Gamma1 Sagittarii, Gamma2 Sagittarii - Alnasl, Nash

The designation Gamma Sagittarii is shared by two star systems in the constellation Sagittarius, γ1 Sgr and γ2 Sgr. They are separated by 0.86° in the sky, and can be resolved by the human eye.

Alnasl

The brighter γ2 Sgr has an apparent magnitude of +2.98, and though given the Gamma designation by Bayer, it ranks only seventh in brightness in the constellation. At the tip of Sagittarius's arrow, it has the traditional names Alnasl, Nasl, El Nasl, and Nash, which are all derived from the Arabic "an-nasl", meaning "arrowhead". The lesser-known name Nushaba comes from a longer phrase that also refers to the point of an arrow.

Physically, Alnasl is an orange class K0 III giant. From a distance of 96 light years, it shines with a luminosity of 64 suns from its 4800 K surface. The star's radius is 12 times solar, its mass about double solar; having given up hydrogen fusion, it is now dying and using up its internal helium. There is also some spectroscopic evidence for a close binary companion about which nothing is known.

Alnasl also serves as a director toward the Galactic center. Follow Sagittarius's arrow to the west by a distance about equal to its length (between Alnasl and Kaus Media, at the center of the bow); then look north a degree and a half. The Galactic center, buried in thick interstellar dust so that it is not visible optically, is almost universally believed to be a massive black hole.

W Sagittarii

Less than a degree to the north of Alnasl is the magnitude 4.66 γ1 Sgr. A Cepheid variable, γ1 is also known by its variable star designation, W Sagittarii. This mid-temperature (5600 K) star regularly varies between magnitudes 4.3 and 5.1 over a 7.59 day period. Its change in brightness is accompanied by a change in spectral class, from G1 to F4. Cepheid variables are massive stars that are unstable and pulsate as a result of advanced age. γ2 Sgr contains some 7 solar masses, and is now probably fusing helium in its core. Direct parallax gives a highly uncertain distance of 2100 light years; the strict Cepheid period-luminosity relationship gives about 1500 light years, and is probably closer to the truth. If so, the star pumps around 2500 solar luminosities into space and is 50 times the size of the Sun.

W Sagittarii may also have up to three companions, one with a period of only 4.9 years (detected spectroscopically), another with a period closer to 100 years (found by sophisticated imaging), and a fainter, more distant one 48" away (seen visually). Nothing is known about any of them.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]