Annual Meeting of the Deep Sky Section, 2011 March 12

Things that Fade in the Night: Variable Nebulae

Mr Privett commented that few deep sky objects changed in appearance from one decade to the next, and that this, combined with the large apertures needed to observe them, meant that amateurs had few opportunities to contribute scientifically significant observations of them which could compete with professional images. A small group of five known variable nebulae provided the only exceptions to this rule, varying in both brightness and morphology on week-long timescales. These were Hind's Nebula (NGC 1555) in Taurus; Hubble's variable nebula (NGC 2261) in Monoceros; NGC 6729 in Corona Australis; Gyulbudaghian's nebula, discovered in 1977 neighbouring the variable star PV Cephei; and McNeil's Nebula in Orion, recently discovered in January 2004 and thought to be associated with the infrared source IRAS 05436-0007.

All of these were reflection nebulae – nebulae which did not produce their own light, but scattered that of nearby stars. Their variability, in contrast to the unchanging appearance of most reflection nebulae, was thought to arise principally because they happened to be illuminated by variable stars, though in some cases the movement of dust lanes might give rise to added variability, casting transient shadows across regions of space measuring light-months across.

That such a small number of nebulae were known to vary in brightness was perhaps surprising given the large number of variable stars which were known to pervade the Milky Way. Was it really so rare for a reflection nebula to find itself close to such a star, or were many more variable nebulae waiting to be found? The speaker found the latter explanation the most likely, especially given the comparatively recent discovery of Gyulbudaghian's and McNeil's nebulae. The latter was absent from pre-discovery images of Orion going back several decades, suggesting that it had been optically faint for many years prior to its discovery in 2004, and so it seemed probable that more variable nebulae would spontaneously appear and be discovered in the future.

Amateur observations of such reflection nebulae could seek to answer a number of questions. Was the variability of each nebula purely the result of fluctuations in the brightness of a nearby variable star, following it exactly, or were there other contributing factors? Were there any colour or morphological changes associated with the variability? What timescales did the nebulae change over? The equipment needed to make observations which could usefully address these questions was modest: CCD photometry of adequate quality to construct a light curve was possible with a 4-inch aperture, while CCD imaging was possible with a 6-inch aperture. When the nebulae approached their peak brightness they were within reach of visual observers using large-aperture Dobsonians.

The speaker concluded by reporting on the Section's work over the past three years to monitor the variation of Gyulbudaghian's nebula – an object ideally placed for observation from the UK. Lying in Cepheus, at a declination of +68°, it was circumpolar across much of Europe, though it could nonetheless be challenging to observe in the spring. A particular stalwart of the project had been the Association's President, David Boyd, who had regularly contributed photometric measurements of sufficient quality to show that the nebula's brightness varied on short as well as long timescales; there could be, for example, considerable brightness fluctuations from one day to the next.

In addition to collating the observations submitted to the Section, the speaker had also scheduled regular observations using the Sierra Stars Observatory 0.61-metre robotic telescope in California. When the weather permitted these to go ahead, the sharp images that could be obtained from such a site revealed morphological changes which accompanied the nebula's brightness fluctuations, though the speaker added that these did not rule out the idea that the nebula's variability stemmed purely from the fluctuating illumination provided by the nearby variable star PV Cephei. The considerable light travel time across the nebula – over a month – meant that different parts of the nebula would be expected to mirror the variability of PV Cephei with differing time lags. As had previously been reported by the Director in his annual review, Dr Boyd had in fact observed the global average brightness of the nebula to closely follow variations in the brightness of PV Cephei, but with a lag of 32±4 days. If this was interpreted as the light travel time between the star and the nebula, it corresponded to a plausible separation of around 0.027±0.003 parsecs.

Following the applause, an audience member asked how Gyulbudaghian's nebula appeared at present. The speaker replied that it had undergone a deep fade, and that no nebulosity was visible even in his latest 300-second exposures from Sierra Stars. However, given its unpredictable nature, it might reappear at any moment, and he was as keen to receive reports of negative observations as positive detections. The Director then invited Geoffrey Johnstone to present a historical perspective on his experiences of astrophotography in the 1980s.