APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

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APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by APOD Robot » Mon Jan 15, 2018 5:11 am

Image Rigel and the Witch Head Nebula

Explanation: By starlight this eerie visage shines in the dark, a crooked profile evoking its popular name, the Witch Head Nebula. In fact, this entrancing telescopic portrait gives the impression that the witch has fixed her gaze on Orion's bright supergiant star Rigel. More formally known as IC 2118, the Witch Head Nebula spans about 50 light-years and is composed of interstellar dust grains reflecting Rigel's starlight. The blue color of the Witch Head Nebula and of the dust surrounding Rigel is caused not only by Rigel's intense blue starlight but because the dust grains scatter blue light more efficiently than red. The same physical process causes Earth's daytime sky to appear blue, although the scatterers in Earth's atmosphere are molecules of nitrogen and oxygen. Rigel, the Witch Head Nebula, and gas and dust that surrounds them lie about 800 light-years away.

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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by Ann » Mon Jan 15, 2018 7:22 am

That's a nice picture! I like the RGB colors, of course!

The Witch Head Nebula is illuminated by Rigel. The length of the Witch Head Nebula is about 50 light-years, according to the caption, and all of it is (presumably) illuminated by Rigel. Does anyone know how far away from Rigel the Witch Head Nebula is located?

I find it strange that many parts of the Witch Head Nebula are as non-blue as they are. Some blobs are downright beige. What is the light source illuminating the beige blobs?

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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by Case » Mon Jan 15, 2018 11:01 am

Ann wrote:I find it strange that many parts of the Witch Head Nebula are as non-blue as they are. Some blobs are downright beige. What is the light source illuminating the beige blobs?
Could it be that the two beige parts (at the 4 o’clock angle) have red nebulosity knots shining through? It is in the path, either behind or in front. I guess you’d have to examine the Hα component to be sure.

alex_555

Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by alex_555 » Mon Jan 15, 2018 11:35 am

What is the distance between Witch Head Nebula and star Rigel ? At least hundred of light years I suppose. Even at such distance Rigel's light can still be reflected by the nebula ?

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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by dkklein » Mon Jan 15, 2018 1:01 pm

Compared to the post from Dec of 2006, the photo would appear to be upside-down making it a bit more challenging to see the witches head.

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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by ta152h0 » Mon Jan 15, 2018 2:33 pm

Clouds lit up by a cosmic streetlamp
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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by rstevenson » Mon Jan 15, 2018 2:34 pm

Ann wrote:... Does anyone know how far away from Rigel the Witch Head Nebula is located? ...
When I look up Rigel in Wikipedia, it says 860 ± 80 ly. A similar look up of IC 2118 says "about 900" ly in the text and 1000 ly in the Observation data box. So... the nebula could be anywhere from 0 to 220 ly from Rigel.

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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by pgp566 » Mon Jan 15, 2018 2:36 pm

Very good, though I've always assumed that "Witch Head" actually refers to the head, as here, and most others on the web:
http://www.astrocruise.com/milky_way/ic2118.jpg

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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by BDanielMayfield » Mon Jan 15, 2018 2:46 pm

rstevenson wrote:
Ann wrote:... Does anyone know how far away from Rigel the Witch Head Nebula is located? ...
When I look up Rigel in Wikipedia, it says 860 ± 80 ly. A similar look up of IC 2118 says "about 900" ly in the text and 1000 ly in the Observation data box. So... the nebula could be anywhere from 0 to 220 ly from Rigel.

Rob
I think we can rule out the low end of that range Rob, since the nebula couldn't exist if it was 0 ly from Rigel!
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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by rstevenson » Mon Jan 15, 2018 2:50 pm

True. How far away must it be to exist?

Rob

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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by BDanielMayfield » Mon Jan 15, 2018 2:55 pm

alex_555 wrote:What is the distance between Witch Head Nebula and star Rigel ? At least hundred of light years I suppose. Even at such distance Rigel's light can still be reflected by the nebula ?
Yes. Consider what Rigel is:
Rigel, also designated Beta Orionis (β Orionis, abbreviated Beta Ori, β Ori), is generally the seventh-brightest star in the night sky and the brightest star in the constellation of Orion—though periodically it is outshined within the constellation by the variable Betelgeuse. With a visual magnitude of 0.13, it is a remote and luminous star some 863 light-years distant from Earth.

Rigel
Orion constellation map.svg

Rigel in the constellation Orion
Observation data
Epoch J2000.0 Equinox J2000.0
Constellation Orion
A
Right ascension 05h 14m 32.27210s[1]
Declination −08° 12′ 05.8981″[1]
Apparent magnitude (V) 0.13[2] (0.05 - 0.18[3])
B
Right ascension 05h 14m 32.049s[4]
Declination −08° 12′ 14.78″[4]
Apparent magnitude (V) 6.67[5]
Characteristics
A
Evolutionary stage Blue supergiant
Spectral type B8 Ia[6]
U−B color index −0.66[7]
B−V color index −0.03[7]
Variable type Alpha Cygni[8]
B
Evolutionary stage Spectroscopic binary
Spectral type B9V + B9V[9]
U−B color index −0.66[7]
B−V color index −0.03[7]
Variable type Alpha Cygni[8]
Astrometry
Radial velocity (Rv) 17.8±0.4[10] km/s
Proper motion (μ) RA: +1.31[1] mas/yr
Dec.: +0.50[1] mas/yr
Parallax (π) 3.78 ± 0.34[1] mas
Distance 860 ± 80 ly
(260 ± 20 pc)
Absolute magnitude (MV) -7.84[11]
Orbit[5]
Primary Ba
Companion Bb
Period (P) 9.860 days
Eccentricity (e) 0.1
Semi-amplitude (K1)
(primary) 25.0 km/s
Semi-amplitude (K2)
(secondary) 32.6 km/s
Details
A
Mass 23[12] M☉
Radius 78.9±7.4[13], 115[14] R☉
Luminosity (bolometric) 1.20+0.25
−0.21×105[13] L☉
Surface gravity (log g) 1.75±0.10[12] cgs
Temperature 12100±150[12] K
Metallicity [Fe/H] −0.06±0.10[6] dex
Rotational velocity (v sin i) 25±3[12] km/s
Age 8±1[6] Myr
Ba
Mass 3.84[9] M☉
Bb
Mass 2.94[9] M☉
Other designations
Rigel, Algebar, Elgebar, β Ori, 19 Ori, HD 34085, HR 1713, HIP 24436, SAO 131907, BD-08° 1063 [15]
Database references
SIMBAD data
The star as seen from Earth is actually a multiple star system of three to five stars, the primary star (Rigel A) being a blue-white supergiant which is estimated to be anywhere from 120,000 to 279,000 times as luminous as the Sun, depending on the method used to calculate its properties. It has exhausted its core of hydrogen and swollen out to between 79 and 115 times the Sun's radius. It pulsates quasi-periodically and is classified as an Alpha Cygni variable. A companion, Rigel B, is 500 times fainter than the supergiant Rigel A and visible only with a telescope. Rigel B is itself a spectroscopic binary system, consisting of two main sequence blue-white stars of spectral type B9V that are estimated to be respectively 3.9 and 2.9 times as massive as the Sun, Rigel Bb may itself be a binary. Rigel B also appears to have a very close visual companion Rigel C of almost identical appearance.
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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by BDanielMayfield » Mon Jan 15, 2018 3:07 pm

rstevenson wrote:True. How far away must it be to exist?

Rob
Good question. But just from the image and the Explanation I'd guess that the separation must be > 60 light years. I think about 100 ly is a reasonable estimate.
"Happy are the peaceable ... "

alex_555

Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by alex_555 » Mon Jan 15, 2018 4:00 pm

BDanielMayfield wrote:
alex_555 wrote:What is the distance between Witch Head Nebula and star Rigel ? At least hundred of light years I suppose. Even at such distance Rigel's light can still be reflected by the nebula ?
Yes. Consider what Rigel is:
Rigel, also designated Beta Orionis (β Orionis, abbreviated Beta Ori, β Ori), is generally the seventh-brightest star in the night sky and the brightest star in the constellation of Orion—though periodically it is outshined within the constellation by the variable Betelgeuse. With a visual magnitude of 0.13, it is a remote and luminous star some 863 light-years distant from Earth.

Rigel
Orion constellation map.svg

Rigel in the constellation Orion
Observation data
Epoch J2000.0 Equinox J2000.0
Constellation Orion
A
Right ascension 05h 14m 32.27210s[1]
Declination −08° 12′ 05.8981″[1]
Apparent magnitude (V) 0.13[2] (0.05 - 0.18[3])
B
Right ascension 05h 14m 32.049s[4]
Declination −08° 12′ 14.78″[4]
Apparent magnitude (V) 6.67[5]
Characteristics
A
Evolutionary stage Blue supergiant
Spectral type B8 Ia[6]
U−B color index −0.66[7]
B−V color index −0.03[7]
Variable type Alpha Cygni[8]
B
Evolutionary stage Spectroscopic binary
Spectral type B9V + B9V[9]
U−B color index −0.66[7]
B−V color index −0.03[7]
Variable type Alpha Cygni[8]
Astrometry
Radial velocity (Rv) 17.8±0.4[10] km/s
Proper motion (μ) RA: +1.31[1] mas/yr
Dec.: +0.50[1] mas/yr
Parallax (π) 3.78 ± 0.34[1] mas
Distance 860 ± 80 ly
(260 ± 20 pc)
Absolute magnitude (MV) -7.84[11]
Orbit[5]
Primary Ba
Companion Bb
Period (P) 9.860 days
Eccentricity (e) 0.1
Semi-amplitude (K1)
(primary) 25.0 km/s
Semi-amplitude (K2)
(secondary) 32.6 km/s
Details
A
Mass 23[12] M☉
Radius 78.9±7.4[13], 115[14] R☉
Luminosity (bolometric) 1.20+0.25
−0.21×105[13] L☉
Surface gravity (log g) 1.75±0.10[12] cgs
Temperature 12100±150[12] K
Metallicity [Fe/H] −0.06±0.10[6] dex
Rotational velocity (v sin i) 25±3[12] km/s
Age 8±1[6] Myr
Ba
Mass 3.84[9] M☉
Bb
Mass 2.94[9] M☉
Other designations
Rigel, Algebar, Elgebar, β Ori, 19 Ori, HD 34085, HR 1713, HIP 24436, SAO 131907, BD-08° 1063 [15]
Database references
SIMBAD data
The star as seen from Earth is actually a multiple star system of three to five stars, the primary star (Rigel A) being a blue-white supergiant which is estimated to be anywhere from 120,000 to 279,000 times as luminous as the Sun, depending on the method used to calculate its properties. It has exhausted its core of hydrogen and swollen out to between 79 and 115 times the Sun's radius. It pulsates quasi-periodically and is classified as an Alpha Cygni variable. A companion, Rigel B, is 500 times fainter than the supergiant Rigel A and visible only with a telescope. Rigel B is itself a spectroscopic binary system, consisting of two main sequence blue-white stars of spectral type B9V that are estimated to be respectively 3.9 and 2.9 times as massive as the Sun, Rigel Bb may itself be a binary. Rigel B also appears to have a very close visual companion Rigel C of almost identical appearance.
Thanks ! So Rigel's system is a big bulb in that corner of the galaxy. Bright enough to light up so distant clouds... Impressive !

Visual_Astronomer

Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by Visual_Astronomer » Mon Jan 15, 2018 5:46 pm

dkklein wrote:Compared to the post from Dec of 2006, the photo would appear to be upside-down making it a bit more challenging to see the witches head.
Actually, this photo is the correct way around, oriented about as Orion would be tonight at 9pm local time. The Dec 2006 APOD is upside down.

I have observed this a couple of times with my 20" scope. It is subtle and very large - it requires panning the scope around a lot to trace out the outline - but it can be seen.

Catalina

Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by Catalina » Mon Jan 15, 2018 6:21 pm

I believe that today's image is "upside down" only in respect to it being a recognizable image of a witch's head.

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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by Chris Peterson » Mon Jan 15, 2018 6:27 pm

Catalina wrote:I believe that today's image is "upside down" only in respect to it being a recognizable image of a witch's head.
There is a convention of showing astronomical images with north up and east to the left. And mirroring an image (e.g. north up east right) has a physical meaning with respect to visual appearance and is arguably a bad idea. But rotation is completely arbitrary. The rotation angle we see an object in the sky at depends on where we are on the Earth and what direction we're facing.
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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by Ann » Tue Jan 16, 2018 12:59 am

Let's say the Witch Head Nebula is a hundred light-years from Rigel. If Rigel was a hundred light-years away from the Solar System, how bright would it shine in the Earth's skies?

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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by neufer » Tue Jan 16, 2018 1:34 am

Ann wrote:
If Rigel was a hundred light-years away from the Solar System, how bright would it shine in the Earth's skies?
As bright as Venus: -4.54 = 0.13 - 5 log10(860/100)
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Re: APOD: Rigel and the Witch Head Nebula (2018 Jan 15)

Post by Ann » Tue Jan 16, 2018 2:23 am

Thanks, Art!

Interesting, though. Venus is not all that bright in our skies. But apparently, Venus should be able to illuminate a reflection nebula in the Earth's night sky. Right?

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