by Ann » Sat Oct 30, 2010 7:18 pm
As a color freak who loves blue things, I was of course immediately fascinated by what looks like a bright blue double star in the picture. Thanks to my astronomy software I was able to identify the brighter component of this double star as HD 202986, a magnitude 7.92 F0 star seen from a distance of 245 light-years, with an absolute visual luminosity of 3.19 times that of the Sun and a color index of 0.42. That makes it yellower than A7 star Altair (color index 0.22) but bluer than F5 star Mirphak (color index 0.48). Altair looked blue to me when I observed it with a 14 inch telescope, but Mirphak looked white. I suspect that HD 202986 would also look white to me, but since it is clearly bluer than the Sun, it is okay to show it as blue in a photograph.
But the other component of the double star is another matter. According to my software it is a magnitude 9.36 star of spectral class G5 and with a color index of 0.68, yellower than that of the Sun, whose color index according to wikipedia is 0.656±0.005. There is no reason to show this star as blue, except that its whitish color might be overwhelmed by the slightly blue color of the brighter component.
It is interesting to see that so many of the stars in the picture seem to have a yellowish-brown color. No doubt they are reddened by the dust that is so prevalent in the constellation of Cepheus. Vad den Berg 141 is not so far away from the Iris Nebula, which is part of a huge dusty complex:
A great picture of all the dust surrounding the Iris Nebula was recently posted here at SA* by Bogdan Jarzyna:
My point is that Cepheus is really a very dusty constellation. At the same time it is home to many hot blue OB stars, but these stars don't look blue to us for two reasons. One, they are far away. The OB stars of Cepheus are generally much farther away than the comparatively nearby numerous bright blue stars in Centaurus, Scorpius and the Southern Cross, which are generally 300 to 400 light-years distant.
The many blue stars of Centaurus are generally "only" about 400 light-years away...
The bright blue stars of Orion are at least twice as far away as those in Scorpius, Centaurus and Crux, but unlike the similarly distant stars in Cepheus their color is relatively unaffected by dust:
But Cepheus is dusty. The intervening dust is between us and the most intensely starforming parts of the constellation, near O stars Lambda Cephei and 19 Cephei.The constellation is full of great stars, but we don't see them well because they are far away and strongly reddened by dust. To me, constellation Cepheus is like an itty bitty tiny baby brother of an ULIRG, an Ultra Luminous InfraRed Galaxy, whose light is smothered by all the dust created by its own intense star formation. So I'd say that it is no coincidence that we find the ghostly nebula of today's APOD in Cepheus; if you are looking for dust in our galaxy, Cepheus is as good a place as any to start.
Finally, I can't keep this truly great annotated picture of Cepheus from you. The picture is big, so I'll only post it as a link, and it is not in English, but you should be able to understand what the designations refer to. "Nebulosa Iris", for example, is obviously the Iris Nebula.
http://www.telefonica.net/web2/astromod ... saico-.jpg
I made a mistake with that link before. but this is the real link, I promise. Do look at it, because it's great!
Ann
As a color freak who loves blue things, I was of course immediately fascinated by what looks like a bright blue double star in the picture. Thanks to my astronomy software I was able to identify the brighter component of this double star as HD 202986, a magnitude 7.92 F0 star seen from a distance of 245 light-years, with an absolute visual luminosity of 3.19 times that of the Sun and a color index of 0.42. That makes it yellower than A7 star Altair (color index 0.22) but bluer than F5 star Mirphak (color index 0.48). Altair looked blue to me when I observed it with a 14 inch telescope, but Mirphak looked white. I suspect that HD 202986 would also look white to me, but since it is clearly bluer than the Sun, it is okay to show it as blue in a photograph.
But the other component of the double star is another matter. According to my software it is a magnitude 9.36 star of spectral class G5 and with a color index of 0.68, yellower than that of the Sun, whose color index according to wikipedia is 0.656±0.005. There is no reason to show this star as blue, except that its whitish color might be overwhelmed by the slightly blue color of the brighter component.
It is interesting to see that so many of the stars in the picture seem to have a yellowish-brown color. No doubt they are reddened by the dust that is so prevalent in the constellation of Cepheus. Vad den Berg 141 is not so far away from the Iris Nebula, which is part of a huge dusty complex:
[img2]http://www.phys.ncku.edu.tw/~astrolab/mirrors/apod/image/0906/NGC7023_jeng800.jpg[/img2]
A great picture of all the dust surrounding the Iris Nebula was recently posted here at SA* by Bogdan Jarzyna:
[img2]http://www.starrysite.com/pliki/galeria/duze/Irys%20-%20ostatni%20starrysite.jpg[/img2]
My point is that Cepheus is really a very dusty constellation. At the same time it is home to many hot blue OB stars, but these stars don't look blue to us for two reasons. One, they are far away. The OB stars of Cepheus are generally much farther away than the comparatively nearby numerous bright blue stars in Centaurus, Scorpius and the Southern Cross, which are generally 300 to 400 light-years distant.
[img]http://www.starrynightphotos.com/constellations/images/centaurus.jpg[/img]
The many blue stars of Centaurus are generally "only" about 400 light-years away...
The bright blue stars of Orion are at least twice as far away as those in Scorpius, Centaurus and Crux, but unlike the similarly distant stars in Cepheus their color is relatively unaffected by dust:
[img]http://www.eaae-astronomy.org/newsletter/images/n_1/orion.jpg[/img]
But Cepheus is dusty. The intervening dust is between us and the most intensely starforming parts of the constellation, near O stars Lambda Cephei and 19 Cephei.The constellation is full of great stars, but we don't see them well because they are far away and strongly reddened by dust. To me, constellation Cepheus is like an itty bitty tiny baby brother of an ULIRG, an Ultra Luminous InfraRed Galaxy, whose light is smothered by all the dust created by its own intense star formation. So I'd say that it is no coincidence that we find the ghostly nebula of today's APOD in Cepheus; if you are looking for dust in our galaxy, Cepheus is as good a place as any to start.
Finally, I can't keep this truly great annotated picture of Cepheus from you. The picture is big, so I'll only post it as a link, and it is not in English, but you should be able to understand what the designations refer to. "Nebulosa Iris", for example, is obviously the Iris Nebula.
[url]http://www.telefonica.net/web2/astromodelismo/Web%20Astronomia/Datos%20web/Tomas%20astronomicas%20propias%20web/Constelaciones/Datos/Reales/Con%20Leyenda/Leyenda-final-Suma-Mosaico-.jpg[/url]
I made a mistake with that link before. but this is the real link, I promise. Do look at it, because it's great!
Ann