APOD: M78 Wide Field (2017 Dec 29)

[APOD Robot]

M78 Wide Field

Explanation: Interstellar dust clouds and glowing nebulae abound in the fertile constellation of Orion. One of the brightest, M78, is centered in this colorful, wide field view, covering an area north of Orion's belt. At a distance of about 1,500 light-years, the bluish reflection nebula is around 5 light-years across. Its tint is due to dust preferentially reflecting the blue light of hot, young stars. Reflection nebula NGC 2071 is just to the left of M78. To the right, and much more compact in appearance, the intriguing McNeil's Nebula is a recently recognized variable nebula associated with a young sun-like star. Deeper red flecks of emission from Herbig-Haro objects, energetic jets from stars in the process of formation, stand out against the dark dust lanes. The exposure also brings out the region's fainter pervasive glow of atomic hydrogen gas.

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[sunson]

How come there are patches of basically no stars?

[Chris Peterson]

How come there are patches of basically no stars?

Dust is very good at blocking background stars. And there's lots of dust. Reflection nebulas, and to a lesser extent emission nebulas also attenuate background stars.

[Boomer12k]

WOW... awesome...

:---[===] *

[ta152h0]

the famous Marilyn Monroe nebula, I think

[Ann]

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