Comments and questions about the
APOD on the main view screen.
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Ann
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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?
Ann
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Case
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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.
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alex_555
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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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dkklein
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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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ta152h0
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by ta152h0 » Mon Jan 15, 2018 2:33 pm
Clouds lit up by a cosmic streetlamp
Wolf Kotenberg
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rstevenson
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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.
Rob
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pgp566
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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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BDanielMayfield
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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!
Just as zero is not equal to infinity, everything coming from nothing is illogical.
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rstevenson
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by rstevenson » Mon Jan 15, 2018 2:50 pm
True. How far away must it be to exist?
Rob
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BDanielMayfield
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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.
Just as zero is not equal to infinity, everything coming from nothing is illogical.
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BDanielMayfield
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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.
Just as zero is not equal to infinity, everything coming from nothing is illogical.
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alex_555
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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 !
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Visual_Astronomer
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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.
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Catalina
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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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Chris Peterson
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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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Ann
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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?
Ann
Color Commentator
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neufer
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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)
Art Neuendorffer
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Ann
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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?
Ann
Color Commentator
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johnnydeep
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by johnnydeep » Thu Oct 29, 2020 2:42 pm
I suck at this. Comparing the Witch Head Nebula pic with this one, I have no idea how this smaller field fits into the larger Witch Head pic. Also which star is Rigel?
I'm not seeing any way to rotate and shrink the left pic to fit over the right one.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
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neufer
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by neufer » Thu Oct 29, 2020 3:53 pm
johnnydeep wrote: ↑Thu Oct 29, 2020 2:42 pm
Comparing the Witch Head Nebula pic with this one, I have no idea how this smaller field fits into the larger Witch Head pic. Also which star is Rigel?
I'm not seeing any way to rotate and shrink the left pic to fit over the right one.
Rotate today's APOD 90º counter clockwise and reduce it to about 20% size.
(Rigel is the really bright star.)
Art Neuendorffer
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johnnydeep
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by johnnydeep » Thu Oct 29, 2020 4:18 pm
neufer wrote: ↑Thu Oct 29, 2020 3:53 pm
johnnydeep wrote: ↑Thu Oct 29, 2020 2:42 pm
Comparing the Witch Head Nebula pic with this one, I have no idea how this smaller field fits into the larger Witch Head pic. Also which star is Rigel?
I'm not seeing any way to rotate and shrink the left pic to fit over the right one.
Rotate today's APOD 90º counter clockwise and reduce it to about 20% size.
(Rigel is the really bright star.)
Thanks! Got it: (I actually wanted to post this on today's 10/28/2020 APOD, but got mixed up!)
Where 'The Ghoul of IC 2118' fits in 'The Witch Head Nebula'.JPG
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"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}