A Variable Star Junkie

Suzaku Catches Retreat of a Black Hole's Disk
Image by NASA Goddard Photo and Video via Flickr

My name is Rod Stubbings and I operate the ‘Tetoora Road Observatory’, a Privately registered observatory for optical research on variable stars in the Strzelecki ranges south of Warragul. The observatory houses a Meade 16-inch reflecting telescope. I visually monitor cataclysmic (explosive and nova- like) variables of interest to professional astronomers and organizations around the world.

Although not a real passion at first, my interest in Astronomy started in 1986 from an advertisement in a magazine which showed a 65mm Tasco telescope for sale to view Saturn’s rings and Jupiter’s belts. Thinking this was going to be exciting, I ordered the telescope. When the scope arrived I read the instructions, assembled the legs and tube and on the first clear night ventured outside. I pointed the scope at a few bright stars but no sign of Jupiter. To be honest, I didn’t have a clue where anything was! Not to be deterred I went to the newsagent and purchased a book on ‘Astronomy without a Telescope’. This enabled me to recognize the brightest stars, their names, the constellations they belonged to and yes, where to find Jupiter and Saturn!

As my interest in Astronomy grew I attended a meeting at the Latrobe Valley Astronomical Society (LVAS) where I met Peter Nelson who introduced me to variable stars. Peter was Director of the Astronomical Society of Victoria, Variable Star Section. Variable stars are stars that vary in their light out-put. The origins of these light variations define the classification system of variable stars.

There are two kinds of variable stars; intrinsic, in which variation is due to physical changes in the star or stellar system and extrinsic, in which variability is due to the eclipse of one star by another or the effects of stellar rotation.

In each constellation, the first variable stars discovered were designated with the letters R through Z. Letters RR through RZ, SS through SZ, up to ZZ are used for the next discoveries. Later discoveries used letters AA through AZ, BB through BZ, and up to QQ through QZ. Once these 334 combinations were exhausted, variables are numbered in order of discovery, starting with the prefixed V335 onwards.

NASA, CHANDRA, 2009

Variable stars can be observed with the naked eye, binoculars or telescope depending on what type of variable interests you. A real contribution to astronomy can be made when a variable star’s brightness is recorded and sent on to the proper organizations. This appealed to me as I could make better use of my hours spent outside at night. Armed with a beginner’s book on “The Observations of Variable Stars” from the Royal Astronomical Society of New Zealand (RASNZ) I learnt how to make an observation on a variable star.

The RASNZ variable star section was founded in 1927 by the late Dr. Frank Bateson, OBE, (1909-2007) and became the recognized centre for Southern Hemisphere variable star research. I made my first observation of a variable star in May 1993. My first month of observing yielded 10 observations, and I was now a member of the RASNZ.

Every time I phoned Peter Nelson for a few variable star charts to add to my list, he would photocopy about a hundred charts and send them to me to keep my interest going. As the variable ‘bug’ was starting to dig in I found I was observing at every opportunity. L Carinae (L Car), a naked eye Cepheid variable, was a handy one to observe because whenever I found myself at a friends place I could always sneak outside and make an observation. L Car has a period of 35.5 days and magnitude range of 3.3 – 4.1. Parties were no exception; all I had to do was slip out the back door, go home and record 50 or 60 observations, and back to the party not to be missed.

The variable bug was now really entrenched! Old White Face (the moon) is the enemy for a variable star observer, so that meant setting the alarm at different hours of the night to be able to observe under dark skies.  My best observing effort was venturing outside at 6.00pm, observing all night until daylight, and then off to work.  The following night the same routine, outside at 6.00pm and right through until daylight then work again. Another clear night followed but I only lasted a few hours before I crashed and went to sleep. Yep! I was a variable star junkie!

Next my attention shifted to cataclysmic (explosive and nova–like) variables (CVs) particularly the U Geminorum (U Gem) type – you never know what to expect on any given night. U Gem stars are interacting binary systems comprising a white dwarf primary, and a mass transferring secondary. The stars are so close to each other that the gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. The secondary is often called the donor star. The falling matter forms an accretion disk around the white dwarf. When a portion of disk material falls onto the white dwarf outbursts occur. Normal outbursts usually last up to 3 days before fading. SU Ursae Majoris (SU UMa) type dwarf nova undergo outbursts which are brighter and of longer duration and are called “superoutbursts” which show ‘superhumps’ in the light curve. These systems are of particular interest to professionals for further study.

Before the onset of e-mail, professional astronomers assigned special program stars that were called targets of opportunity (TOO) for satellite observations. When an outburst was detected it needed to be phoned in to Dr. Frank Bateson so he could alert the professionals. I remember when the SU UMa eclipsing type dwarf nova OY Carinae was a TOO and I caught an outburst at 2.00am one morning. Do I ring at this time of night? I wonder if he will be awake? In the end I decided to make the call and Frank answered instantly, he was very appreciative of the alert which enabled the satellite observations to take place.

This situation changed with the public availability of the Internet. The Variable Star Network (VSNET) is a global professional-amateur network of researchers in variable stars and related objects, such as cataclysmic variables, black-hole binaries, supernovae, and gamma-ray bursts. Instead of keeping these outbursts in my log book I started to send alert notices to VSNET. In 1997 I was contacted by the director of the American Association of Variable Star Observers (AAVSO), the late Dr. Janet Mattei (1943-2004), asking if I could send my notifications of outbursts I had recorded for their newsflash alerts. I was now recording over 1400 observations each month and detecting between 30 and 50 dwarf nova outbursts per month. Outburst notifications were sent on a nightly basis after observing sessions via e-mail to AAVSO, VSNET and RASNZ.

The brightness of a star in the night sky is described as its magnitude. The brightest stars are -0.1 magnitude, then magnitude 1.0, 2.0, 3.0 and so on. The faintest star your likely to see with the naked eye under a dark sky is about magnitude 6.0. Binoculars can see stars to magnitude 9.0.

I started to concentrate on the many poorly studied fainter cataclysmic variables. This involved a lot of stars which were having outbursts at magnitude 15.0 to 15.6. With constant monitoring I was able to record their maximum brightness, frequency, duration and follow the rise and fall of outbursts. All this information was previously unknown. Typically, when I detected an outburst I would send an alert notice which enabled observers with CCD detectors to observe the star. A lot of interesting results were found when notifications of these outbursts were subjected to CCD time-series observations.  The CCD data showed that a lot of these unknown stars belonged to the exclusive SU-UMa type dwarf novae. The CCD data obtained world wide is sent to the VSNET team at the Kyoto University in Japan for analysis with the results announced and published.

The year 1999 turned out to be a very ‘explosive’ year for me as it was the year of many important outburst detections.  I had been observing the variable star V4641 Sgr, in the constellation of Sagittarius near the centre of our galaxy. The star was showing some rapid variations ranging from magnitude 11.1 to 11.9 over a number of nights. On September 15th 1999, in the course of my observing program, I went outside to make a few observations in between clouds as there was a storm approaching. First up was V4641 Sgr. The star had just exploded to magnitude 8.8, so I raced inside and sent an alert message around the world. A three magnitude jump means the star had increased in brightness by 15 times. Within an hour the outburst was confirmed with CCD images at the Kyoto University Japan. This message was passed on to professional astronomers: Visually, this outburst lasted less than 8 hours and was gone but in other wave lengths it was still visible.

Astronomers at the Massachusetts Institute of Technology (MIT) diverted a NASA x-ray satellite to take a look and it showed a rapid rise and fall in X-ray brightness. Within 24 hours, the Very Large Array (VLA) radio telescope in New Mexico was observing V4641 Sgr. Other radio telescopes observing the object were: the National Radio Astronomy Observatory’s Green Bank Interferometer in West Virginia, the Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope (also in Australia), the MERLIN array in Britain, the RATAN 600 meter radio telescope in Russia and radio telescopes at the Owens Valley Radio Observatory in California.

The neutron star at the heart of the well known Crab Nebula is so regular in its rotation and brightness that astronomers use it as a standard candle for measuring the brightness of other x-ray sources. V4641 Sgr became 1,200 times more luminous than the Crab, and for a brief moment was the brightest X-ray source in the sky.

Matter swirling around a black hole

The VLA radio telescope revealed the presence of twin jets escaping the system at nine-tenths the speed of light. The size of the jets was about 125 Astronomical Units, more than three times the distance from the Sun to Pluto. From the information collected, V4641 Sgr is a black-hole binary system which was not previously known. Being only 1,600 lights years away makes it one of the nearest black holes. V4641 Sgr sits around magnitude 13.8 in quiescence and each year exhibits X-ray outbursts and flares up to magnitude 12.0. I am still the only person to ever visually see and record the explosion to magnitude 8.8. This emphasizes the scientific value of visual observations in variable star astronomy.

Always keen to add more stars to my observing list I would search the CV catalogs for more interesting objects that were within the limits of my telescope and also objects that were not well observed.

U Piscis Austrini (U PsA) was only the fourth variable star discovered in this constellation with a variation of magnitude 12.5 to below 14.0 photographic but was still unclassified in the General Catalogue of Variable Stars (GCVS 1985). I read an article on this star where observations attributed possible periods of 117 and 235 days to the star, but it still remained a mystery. I followed U PsA from 1996 to 2000 and noticed slight fluctuations on the rises and falls of this star. Further concentrated observations in 2000 revealed the full magnitude range of this star in one night which was very strange. I enlisted the help of Stan Walker in New Zealand who analysed my observations with period search software and came up with a period of 0.54187 day, just over half a day! Stan obtained CCD measures of this star which confirmed the value of 0.54187 day. The shape, period and amplitude of the light curve indicate U PsA to be an RR Lyrae variable star, subtype RRab. A paper on U PsA was written and published in the Journal of AAVSO Volume 29, 2001.

CG Canis Majoris (CG CMa) was considered to be a possible classical nova. I added this object to my list and after a few months I observed an outburst in February 1999 at magnitude 13.7. This was the first-ever visually recorded outburst since its discovery in 1934 on photographic plates. CCD observations revealed that the outbursting object was slightly off set from the suggested quiescent star which was around magnitude 16.0. It was found that CG CMa was a few arc seconds away from the suggested star and in quiescent is a magnitude 20.0 star. CG CMa was reclassified as a new SU UMa type dwarf nova from the results of the CCD data. No further outbursts have been recorded to date.

V359 Centaurus (V359 Cen) was discovered as a possible nova in 1939 on photographic plates. In July 1999 I recorded the first-ever visually observed outburst at magnitude 13.8. Observations during this outburst showed that the object is likely a dwarf nova. I recorded further outbursts in 2000, 2001 and 2002.  The 2002 outburst received detailed CCD time-series observations which revealed a new SU UMa dwarf nova, and the true classification, of this once suspected nova.

In 2002 I recorded my 100,000th visual observation; the achievement was made on a star called KK Telescopium (a SU UMa type dwarf nova.) This was accomplished in just 9 years of observing variable stars and I was the first Australian observer to reach this total. In that year I was also privileged to receive the Director’s Award from the AAVSO for my contributions to variable star astronomy.

The polar star EF Eridanus (EF Eri) was added to my list in 1997 after a request from Dr. Frank Bateson. The star was also a TOO for satellite observations. I was assured that this star would enter into a highly active state by the professionals needing the observations in the UK and USA.  After 9 years of continuous monitoring and recording negative observations I got the biggest shock when I caught an outburst in June 2006 at magnitude 15.6. I contacted Dr. Steve Howell from the WIYN Observatory, Tucson, Arizona. After detailed study over a number of years and with no activity in this system, Dr. Howell believed that this polar was a cross between a former star, an odd brown dwarf and an extra-solar planet. Steve replied in an email that he had bet it ‘would never brighten again’, and he had just lost!  The following night it rose to magnitude 14.2. He received over 47 e-mails the next two days about the brightening of EF Eri and alerted a number of telescopes to observe it. At the time of writing EF Eri has gone back into hiding once again and remains in an inactive state.

WZ Sge type stars are a sub class of the SU UMa type dwarf novae. The longer and brighter outbursts of SU UMa stars are called “superoutbursts”. The period of time from one superoutburst to the next is called the “supercycle”. The supercycle lengths of SU UMa stars are mostly around a few hundred days, but some systems have shorter or longer supercycles. WZ Sge stars have supercycles that last decades with normal outbursts rarely seen.

GW Librae (GW Lib) was discovered in 1983 as a 9th magnitude outbursting object. The quiescent counterpart was a star at magnitude 18.5. GW Lib has also been known as the “Never – Bursting Dwarf Nova” due to its long dormant period. On April 12th 2007 I noticed GW Lib was visible at magnitude 13.7 so I sent off an alert message.  Returning to the observatory I noticed GW Lib was starting to brighten. The outburst was still in its early stages so I made frequent observations until clouds ended my observing session.  It rose almost 2 magnitudes in 6 hours to 11.8. The following night it was at magnitude 8.5. The full duration of this outburst lasted 70 days and spanned 6 magnitudes. One of the telescopes that professionals used to observe GW Lib was the 10 meter Southern African Large Telescope (SALT) the largest optical telescope in the southern hemisphere. After a 24-year dormant period it attracted a lot of attention from professional and amateur astronomers world wide.   GW Lib is a classed as a WZ Sge type star.

Recently I detected an outburst of the WZ Sge type star HV Virginis (HV Vir) on February 14th 2008 at magnitude 11.5.  This outburst is only the fifth recorded of this UGWZ star – 1929, 1970, 1992 and 2002. The 2002 outburst of HV Vir confirmed the UGWZ status of this object.

As well as the fun and excitement of observing variable stars, seeing the results published, the involvement with professionals and observers all round the world makes this a very rewarding hobby. At the time of writing I have recorded over 176,000 visual observations of variable stars and still counting. I continue to observe these crazy stars at every opportunity and still get a rush of blood every time an outburst is detected. My goal is to observe over 200,000 visual observations, as there is currently only four visual observers world wide who have reached this amount. The observers are from New Zealand, South Africa, England and USA.

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