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Variable Star Of The MonthJune, 2000: SS Cygni"The stars that have most glory, have no rest." -Samuel Daniel in History of the Civil War Over a Century of Observations
SS Cygni Takes the StageThe discovery "of a (variable) star in the constellation of Cygnus" was announced in Harvard College Observatory Circular No. 12 (November 12, 1896) by E.C. Pickering. A photographic range of 7.2 to 11.2, and a period of 40 days was given, which Mr. Pickering noted as "an unusually large range for a variable having so short a period." This new variable was assigned the name SS Cyg in 1897; a name that has become well known and revered in the astronomical community. The 1896 discovery of SS Cyg proved to be only the second star of the dwarf nova type, with U Gem being the first of the class with its discovery in 1855. (In stark contrast, today there are currently over 375 known dwarf novae.) In the Annual Report of the Director of Harvard College Observatory for the year ending September 30, 1896, Mr. Pickering states that "Five new variables have been found by Miss L.D. Wells from a comparison of chart plates. The most important of these is W Delphini, a star of Algol class..." (Hazen, 1997). Clearly SS Cyg's rise to fame was not predicted. Getting to Know SS Cygni
SS Cyg, like all other cataclymic variables, consists of a close binary system. One of the components is a red dwarf-type star, cooler than our Sun, while the other is a white dwarf. Studies suggest that the stars in the SS Cyg system are separated (from surface to surface) by "only" 100,000 miles or less. In fact, the stars are so close that they complete their orbital revolution in slightly over 6 1/2 hours! Astronomically speaking, SS Cyg is also fairly close by, being a distance of 90 to 100 light years from its Earth-bound admirers (Burnham, 1978). The inclination of the system has been calculated to be about 50 degrees, yielding a respective component mass of Mwd = 0.60 solar mass and Mms = 0.40 solar mass (Honey, et al., 1989). SS Cyg's bright appearance, frequent outbursts, and location in the sky seem to be almost a recipe for a successful variable star! Observing SS CygniThis enchanting variable is located in the constellation of Cygnus the Swan. The Cygnus constellation finder chart will help the first time observer locate the section of the sky in which SS Cyg can be found. The "b", "c", "d", and "e" scale charts (available in regular and reversed chart format) may then be used to make variable star estimates. For observers with small telescopes, this is a rewarding variable to watch since its range can be easily covered with such equipment. This star is cetainly a favorite of northern observers, especially those who find Cygnus to be a circumpolar constellation.
Since SS Cyg's behavior is somewhat unpredictable, one observation per clear night is desired. However, the star should be observed several times a night when active, with the time noted to the minute. If you are unsure of the star's current activity, the AAVSO Quick Look files (which may now be access by the AAVSO Light Curve Generator) and the AAVSO News Flash will help to determine SS Cyg's present status. The OutburstsSS Cygni is a cataclysmic variable star, of the dwarf nova class. Such variables are comprised of a close binary system with a white dwarf primary star and a red dwarf secondary star, as mentioned above. Due to the evolution of the system, the main sequence star loses matter which is streamed in the direction of the primary, forming an accretion disk around the white dwarf star. The observed outbursts are believed to be the result of processes that arise in the hydrogen-rich disk. Whether the activity is the result of a sudden transfer of mass from the secondary to the primary (as postulated in the mass-transfer burst model), or if it is the product of instabilities within the disk (as prescribed by the disk-instability model) remains to be determined. (For a further review of this topic, see VSOTM SU UMa and RS Oph and references therein.) During this time, typical dwarf novae rise by 2-6 magnitudes on a timescale of 10-1000 days.
Statistical studies of SS Cygni's light curve over the years have revelaed numerous correlations (Sterne and Campbell, 1934; Bath and van Paradijs, 1983), one of which being the preference of wide and narrow outbursts occurring in an alternating pattern. An investigation of the long-term behavior of SS Cyg was carried out by Cannizzo and Mattei in 1992 concerning the outburst type. In this publication, an outburst is said to occur when the variable exceeds a visual magnitude of 10. The outbursts are then separated by timescale according to long, or wide, outbursts (denoted by the letter "L") with a duration exceeding 12 days, while the short, or narrow, outbursts (denoted by the letter "S") consist of less than 12 day duration. A study of the data yielded the most common sequence as LS (with 134 occurrences), LLS (69), LSSS (14), and LLSS (8). Together these strings represented 89% of the outburst studied. Cannizzo (1993) has suggested that the determining factor as to whether an outburst will be a wide or narrow episode depends upon the amount of mass in the disk at the start of a thermal instability. That is, the narrow outburst may correspond to moderate mass transfer, while wide outbursts may be inspired by a major mass transfer. For a further review of this subject, see the paper by Smak listed below.
Thanks to AAVSO ObserversIt seems that SS Cyg is always at the forefront of breaking new ground. Its involvement in the very first AAVSO Alert Notice, published on June 11, 1974, is no exception.
Why such an intense interest in multi-wavelength coverage? Theory has suggested that the light we see in the visual portion of the spectrum during outburst arises from the cooler, outer-most region of the accretion disk. The inner disk, however, is hotter as a result of the liberated accretion energy, and is the source for ultraviolet emission. The boundary layer between the accretion disk and the white dwarf is even hotter, and as a result emits X-ray and extreme-ultraviolet radition. Thus, in order to investigate the properties of the inner regions of the disk and the boundary layer of SS Cyg, as well as other dwarf nova-type stars, it requires observations with satellites that have instruments that can observe in high-energy bands. Why choose SS Cygni for such observations? There are three reasons why SS Cyg is a favored CV for multi-wavelength obsevations: (1) it is bright, (2) it is nearby, and (3) because of low hydrogen column density, ie, a low number of hydrogen particle/cm2 between us and SS Cyg. With continued observations throughout the spectrum, we will be able to learn more about this fascinating variable. Since SS Cyg's outbursts occur unpredictably, professional astronomers rely of the eyes and instruments of amateur astronomers to help pinpoint the start of an outburst. Thanks to all the AAVSO observers who have spent countless hours heeding the call for observations! SS Cygni Creates a StirTarget-of-opportunity observations of this and other stars become quite exciting when trying to predict the activity of a star. In 1996, Dr. Peter Wheatley, of RXTE, and Dr. Chris Mauche, of EUVE, wished to observe SS Cyg during its October outburst. For this project, both Drs. Wheatley and Mauche needed to be informed precisely when the star would go into outburst. To obtain this information, they turned to the AAVSO. As instructed, AAVSO observers took vigil throughout the night. Indication of SS Cyg's activity was detected by observers in California and Kansas when the variable had achieved a magnitude of 11.5. Later observations of a Hawaiian observer confirmed the activity with a magnitude estimate of 10.9. In the Director's Report presented in the Journal of the AAVSO (JAAVSO) Volume 25, Dr. Mattei shares a bit of the excitement that ensued: Dr. Mauche wrote:
Your Hawaiian observer, Bill Albrecht, called to say that at JD 2450365.779 SS Cyg had reached 10.9, so he said he was positive it was going to go up. He went so far as to say that he would put money on it. So, I called Pete, who'll contact XTE and eventually SAX...Anyway, sounds like we're all set. I hope Bill is right - maybe he's been drinking too much hard pineapple juice! Dr. Wheatley wrote the next day: Chris phoned me at 0700 UT with the news from Hawaii. I got onto XTE and triggered the satellite straightaway. They have an advertised response time of seven hours, so it's possible we'll get onto SS Cyg with 11 hours of the first observer calling Janet. That would be great...I was so excited at the news that I fell off my bike on the way to work - and I'm sitting here covered in cuts and bruises. It's a dangerous business we're engaged in. As a result of this enthusiastic amateur-professional collaboration, the October 1996 outburst of SS Cyg was observed simultaneously for 12 days in the visual, ultraviolet, and X-ray band passes. In light of this excitement, Dr. Mattei will be presenting the results of this unique and important multi-wavelength observing run at the American Astronomical Society meeting in Rochester, NY, June 4-8, 2000! Impressed by the success of the project and to provide more opportunities for amateur astronomers, the director of EUVE granted three day's of observing time of with EUVE to the AAVSO to use on any star of choice! Thanks again to AAVSO observers!
The Excitement ContinuesIn the summer of 2000, SS Cyg will be observed by the Chandra X-Ray Observatory and EUVE with hopes to learn even more about this fascinating variable. Be sure to keep an eye out for AAVSO Alert Notices and AAVSO News Flashes for requested monitorings! As an interesting sidenote, the AAVSO has a special tie to this satellite. AAVSO member and observer Gary Emerson worked as a test engineer at Ball Aerospace on Chandra's Charge Coupled Device (CCD) camera. SS Cygni For Sale
For More Information
This month's Variable Star of the Month was prepared by Kerri Malatesta, AAVSO Technical Assistant. |
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