APOD: M78 and Orion Dust Reflections (2017 Jan 24)

[APOD Robot]

M78 and Orion Dust Reflections

Explanation: In the vast Orion Molecular Cloud complex, several bright blue nebulas are particularly apparent. Pictured here are two of the most prominent reflection nebulas - dust clouds lit by the reflecting light of bright embedded stars. The more famous nebula is M78, in the image center, cataloged over 200 years ago. To its left is the lesser known NGC 2071. Astronomers continue to study these reflection nebulas to better understand how interior stars form. The Orion complex lies about 1500 light-years distant, contains the Orion and Horsehead nebulas, and covers much of the constellation of Orion.

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[Ann]

Yup, that's M78, the brightest known reflection nebula in the sky, as seen from the Earth. (Apart from our own blue skies, which may perhaps be thought of as a reflection nebula.)

I've never taken a great interest in M78, however. But now that I see the picture - great picture, by the way - I have to wonder what kind of star is illuminating this nebula. It can't be an O-type star, that much is certain, because then the nebula would have been a red emission nebula. So it must be a B-type star. A B1-type star? No, that would still produce an emission nebula, although a faint one. A B2-type star? Maybe... Or a B3-type star? That definitely sounds possible. It would be even better if there was a whole little cluster of stars inside M78, illuminating the nebula together and making it so bright.

I think I can see two stars inside it, at least!

Ann

[alter-ego]

I think I can see two stars inside it, at least!

Indeed, the two stars you see may be the primary illuminators. The 2MASS 2um image reveals clusters as well.

[Boomer12k]

WOW... awesome pic... so clear...

The reflection nebula on the right seems to be the Emperor and Empress of the Krull Empire though.... me scared...

:---[===} *

[Chris Peterson]

Yup, that's M78, the brightest known reflection nebula in the sky, as seen from the Earth.

I'm skeptical of that claim. Of course, it might depend on how you choose to define "bright", as the word isn't a technical one and can be interpreted in several different ways when referring to diffuse objects.

[Ann]

Yup, that's M78, the brightest known reflection nebula in the sky, as seen from the Earth.

I'm skeptical of that claim. Of course, it might depend on how you choose to define "bright", as the word isn't a technical one and can be interpreted in several different ways when referring to diffuse objects.

And I'm not insisting on it. I read it somewhere, maybe back in Burnham's Celestial Handbook, that M78 is the brightest reflection nebula.

M78 is the only "pure" reflection nebula among the Messier objects. Yes, there are reflections nebulas associated with some other Messier objects, such as the Pleiades and the Trifid Nebula, but it seems certain that it wasn't the faint reflection nebulas associated with M20 and M45 that caught Charles Messier's eye.

According to this page, M78 is an 8th magnitude object. That seems moderately bright for a reflection nebula to me. M78 may have a higher "surface brightness" than (most) other reflection nebulas, whose light-scattering parts may be more spread out and harder to spot against the background of the sky than the concentrated knot of nebulosity that was Charles Messier 78th discovery (or entry). M78 also stands out particularly well because of the dark dust lane framing half of it. NGC 2023 near the Horsehead nebula may be brighter than M78 for all I know, but the central star there is very prominent, there is no dust lane to frame the nebula, and the background is pretty bright.

And it wouldn't surprise me if there are some bright reflection nebulas in the southern hemisphere. Who's to say that the Rho Ophiuchi nebula isn't brighter than M78, just because Charles Messier didn't spot it? It sure seems big enough as seen from the Earth to be brighter than M78. But again, it has no dust lane to frame it and set it off.

When it comes to the "absolute luminosity" of reflection nebulas, if there even is such a property, I have no idea what nebula would win that prize.

Ann

[Chris Peterson]

Apart from our own blue skies, which may perhaps be thought of as a reflection nebula.

FWIW, taking as typical values an atmospheric particle density of 10¹⁹/cm³ and a nebular particle density of 10³/cm³, our atmosphere is ten thousand million million times denser than than a reflection nebula!

[Ann]

Click to view full size image

Orion nebulosities, with the Horsehead, NGC 2023,
the Flame nebula and M78. Photo: Alan Smallbone.

I have to recommend this page, which discusses the properties of M78 compared with other nebulas. Above all, that page offers some spectacular images. The picture on the left is a small and rather poorly resolved version of one of these pictures. The good thing about it is that you can compare M78 and NGC 2023 (just below the Horsehead). It seems clear that it is the prominent dust lane that makes M78 easier to spot than NGC 2023.

Personally I also think that this picture, by Ignacio de la Cueva Torregrosa, is fantastic.

Ann

[Chris Peterson]

I have to recommend this page, which discusses the properties of M78 compared with other nebulas.

Although that discussion is somewhat inaccurate in its explanation of why reflection nebulas are blue. It may be because they are reflecting the light of blue stars, but it is also because these nebulas preferentially reflect shorter wavelengths and absorb longer ones. So even if lit by more yellow stars, they can appear blue. (Of course, the brightest stars are usually blue, so it's not surprising that so many bright reflection nebulas tend to be illuminated by such stars.)

[neufer]

I have to recommend this page, which discusses the properties of M78 compared with other nebulas.

Although that discussion is somewhat inaccurate in its explanation of why reflection nebulas are blue. It may be because they are reflecting the light of blue stars, but it is also because these nebulas preferentially reflect shorter wavelengths and absorb longer ones. So even if lit by more yellow stars, they can appear blue. (Of course, the brightest stars are usually blue, so it's not surprising that so many bright reflection nebulas tend to be illuminated by such stars.)

<<Distinctly reddish when viewed with the unaided eye, Antares is a red supergiant of spectral type M0.5Iab and is one of the largest and most luminous observable stars.>>

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<<Explanation: The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star Rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark brown regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible here on the upper right, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.>>

[Ann]

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<<Explanation: The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star Rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark brown regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible here on the upper right, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.>>

The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas?

Yes, the red (or magenta) regions shine by emission. We are talking ordinary Hα emission here.

But no, the yellow regions are not shining by emission! What kind of emission would that be? Where in our own galaxy is yellow emission (by sulfur??) so dominant that you get a whole nebula shining yellow from emission? That doesn't happen here.

Click to view full size image

Yellow headlights on a foggy night.

No, the yellow regions (around Antares) are simply yellow reflection nebulas! Why are these reflection nebulas yellow and not blue? That's because Antares emits so much yellow, red and infrared light and so extremely little blue light that there is hardly any blue light to scatter! But there is indeed a lot of yellow light there, and yellow photons from Antares get scattered by the large numbers of dust particles in the vicinity of this supergiant star.

The yellow regions in the vicinity of Antares are like the fluffy yellow puffs of light scattering from yellow headlights on a foggy night.

Ann

[Visual_Astronomer]

Yup, that's M78, the brightest known reflection nebula in the sky, as seen from the Earth.

I'm skeptical of that claim. Of course, it might depend on how you choose to define "bright", as the word isn't a technical one and can be interpreted in several different ways when referring to diffuse objects.

And I'm not insisting on it. I read it somewhere, maybe back in Burnham's Celestial Handbook, that M78 is the brightest reflection nebula.

And it wouldn't surprise me if there are some bright reflection nebulas in the southern hemisphere. Who's to say that the Rho Ophiuchi nebula isn't brighter than M78, just because Charles Messier didn't spot it? It sure seems big enough as seen from the Earth to be brighter than M78. But again, it has no dust lane to frame it and set it off.

When it comes to the "absolute luminosity" of reflection nebulas, if there even is such a property, I have no idea what nebula would win that prize.

Ann

Well, I can tell you that I've seen M78 many times through a variety of different sized telescopes, but I have yet to see the Rho Ophiuchi nebula with my 20", despite repeated tries. So I'd say M78 is the much brighter of the two.

[Chris Peterson]

The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas?

Primarily, no. As you note, most of the light is simply reflected. But images shot in H-alpha demonstrate a great deal of emission around Antares. While that is visually swamped by reflected light, there's no doubt that the apparent color of the nebula is significantly influenced by emissions as well as the reflected light. Of course, that is true for a great many nebulas.

[ta152h0]

When Betelgeuse goes kaboom, will our solar system also light up ?

[neufer]

When Betelgeuse goes kaboom, will our solar system also light up ?

[b][color=#0000FF]Celestia depiction of Orion as it might appear from Earth when Betelgeuse explodes as a supernova[/color][/b]

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Click to play embedded YouTube video.

<<The time until Betelgeuse explodes depends on the predicted initial conditions and on the estimate of the time already spent as a red supergiant. Given the estimated time since Betelgeuse became a red supergiant, estimates of its remaining lifetime range from a "best guess" of under 100,000 years for a non-rotating 20 M☉ model to far longer for rotating models or lower mass stars. Betelgeuse's suspected birthplace in the Orion OB1 Association is the location of several previous supernovae. It is believed that runaway stars may be caused by supernovae, and there is strong evidence that OB stars μ Columbae, AE Aurigae and 53 Arietis all originated from such explosions in Ori OB1 2.2, 2.7 and 4.9 million years ago.

A typical type II-P supernova emits 2×10⁴⁶ J of neutrinos and produces an explosion with a kinetic energy of 2×10⁴⁴ J. As seen from Earth, it would have a peak apparent magnitude of about −12.4. It may outshine the full moon and would be easily visible in daylight. This type of supernova would remain at roughly constant brightness for 2–3 months before rapidly dimming. The visible light is produced mainly by the radioactive decay of cobalt, and maintains its brightness due to the increasing transparency of the cooling hydrogen ejected by the supernova.

Due to misunderstandings caused by the 2009 publication of the star's 15% contraction, apparently of its outer atmosphere, Betelgeuse has frequently been the subject of scare stories and rumors suggesting that it will explode within a year, leading to exaggerated claims about the consequences of such an event. The timing and prevalence of these rumors have been linked to broader misconceptions of astronomy, particularly to doomsday predictions relating to the Mayan calendar. Betelgeuse is not likely to produce a gamma-ray burst and is not close enough for its x-rays, ultraviolet radiation, or ejected material to cause significant effects on Earth.>>

[Chris Peterson]

When Betelgeuse goes kaboom, will our solar system also light up ?

Well, technically, since photons will reach the Solar System and scatter or reflect from planets and interplanetary dust and debris.

I think the most visually interesting phenomenon could be with the Moon. Since Betelgeuse is likely to be of similar brightness after it goes supernova, we could actually end up with a double-phased moon- the primary phase determined by the Sun, but a secondary one visible on the shaded part of the Moon cast by Betelgeuse.

[neufer]

[b][color=#0000FF]Earthshine reflecting off the Moon during conjunction with planet Venus (left). [/color][/b]

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Since Betelgeuse is likely to be of similar brightness after it goes supernova, we could actually end up with a double-phased moon- the primary phase determined by the Sun, but a secondary one visible on the shaded part of the Moon cast by Betelgeuse.

The dark side of the moon is already illuminated by Earthshine which is ~25 times brighter than Betelgeuse supernova-shine.

[Chris Peterson]

The dark side of the moon is already illuminated by Earthshine which is ~25 times brighter than Betelgeuse supernova-shine.

The brightness of Earthshine varies radically with the relative positions of the three bodies.

[ta152h0]

SCHOOLED ! more knowledge than yesterday

[Ann]

The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas?

Primarily, no. As you note, most of the light is simply reflected. But images shot in H-alpha demonstrate a great deal of emission around Antares. While that is visually swamped by reflected light, there's no doubt that the apparent color of the nebula is significantly influenced by emissions as well as the reflected light. Of course, that is true for a great many nebulas.

Antares and the Rho Ophiuchi region. Photo: Tony and Daphne Hallas.
A 512 KB version of the image can be found here.

It is important to understand that the yellow light comes from Antares, and it has nothing to do with emission. The Hα emission that is actually present around Antares - and which is of course red in color - has nothing to do with Antares. As the picture on the left shows, the Hα emission around Antares has two sources: one source is Tau Scorpii, to the southeast of Antares (to the lower left of yellow Antares in the image) and the other source is Sigma Scorpii, to the northwest of Antares (to the upper right of Antares).

Tau Scorpii is a B0V main sequence star with a surface (photosphere) temperature of 30,700 Kelvin, according to Jim Kaler. Sigma Scorpii is a double star, consisting of an O9V main sequence star (just slightly hotter than the B0V star) and an evolved B2 giant. In the picture, you can see that both Tau and Sigma are blue stars surrounded by red "emission halos".

I should perhaps add that Antares has a B2.5V companion buried deep inside Antares' yellow light. I wouldn't think that a B2.5V star is hot enough to produce appreciable emission, but you never know. It could be that the B-type companion is producing some emission that would appear to come from Antares itself, although of course Antares itself is no source of emission.

My point, which I must repeat, is that the yellow light around Antares is not produced by emission, and the emission that is present around Antares is produced by other stars.

Ann

[Ann]

When Betelgeuse goes kaboom, will our solar system also light up ?

Art and Chris already gave you good answers to this question. But please note that Antares is also going to explode as a supernova, and Antares may be closer to us than Betelgeuse, although that is not certain.

But the Rho Ophiuchi region is going to look dramatic when Antares goes KA-BOOM!

Ann

[Chris Peterson]

It is important to understand that the yellow light comes from Antares, and it has nothing to do with emission.n

Yes, the yellow light comes from Antares. But the yellow color of the nebula around Antares only partly comes from Antares. The actual color of the nebula is a mix of reflected light from the star and various emission lines stimulated by nearby hot stars.

[neufer]

[b][color=#0000FF][size=150]Pluto’s large, Texas-sized moon Charon:[/size] NASA/JHUAPL/SwRI[/color][/b]

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The dark side of the moon is already illuminated by Earthshine which is ~25 times brighter than Betelgeuse supernova-shine.

The brightness of Earthshine varies radically with the relative positions of the three bodies.

<<This image obtained with the Ralph/Multispectral Visible Imaging Camera aboard NASA’s New Horizons spacecraft shows the night side of Pluto’s large, Texas-sized moon Charon, against a star field, lit by faint, reflected light from Pluto itself. The bright crescent on Charon's right side is a sliver of sunlit terrain; it is overexposed. New Horizons was already about 100,000 miles (160,000 kilometers) beyond Pluto when the image was taken on July 15, 2015.>>