MOTHALLAH (Alpha Trianguli). Triangulum: not a very exotic name for an ancient constellation. Its origin, however, is obvious, the figure a long, thin triangle between Aries and Andromeda. Our star, Mothallah, is the only named star within the constellation, the name from Arabic meaning simply "the triangle." In Latin it is referred to as "Caput Trianguli," the "head of the triangle," from its position at the sharp southwestern apex. That special location is probably why Bayer gave the star the Alpha designation, even though at dim third magnitude (3.41) it is 50% fainter than the Beta star at the northern apex. Most naked eye stars are much brighter than the Sun, the reason we see them. Modest stars like the Sun make little impact on the outlines of the constellations, Chara in Canes Venatici a notable exception. The huge numbers of stars much fainter than the Sun are hardly visible at all without a telescope. It is rather refreshing then to find one that, while certainly brighter than our Sun, is at least modestly similar. Residing in yellow-white class F (F6), Mothallah has a temperature of 6350 Kelvin, only 10% greater than the solar temperature. From its modest distance of 64 light years (which is why it appears even as bright as it does), we find Mothallah to shine with only 13 solar luminosities, and from that and temperature that it is only three times larger than the Sun. More advanced in relative age than the Sun, Mothallah is a 1.5 solar mass "subgiant," that is close to shutting down its core hydrogen fusion, if it has not done so already, as it prepares to be a giant star. Yet because stars of higher mass live shorter lives, Mothallah is only about 2.7 billion years old, as compared with the 4.5 billion year age of the Sun. Mothallah is observed to shift back and forth, telling us that it is accompanied by a small companion about which nothing is known. The system is odd because the orbital period of the companion around Mothallah proper is very short, a mere 1.74 days. (Even Mercury takes 88 days to go around the Sun). The separation between them must be very small, perhaps around 0.04 times the distance between the Earth and Sun. Given Mothallah's size, the companion is only around three million kilometers from Mothallah's surface. Seen from the companion, Mothallah would be an immense 45 degrees across. The closeness would probably not preclude planets, which could orbit at such a distance that any residents would see a "double sun." Mothallah is also a fine guide to the "Triangulum Galaxy," M 33, a dim patch of light that contains billions of stars that lies between Mothallah and Mirach in Andromeda. The galaxy, over two million light years away, can be seen with binoculars in a dark sky and under perfect circumstances even with the naked eye.
Written by Jim Kaler. Return to STARS.