The night sky Including a sky tour for double andvariable star observers

摘要

Capella itself will not transit until after 3am andDecember, not October, is its best month, but from the UKit is actually visible all year round because, at +46 degrees,it never drops below seven degrees altitude from London.By midnight it is already 55 degrees above the easternhorizon. Capella shines at magnitude +0.1 making it thethird brightest star in the northern celestial sphere. It is nota 60,000 solar luminosity cosmic lighthouse like Deneb,but simply a very bright nearby system just 42 light yearsdistant. Capella's light comes mainly from two giant stars,roughly ten times the diameter of our own Sun and a 'mere'80 times its luminosity. Their separation of only 100 millionkilometres means the two stars are only a twentieth of anarcsecond apart as seen from Earth and so not resolvablein amateur telescopes. The stars do not eclipse from ourviewing point, but simply orbit each other every 104 days.Capella will be a prominent marker in the skies for manymonths to come.