Shallow Sky Object of the Month: W UMa - Eclipsing Binary

Original article appears in GuideStar May, 2013.

Finder chart and 1 degree circle chart. North is up.
Star chart generated by TheSkyX © Software Bisque, Inc. All rights reserved. www.bisque.comObject: W UMa
Class: Double star
Constellation: Ursa Major
Magnitude: 7.75-8.48
R.A.: 9 h 43 m 46 s
Dec: 55 deg 57 min 09 sec
Size/Spectral: F class (white)
Distance: 170 ly
Optics needed: Small telescope

Why this is interesting:

Comparison stars (not from AAVSO, from TheSky Decimal point ommitted
Star chart generated by TheSkyX © Software Bisque, Inc. All rights reserved. www.bisque.comThe name of this star gives it away as a variable star. (A star that has a letter of the alphabet, or two, starting with R preceding the name of the constellation identifies a variable. This one, with W as the designator is the 6th variable identified in this constellation). One of the things that scientists of all kinds do is to put items in categories. It helps these scientists understand and organize information. This happens with variable stars, too and there are lots of categories for variable stars. The star that defines that category is called the ‘prototype’ star. This star is the prototype for other variables which are thus called W UMa variables.

W UMa is an eclipsing variable with a period of .3336 days, so the star goes through a complete cycle from dimmest, to brightest, to dimmest three times in one day. Could you detect this variability by watching it with your small telescope. Yes, you can. It is generally agreed that you can estimate star brightness within .1 magnitude visually.

Finder charts can be produced at AAVSO.org although comparison stars are rare in this area.

So, what is the source of the variability for this star? It turns out that this star is actually a double star with the two stars so close that they are touching each other. They are both known to be F (color) stars so we should not expect a change in color of the star as it goes from brightest to dimmest. There is thought to be a significant amount of mass transfer between the two stars. The stars are believed to be just over 1 solar mass for the larger star and just over .5 solar masses for the secondary star. A .5 solar mass star is too small to be a F star, but this color is accounted for by the mass transfer between the stars. The period of variability has changed over time; it is now longer due to the braking affect of the mass transfer between the two stars.The original light curve of the discoverers (Muller and Kempf) in 1903 — from AAVSO.org

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