VARIABLE STARS : Part 3

ARTICLE PAGES

Eruptive or Cataclysmic Variables

Of all variable stars this group is the most violent and dramatic, exhibiting huge changes in brightness with a short but fantastic and massive outpouring of energy. The existence of these variable types make life for the observer exciting and unpredictable.

(i.) Novae

The most spectacular of the variables are the novae, that brighten by between 7 and 16 magnitudes over a time of about a day to about a month. In some rare cases, maximum brightness may take several hundred days. However, the fall is far more gradual. It may take several years to decades until the initial magnitude is reached. After this time, the star will fluctuate irregularly in brightness for many more years.

It is possible, that these stars are all close binary systems. It probably comprises of a single white dwarf that has material transferred from a single star. This material produces an atmosphere around the white dwarf that heats up overtime. Temperature increase is caused by the hot white dwarf having its energies confined by the atmosphere. This temperature increase reaches millions of degrees, until it rises to some critical point where thermonuclear reactions can start. The dense atmosphere is comprised almost totally of hydrogen and helium, and in an almost instantaneous and spectacular flash, the atmosphere ignites. The material, infused with energy, immediately ejects the material violently into the surrounding space. The area of the cloud increases, and produces a subsequently dramatic increase total luminosity. It is this increase in luminosity, that causes the novae to brighten as typically seen in the light curve. After weeks or months, the material disperses sufficiently to become transparent, and then the magnitude begins to decline. The white dwarf is soon re-exposed, slightly hotter before the mass transfer began, but this quickly cools until the normal surface is reached.

Characteristics of the light curve determines the subclass of the many novae types. Most are determined by the appearance of the light curve. All novae, however, are unique in their individual light curves in some way or another. Some novae have been known to reach 1st magnitude, with the brightest novae being V603 Aquilae in 1918. This star reached a maximum brightness of −1.1 — much brighter than the first magnitude star Altair, and almost equalling Sirius.

RECURRENT NOVAE are a subgroup of the novae, that have been known to flare in brightness several times, after several decades have elapsed. The brightness rises with these stars are not a large as for novae, most only increasing by between six and eight magnitudes. The brightest example is T Corona Borealis that has gone nova in the years 1894, 1946. Another is the star TX Pyxis that has gone nova in the years 1890, 1902, 1920, 1944 and 1966. Recurrences of the brightening is attributed to the companion star in the binary replenishing the material that is flow onto the white dwarf. This is believed to continue, as long as the primary star feeds material to its companion. The rate of replenishment to cause another nova outburst is between 10 and 80 years, though the maximum figure is likely biased because of the short time that we have known about these recurrent novae.

All discoveries and observations can be made by CCD imaging or photography — estimating their brightness or discovering them. Naked-eye or binocular sky surveys are common methods to discover new novae. Novae are normally restricted to the Milky Way, whose observed frequency seem to occur along its borders. Variable star observers focus on the familiar stars within the constellations. This is typical is made by a quick visual scan of the sky, followed by more detailed scans 7×50 binoculars. Some organised astronomical societies allot amateurs areas of the sky to observe as often as possible. A new discovery is required as early as possible is highly desirable. Professional astronomers, when alerted, will turn their large telescopes to the novae so they can then investigate the ejection of material and the nature of the star. This must occur prior to maximum brightness. After the first few days, little detail can be obtained except for the details of the magnitude changes over time.

(ii.) UV Ceti Types or Flare Stars

These are late ‘M’ spectral class stars that may flare very quickly by as much as one to six magnitudes. As a class of variables, it was first discovered by Luyten, as recently as 1948. They are commonly referred as flare stars, and brighten not only visually, but both in radio and X-Ray frequencies.

Some flares can burst, by increasing by several magnitude in several seconds to about one hour. Sometimes visually, they can be seen to flash for a few moments, before returning to their original brightness. The cause of these flare outbursts has not yet been properly described, but it is somehow related to strong chromospheric activities, in the atmosphere lying just above the surface.

Observations of flare stars has been undertaken for many years, and continued telescopic surveillance is still deemed as important. Photoelectric and visual observations are of equal importance. Sometimes radio telescopes are made simultaneously with optical ones. An example of this type is Proxima Centauri.

(iv.) BY Draconis Types

These are emission stars of the spectral classes of late-G, K or M, typically varying by 0.3 to 0.5 magnitudes in periods up to 120 days. The variations are thought to be caused by high axial rotations, that causes strong starspot activities, non-uniformity in brightness and very active chromospheres. The brightest example it BY Draconis, though it is really a prototype of this class.

(v.) R Corona Borealis Types

R CrB’s are truly fascinating objects, as they can drop in a period of several weeks by as much as 10 magnitudes. When the light diminishes at irregular intervals and is totally unpredictable. The frequency of falls in magnitude varies from star to star. Only twenty-six are known (2000). Latest discovered of the class in 1987 was V842 Cen / NSV 6708 in Centaurus (Position: 14h 34.8m −39°33″) by the Sydney amateur Glenn Dawes. Originally, it was listed as a suspected variable, but no one followed it up, nor determined what was the type of variable, until it had a sudden drop in magnitude. In 1964, the maximum and minimum was only suspected to be between 9.7 to 10.5. Surprisingly, V842 Cen turns out to be the third brightest of its class! The brightest is the star R Corona Borealis, though it is not a typical example because it shows peculiarities not observed in any of the others.

Spectral classes for stars in this group are usually either F, K, M or R. Prior to the sudden drop in magnitude is typically about four days, though one has faded over 2½ years. Some variations are suspected to be on a semi-regular basis. The star SY Sagittarii has secondary changes every forty days or so, while some of the others show similar periodicity. This fluctuation effect was first detected by amateurs from visual observations.

Observations are still required to explain the variations. Professionals and spectroscopists depend on the amateurs to advise of the initial falls in brightness. Spectra taken during the fall and at minima is highly desirable, so the structure and nature of the causes can be determined.

A probable cause of the drop in brightness is thought to be due an huge obscuring cloud of carbon material likely in the form of small particles of soot. When the orbiting material passes across the line of sight between us and the star, the magnitude suddenly drops. As the cloud moves on, the trailing carbon decreases in density and so the magnitude slowly increases until the star again fully shines. The carbon may be considered to be like a comet with a denser nucleus and a less dense coma and tail. How this material ends up orbiting around the star is not known. Some astronomers suspected the star maybe a merging of two small white dwarf stars, thus producing a gas disk around the star. The forces between the two object combined with the end-evolution mass loss, causes the carbon to be continuously ejected into orbit.

For the amateur, these anti-novae, are exciting types of variables to observe because of their sudden and unpredictability fall in brightness. Any visual observations are always considered useful.

Examples include R Corona Borealis, V842 Cen, UW Centaurus, S Apodis, RY Sagittarii and V Corona Australis.

(vi.) Gamma Cassiopiae Types

γ Cassiopiae types are irregular variables of the B-class, in the luminosity classes of III IV or V. All typically show emission lines and all are rapid rotators. The light curve fluctuates between 1 or 2 magnitudes but occur at unpredictable times. Causes is attributed to the massive ejection of material from the equatorial regions of the stellar surface. Visual observers unfortunately cannot produce magnitude estimates because of the swift rate of the change, so photometry is the only option.

Examples of these stars include Gamma Cassiopiae, Omega (ω) Canis Majoris, Nu (2) (ν²) Crucis and Nu (ν) Centauri.

(vii.) U Geminorum (or SS Cygni) Types or Dwarf Novae

U Gem’s are dwarf stars that normally show small fluctuations in brightness. Sometimes, unpredictably, the star will flare by between 2 and 7 magnitudes. Because of the suddenness of the increase they are often referred to as dwarf novae. The flaring takes between 1 or 2 days, quickly returning to the original small magnitude fluctuations. Intervals between the outbursts are irregular. Sometimes, it is after periods averaging about 10 days, though some tend to have them over a period of 5 or so years. Many, though not all, are thought to be contained in close binary systems.

Another sub-group of the class is the Z Camelopardi types. These are similar to the U Gem variables, except that the light curves have periods of negligible change. This constant light may suddenly change to small fluctuations that may persist for many cycles, and then as abruptly they stop. The flaring for al Z Cam stars may change between 2 and 5 magnitudes in rough periods, somewhere between 10 and 40 days.

Observations of these stars are deem useful either being made photoelectrically or visually.

(viii.) S Doradus Types

Very luminous blue and yellow stars are frequently variables. They are believed to be, like their archetype of the class S Doradus in the Larger Magellanic Cloud, super-massive stars, typically very young. Each have absolute magnitudes between −8 and −10.

Variations mostly range between 1 and 3 magnitudes, with the periodic changes being either regular or irregular. Some have been known to exhibit nova like properties, like the star Eta Carinae.

Light changes are probably caused by instabilities from the tremendous energy production, especially when the star changes the energy producing modes within the star. Of late, some of these stars have proved to be unresolved mini-clusters or multiple stars. The Hubble Space Telescope (HST), for example, was first to resolved the heart of S Doradus, to reveal a small cluster of about twenty stars. Variability of the brightest star, S Doradus, has been determined to have highly variable components — mimicking the effects the total light of the seemingly stellar source.

Examples include Eta (η) Carinae, P Cygni, S Doradus and HR Carinae.

Eta Carinae 10451-5941

Eta Carinae is a most unusual star, and is classed as either a peculiar type of novae or a bright and luminous S Doradus type variable. It is also believed to be a young star. It is associated with the star cluster and nebulae, NGC 3372. In actuality, it is placed in the 10 arcmin sized star cluster Trumpler 16, that is estimated to have an average O5 spectral type star, containing fourteen stars. Age of this cluster is estimated to be about 10 million years old. η Car appears as an orange star, in a starry field. The star is surrounded by an orange coloured nebulae about 3.5 arcsec across. The colour I have not been able to see below 30cm. telescope, and the reality of the colour is likely to be strongly influenced, physiologically, by the colour of the stars. Hartung states the nebulae is visible with a 10.5cm. telescope, though I have only been able to see it clearly using 20cm.

The star has a remarkable history. In 1603, Bayer indicates that it was probably not less than 5th. 1677, Halley listed it as a 4th magnitude star. By 1751, the brightness increased to about 2nd magnitude, only to drop again down to 4th between the years 1811 to 1815. It again brightened between 1822 and 1826 to 2nd, then to 1st in 1827. Between 1828 and 1843, it fluctuated between 1st and 2nd magnitude, until April, 1843 Eta Carinae reached its maximum brightness of −0.8, only being rivalled by Sirius. In this period, strange fluctuations were observed in both magnitude and spectra. By 1845 and 1848 it disappeared from naked eye visibility, until 1864 it changed to 5th, only to disappear again from view in 1892, where it stayed at magnitude 7.8, showing no variability. By 1914, the magnitude again started to slowly decreased. By 1935 it had ‘bottomed out’ to magnitude 8.5.

From 1935 to 1941, it started to show variability between magnitude 7.4 and 8.0. By 1951, it steadied in brightness to 7.3. Since then, it has continued to brighten. Magnitude was during 1995 was about 6.0, though Sky Catalogue 2000.0 states it as 6.21 through a photometric V-filter. The more recent observations place it now at 4.7 magnitude (2012), with variations between 4.3v and 5.2v. The star also displays rapid very short period variability, as first shown by de Vaucouleurs and Eggan in 1953. History for this star is highly not yet over, with a pretentious certainty, that more exciting things are yet to happen. At the current rate of rising, it could be 3rd by 2015 or 2020 and 2nd magnitude by 2050. It will be interesting if my own prediction turns out to be correct!

Prior to the maximum brightness in 1843, the star was a known variable, with a rough period somewhere between 15 or 16 years. Spectral history of this star did not start until the 1870s, and the development of the evolution of the spectra is unfortunately only mostly guess work.

Eta Carinae is a known multiple star, called Innes 1092. It has two main companions, 1.6 and 1.3 arcsec away, being magnitudes 11.1 and 10.9, respectively. I have never bee able to separate these two stars in 30 cm as they are partly obscured by the nebulosity. (Note: The Hubble Space Telescopes shows both these stars, and other fainter ones, in recent high resolution photographs of the area.) Since the discovery in 1915 of the two brighter stars, it is likely the expanding nebula was small enough to resolve the two stars cleanly. Prior to this time the brightness of Eta Carinae likely obscured both stars. They were discovered only when Eta Carina reached 7.8 magnitude. As time has progressed, the ability to see these two stars is likely to become more difficult, as the nebulosity expands.

Since the outburst of 1843, a gaseous shell(s?) of material seems to have been ejected violently from the star, at an average velocity of 400 kms.^-1. This nebula was first discovered by Innes in June 1914, seventy years later. An outer shell is about 28 arcsec. across, with a possible inner shell 3.5 arcsec in size. Its shells exhibit a typical material ejection of a “shelling novae”, showing common elements like Hydrogen and Helium, dispersed with highly ionised materials of elements like Oxygen, Nitrogen and Calcium.

If this was a novae explosion, it is now unique. Such objects are called slow novae, but a thirty-five year peak is longer by a factor of 10, than the longest of any novae. At the time of maximum brightness, the absolute magnitude was grater than -10. This also indicates that it is significantly brighter than the typical novae. Due to this, F. Zwicky in 1953, and others, believe instead that it is among the supernovae. While some say that it is the prime supernova candidate, just about to blow the powder-keg. If it does go supernova, it would reach a maximum brightness of about −7.5 or brighter. G. Burbridge in 1962 stated in the Astrophysical Journal (136,304) that η Car was the next nearby candidate for stellar detonation.

Others think it is a very unique variable. As Burnham frankly states in his hand book, Vol. 1 pg.471, Eta Carinae is one of the “highly luminosity ejection variables”. Current absolute magnitude is estimated to be between −4 and −5. Distance is estimated to be about 1,100 parsecs or 3 580 ly., the same distance to the cluster and the nebulae, in which Eta Carinae is immersed.

(ix.) Z Andromedae Types or Symbiotic Stars

These types are close binary systems that contain a very large and cool red giant and another small but very hot companion. Brightness variations are thought to be caused by a combination of the cool stellar pulsations and perhaps the material that is interacting with some mass transfer between the two stars.

All Z And’s are surrounded by nebulosity and have been known to flare like novae, as R Aquarii did in 930 A.D.

A subgroup is known as RR Telescopii types are represented by their rises by between 4 to 6 magnitudes. Oddly, these stars do not return to their original brightness. Many astronomers who specialise in variables, believe that they maybe an indication of the start of the Planetary Nebula phase of their evolution.

Examples include V366 Carinae, AG Pegasi, RR Telescopii and the brightest R Aquarii.

(x.) Flashes

This group is not very distinct but reflects the unpredictability of variable star observing. Flashes, or Visible Light Bursters, are star that increase in brightness very suddenly. The bursts of light can last several seconds to a few minutes. The bright northern star Beta (β) Camelopardi is an example that has been known to increase by three times its normal brightness in 0.25 of a second! Their seems nothing very unusable about this G-type star, with no peculiar characteristics. Some other single stars, are also known to do the same thing. Discovery of these stars has been made either by visual observation by celestial reconnaissance using video or CCD cameras. Explanation of the reasons or the mechanisms for the increase in brightness is presently unknown. A relationship to X-Rays or γ-rays bursts have never been observed.

Examples include the star β Camelopardi and the star BD+58°349, near the double star Ross 15, in the constellation of Cassiopeia.

(xi.) Secular Variables

Another group is the secular variables, which are believed to be stars that have changed in brightness or colour over the past centuries or millennia. Known examples include the brightest star in the sky, Sirius, that has been stated in some sources, as “ruddier than Mars“, suggesting that this star was once a red star. The explanation for this has been suggested to be a ancient transcript error, passed down to our times. Others have postulated that the white dwarf, orbiting the brightest component, has changed from a red giant into a white dwarf in the intervening centuries. For this to happen would be difficult against the current theories of stellar evolution.

A more recent example is Delta Scorpii (δ Sco) or Dschubba the central star at the head of the Scorpius. This star has since records o remained at 2.3 magnitude, but suddenly rose in June 2000 to 1.6 to 1.7 magnitude. Since then it has stayed at this elevated magnitude, and continues to slowly fluctuate by 0.1 magnitudes. The Gamma Cas B0.2IV variable is believed to have expelled gaseous material from its surface which continues to be thrown off into interstellar space. It remains uncertain if a very close companion star is responsible for the rise in brightness, or even another third companion star orbiting in just a little over over ten years whose orbit seems to be fairly elliptical. As the years continue to progress, it will be interesting to see if the star again falls back to its pre-2000 magnitude.

Pleione, contained within the star cluster of the Pleiades, is said to be another example. The Pleiades is known as the “Seven Sisters”, yet only six stars are visible to the naked eye. It is believed that this star may have faded over the centuries, though some say that it has been written to justify the Greek legends. However, it is definitely possible that this, or another has faded from view. Again, in terms of stellar evolution theory, this cannot be easily explained.

Many possible and unusual examples have been recorded, either photographically or by close examination of the early stellar catalogues.

(xii.) Unusual Variables

Some stars show variations that are unclassifiable. Such stars do not appear to have any particular known cause for the changes. The investigation of their natures is important, so we can classify them into the established systems. Some are unique or a variable star that fits into several different categories, as seen from their light curves.

Observations of certain examples of this class are requested from time to time by either the RASNZ or the AAVSO.

Note: Many other classes exist that have not been specified in the text. A relevant book or two on variable stars maybe useful for further reading or in visiting the more than useful AAVSO Website.

Last Update : 13th November 2012