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AL NASL

(Gamma Sagittarii). Al Nasl, sometimes called Alnasl or El Nasl, gives us two stars for the price of one. The bright one you readily see makes the tip of Sagittarius's arrow, the name coming directly from the Arabic phrase that means "the point." It is one of several stars that also goes by two names, the lesser-known one "Nushaba," which comes from a longer phrase that also refers to the point of an arrow. Alnasl almost exactly defines mid-third magnitude (2.99), and though given the Gamma designation by Bayer, ranks only seventh in a constellation in which the Greek letters are wildly out of their expected brightness order. Physically, Alnasl is an almost-too-ordinary orange class K (K0) giant, of the kind that populates so much of the naked-eye sky. Having given up hydrogen fusion, it is now dying and using up its internal helium. From a distance of 121 light years, it shines to us with a luminosity 68 times that of the Sun from its 4800 Kelvin surface, the star's radius a dozen times solar, its mass about twice solar. There is also some spectroscopic evidence for a close binary companion about which nothing is known. More important is Alnasl's service as a director elsewhere. First, follow Sagittarius's arrow to the west by a distance about equal to its length (between Kaus Media at the center of the Bow and Alnasl), and then look up a degree and a half to locate the center of our Galaxy (almost as if the ancients deliberately constructed the constellation to do so). The Galactic center, so buried in thick interstellar dust that it is not visible optically, is almost universally believed to be a massive black hole. Second, the "Gamma" designation refers to TWO stars. Alnasl itself, the eastern one, is actually Gamma-2 Sagittarii. Less than a degree to the north is fifth magnitude (4.7) Gamma-1, which is much the more interesting star. A Cepheid variable (like Mekbuda, Zeta Geminorum), Gamma-2 is also known by its variable star name, W Sagittarii (indicating it to be the 6th variable found in the constellation, the names beginning with "R"). The mid-temperature (about 5600 Kelvin) star regularly and obviously varies between magnitudes 4.3 and 5.1 over a 7.59 day period. Cepheid variables are massive stars (Gamma-2 Sagittarii having some 7 solar masses) that are unstable and pulsate as a result of advanced age (the dying star now probably fusing helium in its core). The distance is great (showing that W (Gamma-1) and Gamma-2 have nothing to do with each other). Direct parallax gives a highly uncertain 2100 light years. However, a strict relation between luminosity and pulsation period also allows a distance determination (used to find the distances of galaxies). The distance of Gamma-2 so found is notably less, about 1500 light years, and is probably closer to the truth. If so, the star, 50 times the size of the Sun, pumps around 2500 solar luminosities into space. W Sagittarii may also have up to three companions, one with a period of only 4.9 years (detected with the spectrograph), another with a period closer to 100 years (found by sophisticated imaging), and a more distant faint one 48 seconds of arc away (as seen from Earth), which would take a million years to make the journey. Nothing is known about any of them.