A FAINT ENVELOPE AROUND THE RING NEBULA IN LYRA

Abstract

On a photograph of the ring nebula in Lyra (NGC 6720) which I made with the 100-inch Hooker telescope on the night of August 6, 1935, there appears around the famous ring a faint envelope which seems not to have been recorded before. The aperture used was 84 inches, the plate a backed Imperial Eclipse, and the exposure time 30 minutes. The seeing was unusually good and the sky very clear, but there was scattered light from the 7-day-old Moon. The existence and appearance of the faint envelope are confirmed by a second photograph, obtained on the night of August 27 with the full 100-inch aperture, a similar plate, and an exposure of 58 minutes. The sky on this night was clear and moonless, with fog covering the valley lights, but the seeing was poor. The first photograph, enlarged, and intensified by successive copying on slow plates, is reproduced in Plate XVI. In order to show the faint envelope, the bright ring is of course so overexposed as to lose most of its familiar appearance. The fine little spiral in the lower left (southeast) corner has been shown on a number of earlier photographs. The newly discovered envelope appears rather sharply defined and roughly circular in form, with a diameter of about 145 or nearly twice the mean diameter of the familiar bright ring. It shows a structure of looped filaments, the brightest of which is on the preceding side. Microphotometer tracings made on the negative of 'August 27 give some indication of a still fainter extension which fades gradually to a diameter of some 200, but this has not been detected by direct inspection of the plates. This faint envelope gives to the Lyra nebula an appearance suggestive of that of the* well-known planetary nebula in