MEROPE (23 Tauri). There is little in the sky more attractive than
the marvelous cluster of the Seven Sisters, the Pleiades of Taurus, a
bright compact, fairly young "open cluster" that is filled with
bright stars of blue class B. Most eyes see 6 stars, some 8 (or
even more). Seven of the stars are named for the sisters, who in
Greek mythology were the daughters of the god
Atlas and mortal
Pleione, who appear there as well.
Among the classic six visible
stars is fourth magnitude (4.18) Merope, which ranks fifth within
the striking group. Of the Pleiades, only Alcyone carries a Greek letter (Eta), the
rest designated only by Flamsteed
numbers (Merope otherwise called 23 Tauri). Merope, a mid-
class B (B6) subgiant lies, along with the rest of the cluster, at
a well-determined distance of 385 light years. From its hot, blue-
white 14,000 Kelvin surface, it shines with a total luminosity
(allowing for ultraviolet light) of 630 times that of the Sun. It is a particularly fast rotator,
spinning with an equatorial speed of at least 280 kilometers per
second, 140 times that of the Sun. Given that the star is 4.3
times the solar size, it makes a full rotation every 18 hours. The
fast rotation affects the star's surroundings and the spectrum.
Like others in the Pleiades crowd, Merope is an emission-line (Be,
class B emission) star, its rapid rotation flinging out a disk of
bright, emitting gas though Merope's is quite thin compared with
others such as Pleione. Nevertheless, it is sufficiently dense and
hot (from shock waves) to produce observable X-rays. Merope's
greatest claim to fame, however, is not the star itself, but its
surroundings. The Pleiades is enmeshed in a cloud of dusty gas.
The stars are not hot enough to make the gas glow (as they are to
make the Orion Nebula). Instead, the tiny dust grains embedded in
the cloud scatter and reflect the starlight to make the quite-blue
Pleiades reflection nebula. It is at its brightest around Merope,
so bright as to have special names, the Merope Nebula and "IC 349,"
the "IC" standing for Index Catalogue, a large addendum to the
standard "NGC" or" New General Catalogue" of celestial objects,
which in turn is an updating of the great Herschel "General
Catalogue." (The NGC and IC together contain over 12,000 clusters,
nebulae, and galaxies.) Long thought to be the remnant of the
Pleiades birth, the nebula is instead a chance occurrence, as the
cluster is merely passing through an interstellar cloud. (The
cluster actually leaves a wake as the cloud blows past.) The
bright class B stars of the Pleiades are all relatively massive,
Merope (which appears to be beginning to evolve) containing some
4.5 solar masses. Since higher mass stars evolve first, the
cluster and its stars must be relatively young, Merope and the
other Pleiads a mere 100 million years old.