Propus

[Lady Isabella]

Hi

Does anyone observe the tricky double star eta Geminorum (Propus). This naked eye star is a semi-regular

M3 variable star that is on its way to becoming a Mira type variable star. Its brightness tends to fluctuate

between mag. 3.1 and 3.9 over a period of around 233 days. It has 2 companion stars, the first is around

7 Astronomical Units away and cannot be observed in telescopes. However it orbits Propus in a period of

8.2 years, in doing so, it produces an eclipse which causes the naked eye star to drop in brightness.

The second companion is a magnitude 8.8 star that is around the size of our sun. WDS (1990) gives its

P.A. as 258° and separation as 1.5 seconds of arc. Other sources give the separation as 1.4".

It has an orbital period in excess of 700 years.

Last night I was able to split the two with a 102mm f/8 refractor at 218x. Propus has a beautiful orange

colour in the eyepiece, while the fainter companion was observed sitting in the first diffraction ring.

Several sources list this star as a tough test for a 5 or 6 inch scope, due to the glare of the primary.

It is quite a tough one but well worth seeing.