NGC 7635, Bubble-Nebula
One of the most beautiful objects in the northern sky is definitely the Bubble Nebula in the constellation Cassiopeia, which was discovered by F.W. Herschel
in 1787. "NGC 7635" (this is its catalog name) is located very close to the open star cluster Messier 52 and is therefore easy to find.
Inside the gas masses there is an extremely luminous and hot star (BD +60°2522). At the surface the star (which belongs to spectral class O6.5) is nearly 38,000
Kelvin hot. Its mass is estimated to be 44 solar masses - so it is a truly gigantic blue hypergiant, which with a diameter of 21 million kilometers is about 15 times
larger than our sun. With a luminosity of 400,000 suns it is one of the brightest stars in our Milky Way, even if you wouldn't expect that because of its low apparent
brightness of about 8mag. But its absolute magnitude is M=-5.5mag, so at the distance of Sirius it would be visible with the naked eye at an apparent magnitude
of -8.7mag even in the daytime sky. The star is very young with an estimated age of 2 million years, but nevertheless it will most likely end as a supernova in
the next few million years.
About 40,000 years ago "BD +60°2522" started to eject parts of its surface into space, which since then have been moving outward at a speed of 28 kilometers per
second. Where this matter collides with the surrounding molecular cloud, it is heated and begins to glow, creating this wonderful gas bubble.
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I have never seen the Bubble Nebula as beautiful as I did with my 20" Dobsonian under the dark mountain sky at 1000mNN.
With 20" aperture the crescent is beautiful to look at - at a magnification of 200x in combination with an UHC- filter. If you increase the magnification to 270x,
you can even see some structures in the central node of the nebula. But the seeing has to be quite good.
Above the bubble nebula you can find more faint gas masses, which are best seen at low magnifications in combination with a UHC filter.
