Variable Star Work


The star scanner has helped discover one of the brightest eclipsing binary stars known and it has made observations on another irregularly behaving star when no earth based observers or satellites could see it.


Delta Velorum


On Nov 21, 1989, Galileo had just been launched four weeks before and was a brand new spacecraft. On that day, the star scanner happened to be observing a star called delta Velorum in the southern constellation of Vela. The star dimmed for about half a day and then returned to its normal brightness. However, apparently no one on the spacecraft team took any notice of this event. The problem did not repeat immediately and was forgotten over the years.


In July of 1997, an Argentine amateur astronomer named Sebastian Otero, was making naked eye observations of the Southern hemisphere night sky. He saw this same star dim and then return to its normal brightness the next night. He checked various catalogs to see if this star was known to be variable – it wasn’t. Over the next few years, he saw the same event happen several more times. He was the first to understand that this star was variable but Sebastian kept his find quiet – who would believe that there was a star, about the 45th brightest in the sky, that was a variable and no one had noticed before…..


On June 19, 2000, the star dimmed while Galileo was again using this star to determine its attitude. The star’s intensity dropped only about 20%, but that was enough that the sensitive star scanner could no longer recognize it. Yet during this time, it was seeing several other stars as rock steady as ever. Confused by the situation, the spacecraft issued a complaint in its telemetry stream.  Eight hours later, delta Velorum returned to its normal brightness and all was quiet again.


I had been working with the star scanner for several years by this point and, in my experience, the star scanner was unfailingly reliable. This mysterious dimming was trying to tell me something – but what? I spent a week eliminating such unlikely events as an occultations by some passing asteroid or some little outer Jovian moon or even a piece of the spacecraft falling off. Rather unhelpfully, even the influence of Arthur C. Clarke’s monolith was suggested! In the end, I felt the star must be variable even though a half dozen catalogs said it wasn’t. I shipped an e-mail to the good folks at the American Association of Variable Star Observers. The e-mail was forwarded around the internet until it fell into the hands of Sebastian Otero who, in an excited e-mail, contacted me.


Within days, Sebastian and amateur astronomers in South America and Africa started monitoring this star nightly. Discussion of a possible period for these dimming events were carried out through the Japanese led Variable Star Network and e-mails. At the same time, I started looking through megabits of old Galileo data only to find that our spacecraft had been trying to tell us about this star way back in 1989. With all this data starting to accumulate, Christopher Lloyd, an astronomer at the Rutherford Appleton Laboratory in the United Kingdom and Sebastian were able to compute a primary period of 45.15 days. With the period in hand, the next dimming event was prepared for and Sebastian saw it exactly at the predicted time. Paper number 4999 was prepared for publication[7] in the Information Bulletin on Variable Stars.





The Galileo star scanner’s 1989 observation of delta Velorum’s variability with 5 other reference stars shown. This data would go unnoticed for 11 more years.




In the end, the star’s variability has been re-confirmed by Galileo as well as by Sebastian and subsequently, other observers.  The delta Velorum system is composed of up to five stars orbiting one another. Only the central star is visible to the naked eye and that star, we now know, is actually two similar stars, each one in turn eclipsing the other every 20 and then 25 days. The dimming is just at the range that a human observer with excellent eyesight can see it – if he or she is looking for it. The AAVSO called delta Velorum the brightest variable star discovered visually in the 20th century [8] and, since it is brighter than the famous eclipsing binary star Algol, it is arguably the brightest eclipsing variable now known. Look for it in your night sky if you live south of about 30 degrees north Latitude.




Delta Scorpii


The same summer that the delta Velorum discovery was made, the same sharp-eyed Argentine amateur astronomer also helped discover the variability of the star delta Scorpii [9]. This bright star happened to be in Galileo’s field of view from Oct 12, 2002 to Jan 15, 2003. Galileo was not able to observe this star with the same accuracy as ground based observers but Galileo is a very patient witness able to provide a more or less continuous stream of data lasting for several months. Luckily, it was also observing this star during a period while it is lost behind the sun for observers on Earth. Below is a graph of data for delta Scorpii during a part of this time period. It appears that on or about November 2 (DOY 306), the star entered “lull” as described by Gandet, et al. [10]. Prior to this, the star was rapidly increasing its intensity.



         Star scanner data showing data gathered on the star delta Sco. from late October to mid-January.