Messier 5 and its Variables

by Leos Ondra

         `A noble mass, refreshing to the senses after
         searching for faint objects'
                                         Admiral Smyth

The oldest known stars have survived chiefly in the rich and wonderfully symmetric formations called globular star clusters. Unlike open clusters or loose associations of hot and massive OB stars, that are apparently being created even today, globular clusters were born along with or just shortly after the Galaxy itself some 16 billion years ago. This is not a general rule, since there are also galaxies, like the Large Magellanic Cloud or bizarre NGC 1275 in Perseus, where blue, luminous and thus young clusters of this type are observed. In the Milky Way, however, they are living fossils and as such quite rare beasts. Although astronomers have been discovering them since antiquity (Omega Centauri was recorded, as a star, by Ptolemy), only about 150 examples can be found in today's lists.

One of the jewels coming with spring constellations is my favorite, Messier 5 (NGC 5904). It is at least equally bright as the famous M13 in Hercules, is more attractive in larger telescopes, has a richer history, and among its stars one finds a few bright variables, the most prominent of which is easy to spot with 25x100mm binoculars.

First nights' honors

All begins with a small indiscretion. It is certainly improper to look into a lady's diary. But if it were not for historians of astronomy who have ventured to do it, we would never know the real discoverer of M 5.

The cluster was naturally entered in the Messier catalogue. In its last version [1] it states: "Fine Nebula discovered between Libra and Serpens, near the star in Serpens, of sixth magnitude, no. 5 in Flamsteed's Catalogue: it contains no star, it is round and can be seen very well in a clear sky with an ordinary [i.e. nonachromatic refractor] telescope of one foot [focal length]. Mr. Messier reported it on the Chart of the Comet of 1763 - Mem. Acad. year 1774, p. 40. Reviewed 1780 September 5, 1781 January 30 and March 22." [2]

Charles Messier revealed the globular cluster on May 23, 1764. The royal comet hunter of King Louis XVth was not, nevertheless, the first mortal to see the object. The credit for discovery has to be given to the German astronomer Gottfried Kirch (1639 - 1710). Kirch began his astronomical career as an assistant of Hevelius, but then worked independently and eventually even became the director of the Berlin Observatory. Well-known is his addition to the list of variable stars (very short in that year of 1685), namely the mira variable Chi Cygni, as well as the discovery of the spectacular Comet of 1680.

Regarding what are now called deep-sky objects, Kirch found the open cluster M 11 in Ganymede (now Scutum). This discovery got a wide publicity and was mentioned in subsequent compilations of nebulous objects. The original paper, including a sketch, was reprinted and discussed in an excellent series of articles by Kenneth Glyn Jones. [3]

In contrast to this fame, it seems that Kirch's priority in discovering the globular cluster M 5 was not recognized for a long time. This was most likely due to the intimate nature of the source from which the notice about the discovery came - the diary of Kirch's wife Maria Margaretha (it was first quoted by Dreyer in his supplement to John Herschel's General Catalogue [4]). She had just discovered the Comet of 1702, which was watched afterwards closely by Gottfried. Looking for it on May 5, he came across a `nebulous star' near 5 Serpentis. Next night, the existence of this object was verified by Maria Margaretha, and she made a sketch of its environs. Her original account (in German, of course) can be found in an article by Helen Sawyer Hogg. [5]

Painting the portrait

The fifth object of the Messier catalogue is easy to find in Serpens Caput, halfway from Arcturus to Antares, and about 0.5 degree north of the bright star 5 Serpentis. Various sources, including the most recent papers, differ in total visual (V) magnitude of the cluster. Webbink, in his compilation of many original sources [6], gives 5.69 mag, van den Bergh and Morbey [7] suggest a slightly different value 5.58 mag (for comparison, the same sources have 5.68 and 5.82 mag for M 13, respectively). Under clear and dark skies the cluster M 5 should be therefore visible with the naked eye, although I must confess that I am not aware of any such report.

With a pair of giant binoculars (25x100mm) the globular is too splendid to describe and strikingly brighter towards the middle. About one third of the total diameter (as is visible with averted vision) is occupied by a bright central part that suddenly changes into much fainter, diffuse lacy edges. Individual stars, with a single exception, are not discernible. The brightest members are about V magnitude 12.2.

To enjoy more details of Messier 5, I used, together with my friend Jirka Dusek, a 6-inch Carl Zeiss refractor, a telescope that already enables one to notice many charming forms of this stellar gathering. At small power the double star 5 Serpentis (ADS 9584) appears in the same field. It is easy to split because the angular separation of the very unequally-bright components (a bright yellow-orange mag. 5.2 star accompanied by a mag. 11 one) is as large as 11 arc seconds. Wilhelm Struve's remark about his seeing the main component elongated, that has handed down to posterity thanks to Admiral Smyth [8], is only a historical curiosity. Micrometric measurements of the pair have not shown any relative motion since the discovery, but the common and fairly large proper motion is a proof of the physical connection between both stars.

The globular cluster itself is magnificient and granular at 60x. Individual small stars are scattered around the edges, and one or two of them can exceed the others in brightness. The cluster proper is circular, but an extended halo spreads first of all to the northwestern quadrant and makes the entire globular to be rather triangularly shaped (under the best skies it was so seen by some observers in large binoculars).

At 90x, the field gets still darker and crowds of faint stars, bordering the soft edges of the cluster, appear. A few of the brightest stars, when seeing is good, are glimpsed even in front of the disk. The highest power diminishes the central brightness allowing you to see a number of stars in the middle part and a small, nearly stellar nucleus in the center. Tens of widely scattered outliers form tiny groups and short chains.

To continue a journey into the depths of this springtime globular cluster would require considerably larger instruments that unfortunately are not at my disposal. I have therefore visited astronomical libraries. One of the most remarkable papers was published at the end of the 19th century by Emerson E. Barnard [9], who visually studied variable stars in M 5 with the largest refractor ever made, the 40-inch one at the Yerkes Observatory.

Barnard cared not only about behavior of variable stars, but in the course of the research looked at the whole cluster. According to him, it is "much finer than M.13 Herculis, which is more suitable for smaller apertures." The giant refractor revealed some very remarkable details as well: "A striking feature of Messier 5 when seeing is good, is a number of inky black spots or holes, not in the densest part, but close south-preceding and south-following. Under best conditions these look almost like black occulting masses." At another point Barnard wrote: "Apparently near the middle of the cluster is a group of six or seven small bright stars which in a small telescope give the appearance for nucleus to Messier 5." It was most likely this false nucleus what we spotted in the 6-inch refractor.

The story of the cluster variables

Some observers scrutinizing Messier 5 with 25x100 binoculars report a bright solitary star wrapped in its soft edges and lying some 3 arc minutes southwest of the center. On other occasions there is, however, no trace of its existence. This is due to its variability, when the visual magnitude of the star - designated Variable 42 at the Harvard Observatory - changes between 10.6 and 12.1. It is, as far as I know, the easiest variable star in a nothern globular cluster to estimate. There is still one variable within M 5 that would be worth watching if it were not for one drawback. Variable 84 (visual magnitude 10.8 - 12.3) is unfortunately placed at the edge of a dazzling central part of the cluster, and thus is lost in it at low power needed to fit comparison stars in the field of view. For this reason we estimated only Variable 42 - the results are to be seen in a diagram.

Both these details were added to the cluster's portrait by D.E. Packer, who observed M5 with a 4.5-inch dialyte refractor at the end of the last century. Comparing records of April 22 and May 14 in the spring 1890, he noticed a small changing star (#42), and subsequently he found still older observation of it, dated May 31, 1889. [10] One year later Packer announced the discovery of Variable 84. [11]

A nice, well-founded, and thorough paper on both our acquaintances was published by Coutts and Sawyer Hogg of the David Dunlap Observatory. [12] The variables belong to the W Virginis stars, otherwise called the Cepheids of Population II, a sort of evolved luminous supergiant. Light changes of these stars reflect their pulsation, and a gradual shortening or lengthening of a period, if detected, can tell us much about stellar evolution. As concerns Variable 42, the Canadian astronomers showed that its pulsations have been keeping the strict regularity of 25.738 days since the year 1895, when the first suitable pictures of M 5 were taken by Solon I. Bailey. In contrast, the period of Variable 84, today at 26.42 days, has changed dramatically. For instance, it increased sharply by about 0.2 day during the 1950s. This behavior does not seem, however, to be associated with ageing of the star.

Fruit of the Tree of Knowledge

To learn something about the evolution of stars in a globular cluster, it is very useful to plot the relation between the visual magnitude (V) and the color index (B-V) for individual members. If you do it for Messier 5, you will obtain a picture that resembles an old distorted apple tree with only two branches left (see figure).

The lower part of the leaning trunk is formed by the main sequence stars (A) that perform the basic alchemic transmutation in the Universe, namely the conversion of hydrogen into helium, in their cores. Hydrogen is a very caloric fuel in thermonuclear reactions, so the stars manage with it for a long time. But nothing lasts forever. Once a burned-out core reaches about one tenth of the total mass, the star embarks on a rebuilding of its interior hoping that also helium ash can be ignited. The core shrinks and its temperature increases, while a shell of burning hydrogen produces the energy, also expanding the star's envelope. In the color-magnitude diagram the star leaves the main sequence and climbes first through the region of subgiants (upper part of the trunk, over the bend) and later along the right nearly vertical bough of the apple tree, called the red giant branch (B).

More massive stars, which evolve faster, conclude their stay in this part of the diagram by an explosive ignition of the helium core (at C), but this explosion is well hidden inside the star. A plentiful crop of still more evolved stars, which are already changing helium into carbon, can be found at the left branch called the horizontal branch (D). The continuous sequence of stars on this branch is interrupted by the Schwarzschild space (E), where we find only the variable stars of the RR Lyrae type, which cannot be represented by just one dot, and are therefore usually omitted.

Eventually, the helium core is used up as well and the star becomes a giant once again. At this moment, thermonuclear reactions go on in two separated shells surrounding the inert carbon core: the old hydrogen-burning shell and an inner helium-burning one. In the diagram, the star finds itself at what is called the asymptotic giant branch (just left of the red giant branch and parallel to it). These stars lose mass at a remarkable rate and are believed to be ancestors of planetary nebulae. However, the delicate bubble of a planetary is a very short episode in the life of star. It gets out of CCD's, gets out of mind, and what remains is only a cooling nucleus of the former nebula, a hot subdwarf quickly changing into a degenerate white dwarf, too faint to be included in our figure (below the arrow F).

Wandering through the color-magnitude diagram, a star may happen to come in a well-defined region dubbed the instability strip. Then, its outer layers acquire a remarkable property: the continuous flow of energy from the interior makes them pulsate, changing diameter as well as the effective temperature of the star. RR Lyrae stars in the Schwarzschild space mark the place where the instability strip crosses the horizontal branch, and the W Virginis variables are nothing but stars that got into the pulsational wonderland at a still more advanced phase of evolution. The length of the pulsation period depends on the average density of a variable star. This gives astronomers a fine tool for studying the evolutionary directions and rates, for a period can be determined much more precisely than the V or B magnitudes.

A fast intruder and a doomed cluster

I would like to round out this article by a few sentences about the surprising results that I have found in a paper sent to me recently by Kyle Cudworth of Yerkes Observatory. [13]

In the sky, there is another globular cluster nearby to M 5, named Palomar 5, first noted by Walter Baade on plates taken with the 48-inch Schmidt camera still before the famous sky survey. Nevertheless, Pal 5 is quite similar to the other clusters found in the POSS: it is sparse, has a very low surface brightness and a low mass. One would expect that the relatively nearby (25,000 light years from the Sun), rich, and dense Messier 5 is confined to the inner region of the galaxy, while the distant (70,000 light years), poor, and ghostly Palomar 5 comes from the periphery and is around its perigalacticon now.

However, refined proper motion studies that Cudworth took part in show quite the contrary. Messier 5 moves, relative to the center of the Galaxy, at an extremely high speed, about 500 kilometers per second, which is comparable with the escape velocity. "It appears", remarks Cudworth, "that M 5 may be an outer halo cluster briefly visiting the inner halo." Palomar 5 turned out surprisingly to be near its apogalacticon and is very likely on its last, or nearly last, orbital cycle before dissolution by tidal forces of the galactic disk.

References:

 [1] Messier, C., 1784, Connoissance des Temps for 1787, Paris, p. 239
 [2] Glyn Jones, K., 1969, Journal of the BAA 79, 359
 [3] Glyn Jones, K., 1968, Journal of the BAA 78, 367
 [4] Dreyer, J. L. E., 1878, Trans. Roy. Irish Acad. 26, 397
 [5] Sawyer Hogg, H., 1949, Journal of the RASC 43, 45
 [6] Webbink,  R. F., 1985,  in Dynamics of  Star Clusters, IAU  Symp.
     113, ed. J. Goodman and P. Hut (Dordrecht, Reidel), p. 541
 [7] Van den Bergh, S. and Morbey, C., 1991, Astrophys. J. 375, 594
 [8] Smyth,  W.  H.,  1844,  The  Bedford  Catalogue, John W. Packer,
     London, p. 339
 [9] Barnard, E. E., 1898, Astron. Nachr. 147, 243
[10] Packer, D. E., 1890, Sidereal Messenger 9, 381
[11] Packer, D. E., 1891, Sidereal Messenger 10, 107
[12] Coutts, C. M. and Sawyer Hogg,  H., 1977, Journal of the RASC 71,
     281
[13] Cudworth,  K.,  1992,  preprint   (to  be  published  in  Galaxy
     Evolution: The Milky Way Perspective, ASP Conf. Ser., ed.
     S. Majewski)

Figures:

M5's position in Hevelius' atlas, Firnamentum Sobiescianum sive Uranographia (1690)
Finder Chart for Variable 42 in M5
Central part of M5 with Variables #42 and #84
CMD of M5
Variable 42 light curve.

(Version: June 15, 1994)

Appendix -- notes and open questions:

There are no colorful photos of Messier 5 among the figures. This is because I have no ones at my disposal. By the way - I noted that globular clusters in general are blue (with overexposed parts in white) at most pictures, though one expects them to be reddish. Why?
I don't know (but would like to) if there are reports about naked-eye observation of the globular cluster.
The most recent (July 1993) compilation of data on galactic globular clusters appeared as ESO Scientific Preprint No. 932 (Appendices and Tables, to appear in Structure and Dynamics of Globular Clusters, ASP Conference Ser. #50, ed. Djorgovski & Meylan). In particular, total visual (V) magnitudes of 5.68 and 5.84 are given for M5 and M13, respectively, by Charles Peterson (Univ. of Missouri). According to the proceedings, Messier 5 is 7.6 kpc away.
Does anyone knows about the constellation Mons Menalus more than is is given in Allen's unreliable, confused, and dated 'Star Names' ?
It may be that the discoverer of Var #84 in Messier 5 is A. A. Common (what's his full name ?). Before Packer's announcement, he published a short article 'Note on some Variable Stars near the Cluster 5 M' (MNRAS 50, 517, 1890). While each of five 'variable stars' noted by him was captured on a single plate only (and was most likely just plate fault), the sixth star ('about 3 s f [i.e. 3 sec in right ascension east of] the centre of Cluster', which agrees well with the position of Var #84) appeared on all four plates. It was 'an unmistakable object' on the plates taken on April 22 [1890], May 15 and June 9, but 'possibly two magnitudes less' on the plate of May 9. This also is consistent with character of Var #84's light changes and its period, though I haven't check the phase yet.

Where is Margaretha Kirch's diary today ? Dreyer, writing about this source in the supplement to General Catalogue, noted that it was in the possesion of Lord Lindsay; later, preparing the New General Catalogue, he noted Lord Crawford as the owner of the diary. Here is the original text (in German), as quoted by Dreyer and Hogg (remarks are Dreyer's):

   May 5

   "Durch  solches  Suchen  [for the comet then visile] fand mein Mann
   durch  eben  diesen 3 Sch. Tub. hoch ueber mu [Serpentis, mentioned
   in  the  foregiong] em neblicht, aber doch deutliches Sternchen, es
   hatte  viel  feine  andere  Sternchen  um  sich,  doch  eins  stand
   sonderlich  per  Tubum  ueber diesen ungefaehr also [then follows a
   rough sketch of a star and the 'nebulous star' below it]."

   May 6

   "Das  nebliche  Sternchen  haben  wir  deutlich  auf seiner vorigen
   stelle gefunded."

What is DIALYTE refractor used by Packer ?
I would much appreciate receiving of a photocopy of Packer's original articles, or info how to get them. Until now, I have failed to find Sidereal Messenger in libraries.
Also, I'm searching for the last edition of the catalog of globular-cluster variables by H. S. Hogg (Publ. David Dunlap Obs. Vol. 3, No. 6, 1973). Could someone possibly made me a copy of pages related to M5 ?
Speaking about variables in globular clusters, Messier 5 is the only object of this type which (most likely) hosts a dwarf nova. It was spectroscopically confirmed by Margon et al. (ApJ Lett. 247, L89, 1981) and its long-term photometric properties were discussed by Shara et al. (Astron. J. 94, 357, 1987). I haven't consulted the papers yet, but dwarf novae are quite faint, and this one is moreover 24,800 light years away ...
Any report about amateur observation of Palomar 5 is welcome!

Thanks in advance for any help, info, comments and correction, and clear skies!

Leos Ondra (leo@sky.cz)

Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)

Last Modification: 10 Feb 1998, 21:30 MET