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Sierra NightSky

"Let your soul stand cool and composed before a million universes" Walt Whitman

Sierra NightSky for the period starting October 17, 2014 by Jim Kaler

Coming off third quarter last Wednesday, October 15, the Moon fades during most of the first week of our fortnight in the waning crescent phase, passing new Moon on Thursday, October 23, when it will cross in front of the Sun to produce a partial solar eclipse visible throughout nearly all of North America. (The dark part of the lunar shadow completely misses Earth, so there is no total eclipse anywhere).

The western part of the continent will see the whole event beginning in late afternoon, while those in the east will see it in and around sunset. The times depend strongly on location: check local sources.

Be sure to use proper eye protection, as the Sun in any stage of partial eclipse is much too bright to look at directly. Use a professionally made filter only. A safe way to view the eclipse is by projection, in which sunlight falls through a pinhole in a piece of cardboard onto a piece of paper or even the sidewalk. At maximum about half the Sun will be covered.

The waxing crescent will become barely visible in western twilight the evening of Friday the 24th as the Moon heads toward first quarter on Friday the 30th in late evening in North America, allowing the near-perfect phase to be seen. The Moon passes apogee , where it is farthest from Earth, on Friday the 17th.

On the morning of Saturday the 18th, the waning crescent will shine just below Jupiter with the star Regulus to the left. The following morning sees Jupiter below the star.

After the Moon flips to the other side of the sky, the waxing crescent will make a nice pairing with Mars (the planet to the left) the evening of Monday the 27th, while the following evening it will have moved to a position above and a bit to the left of the red planet.

Jupiter, rising in western Leo half an hour past local midnight (1 AM Daylight) at the beginning of our two-week period, shortly before local midnight by the end of it, dominates the planetary sky. But Mars still glows redly in southwestern evening skies, setting around 9 PM Daylight.

Between the classical figures of Scorpius and Sagittarius, Mars' westerly motion toward the latter can easily be followed. In the second week of the fortnight, Mercury makes a nice appearance in eastern morning twilight, the planet passing greatest western elongation on November 1.

Venus, second from the Sun, goes through superior conjunction with the Sun on Saturday the 25th. While becoming an evening planet, it will not be readily visible until early next year.

Watch for the Orionid meteor shower, which peaks the mornings of October 21 and 22. The leavings of Comet Halley, the shower (which appears to come from the direction of Orion) produces 20 or so meteors per hour.

The summer stars slip off to the west to be replaced by those of autumn. To the east and a bit north of the Little Milk Dipper of Sagittarius is faint Capricornus, which looks like an old-fashioned upside-down cocked hat. The two make a triangle with Aquila (the Eagle) and the star Altair above them. Altair is quickly recognizable for its two flanking stars, Alshain (to the southwest) and Tarazed. Well to the east, Fomalhaut, in Piscis Austrinus (the Southern Fish), begins to sail across far southern skies as seen from the temperate north.


Aquila is known for many things, among them a beautiful part of the Milky Way and the first-known Cepheid variable, Eta Aquilae, discovered by Edward Pigott in 1786.

If things were fair, Cepheids (class F and G supergiants that typically vary by a magnitude or so over periods of several days) should be called Aquilids instead of being named after the second to be found, Delta Cephei (by John Goodricke shortly thereafter). But things aren't fair and the variables remain "Cepheids," which are critically important to the establishment of the cosmic distance ladder and the structure of the Universe.

A Cepheid's variation period is strongly linked to its luminosity, or absolute magnitude, the magnitude a star would have at a standard distance of 32.6 light years. Comparison of absolute and apparent magnitudes then gives the precious distance of the Cepheid and of the assembly to which it belongs. The relation was discovered in 1912 at Harvard by Henrietta Leavitt.

While fourth magnitude Eta Aql, eight degrees south of Altair, dominates Aquila's Cepheid scene, it's not the only one visible to the naked eye. In the far northwestern corner of the constellation lies the fifth magnitude (averaging 5.38) class F (nominally F8) supergiant FF Aquilae, known best by its two-letter variable-star name.

The star's a bit of a curiosity as it varies by only a few tenths of a magnitude (5.20 to 5.55, the class going from F5 to G0) over a period of 4.471 days (increasing at a rate of 0.07 seconds per year), the size of the variation notably less than usual. While most Cepheids have light curves (plots of magnitude vs. time) characterized by a rapid rise in brightness followed by a longer fall, FF's curve is more sinusoidal, making it into an "s-Cepheid" that does not fit the usual period-luminosity relation.

One of the serious problems in the use of Cepheids to get distances, or to calibrate the period-luminosity relation using Cepheids of known distance, is the contamination of their light from companions or stars in the line of sight.

FF Aql has three of them, a visual ninth magnitude neighbor at a separation of 6.8 seconds of arc, a sixth magnitude companion at around 0.2 seconds, and a spectroscopic one (ninth magnitude?) with a period of 3.93 years.

After removing their light and correcting for 0.73 magnitudes of dimming by interstellar dust, FF Aql's visual magnitude is actually 4.78. Two disparate distances, 1545 light years from Hipparcos satellite parallax and 1160 from Hubble's Fine Guidance Sensor, give the adopted distance of 1355 light years.

A temperature of 6195 Kelvin indicates little correction for infrared or ultraviolet. The star's mean luminosity then sits right at 1600 Suns, which together with temperature yields a radius of 35 times solar.

A direct measure of angular radius shows 39 times solar, not a bad match given the uncertainty in distance and the variable radius of the pulsating star. Application of theory yields a mass of six Suns. The removal of the contamination reveals the asymmetry of a classical, non-s-Cepheid.

The standard period luminosity relation is then applicable and yields an absolute magnitude of -3.25, which with the corrected apparent visual magnitude shows the star to be 1315 light years away, very close to the above average and a highly satisfactory result. Meanwhile, back at the system, the three companions from inside out should be close to dwarf classes M2, K2, and M1 with masses of 0.8 solar for the K dwarf and 0.5 for the M stars.

We might expect a separation of 5 AU from FF proper for the spectroscopic companion, at least 83 AU and a period of 280 years for the next one, and at least 2800 AU and 120,000 years for the visual neighbor. This star is relatively nearby. Just think of the contamination problems facing the astronomer trying to get a distance from a Cepheid in another, far more distant, galaxy.

(Thanks to D. G. Turner et al. for discussion of FF Aql in a paper that appeared in Astrophysical Journal Letters, 772:L10, 2013.)

Do you have a favorite star or one you would like to see highlighted on the Star of the Week? Send a suggestion to Jim Kaler

Sierra NightSky thanks to Jim Kaler.
Check out his site for more




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