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Organization:Alexa Crawls

Starting in 1996,Alexa Internet has been donating their crawl data to the Internet Archive. Flowing in every day, these data are added to theWayback Machine after an embargo period.

Collection:35_crawl

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TIMESTAMPS

The Wayback Machine - https://web.archive.org/web/20061216203451/http://www.astro.uiuc.edu:80/%7Ekaler/sow/phiand.html

PHI AND (Phi Andromedae). At the eastern end of the northernstring of stars that makesAndromeda, fourth magnitude (4.25) Phi Andromedaeshines modestly from a large distance of 735 light years, showingthat it is luminous indeed. Not once, but twice, as the star is atightdouble with aseparation of only half or so of a second of arc, making it a verydifficult target to resolve visually. Taken as one, the starappears as a class B (B7) dwarf.

Phi Andromedae sits in the middle of this seven-degree-wide portionof northeastern Andromeda. The four brighter stars to the leftare, from top to bottom, 51, 49, Omega, and Xi Andromedae, (Xi theone on the right). The trio of stars to the right of Phi are inCassiopeia, the top one Omicron Cas, thelower one Pi. Ahigher resolution view reaches to mid-tenthmagnitude.

Separately, the brighter, Phi And A, seems to be a fifth magnitude(4.54) B6 subgiant, while the fainter, Phi And B, is a sixthmagnitude (5.55) B9 dwarf (the magnitudes uncertain and scaled tofit to the measure of the combined stars). There are notemperature measurements as the stars are too close for readyobservation. Assuming 14,000 for B6 Phi And A, correcting for 0.2magnitudes of interstellar dust extinction as well as for a lot ofultraviolet light, gives a luminosity of 1980 times that of theSun, a radius 7.6 solar, a mass of about 5 1/2solar, and confirmation that the star is indeed a subgiant and isabout to give up core hydrogen fusion. An assumed temperature forPhi And B, with the same kinds of corrections, gives 350 solarluminosities, 5.1 solar radii, 3.7 solar masses, and confirmationof dwarfhood. As best we can tell, given the difficulty ofmeasure, the pair orbit each other with a fairly long period of 372years at an average separation of 101 Astronomical Units, with aneccentricity that takes them between 140 and 80 AU. They wereclosest together (periastron) in 1886 and will be farthest apart(apastron) in 2075 (a long wait). Application of Kepler's lawsyields a total mass of 7.5 solar, short of the sum of 9.2 obtainedfrom stellar structure theory. Given that only a partial orbit isavailable (we have yet to see them make a full orbit), the orbitalparameters surely contain errors, so the difference is notsurprising. In fact it is satisfying that the two determinationscome that close. Phi And A -- likeGammaCas and Zeta Tauri -- is a "B-emission" ("Be") type star, one with a circulating circumstellardisk that radiates emission lines (the origins of which are not atall clear). Be stars are fast rotators, in the range of 200-300kilometers per second (which clearly has something to do with thephenomenon). Phi And A's observed rotation speed is relativelylow, however, only 81 kilometers per second, implying that therotation axis is rather well tilted toward us (such speeds beingprojections against the sky and therefore lower limits). Such atilt is consistent with the nature of the disk, which does not seemto superimpose itself against the star (that is, Phi And A is nota "shell star" likeAlpha Arae,Eta Centauri, and several others that dotthe sky).

Written byJim Kaler. Return toSTARS.

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