Constellation of the Month - Sextans

Ross Gould

Sextans is one of those obscure and easily overlooked constellations added in relatively modern times (as the historian counts time). It is attributed to Hevelius about 1680, as Hartung remarks "in honour of the observatory instrument which he had used since 1658". Hevelius'.s sextant was a large device used for measuring star positions from an observatory, quite different in scale and practicality from the more recent and hand-holdable marine sextant, which is a descendant of Hadley'.s small octant of 1730.

Sextans is rather small, and is found between Leo and Hydra - it has no bright stars, Alpha Sextantis being only magnitude 4.5, but Alpha is easily found 12 degrees south of Regulus. The April 1998 Sky & Telescope has an item by Sissy Haas on double stars in Sextans and nearby, titled "Gems of the Desert", a suitably descriptive title. Sextans, like Camelopardalis in the far North, could be described as "the absence of a constellation".

Starhopping in this region I found not too difficult, given a finder of good aperture and star charts to 9th magnitude, despite fairly bright moonlight. I used Alpha Sextantis as the starting point, and found sufficient 6-8 magnitude stars to navigate the region.

Being distant from the Milky Way, there are no nebulae or clusters here except for the very dim globular Palomar 3 - the main items of interest are some galaxies and double stars, some of them rather good.

Brightest of the galaxies is NGC 3115 (RA 10h 05.2m, Dec -07deg 43'.) a spindle-shaped galaxy 7 degrees south and just west of Alpha. This is an easy object for moderate apertures, readily seen on moonless nights with 15cm.

Again using Alpha Sextantis as the starting point, the pair of galaxies NGC 3166-9 can be found some 4 degrees north and slightly east. NGC 3166 is at 10 13.8, +03 26; mv (visual magnitude) is 10.4, listed as 4.5'.x2.5'., type SABa. NGC 3169 is at 10 14.2, +03 28, mv 10.2, 5'.x3'., SAa. Hartung describes them as being 8'. apart and of similar appearance, both round, about 1.5'. across (with 30cm), though 3169 has "evidence of a larger faint envelope"... 15cm shows both fairly well. Large scopes will show the much fainter NGC 3165 in this field , SW of 3166 (at 10 13.5, +03 23) - 3165 is mv 13.9, type SA and 1.5'.x0.7'. on photographs - a test object for large amateur scopes.

Less than 1 degree SW from 3166-9 is the fainter elliptical galaxy NGC 3156, at 10 12.7, +03 08: mv 12.3, 2'.x1'., S0.

Most of the other galaxies in Sextans are quite faint.

Brightest of the doubles is Gamma (RA 09h 52.5m, Dec -08deg 06'.), a testing object, though discovered by Alvan Clark with only 12cm aperture. This is a 77.5 year binary which Hartung said he had never seen "even elongated" with 30cm aperture, but it was around minimum separation in the early 60s when he was observing. It is now near maximum separation, though only 0.6" separation (in PA 62 at present). Haas has used van den Bos'. 1960 orbit calculation, given in Sky Catalog 2000; I have used the later orbit of WD Heintz, which gives a slightly longer period.

Magnitudes for Gamma are 5.6 and 6.1, so in theory a good 20cm scope will separate it. Haas mentions elongation in an 8-inch refractor, and a split in a 9-inch. To see this will take high power - at least 300x for most observers, and 400x might help for those not in practice on observing fine details. Gamma is variously reported as white or yellow. There is also a mag 12.0 comes at 35.8" in pa 325 1940 (and 1880).

Viewed recently with a Celestron 14, the mag 12 companion was seen well separated NW at 110x. Going to 240x showed the main star barely double in pa 60 - more power would have improved the separation. I saw the colour as pale yellow to cream tone.

There are some easy doubles available as well. Among these are

35 Sextantis (10 43.3, +0445) is an easy pair of mags 5.8 and 7.6. Measured at 6.8" in pa 240 in 1958 and 1832, it should be seen even with a 6cm aperture. Spectral types are K0 and A2, so colours of orange and white might be expected - Haas calls them "golden orange" and "blue-green", perhaps a contrast effect, or the result of using an achromat refractor. I found 35 Sex to be a lovely uneven coloured pair, with a few faint stars near. The brighter star I see as clear gold, the less bright a dull brown/green tone.

Fainter and closer but not difficult is Burnham 25 (10 21.8, -09 46) mags 8.2 and 8.8 at 1.8" in pa 153. Haas both stars "pale orange". Bu 25 is a binary with decreasing angle and constant separation. With the C14 at 110x it was a slightly uneven close pair in a thin field.

A somewhat similar pair to Bu 25 is STF 1457 (10 38.7, +05 44) mags 8.0 and 9.0 at 1.8" (pa 326). This is a binary, pa inc from 288 in 1829 to 326 in 1959, and the separation increasing from 0.7" to 1.8" in the same period. Both stars white according to Haas. With the C14, STF 1457 was a neat fairly bright little pair, even and close, both stars pale yellow. A nice object.

A very wide pair is 9 Sex (09 54.1, +04 57) mags 7.0 and 9.2 at 52.5" in pa 290. The spectral types are K5 and K0. Haas refers to "A" as "tangerine-colored". Pairs this wide often don'.t change much in a century - this one was the same in 1825 and 1924. The C14 showed a bright wide easy pair, bright orange and dull orange, with a few faint stars about.

40 Sex (10 49.3, +-04 01) is a neat double of mags 7.0 and 7.8, moderately close at 2.2" in pa 010 (nearly N-S). 8cm should make it clear - in larger scopes it will be a fairly bright easy pair. Both stars looked white to me.

STF 1377 (09 43.5, +02 38) mags 7.4 and 10.6 at 3.9" pa 136 in 1944 - a binary with slow pa decrease and distance increase. Should not be difficult with 15cm despite the brightness difference. The C14 showed it well at 110x, a yellowish star with a small companion SE.

Palomar 3 is one of those very obscure globulars beloved of Steve Crouch and others who like to achieve the near impossible. It can be found at 10 05.5 +00 04, mv 13.9 (!!), and 2.8'. diam on photographs. Most observers will find it easier to image (CCD or film) than see. It is NW of Alpha Sex.

HDE 331015: A short period eclipsing binary

Len Williamson and Peter Williams

In the 1996 March issue of the Southern Cross a note was published of the discovery of a new variable star. The discovery arose from examination of routine patrol photographs made by Paul Camilleri of the Nova Search organisation. One of us (P.W.) confirmed the finding and suggested that the object was an eclipsing binary. Based on photographs taken in 1990/1991 Paul Camilleri had suggested the period was 210 days.

In May of that year the Southern Cross published another note advising that it had not been possible to confirm the 210 day period. However as more estimates were made by the group - a part of the Royal Astronomical Society of New Zealand variable star section - it was found that the period between eclipses was very much shorter. All attempts to predict the times of eclipses were unsuccessful.

In March 1997 a further paper was published in the Southern Cross which embodied all the accumulated estimates of the group which now included Albert Jones in New Zealand. As it later turned out, since the night skies over Nelson were more often clear, Albert's estimates were to prove most valuable in elucidating the period and duration of eclipses.

From the raw data given in the 1997 paper one of us (L.W.) worked on the figures and found that a period of 9.5 days appeared to fit reasonably well {see Table 1}. However there was some evidence to suggest the period was shorter. Could it be a fraction of 9.5 days such as 4.75 days? The only way this could be settled was to predict a series of eclipses and hope that clear skies would allow estimates to be made.

        

               Table 1: Eclipses of HDE 331015

              Correlation with different periods

Dates of eclipses  Interval Possible  Deviation from exact fit for
J.D.+ 2450000      Days     number of          periods of:
                            eclipses   9 days  9.15 days  9.5 days
__________________________________________________________________

199 - 246          47       5          -2 d.   -1.25 d.   +0.5 d.
246 - 312          66       7          -3 d.   -1.95 d.   +0.5 d.
312 - 321          9        1           0      -0.02 d.   +0.5 d.
321 - 340          9        2          -1 d.   -0.7 d.    0
__________________________________________________________________

In the early evening of the 4 June 1997 an eclipse was seen by several experienced observers to reach totality at 20.00 hrs. This gave a Julian Date of 2,450,603.9165. In Table 2 multiples of 4.7 and 4.8 days have been added to this time to give two times for 11 predicted events.

  Table 2: Predicted times of eclipse of HDE 331015

Times for mid eclipse calculated from JD. 24500603 9165

   4.7 day period    4.8 day period
 _____________________________________________
      608.6165         608.7165
      613.3165         613.5165
      618.0165         618.3165
      622.7165         623.1165
      627.4165         627.9165
      632.1165         632.7165
      636.8165         637.5165
      641.5165         642.3165
      646.2165         647.1165
      650.9165         651.9165
      655.6165         656.7165

The usual nightly viewing hours in winter for backyard astronomy are in the range 19.30 hrs. to 23.30 hrs. or 0.9 day to 1.05 day - the Julian Day commences at 22.00 hrs. E.A.S.T. - and only when an event is happening do most of us stay up to follow it through. Notice that of the 22 predictions in Table 2 only four mid eclipses occur in the normal time slot, and a further three around 01.00 hrs. In earlier work it was found that the duration of an eclipse of HDE 331015 was less than 10 hours (currently thought to be approximately 8 hours) and this compounded the difficulties in elucidating the period. In actual fact the group were able to observe an eclipse at totality on .613.33 (observer Rod Stubbins, not confirmed), 632.12; and another at 650.97 . Note these times are all approximate. Consequently we know the period is a multiple of 4.7 days - probably 2 x 4.7. The evidence for this is derived from the following:-

(a) No eclipse was observed on 618.0165 {see Table 2}. One of us (L.W.) estimated the magnitude to be 11.0 at 617.917.

(b) Three members of the group made a concerted effort to observe the eclipse predicted at 646.2165 (03.12 hrs. in the morning) and observed the object through midnight. If the duration of the event is 8 hours the eclipse would be expected to commence at 23.12 hrs. and consequently a fall in brightness would have been detected after midnight. No change was observed.

(c) In Table 3 the predictions in Table 2 have been projected still further from the eclipse on 650.9165:-

       Table 3: Predicted times of eclipse of HDE 331015

     Times for mid eclipse calculated from 2450650.9165

      Period 4.7 days

        655.6165
        660.3165
        665.0165
        669.7165
        674.4165
        679.1165
        683.8165

At 669.878 one of us (L.W.) detected in twilight (unconfirmed) what appeared to be the tail end of an eclipse. If the duration was 8 hours then totality was calculated to have occurred near 669.75 which approximates to the projected date of 669.7165. Clouded skies prevented further observations until the 23 August which was clear. If the period of HDE 331015 was indeed 4.7 days then an eclipse should have occurred (Table 2) that evening at approximately 18.00 hrs. One of us (L.W.) estimated the magnitude at 20.00 hrs. (683.892) to be 10.85. If there had been an eclipse at 683.8165 the magnitude would have been near 11.5 (see Fig.1).

In Fig.1 the light curve is given for an eclipse of HDE 331015. This curve was plotted by combining the estimates made by the group for events numbers 10 and 17 - in all the group observed 20 eclipses. So far it has not been possible to follow an event from start to finish in the hours of darkness and it is for this reason that the curve in Fig.1 embodies two events which overlapped at totality. Note that there is perhaps an indication that the curve is not symmetric. These estimates are raw data and as such are subject to the following considerations:-

(i) The size of the mirrors varied from observer to observer and whilst the reproducibility for each would be similar the magnitude of their estimates would differ slightly. This factor is well known and Dr Frank Bateson, Director of the VSS, applies a correction for the bias of all VSS members. The data used in Fig.1 has not been corrected for individual bias.

(ii) It is not yet known if the light curve of HDE 331015 is precisely the same from event to event - some binaries have a diffuse secondary component which is subject to convection at and near the surface. This can give rise to small differences both in the duration of an eclipse and the symmetry of the light curve.

Most binaries of the Algol class exhibit a secondary eclipse formed when the bright component passes in front of the line of sight of the less bright component. So far we have not been able to detect this event - perhaps because of the difficulty in isolating a small loss of brightness against background scatter - especially since it is possible that the object has an intrinsic short term variation of +/- 0.15 magnitudes. Alternatively if the less bright component of the binary has a low relative luminosity the magnitude change of the secondary eclipse may be too small for visual estimation.

The catalogued data on HDE 331015 gives it an objective prism spectrum of class A, a magnitude of 11.1, and proper motions of -0.002 and -0.005 arc sec/year. Is the primary a white dwarf? This seems possible since this class of star has an AO spectrum. However a white dwarf as bright as HDE 331015 would be close to the solar system and would be expected to show large proper motions. The coordinates for the object are given as 16hr.37min.23.3sec., -48deg.42min.10.7sec for epoch 2000 which gives galactic coordinates of 336,-1 i.e. right in the plane of the galaxy. Since a close white dwarf could be projected on any part of the sky the odds are against it being confined to the galactic plane.

What kind of model can we put forward as to the structure of HDE 331015? An astronomer at MSO compared this object with two other well measured binaries of similar period (YZ Cas 4.47d., Alpha CrB 17.36d.) and similar spectra (YZ Cas A3 & F3, Alpha CrB A0 & G6) and accepting the 9.5 day period suggested that HDE 331015 might have intermediate values. He came up (crystal ball!) with A1 & G0 for the spectrum of this object and calculated a mass of 2.3 solar masses for the primary, whilst for the secondary a GO spectrum with a mass of 1.0 s.m. The distance between the pair could be near 30 solar radii. Bear in mind that the primary of HDE 331015 on this model could be 2.5 times larger than our sun whilst the secondary is the same size as the sun.

Compared with other Algol stars studied by the group the light curve in Fig.1 appears unusually flat at minimum light. There could be several reasons for this. The obvious one is that the secondary is much larger than the primary. However if the suggested model above is correct the smaller diameter of the secondary will result in only a partial (annular)eclipse of the A star (even if both components are in the line of sight) and this could give rise to a flat light curve at minimum.

We express our thanks to the Director of MSSSO for discussions and background input from MSO .

Note (i) A formal paper is being submitted for publication in the Journal of the Variable Star Section of the Royal Astronomical Society of New Zealand.

(ii) An astronomer at MSO has suggested that, since it is now possible to predict an eclipse, any amateur astronomers with CCD or other photoelectric device should observe an eclipse of HDE 331015 to obtain accurate magnitudes and colours then model the light curves and calculate (and publish?) the masses and radii of the binary pair. A project for the CAS?

When Alpha isn't the Brightest

Greg Bryant

During autumn evenings, a bright chain of stars crosses the sky from south to north. Low on the south-east horizon lies Alpha Centauri (magnitude -0.10), part of the Pointers that point to the Southern Cross, which includes the star Alpha Crucis (Acrux : magnitude 0.87).

From there, we move north to Alpha Carinae (Canopus : magnitude -0.72), to Alpha Canis Majoris (Sirius : magnitude -1.46), Alpha Canis Minoris (Procyon : magnitude 0.38), Alpha Tauri (Aldebaran : magnitude 0.85), and finally Alpha Aurigae (Capella : magnitude 0.08), low on the northern horizon. It's natural that these stars, the brightest in their constellation, are given the designation of "Alpha". Right?

When I was researching an article last year on Uranus' movements through the constellation of Capricornus, I was struck by the fact that Alpha Capricorni was not the brightest star in that constellation. Knowing that there were other constellations that shared a similar characteristic, I ploughed into the catalogs to come up with a listing of exceptions to the "rule" that the brightest star in a constellation is designated "Alpha". Halfway through my research, I realised that the Belgian astronomer Jean Meeus had already discussed the problem, so the below listing represents his work.

Firstly, there are four constellations that have no star designated Alpha. They are Leo Minor, Norma, Puppis, and Vela. For the latter two, the reason is simple. They formed part of the old constellation Argo, which was subsequently divided into Carina, Puppis, and Vela. The original "Alpha" star (Canopus) became Alpha Carinae, leaving Puppis and Vela with no Alpha.

Norma had a star designated Alpha when it was conceived in 1752, but since then, the star has crossed the border into neighbouring Scorpius. I have yet to unearth a reason why Leo Minor is without an Alpha designation. Anyone?

The following constellations have one star brighter than Alpha:

Aquarius (Beta)
Ara (Beta)
Camelopardalis (Beta)
Cetus (Beta)
Coma Berenices (Beta)
Delphinus (Beta)
Gemini (Beta)
Hydrus (Beta)
Libra (Beta)
Lupus (Beta)
Mensa (Gamma)
Monoceros (Beta)
Orion (Beta)
Pegasus (Epsilon
Triangulum ( Beta)
Vulpecula (13 Vul)

The following constellations have more than one star brighter than Alpha:

Cancer (Beta, Delta, and Iota)
Capricornus (Beta and Delta)
Cassiopeia (Beta and Gamma)
Corvus (Beta, Gamma, Delta, and Epsilon)
Crater (Gamma and Delta)
Draco (Beta, Gamma, Delta, Zeta, Eta, and Iota)
Microscopium (Gamma, Epsilon, and Theta 1)
Octans (Beta, Gamma 1, Delta, Epsilon, Theta, Nu, and Chi)
Pisces (Gamma, Eta, Iota, and Omega!)
Sagitta (Gamma and Delta)
Ursa Major (Epsilon and Eta)
Volans (Beta, Gamma, Delta, and Zeta)

In the constellation of Hercules, Beta is always brighter than Alpha, which is itself a variable. Depending on the brightness of the variable Alpha, the following stars in Hercules can be brighter : Delta, Zeta, Eta, Mu, and Pi.

Finally, we come to the constellation of Sagittarius where THIRTEEN stars are brighter than Alpha (Gamma, Delta, Epsilon, Zeta, Eta, Lambda, Xi 2, Omicron, Pi, Rho 1, Sigma, Tau, and Phi)! What were they thinking of when they designated a star Alpha Sagittarii.

All up, there are 34 out of 88 constellations (39%) where the brightest star is not given the designation Alpha. Why not seek out some of the above constellations, and see if you can tell the difference.