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A Mira to admire.

One of the things I like about amateur astronomy is that so much is going on over everyone's heads that's so easy to see--for the few who know how. The constellations (at least the brightest ones) are always old friends. And with binoculars you can spot things from your back porch that are known only to a very tiny elite of the world's 7 billion people, most of whom never really look up.

Big Ophiuchus eternally holds his snake Serpens in the southern sky on June and July evenings (seen from northern latitudes). A diagonal row of four 2nd-and 3rd-magnitude stars marks his hands and the part of the snake between them (illustrated on pages 39-40). The lower left of these stars is Eta ([eta]) Ophiuchi, or Sabik, magnitude 2.4. And did you know what lies just 3/4[degrees] southwest of it?

The deep-orange, Mira-type star R Ophiuchi won't catch your eye in binoculars unless you're looking for it. Often it won't be there at all; it's a long-period variable that spends some of its time as faint as 13th magnitude. But every 10 months, it rises into binocular visibility for several weeks. One of those times is now. R Ophiuchi should have a maximum centered around June 20th, predicts the American Association of Variable Star Observers (aavso.org).

How bright it will become is not very predictable. In recent years R Oph has peaked as bright as magnitude 6.8 and as faint as 8.5, a factor-of-five visual brightness difference, with no apparent rhyme or reason.

Use the chart below to identify it and estimate its magnitude when you take binocs out to have a look around. The chart gives nearby comparison stars' magnitudes to the nearest tenth with the decimal points omitted.

Mira-type stars are pulsing red giants in a late stage of life. As they expand and contract they cool and heat, and this causes light-blocking molecules in their outer atmospheres to form and break.

And why do some stars pulse? The basic mechanism is simple. Deeper below the surface, a layer develops that becomes ionized as it heats, which turns the layer more opaque. This causes it to bottle in the heat coming from below, which drives the star to expand, which cools the critical layer, which loses its ionization and becomes transparent again, letting the heat out. Think of a flapping lid on a pot of boiling water.

This process is simple in orderly, highly regular pulsating stars such as Cepheids and RR Lyraes. Matters are more complicated in the giant, cool Miras. They have those atmospheric molecules. Their surface gravity is weak, so they can become irregular blobs rather than clean spheres. They cool enough at minimum to shift almost all of their visible light into the infrared. And they may throw off smoky dust.

Take a look this evening and make a faithful, if odd and sometimes elusive, new friend that you can keep for life.

JULY METEORS

Several minor, long-lasting meteor showers with radiants in the southern sky are active during July, including the Alpha Capricornids, Piscis Austrinids, and Northern and Southern Delta Aquarids. All are weak, but together they increase the chance that a meteor you see late on a July night will be coming out of the south.

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Title Annotation:Celestial Calendar
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jul 1, 2013
Words:551
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