Sierra NightSky

Sierra NightSky for the period starting January 27, 2017 by Jim Kaler.

The next Skylights will appear Friday, February 10.

  • The Moon
    • Having just passed new, during the next two weeks the Moon runs through its waxing phases.
    • New Moon, Friday, January 27.
    • First quarter Moon, Friday, February 3, around midnight with the Moon high in the sky
    • Full Moon, Friday, February 10. Penumbral eclipse will slightly dim northern limb of Moon; visible only in far NE US and Canada. Mid eclipse 6:43 PM CST.
    • Moon at perigee, closest to Earth
    • Spectacular grouping of Mars, Venus, and waxing crescent, Tuesday, Jan. 31, enhanced by earthlight on the Moon
    • Moon makes fine triangle with Pleiades to north and Hyades to east, Wed., Feb. 4. Moon east of Aldebaran the following night.
  • Planets
    • Venus, visible in bright evening twilight, dominates southwestern skies.
    • Just to the east of Venus, Mars sets about 9:30 PM.
    • Saturn rises about 4AM, while much brighter Jupiter transits the meridian a half hour later.
    • Jupiter, just above Spica in Virgo, begins retrograde (westerly), motion against the stars
  • Sun and Earth
    • Groundhog Day, Thurs.,Feb 2;Sun midway from winter solstice to vernal equinox.

The Sky

Orion, with his three-star belt dominates the stellar sky, Betelgeuse to the upper left, Rigel to the lower right. Just up and to the right of Rigel, is Cursa, Beta Eridani, which begins Eridanus the River. The River then meanders off to the south and west, ending in bright Achernar. Sirius, the sky’s brightest star, lies southeast of Orion. If you live in the far southern US, you might spot Canopus, the second brightest. Then see the Big Dipper rise in the northeast as the stars of the Andromeda myth fall to the northeast.

STAR OF THE WEEK: RHO AND (Rho Andromedae)

If you look about 5 degrees southwest of the Andromeda Galaxy (Andromeda‘s greatest feature), roughly between it and the Great Square of Pegasus, you’ll find a small triangle of fifth magnitude stars a couple degrees across. At magnitude 5.18 the faintest of the three, Rho Andromedae is also the most easterly, while brighter Theta is at the northern apex and Sigma is at the southern. Classed as an F3 giant, the first impression would be that it is well along in its evolutionary track on the HR diagram (a plot of brightness vs, temperature, wherein absolute magnitude and spectral class are used as proxies).

Andromeda is rather well off the Milky Way, so dimming by interstellar dust is not much of a problem and we’ll ignore it for now. Rho’s temperature is rather well-defined by a number of observations at around 6720 Kelvin, so most of the light is in the visual spectrum and we need make little correction for either infrared or ultraviolet light.

At a distance of 158 light years (give or take 2 and in between Theta and Sigma), the star shines with the light of 16.6 Suns, which with temperature gives it a radius of only 3.0 times that of the Sun, not much for a so-called “giant.” The projected equatorial rotation velocity of 42 kilometers per second gives a rotation period of less than 3.6 days. In spite of the relatively small size, the angular diameter has been measured by interferometry at 0.600 thousandths of a second of arc to one percent precision. Given the distance, the star then has a radius of 3.13 times that of the Sun, just four percent higher than that calculated from temperature and luminosity, not a bad fit at all. A little juggling of parameters cold bring the two into exact agreement. While Rho And’s spectrum may be that of an F3 giant, the theory of stellar structure and evolution shows that the star is really a 1.75 (Or a bit higher) solar mass subgiant that has just taken on the role and is more a dwarf that has just given up core hydrogen fusion and is some 1.8 billion years old.

The expected color of a dwarf is just what we see, so there indeed appears to be no need for correction by interstellar dimming, not that it makes too much difference. Rho And appears to be all alone with no companion to witness its expansion to a real red giant, which will happen before long, though the pace of evolution is so slow (barring explosions) that we will not see it.

Nowhere nearly massive enough to blow up as a supernova (at least 8 or 9 Suns is required), Rho And will slough off its outer envelope, maybe produce an ephemeral planetary nebula, and die as a white dwarf of about 0.6 solar masses. The star teaches a nice lesson in that there is not a one-to-one relation between spectral class and actual evolutionary class, the dichotomies more noticeable among the class B stars.


Sierra NightSky thanks to Jim Kaler. Read Dust to Dust in Stellar Stories.
Check out his site for more.