VARIABLE STARS : Part 1

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INTRODUCTION

Variable Stars, or sometimes simply termed variables, are stars that change in apparent brightness over time compared to the background field stars. All stars during their evolution undergo changes during their lifetimes which are commonly caused by imbalances in the stellar interiors - hence the term intrinsic variable stars - due to the star and not by some obscuration of material passing somewhere between us and the star in the vast areas of interstellar space.

Some other types of variables do change brightness due to the transit or occultation by companion stars, and these are termed eclipsing binaries. Others are highly explosive and include the familiar novae and supernovae, but there are also m any subclasses, depending on the circumstances that causes the violence. These are generally known as cataclysmic variables.

Observations of all these stars can therefore provide both physical data and an explanation of the mechanisms that cause the light variations. This can be further applied to deduce the structure and properties of stars. Understanding stellar structure through variables is often the only means that astronomers can find the internal natures of the stars. Variable are also givens a high priority among the professional astronomers. This task ca n be aided by amateurs who, if they decide to pursue such a course of action and can contribute to variable star observations. These are deemed to have real and significance importance, even with the naked-eye or the small telescope.

A Short History of Variable Stars

The Chinese were the first to record “new stars” and some of these are now known to be highly luminous objects — either as novae or supernovae. Many were thought to be wrongly identified and some have been attributed to comets. Others have been identified to be explosive stars by their remnants. In Roman times the historian Pliny reported a bright novae in July 134 BC in the constellation of Scorpius. Another was reported in February, A.D. 78. The discovery of another of these ‘new stars’ also was said to inspire the astronomer Hipparchus of Rhodes to produce the first star catalogue of the heavens.

From the beginning of the second millennia AD, several more new stars were identified. One appeared in the constellation of Taurus and nothing like it had been seen in recorded history. It was extensively observed by the Chinese between July and November 1054 AD, later to be classified to be a supernova. It reached the brightness equal to that of Venus, and some now believe it may have peaked nearer -6 or -7 magnitude! This star was so bright that some of the Chinese records say that it was even visible during the daylight hours. The remanets of the event were rediscovered by Doctor John Bevis in 1731, which he named because of its presumed telescopic form — the famous Crab Nebula. Later Charles Messier on 12th September 1758 placed it as the first object in his catalogue of deep-sky objects.

Another supernova was discovered by Tycho Brahé in November 1572. Lying in the constellation of Cassiopeia it reached a maximum of -4 magnitude.

The reality of true variable stars prior to the Renascence was the Aristotelian belief that the stars were eternal. First of the true periodic variable stars was Omicron (ο) Ceti was discovered by David Fabricius on the night of the 13th August 1596. The ancients gave the 2nd magnitude red star the proper name of Mira meaning “wonderful star”. Occasionally the star was at naked-eye visibility, but at other times, over several months became totally invisible. In 1638, Helvetius’s subsequent observations revealed Mire varied in an eleven month cycle. With the advent of the invention of the telescope, Mira when at minimum was revealed to be just below 10th magnitude.

Another early prominent star was also found to be a variable star. Lying in the constellation of Perseus, in the head of the Medusa held by this mythical hero, Montanari became first to discovered the regularity of the fluctuations in the year 1669. This star, Beta (β) Persei was given the proper name of Algol by the Chaldaeans. The name’s meaning derives from the Arabic word for ‘The Demon’ or ‘Evil One’. Roughly twice every two days and twenty hours, Algol drops by either two or four magnitudes. Each time it returns in about then hours to the original brightness. Later this star was realised to be an eclipsing binary, dropping during the main eclipse precisely in the clockwork period of 2.8673075 days. This period was discovered by the brilliant mathematician Goodricke on 12th November 1782, and published the times in May 1783.

By the year 1800, only thirteen periodic variables were discovered, and by 1850, this number had increased to twenty-two. Then the discovery of variables dramatically increases when Argenlander compiled his 300 000 star catalogue — the Bonner Duchmusterung (B.D.). Later Chandler discovered even more, making the total by 1896; 393 variables. Introducing the astronomical photographic plate then further hastened the number of discoveries. Several surveys were specifically undertaken to discover variables. Today, the number of variables now totals over 40 000 (2002), with an additional 20 000 suspected variables and more are discovered each year.

The Call of Variables

The true modern history of variable stars did not begin until the late 1800s, when the usefulness of the observations of these types of stars was finally realised. Most important and significant instrument was the introduction of instrumental photometry and later the photoelectric photometer. Details revealed from this device show magnitude variations down to the order of 0.001!

These types of light detectors in use vary significantly. Firstly, this was accomplished by repeated short photographic exposures, where the brightness fluctuations could be then plotted. The introduction of electronic photometers improved the accuracy and simplified the measurements. Today, photomultiplier tubes, photo-sensitive diodes and charged couple devices (CCD) are all in use. These have enhanced features, such as the detection in other wavelengths of light beyond the sensitivity of the human eye. Today’s observations are made by numerous techniques and methods. Professional astronomers now has at his disposal an arsenal of equipment for such purposes. This may range from spectroscopy, photography to automated blink comparators.

Spectroscopy has improved our understanding of other stellar properties such as atmospheric phenomena, chemical composition, mass and radii, and stellar surface features. Spectroscopic investigations via doppler shifts can also reveal changes in the expansions or contractions of some stars. This reveals changes in the stellar photospheres.

However, for all the observing time with the finite number of telescopes, all this wondrous equipment is precious demand. A professional is only allocated certain restricted times to do their observations. If they miss out on a significant changes in the star, often nothing can be done about it. Unlike the professionals astronomers, amateurs often can observe when they want to, and although we don’t have the aperture, we can quickly learn of any sudden changes — thus alerting the professionals to examine the changes.

Amateurs have observed variable stars for many years, either by eye estimations or by instrumental techniques. In 1982 for example, over 200 000 individual observations ere made. Most of the observers used modest equipment, many with rough mounts and small apertures. By plotting the results of the light changes against time, or the so-called light curves, will reveal the natures of the component stars. Contributions of these amateurs can not be understated but it certainly has been significant. Although individual observations count for little, the combination many observations have provided powerful information that can change theories about variables. Such is the usefulness of the observations.

Nomenclature

Nomenclature of variable stars is slightly different than for normal stars. A system that is used for the designations began with the observer Argenlander in the mid-1850s. He gave the first variable discovered in each particular constellation the Roman letter ‘R’, followed by the next letter ‘S’, down to the last letter in the alphabet, ‘Z’. We find that none of these ordinary southern stars contain these Roman letters. The number of variables that were known at the time this system was introduced was small enough to make this classification adequate. When the time came, somewhere in the 1880’s, when more variables were discovered, the letters were then doubled up. So after the letter ‘Z’, we have RR,RS,RT…RZ; followed by SS,ST…SZ; TT,TU…TZ, to ZZ, producing fifty-four different combinations.

As discoveries continued to increase, other letter began to be doubled up from the beginning of the alphabet. Hence follow ZZ, comes AA, AB…AZ; BB,BC…BZ; then CC,CD…CZ, on down to QZ. Other than this, the letter ‘J’ was omitted from each of these groups which was used for other purposes. This extends the number of variables to 334 for each constellations. However, even this was found lacking in some of the larger Milky Way constellations.

These letters still remain exclusive to variables. Today this series is still used, but it is now concurrently used with the system introduced by C. Andre. This simply lists each variable in order of its discovery. So variables in each constellation starts with V1, then V2,V3,V4… etc. Hence, V1=R; V2=S, V3=Y,… V9=Z, V10=RR, etc.

This numbering system should have been adopted in the first place. In the constellation of Sagittarius the variable V4000 has been reached, while Ophiuchus has up to V2000. Cygnus, Aquila and Orion have over 1000, wile 15 others have passed the V334 mark. As a comment, the poorest to contain variables is the southern constellation of Caelum, where the highest variable so far reached is Z Cae (V9). Most of these variables are now listed in GVSC updates.

Another method designates the variable by the position in the sky. A variable is given its number based on the positions in right ascension and declination. Hence the number maybe 10231-2343, indicating the R.A. as 10 hours 23.1 minutes and Dec. of −23° 43′.

Brighter variable stars traditionally have their names designated by the normal constellation naming system. Some still retain their common named, like Mira and Algol, but few exist. Other are labelled by the Bayer letters I.e. alf Vir or Alpha Virginus.

All four of these designation are now used in naming a variable star.

Discovery of variable stars after about 1950, now do not receive an official designation until the type and nature of the variable has been determined. Most stars receive provisional designations until the discoverer and certain checks into the variability of the star has been ascertained. These stars are commonly listed in sequential order within the New Catalogue of Suspected Variable Stars, termed by the acronym NSV. I.e. NSV, as of 1990 contains about 14,900 stars. When the type and nature has been figured out, the star receives its official designation.

Variable Star Catalogues

Presently all known variables is published in the Russian reference known as the “General Catalogue of Variable Stars” 4th Edition (GVSC4) entitled “The Combined Table of General Catalogue of Variable Stars” Volumes I-III, 4th Ed. (GCVS4) (Kholopov, et al.+ 1988) and the “Namelists of Variable Stars Nos.67-76“ (Kholopov,et al., 1985-2001). The Catalogue, like the “Washington Double Star Catalogue” or (WDS) is now update continuously. New entries are produced one every year or so, in the IAU Inform. Bull. Var. Stars., the latest being in 2002.

All these are is available to download from several placed on the Internet. The final variable star listings are controlled by the International Astronomical Union or IAU’s Variable Star Commission. This reference is used by all variable star observers.

This text is designed to;

1) Introduce variable stars to observers for observational contributions by serious amateurs
2) Understand the natures and structure of variable stars
3) Promote observations by amateurs.

I would like to acknowledge the late Mr. Edward Lumley of the Astronomical Society of New South Wales Inc. (A.S.N.S.W.) He was a long-time member of the Society (49 years) and the Leader of its “Variable Star Section Leader” for most of it. He provided much of the information on visual photometer and the Selected Variable Star List. He was an inspiration to me to many on the fellow member of the ASNSW Inc. Ted will surely be missed and will never be forgotten.

Last Update : 13th November 2012