Date: September 2, 2010
Title: Exótico Cielo Profundo: By the border of Vulpecula-Sagitta
Podcaster: Julieta De Simone Shecre
Organization: Sur Astronómico
Description: An observational proposal in which we will be moving by the surroundings of the equator of our galaxy, driving along the shared border of the small constellations called Vulpecula and Sagitta, studying visual characteristics, astrophysical data and some history of selected objects, where the naked eye and different instruments will offer us different perspectives and details. We will notice different aspects and colours and we may even note familiar shapes…
Bio: Sur Astronómico is a website in Spanish managed by Enzo De Bernardini. The Exótico Cielo Profundo (Exotic Deep Sky) section was created by Rodolfo Ferraiuolo and Enzo De Bernardini and it is devoted to the visual observation of select objects from the deep sky. Having started in January 2008 and having received great approval from the amateur public, getting a lot of comments in the forum of the group, the periodic articles of the section were shaped to be written on a book.
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By the border of Vulpecula-Sagitta
Area: Sagitta (the Arrow), Vulpecula (the Fox)
On this observational proposal, we will be moving by the surroundings of the equator of our galaxy, driving along the shared border of the small constellations called Vulpecula and Sagitta. These constellations have a lot to show and that is why we chose them. We hope that by exploring this area of varied difficulty you’ll be surprised by the richness of objects and you’ll enjoy many nights of observation.
Our starting point will be CR 399. In 1931 the Swiss astronomer Per Arne Collinder introduced this object as the 399th open cluster of his catalogue but, even though it is still on discussion, it looks as if it was just a fantastic asterism of ten stars between the 5th and the 7th magnitudes. It is situated on an area of low, irregular absorption, which is full of stars, by the south-western end of Vulpecula, limiting with Sagitta, almost halfway between the white star of 5th magnitude – 9 Vulpeculae – to the east-south-east, and the blue/white star of magnitude 4.8 – 1 Vulpeculae – to the north-west.
It is commonly called in three different ways. Al-Sufi Nebula after the Persian astronomer Abu’l-Husayn al-Süfï (reduction and adaptation of his real long name). He registered it in the middle of the 10th century, as a small blot. Brocchi’s Cluster because of the amateur astronomer and great variable star observer, Dalmiro Francis Brocchi, who stated it in one of the 1920’s important charts for variable star observation that he was preparing for the AAVSO, the American Association of Variable Star Observers (his mapping work earned him the Merit Award of the AAVSO in 1942). And the most accepted one, Coathanger, because of its distinctive star pattern.
It turns out to be an interesting asterism for small apertures and even better for some simple binoculars, with which we will be able to observe, even with the presence of the moon, its ten colourful component stars contrasting and standing out among fifteen other fainter stars. Its coathanger shape, observed in the position it should be hung from the southern hemisphere, is made up of six practically aligned stars oriented towards the west, and four other stars with the shape of a hook, by the centre, to the south.
With a 3.6 combined magnitude, it can be easily spotted from dark places as a faint nebulous patch. By observing it with a 4” telescope, we will miss part of its structure because of its size, and we will see about forty fainter stars around its components, forming pairs, triplets and lines.
Going to the east, at about 17 arcminutes from 7 Vulpeculae, the star that is part of one of the ends of the Coathanger, we will find the open cluster NGC 6802, our second stop. With a magnitude of 8.8, a diameter of 3.2 arcminutes and classified as L1, this small galactic cluster was discovered by William Herschel in 1784. It is 3600 light years away from the Earth and it contains more than 60 stars with a magnitude from 13 onwards.
Visible with a 2.3 inch, 30x refracting telescopes as a faint luminosity, by observing it with a 5 inch Schmidt-Cassegrain telescope it will have little change, appearing as a dim oval blot. With an 8” telescope, we will see it with low surface brightness, starting to become resolved, and virtually oriented from north to south. With a 12” we will be able to observe about 20 stars over a misty background, of about 3 x 1.5 arcminutes. The cluster, which is situated in an area of high absorption, is found in the middle and to the south of two nice stellar pairs: one of 9th and 11th magnitude, 6 arcminutes to the north-east, and the other of 9th and 10th magnitude, 6.3 arcminutes to the north-west of the centre of the cluster.
We are now going to look for M27. In order to find it, we must practically look 3 degrees to the north of the red giant star of magnitude 3.5, Gamma Sagittae, and 23 arcminutes to the south and a little to the east of the yellow star of magnitude 5.7, 14 Vulpeculae.
It is the first planetary nebula to be discovered in July 12, 1764 by the well known French astronomer Charles Messier, who discovered it as a starless oval nebula. It is known by the name given in John Herschel’s description in 1833, Dumbbell, because of its shape. It is also known as Apple Core since its appearance recalls that of a bitten apple around its outline. It is of 7.2 magnitude, surface brightness of 11.1 and apparent size of 8 x 5.7 arcminutes which expands vaguely in photographs up to a diameter of 17 arcminutes.
Classified as 3+2, it appears with a position angle of 119 degrees, a negative heliocentric radial velocity of 42 km/s has been calculated, its real diameter would reach almost 6 light years, and it is found at an arguable distance of about 1250 light years. Its minimum estimated age is of about 3500 years, since the gas of the shell, in its central area, expands almost 7 arcseconds per century.
We observe the nebula practically on the equatorial plane, similar to an hourglass, and if we observe it from one of its poles, its shape would probably be annular.
In the 1970’s, a faint companion of the warm central star of 17th magnitude was discovered, with a separation of 6.5 arc seconds, and a position angle of 214 degrees. The real separation between both stars would be about 2500 AU.
Being visible from a rural place with 7 x 50 binoculars or finder scope, as a tiny faint dim blot, with a small refracting telescope it will be small, quite rectangular and faint. However, with 15 x 70 binos, it will surprise us as a more resolved pale-grey blot, very pretty and with a diameter of about 7 arcminutes.
With a 4”, it will start becoming one of the best planetary nebulae for visual observation, appearing with its typical dumbbell or weight shape, oriented from north-north-east to south-south-west, with a pale-grey shade and even brightness, contrasting with the rich stellar field, improving its borders and enlarging its size by using avert vision. With an 8” refracting telescope, we will notice its bitten-apple shape better, its south lobe will be brighter and more to the west. At low power, both lobes will widen irregularly, following the curves
to each side. The nebula, which has a good response to high magnification, reaches a size of 6 x 3.5 arcminutes, enlarging to 7 x 5 arcminutes by using an Oxigin III filter. With more than 170x, we will detect its extremely hot central star, a blue dwarf of 13.5 magnitude. This star is responsible for provoking the brightness of the nebular gas, with its UV radiation emission. Two faint overlapped stars will also start to be seen. By observing it with an 11”, its beauty will keep on surprising us, with a green-grey shade, irregular brightness, as if it were spotted, making the lobes and its thin side extensions stand out, appearing as a central rod crossing the nebula from north-east to south-west, and detecting about six or seven faint stars over it. When using an Oxigin III filter, it will get more contrast, appearing as turquoise-green, and the gas shell will vaguely expand to the east and west, almost completing its elliptical shape, with a gas of lower density, becoming a rugby ball.
In 1991, the amateur Czech astronomer Leos Ondra discovered a Mira-type variable star by matching photographs, with a magnitude varying from 14 to 18, situated at the north-west end of the nebula. He gave it the name of Goldilocks Variable.
We have travelled through some of the many objects in the border of Vulpecula-Sagitta, a rich area in which loads of open clusters, nebulae and asterisms may be found. We encourage you to keep on exploring the wonderful views of our exotic deep sky.-
End of podcast:
365 Days of Astronomy
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