 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
| HomeDiscussion GroupsSpace ScienceAstronomyAmateur AstronomySpace FlightSpace StationShuttleSpace HistorySpace PolicySETI SpaceKB.com Contact UsLink To UsSearch & Site Map |
Space Forum / Astronomy / February 2007Andromeda and other nebulae
Why has Andromeda nebulae not been known from ages immemorable? It is magnitude over 4. It is in northern sky. Shouldn´t it be a naked eye object? Compare with Presepe, also magnitude over 4. Presepe was mentioned by both Hipparcos and Ptolemy, and even before. It is a nebula, because the brightest individual star is magnitude 6,5, so cannot be resolved by naked eye. But Andromeda, though it was mentioned by Al-Sufi, was somehow missed by Hipparcos, Ptolemy, Tyge Brahe and other naked eye classics. It was found in 17th century by several observers unaware of each other and previous naked eye observations. Why was Andromeda so little known? Unlike Magellanic Clouds, Omega Centauri and 47 Tucanae, it is a northern object... Wasn't it who wrote: >Why has Andromeda nebulae not been known from ages immemorable? > [quoted text clipped - 13 lines] >Why was Andromeda so little known? Unlike Magellanic Clouds, Omega >Centauri and 47 Tucanae, it is a northern object... M31 is a naked eye object, I've seen it, but it doesn't look fuzzy to the naked eye. Much of the luminosity is centred at the core which looks like a point to my eye. If the luminosity were spread evenly over the whole disk it would be far too faint to see, because the magnitude per "pixel" would be very low. For fuzzy objects it's the (inverse) magnitude divided by area that counts for visibility, not the total magnitude of the whole object.  SignatureMike Williams Gentleman of Leisure Mike Williams kirjutas: > Wasn't it who wrote: > >Why has Andromeda nebulae not been known from ages immemorable? [quoted text clipped - 18 lines] > the naked eye. Much of the luminosity is centred at the core which looks > like a point to my eye. If it looks like a point, why was it not known, and named, as a fourth- magnitude star? Like Omega Centauri and 47 Tucanae were? Wasn't it who wrote: >Mike Williams kirjutas: >> Wasn't it who wrote: [quoted text clipped - 22 lines] >If it looks like a point, why was it not known, and named, as a fourth- >magnitude star? Like Omega Centauri and 47 Tucanae were? Not all of the brightness is at the core. Some of it is spread across the disk. The central point is considerably dimmer than a Mag 4.4 star. SignatureMike Williams Gentleman of Leisure > Mike Williams kirjutas: >> Wasn't it who wrote: [quoted text clipped - 22 lines] > If it looks like a point, why was it not known, and named, as a fourth- > magnitude star? Like Omega Centauri and 47 Tucanae were? Perhaps because it doesn't look quite as bright as a 4'th magnitude star to the naked eye? The magnitude of M31 is 4.4, i.e. about halfway between 4th and 5th magnitudes. Since M31 is an extended object, it looks less bright to the naked eye than a mag 4.4 star - perhaps more like a 5th magnitude star? You can go out some clear night and make a naked-eye magnitude estimate yourself. It appears that M31 was observed by few observers between Al Sufi's observation in 905 and Simon Marius first telescopic observation in 1612. It is for instance missing from Bayer's star atlas from 1603. When Messier catalogued M31, he believed that Marius was the discoverer. Not all 5th magnitude stars received a Bayer letter or a Flamsteed number. One can always wonder why early observers overlooked some celestial object they ought to have noticed. Why did for instance Messier neglect to catalog the Double Cluster in Perseus? That too is an easy naked-eye object, and very obvious even in small telescopes. We'll never know why Messier overlooked the Double Cluster - we just know that he did, but not why. It's the same with why M31 was noticed by so few pre-telescopic observers: we'll never know why. Signature---------------------------------------------------------------- Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN e-mail: pausch at stockholm dot bostream dot se WWW: http://stjarnhimlen.se/ |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2009 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.