There are many different kinds of stars, with the primary determining factors being a star's mass and age. The naked-eye star Phi Herculis (Phi Her), located near the left foot of the constellation of Hercules, is a perfect example of the exotic variations possible in stellar types. Phi Her is a hot "mercury-manganese" star. It has a surface temperature of about 12,000 kelvin, is orbited by a smaller, binary companion star, and boasts a whopping excess of the elements mercury and manganese -- about 100 times more of each than is seen in the Sun. Nuclear fusion powers stars, and systematically consumes hydrogen (and subsequently other elements) while leaving behind heavier elements like carbon, oxygen, nitrogen, and so on, including mercury and manganese. Astronomers trying to better understand our Sun look to exotic stars like Phi Her to test and refine their models of stellar processes.
It has been suspected for several decades that the extraordinary abundances of mercury and manganese seen in Phi Her are due to the internal stellar radiation field that selectively pushes these elements from the star's central furnace up into the atmosphere where they can be detected and their abundances measured. Mass is the single most important determinant of a star's nature, and this radiative process, too, depends on stellar mass. Unfortunately, the mass of Phi Her has been poorly known. SAO astronomer Guillermo Torres has now carefully re-examined a decade of data on Phi Her, and has for the first time been able to pin down its mass: 3.04 +- 0.24 solar-masses, which exactly overlaps the theoretical model predictions for the star's properties; the model also implies that Phi Her is only 210 million years old. Torres also has determined the mass of the companion star, 1.614 +- 0.066 solar-masses. The new results add another piece to the remarkable success story of explaining stellar properties from starlight.