NU HER (Nu Herculis). The standard wisdom is that in his Uranometria of 1603, Bayer gave out Greek letters in order of decreasing brightness. However, he also used position, Hercules providing one of many examples. To the east of Delta Herculis are Lambda, Mu, Nu, Xi, and Omicron Her, fourth magnitude Nu and Lambda tied for faintest at 4.41, following which we bounce back to Pi Her in the Keystone at 3.16. Of much more interest, Nu Her is at the northwest corner of a small box formed also by Xi (southwest corner), Omicron (southeast), and 99 Her(northeast)and that contains the "Apex of the Sun's Way," the point near the Hercules-Lyra border toward which the Sun appears to be moving at some 20 kilometers per second amongst the brighter local stars (the exact position depending on your definition of "local"). The Antapex, from which the Sun is fleeing, is in Columba, south of Lepus, 3.5 degrees due east of Zeta Canis Majoris. Reflecting solar motion, Nu Her has only a small annual angular displacement across the sky (its "proper motion"), but is coming at us (or we going toward it) at 22 kilometers per second as found from the Doppler effect on its spectrum.

Aside from location, Nu Her quite nicely stands on its own as a rare class F (F2) yellow "bright giant," the only other of which amongst these stories is Pi Sagittarii. After adjusting for 0.13 magnitudes of dimming by interstellar dust and allowing for some light from a companion (see below), from a distance of 861 light years (known only to within about 90 light years) and with a temperature of 6620 Kelvin, Nu Her shines with the light of 1070 Suns (somewhat less than does Pi Sagittarii), which leads to a radius of 25 times solar. Adopting an old value of 28 kilometers per second as an equatorial rotation velocity, the star must turn once in under 45 days. Theory gives a hefty mass of 5.0 times that of the Sun. Nu Her is a star in transition. With a shrinking helium core, it's swelling and cooling at the surface as it turns itself into a red giant, whereupon it will fire up its helium to fuse to carbon and oxygen. As it evolves, it will for a time turn into a pulsating Cepheid variable, such stars beloved of astronomers since they give a way (through the period-luminosity relation) of finding distance. Even now, for other reasons Nu Her seems to jitter by a few hundredths of a magnitude. The star has a reputation of being rich in manganese, but low (60 percent solar) in iron, but nobody really seems to know. Half a second of arc away is a seventh magnitude (nominally 7.49) companion that, if really bound to Nu proper, would be a class A0 dwarf with a mass of around 2.5 times solar. With a mean separation of at least 130 AU, it would take more than 550 years to make a complete orbit of the yellow giant toward which we are moving (and that will have left by the time we get there). Not massive enough to blow up, Nu Her will eventually lose its outer layers and die as a white dwarf of about 0.85 solar masses.

Written byJim Kaler 8/29/14. Return to STARS.