ALNASL (Gamma-2 and Gamma-1=W Sagittarii).
Alnasl, sometimes called Al Nasl 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 96
light years, it shines to us with a luminosity 64 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-1 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.