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.