September 30, 2014

The Scarlet Hues of TX Piscium.

U Camelopardalis, a carbon star with the same fate as TX Piscium.

(Credit: ESA/NASA/Hubble).

Oh, pretty! Is a frequent exclamation surrounding that rarity of celestial beasts, the carbon star. Fans of this space will recall our exploits tracking down such favorites as Hind’s Crimson Star, UU Aurigae, and V Hydrae. These ruddy stars come as a welcome surprise in the often monotone universe and can serve as a star party “secret weapon” when every other ‘scope is pointed at Albireo. This week, we’ll look at just such a treat that is well placed for fall viewing for the northern hemisphere. And October is an ideal time to look for it, as Mercury, Mars, and Saturn huddle low in the dusk, Jupiter hasn’t yet reached its evening prime time, and Venus remains high in the dawn.

TX Piscium is where it’s at, the easternmost star in the asterism known as The Circlet of Pisces. Just above naked eye visibility from a good dark sky site at magnitude +5, TX Piscium (sometimes referred to as 19 Pisces) is an irregular variable fluctuating between magnitude +4.9 and +5.5 & is one of the brightest carbon stars in the sky. At a distance of 760 light years, this old C-class giant is also huge; its estimated to have a diameter of 240 times that of our own Sun. In fact, the orbit of the Earth would fit comfortably inside its radius! With a B-V Index of +2.6, TX Piscium is also one of the few carbon stars easily discerned with binoculars, and while not as red as Hind’s or R Leporis (both at B-V +1.8), its ruddy overtones are red-dilly (bad pun) apparent.

TX Piscium: a finder chart. (Created by the Author in Starry Night & Paint).

The position of TX Piscium is;

Right Ascension:23 Hours 46’ 24”

Declination:  +3° 29’ 13”

TX Piscium transits the local meridian at around midnite in early October and is well placed for early evening viewing by the end of the month. Burnhams’ lists TX as “One of the few N0 –type stars within naked eye visibility,” and general stellar classification for the star for the astro-physically inclined runs from a C62 to a C53. Burnhams also mentions that the nearby +6 magnitude stars of 21 & 25 Piscium make an excellent “offset” to compare with TX Pisciums’ splendor. Move TX into absolute magnitude range at around 10 parsecs (32.6 light years) distant, and it would be the brightest star in our sky at magnitude -2!

Carbon stars appear red because the light that they emit is shining through a layer of CH4 (methane) CN (cyanogen, including that favorite comet hysteria-inducing compound, cyanide) and plain old molecular carbon that have accumulated in its outer shell.

And of course, it’s sobering to think that such is a similar red giant fate to many stars such as our own Sun; eventually our star will ‘puff out” in a short lived (at least on astronomical time scales) effort to squeeze energy by fusing heavier elements on the periodic table. Hey, thank goodness for all that “carbon-soot” precipitating into the atmosphere of stars like TX Piscium; their death will shed much of that life-giving material throughout cosmos!

Comments

  1. Ed Kotapish says:

    Dave,

    This was the last of my “Red Star” list I could observe before the clouds rolled in and killed te rest of the weekend. Itis part of the Circlet in Pisces (The Western Fish – very easy to locate under Pegasus) and very reddish orange. V Aquilae is another neat red star to check out!

    Ed

  2. David Dickinson says:

    Hi Ed,
    Thanks; V Hydrae & Hinds Crimson star are two of my faves. Carbon stars are a cool and unique thing to show folks at star parties… Hinds Crimson Star is one of my wife’s faves!

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