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.