Two nebulas are of interest in today's APOD (in my opinion): Bright reflection nebula M78 and dusty starforming pillar LDN 1622.
Reflection nebula M78 with dark dust lane. Photo: Terry Hancock.
M78 is said to be the easiest-to-spot reflection nebula in the sky. Why is it so bright? I'd say it's because of the dark dust lane that frames it. Note how the very brightest part of M78 is situated right next to a very dark dust lane. It must be the contrast between dark and light that makes this nebula so visible. Note that another reflection nebula "above" M78 has no dark dust lane to outline it. That reflection nebula is probably almost (or totally) impossible to spot visually.
Dusty starforming pillar LDN 1622. Photo: Terry Hancock.
LDN 1622 is a starforming pillar, because we can see a star forming at the tip of it (just below center right). We can also see that it is a cosmic pillar, because it is dusty and elongated, which is what we expect from windswept dust structures being eroded by strong stellar winds from hot bright stars. Fascinatingly, it is elongated from lower right to upper right, which is to say that it is "pointing" in the general direction of Orion's Belt and the hot bright stars Alnitak, Alnilam and Mintaka. But in this general direction we also find Sigma Orionis, which is responsible for sculpting the Horsehead Nebula.
The LDN 1622 region in Orion. Photo: Stanislav Volskiy.
Note in the picture at left that LDN 1622 (arrowed) is elongated in such a way that it is pointing in the general direction of Orion's Belt, Sigma Orionis and the Horsehead Nebula
. (The Horsehead nebula is barely visible as a tiny dark protrusion near the bottom edge of the picture close to 6 o'clock (closer to 5.30). Orion's Belt and Sigma Orionis are bright blue stars near bottom right.
M78 is located on the inner side of broad red Barnard's Loop, while LDN 1622 is on the outside of if. You could actually draw an almost straight line from LDN 1622, to M78, to the Horsehead nebula, and find that these three nebulas are at (apparently) equal distances from one another.
The appearance of LDN 1622 suggests to me that it is actually being sculpted by stellar winds from Orion's Belt or Sigma Orionis.
So the point I'm trying to make is that LDN 1622 looks pretty much like a dusty starforming pillar that is being sculpted and eroded by stellar winds from either the bright stars of Orion's Belt, or, perhaps even more likely, by stellar winds from Sigma Orionis. But today's caption does not agree with me:
APOD Robot wrote:
While M78 and the complex Barnard's Loop are some 1,500 light-years away, LDN 1622 is likely to be much closer, only about 500 light-years distant from our fair planet Earth.
Note in the picture by Stanislav Volskiy that LDN 1622 appears to be located at the bottom of the "wake" that has apparently been left by Betelgeuse, which is in fact a runaway star:
wrote about Betelgeuse:
Having been ejected from its birthplace in the Orion OB1 Association—which includes the stars in Orion's Belt—this runaway star has been observed moving through the interstellar medium at a speed of 30 km/s, creating a bow shock over four light-years wide.
But I don't think that Betelgeuse has anything to do with LDN 1622. This dark dusty pillar is elongated in the wrong direction to have been sculpted by Betelgeuse.
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