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The Sky This Week, 2019 October 8 - 15

Time for the Hunter's Moon
Epsilon Lyrae, the 'Double-Double' star, imaged 2015 November 16 from the U.S. Naval Observatory, Washington, DC.
Epsilon Lyrae, the 'Double-Double' star
imaged with the USNO's 30.5-cm (12-inch) f/15 Clark/Saegüller refractor
and a Canon EOS Rebel T2i DSLR.

The Moon waxes as she climbs northward along the ecliptic, washing out the feeble light of autumn’s constellations.  Full Moon falls on the 13th at 5:08 pm  Eastern Daylight Time.  October’s full Moon is almost “universally” known as the Hunter’s Moon, and its lore is closely tied to September’s Harvest Moon.  Luna’s orbital geometry makes her appear to rise at relatively closely spaced intervals as she approaches the full phase for northern hemisphere observers, and the farther north you go the more this effect occurs.  Just as the Harvest Moon seems to provide a little extra light for farmers to bring in their crops, the Hunter’s Moon was thought to aid hunters as they pursued game across the stubble of the harvested fields.  Here in Washington successive moonrises occur less than half an hour apart from the evenings of the 10th through the 14th.  Typically the interval is closer to an hour.

While the Moon’s bright light washes out much of the autumnal constellations, we can still enjoy the view of the summer’s bright stars and constellations for much of the evening hours.  As evening twilight fades the three stars of the Summer Triangle asterism are almost directly overhead.  The Triangle’s three bright stars, Vega, Deneb, and Altair, beam down from the bright sky and help you to find some bright stellar targets that you can enjoy with a modest telescope.  Near the center of the Triangle you should be able to make out a third-magnitude star, Albireo, that marks the “head” of Cygnus, the Swan.  Point almost any small telescope at Albireo and you will be treated to one of the sky’s true delights.  Instead of the single star that you see with the naked eye, the telescope presents two bright luminaries with a distinctive color contrast.  The brighter of the two stars shines with a golden yellow hue, while the fainter companion sports a distinctive blue tint.  This color contrast is especially pronounced in telescopes of small aperture; I find that larger apertures tend to wash out the color contrast, so I tend to look at Albireo with my 3-inch refractor telescope for the best view.  The color contrast tells us something about the individual stars themselves.  The yellow component is an evolved star that is fusing hydrogen in an expanding shell around a growing helium core.  It is similar in many ways to Arcturus, but it is over 100 times farther away from us than the bright springtime star.  Albireo’s blue component is a relatively young star that is about 30 light-years closer to us than its brighter cohort.  It is considered to be a “normal” star that is about four times as massive as the Sun.  This means that it is fusing hydrogen in its core at a much faster rate than our star, resulting in a much hotter surface temperature and hence its blue tint.  Albireo is an example of a chance line-of-sight alignment of the two component stars and thus not a physical binary pair.  

We find a true binary system just northeast of the bright star Vega in the Summer Triangle.  Binoculars will show two closely-spaced stars basking in Vega’s glow.  Point a modest telescope at this pair and you will find that each component is itself a close double star.  Each of these close pairs represent a physical binary star system, orbiting each other with periods of several hundred to about 1000 years.  These two binary stars form the pair that you see in binoculars, and since they share a common distance and motion through space they are considered to be physically associated, thus making Epsilon Lyrae a quadruple star system!  If you could stand on the surface of a hypothetical planet orbiting one of the close pairs the sky would be radically different from anything we can imagine here on Earth!

You can still find the bright glow of Jupiter in the southwestern sky as twilight fades into darkness.  The giant planet now sets well before 10:00 pm, so you have limited time to view him.  By the end of evening twilight Old Jove is just over 15 degrees above the horizon, so your chances of getting a good view of him without the effects of turbulence in the atmosphere are pretty low.

Saturn offers a more pleasing target for the telescope in the early evening.  By the end of twilight the ringed planet is just west of the meridian, and he is high enough to still get a decent view under typical atmospheric conditions.  Saturn’s family of moons just grew by leaps and bounds this week with the announcement of the discovery of 20 more objects circling the distant planet.  That brings the total number of Saturnian moons to 82, surpassing Jupiter’s 79 known moons.  That said, only seven of Saturn’s moons are visible through large amateur telescopes.

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