by Ann » Sat Dec 30, 2017 6:20 am
It's a lovely picture with great colors and details. It is interesting that the background - which is
Barnard's Loop - is red, likely because it's ionized by the ultraviolet light and strong stellar winds from a lot of hot, bright and energetic stars in Orion. (Or, as Wikipedia suggests, it might be a supernova remnant! I didn't know that!)
But M78 is a blue nebula set against a sea of redness, which is a dead giveaway that none of the stars inside M78 is an O-type star. It seems certain that M78 has given birth to at least two B-type stars, at least one of which must be of a spectral type probably no later than B2. That would explain not only why M78 is as bright as it is, but also why the stars inside are not hot enough to turn this reflection nebula into an emission nebula. Note, however, that there are faint patches of pink in M78, which suggests that at least one of the B-type stars inside is not far from being hot enough to ionize an emission nebula. Again, this suggests a spectral type of around B2.
And indeed,
Simbad's Astronomical Database does say that the binary star in M78 is of spectral class B2 (B2II-III). I checked Simbad only after I had identified the spectral class of the hottest star inside M78 as B2.
An important reason as to why M78 is reportedly the most visible of all reflection nebulas in the sky is the thick, dark, arc-shaped dust lane framing the bright reflection nebula and setting it off against the background.
Ann
It's a lovely picture with great colors and details. It is interesting that the background - which is [url=https://en.wikipedia.org/wiki/Barnard%27s_Loop]Barnard's Loop[/url] - is red, likely because it's ionized by the ultraviolet light and strong stellar winds from a lot of hot, bright and energetic stars in Orion. (Or, as Wikipedia suggests, it might be a supernova remnant! I didn't know that!)
But M78 is a blue nebula set against a sea of redness, which is a dead giveaway that none of the stars inside M78 is an O-type star. It seems certain that M78 has given birth to at least two B-type stars, at least one of which must be of a spectral type probably no later than B2. That would explain not only why M78 is as bright as it is, but also why the stars inside are not hot enough to turn this reflection nebula into an emission nebula. Note, however, that there are faint patches of pink in M78, which suggests that at least one of the B-type stars inside is not far from being hot enough to ionize an emission nebula. Again, this suggests a spectral type of around B2.
And indeed, [url=http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+38563&submit=SIMBAD+search]Simbad's Astronomical Database[/url] does say that the binary star in M78 is of spectral class B2 (B2II-III). I checked Simbad only after I had identified the spectral class of the hottest star inside M78 as B2. 8-)
An important reason as to why M78 is reportedly the most visible of all reflection nebulas in the sky is the thick, dark, arc-shaped dust lane framing the bright reflection nebula and setting it off against the background.
Ann