Star of the Month
AKA: Sheliak (official name), Shelyak or STFA 39.
Position: 18 hrs 50 min 05 sec +33 degrees 21 min 46 sec
Due south at 22:12 (BST) on 15 August 2020
Beta Lyrae
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
There are many archetype variable stars, the best known probably being Delta Cephei and last month, in July, we had RR Lyrae, the archetype of the RR Lyrae variables. This month we have another prototype variable in Lyra, Beta Lyrae. It is what is known as a contact binary variable. The period is almost thirteen days (12.94 days to be precise) and it varies between magnitude 3.2 and magnitude 4.4. It is easy to check if Beta Lyrae is at maximum or minimum. First make out the parallelogram under Vega which makes up most of the constellation of Lyra. The top two stars are Zeta Lyrae (on the right or west) and Delta Lyrae (on the left or east). Zeta Lyrae is 4.3 and Delta Lyrae is 4.2, both slightly brighter than Beta Lyrae’s minimum. If you look at the bottom of the parallelogram, Beta Lyrae (Sheliak) is the star on the right or west side and Gamma Lyrae (Sulafat) is on the left or east side. Gamma Lyrae has a magnitude of 3.25 or about the same as Beta Lyrae’s maximum. Incidentally last month’s DSO of the month, the Ring Nebula (M57), is between Beta and Gamma Lyrae. The variability of Beta Lyrae was first discovered in 1784 by the deaf astronomer John Goodricke at York, the same year he discovered the variability of Delta Cephei.
What do we mean by a contact binary? Surely two stars cannot actually touch each other? Well, not exactly, but there is a sphere around a star called a Roche limit and if another star crosses the Roche limit, the two stars will start to transfer material from one to the other. The two stars rotating round their mutual centre of gravity are both B stars which is the second hottest type of star (after O stars). Such stars quickly burn through their nuclear fuel and are thus short-lived compared with a cooler star such as our Sun (fortunately for us). One of the B stars has already left the main sequence and become a giant star which easily loses the material in its outer layers to the main sequence B star which is its close neighbour. The material being transferred cannot simply go straight from one star to the other like a stone falling to the ground, but it swirls round the accreting star just like water going down the plug hole in your bath. It is this accretion disk which is blocking our view of the secondary giant star and lowers its apparent brightness. As it passes between us and the main star, it thus lowers the brightness of the star as we see it here on Earth because the orbital plane is almost edge on. In addition to this variability caused by the eclipsing binary (in a similar manner to Algol) there is a longer period variability of about 282 days caused by the accretion disk itself. So look up at Beta Lyrae and imagine all the material swirling away from a dying giant star to its greedy neighbour.