Mining Hipparcos's Buried Treasure
Here are 11 stars that need watching.
These words, coming from the Hipparcos project scientist, gave Sky Publishing's Millennium Star Atlas team its first inkling of yet another windfall from that satellite (Sky & Telescope: June 1999, page 40). Now we could label thousands of newly discovered variables on our charts using the names by which astronomers of the future would refer to them even though little was known about these stars yet!
Practically speaking, this type of research can't be left to the professionals. Telescope time is in great demand at the major observatories; to divert those instruments to the long-term monitoring of individual stars would be downright wasteful. That's why variable-star work remains a field where dedicated amateurs can do some real science.
American Association of Variable Star Observers (AAVSO), we at Sky & Telescope are pleased to present our first selection of new Hipparcos variables. The finder charts and comparison-star magnitudes on these pages are just what observers need in order to start figuring out the fluctuations we already know take place. Because these stars vary by a half magnitude or more, even visual estimates can tell a lot. The table on page 5 gives each star's designation, coordinates, and brightness clues from the European Space Agency's Hipparcos and Tycho Catalogues. Every star on this list is well placed for examination on May and June evenings. (Another list of Hipparcos variables can be found in the article "A Treasure-Trove of Variable Stars.")
What to Expect
For example, a well-known star that changes only rarely is Epsilon (e) Aurigae, the eclipsing binary whose companion comes around to hide part of the primary star only once every 27 years! The system then loses half its normal luster for 13 months at a stretch. These episodes, first noticed in 1821, happen only once for each new generation of astronomers. (Don't hold your breath; Epsilon's next eclipse isn't due to start until 2009.)
Other new variables behave quite differently. The light-curve fragment for HS Ursae Majoris, for example, shows a gradual rise in brightness (with minor ups and downs) from magnitude 8.9 at the start of 1990 to an apparent peak of 8.3 in November 1991. The star then faded back to the middle of this range by November 1992. Clearly it's an intrinsic variable, probably irregular, but only by continued monitoring can anyone really know.
A Few Observing Tips
The techniques for making visual magnitude estimates are not very demanding. (For more detailed information, see "The Lure of Variable Stars.") Since the stars on our list are 7th to 9th magnitude, they can all be seen in a good pair of binoculars. But if a star is within a couple of magnitudes of the limit for a particular instrument, it can be seen clearly only by "averted vision" directing your gaze a little to one side.
Try to select comparison stars that bracket the variable star in brightness, differ from it by less than 0.5 magnitude, and have a similar color. The colors on our charts are an excellent guide for they, like the magnitudes of the comparison stars, are derived from Tycho photometry of the Hipparcos mission. The colors are more vivid than those you will see at night. By convention, comparison-star magnitudes are indicated to tenths with the decimal point omitted. Thus, a star labeled "79" is magnitude 7.9 visually.
What to Do Next
The best way to share your results with professional astronomers and learn from more experienced observers is to join an organization like the American Association of Variable Star Observers, 25 Birch St., Cambridge, MA 02138 USA. The benefits of membership include being in the pipeline to receive the AAVSO's Alert Notices and News Flashes of novae and other stars showing unusual activity, as well as the biannual Journal. To find out how to join, call 617-354-0484 or visit its Web site. This site has links to other variable-star groups around the world.
In recent years, some 600 active amateurs in 40 countries have been sending their observations regularly to the AAVSO. Its database now bulges with nine million magnitude estimates going back to 1902 a rich resource that professional astronomers consult whenever they need the history of a particular star's variations.
Comparison charts for the four variables listed in the table but not included in this Web article are contained within the original article, available in Sky & Telescope's online archive.
|Eleven "Unsolved" Hipparcos Variables|
|FM Leo||EA||11h 12.8m||+00° 21'||8.4||0.5|
|HS UMa||I||11h 35.5m||+34° 52'||8.5*||0.6|
|TX Crv||||12h 00.8m||-12° 09'||8.0*||0.7|
|KM Com||||12h 38.9m||+13° 48'||8.8*||0.6|
|DK Boo||||13h 44.0m||+21° 49'||8.2||0.7|
|V353 Hya||EA||14h 10.2m||-25° 24'||7.4||0.5|
|DR Boo||I||14h 16.0m||+34° 26'||8.3||0.5|
|FG Boo||L||15h 11.6m||+49° 54'||7.6||0.6|
|KZ Lup||SR||15h 51.7m||-38° 09'||9||0.5|
|V939 Her||SR||17h 10.3m||+40° 41'||7.3||0.7|
|GV Dra||EA||17h 53.5m||+52° 23'||8.5||0.5|
|Variable types tentatively assigned in the Hipparcos catalog based on a partial light curve are EA, Algol-type eclipsing binary; I, irregular; L, long-period; SR, semiregular. The magnitude given is the median of the values measured during the mission, adjusted to the V (visual) photometric band; asterisked values refer to the combined brightness of a close pair. The amplitude is the difference between the faintest and brightest magnitudes measured for the star (the actual amplitude could be even greater).|