STARS OF THE WEEK: EU and U DEL (EU and U Delphini.) The Roman letters
tell us immediately that we are dealing with variable stars, the naming system odd and thus readily
recognizable. The(usually) sixth magnitude (6.25 and 6.38, almost
seventh) stars are separated by only a couple degrees along an
east-west line roughly 20 and 3/4 degrees north of the celestial equator in the ancient
constellation Delphinus (the Dolphin) in
the heart of the Milky Way. Oddly, both are
semi-regular "SRb" semiregular red giant class M variables
(respectively M6 and M5) with periods of 60 and 110 days and listed
apparent magnitudes of 6.25 and 6.38. There seems to be no or little
interstellar dimming, so we'll ignore it. With temperatures of 3300
and 3330 Kelvin and distances of 380 and 1570 light years (give or
take 22 and 540), the luminosities are 600 and 8200 Suns. (For unknown reasons, the literature
gives a luminosity for EU Del that is almost three times as high.)
On the visual magnitude scale, in spite of its listed magnitude, EU
Del flips between magnitudes 5.84 and 6.9, while U Delphini goes
between 5.8 and 6.9 (the latter on a photographic scale, which makes
red stars look fainter). EU's radius is calculated to be only 15 times
that if the Sun, but the luminosity is suspect. The radius of EU is
calculated at 420 solar radii, while interferometer measures give
275 in the infrared part of
the spectrum). In spite of their generic name, SRb's are quite regular
(whereas the semiregulars among supergiants (the SRc stars) are not).
SRb's are thought to be stars in beginning stages of the collapse
of the carbon core, which has now run out of helium fuel, and as they
age and brighten will eventually become Long Period (Mira) variables. U is moving along at a
marginally rapid 37 kilometers per second relative to the Sun. EU,
however it is going much faster, at 77 km/s, which may be related
to a high metallicity, the star coming from a different part of the
Galaxy. The most interesting
part of the story is that the variability periods are is 50.5 days
for EU and 110 days for U, which bracket a seeming 60-day period,
longer than which the stellar wind and mass loss increase as the stars
eventually lose all their outer envelopes, the carbon/oxygen cores
becoming white dwarfs.
Most of the dust in the Milky Way comes from stars like these and
their Long-Period" successors as they whittle themselves away, U Del
losing mass at a rate of a million times that of the Sun In the solar
wind).
Written byJim Kaler 8/11/17. Return to STARS.