MEGREZ (Delta Ursae Majoris). The faintest star of the Big Dipper, Megrez is in the Dipper's middle, linking the handle to the bowl, and in the bigger picture linking Ursa Major's tail to the Bear's hindquarters. The name appropriately refers not to the Dipper, but to the Bear, and straightforwardly comes from a long Arabic phrase that means the root of the Great Bear's tail. (The Greek letters were assigned from west to east, not in order of brightness.) Megrez is also a straightforward star. Shining toward the fainter end of third magnitude (3.31), this rather ordinary class A (A3) hydrogen-fusing dwarf glows whitely with a temperature of 8630 Kelvin, about half again hotter than our Sun. From its distance of 81 light years, we find a luminosity of 23 solar, from which we calculate a radius double that of the Sun. Like the middle stars of the Dipper, Megrez is part of the sprawling Ursa Major Cluster, all five moving through space together at about the same distance, Megrez just a half light year farther than the average of the five. From Megrez, the Dipper's handle stars Alioth and Mizar (as well as Alcor) would be almost aligned, Alioth appearing almost as bright as Venus in our sky. Yet for all its seeming ordinariness, Megrez still tells a tale of how sensitive the stars are to surface temperature and mass. Though Megrez carries only twice the mass of the Sun, it is over 20 times more luminous, the result of higher internal gravitational compression and temperature. Comparison to the Dipper's brightest star, Alioth, tells a similar story. Though Alioth has only half again the mass of Megrez, it radiates almost five times the luminosity. The comparisons vividly show the effect of the "mass-luminosity" relation that is derived from the masses of double stars. In the middle of the stellar temperature range, the luminosity is sensitive to greater than the third power of the mass. Though Megrez has exactly the same spectral class as Denebola, it is 60 percent more luminous, the result of slightly higher mass and greater age (age slowly brightening these stars). Actual age measurement agrees, by showing Megrez (and the other central Dipper stars) to be about 50 million years old (roughly half way through its hydrogen-fusing lifetime), whereas Denebola is around 20 million years old. Megrez has been searched for a surrounding dusty disk of the kind found around several stars of its class (from which planets might presumably have formed), including Denebola and the Dipper's Merak, but nothing has been found. Megrez's only claim to any sort of distinction is that it has been accused of having faded over the centuries, as Tycho Brahe called it second magnitude. Several stars are steeped in such lore, which is most likely false, as the evidence is so thin.
Written by Jim Kaler. Return to STARS.