me when I first saw it, mainly because it seemed if Algol had an easily visible companion of that magnitude, I would have come across it before now.   And on my first telescopic sighting of Algol that evening, the 4.60 magnitude companion was nowhere to be seen.  Back into the house I went to pull up the data in the Washington Double Star Catalog, and found it listed there with a distance of 0.1” –– which explained why I couldn’t see it.

Having now had my first glimpse of what was a very obnoxious Algolian glare, I realized I was in for real a marathon struggle.  It’s amazing how much glare a 2.0 magnitude star can produce in a six inch refractor, but I suspect the temperature inversion made it worse than it would have otherwise been.   Suffice it to say I started with an 18mm Radian (84x) and immediately got nowhere.  There was a host of faint stars surrounding Algol, but all of them were in the wrong place.  Just on a hunch, I switched to a 20mm TV Plössl (76x) in hopes it would do a better job of containing the scattered light.  I peered hard with perseverance and averted my vision with abandon, but there was just nothing to be seen in all that glare.

Now I remembered when I fought this battle before in the glare of another star, I had done rather well with some Astro-Tech High Grade Plössls (which I believe are the same as the Sterling brand).  So I reached into my eyepiece stash and came up with a 12.5mm version of that eyepiece.  And even though 122x seemed like it might be a bit much given the horrendous seeing conditions, I dropped it into the scope’s diagonal and brought it to a careful focus.  And there, just as clear as a weather-worn white shard of clam shell on a beach of damp gray sand, was a pleasantly welcome point of light that wasn’t there before.   Mother of Menkib!

I moved the telescope west and south with the hand controller in order to get a bearing for the position angle, and found its welcome point of weak light was very slightly west of the 180 degree mark.  Looking at my notes, I found the 10.50 magnitude “D” companion listed with a PA of 193 degrees at a distance of 81.70”, which looked like a pretty darn good fit.  But if the star I was looking at was “D”, then 12.50 magnitude “E”, at a PA of 186 degrees and a distance of 85.50”, should be visible right beside “D”.  It wasn’t, despite running through every visual sleight-of-hand trick I could think of.

Lots of faint stars near Algol, but the only visible companion was 10.5 magnitude "D". (East & west reversed to match the telescope view, click for a better view).

Lots of faint stars near Algol, but the only visible companion was 10.5 magnitude “D”. (East & west reversed to match the telescope view, click for a better view).

I ratcheted the battle up several high-powered notches to a 6mm AT Plössl (253x) – but no luck, wouldn’t even come to focus.  I retrieved the 20mm TV Plössl and parked it in a 2.4x Dakin Barlow (183x) – zilch.  Then I tried a 24mm Brandon in the 2.4 Barlow (152x) – and still nothing to be seen except “D”.

So.  The Al-ghoulish glare had cast a veil of impenetrable diffuse white light over its circle of companions.  Judging by the waveringly weak light of “D,” I suspect I could have penetrated the glare to a magnitude of about 11.0, maybe a bit beyond, but since all of the other companions were listed with magnitudes of 12.5 and 13.7, I was out of business.  I should have known when I saw S.W. Burnham‘s initials (BU) under the discoverers’ column that this would be more difficult than it looked.   In fact, when I looked up S.W.’s observations (see page 36) , I found he had discovered the “C,” “D,” and “E” companions with the 18½ inch refractor at the Dearborn Observatory in Evanston, Illinois.  Maybe I need a bit more aperture.

I stuck it out under the unstable skies until a little after midnight, as oblivious as possible to the bloated star images I was stuck with, and then after hauling the astronomical gear back into the house, my four-legged companion grabbed his leash and took me on our customary post-observational walk.  Off we went down a dark street toward the east, catching sight of a third quarter moon breaking over the trees in the southeast, turned north for a short stretch, then rounded the corner and started uphill into the west.  As we came out at the top of the hill, I looked up into the western sky where Perseus was just beginning to start it’s low swing into the northwestern haze, and suddenly felt a shudder race up my spine.  Algol was gone.

I stopped, looked again, and finally caught sight of it.  Barely.

The haze had swallowed up a lot of its light, but it really looked like Algol had dimmed down to the bottom of its magnitude range.  My first thought was darn, I should have looked up the times of its maximum and minimum magnitudes because I might have been able to catch a few more of its companions as the glare dimmed down with the magnitude.  Rats! – a lost opportunity!

But about that time the wind came barreling up the hill behind me, blew the hood on my coat up over my head, ruffled the hair on Klaus’s back, and rattled every branch it could find within several square blocks.  A booming and dry rattle it was, too, almost a screech when it roared through the trees.  Then there was a short, very quick burst of light from Algol, probably because of a momentary hole in the haze I had been looking through, although that thought didn’t come to me until later.  Instead, another shudder shuddered up my spine with enough energy to raise the hair on my head and lift the top of my hood.  In the weak scattered moonlight at the top of the hill, it was downright eerie.

The ghouls were out tonight.  It was time to head for home.  Argh.


Just as added information, if you’re interested in the history of Algol, you should look up the amazing John Goodricke, who pinned down Algol’s period of variability in 1783.  At the time, he was only eighteen years old, and did it entirely visually by using nearby stars for comparison, coming within just a few minutes of currently accepted figures.  He authored two papers describing his observations in precise detail, both of which were published in the Philosophical Transactions of the Royal Society.  Fortunately for us, both of those papers are available on-line.  The paper of May 12th, 1783, is available here (scroll down to the bottom of the list), and the December 8th, 1783 paper can be found here (about halfway down the list).  And if you scan the titles of the other papers in those two tables of contents, you’ll find lots of other good stuff as well.

Clear Skies!   😎


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: