APOD: Rigel Wide (2023 Apr 07)

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APOD: Rigel Wide (2023 Apr 07)

Post by APOD Robot » Fri Apr 07, 2023 4:05 am

Image Rigel Wide

Explanation: Brilliant, blue, supergiant star Rigel marks the foot of Orion the Hunter in planet Earth's night. Designated Beta Orionis, it's at the center of this remarkably deep and wide field of view. Rigel's blue color indicates that it is much hotter than its rival supergiant in Orion the yellowish Betelgeuse (Alpha Orionis), though both stars are massive enough to eventually end their days as core collapse supernovae. Some 860 light-years away, Rigel is hotter than the Sun too and extends to about 74 times the solar radius. That's about the size of the orbit of Mercury. In the 10 degree wide frame toward the nebula rich constellation, the Orion Nebula is at the upper left. To the right of Rigel and illuminated by its brilliant blue starlight lies the dusty Witch Head Nebula. Rigel is part of a multiple star system, though its companion stars are much fainter.

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Re: APOD: Rigel Wide (2023 Apr 07)

Post by Ann » Fri Apr 07, 2023 5:42 am

Rigel_wide1200[1].jpg
Rigel Wide. Image Credit: Rheinhold Wittich

What are we seeing in today's APOD? It looks as if B8Ia-type blue supergiant Rigel is creating its own red hydrogen emission nebula.

But that's not possible. At spectral class B8Ia, the temperature of the photosphere (the outer visible layer or "edge") of Rigel is ~12,000 K. That's hot, and it is more than twice as hot as the Sun, but it is nowhere near hot enough to ionize hydrogen and make it glow as a red nebula. But Rheinhold Wittich's image shows us that there is red nebulosity around Rigel. How can that be?


Well, as Stanislav Volskiy's image demonstrates, the red nebulosity near Rigel is an extension of Barnard's Loop, the very large red semicircular feature seen to the left (east) of the Orion Nebula and Orion's Belt.

Let's check out Barnard's Loop and the names of the principal stars of Orion:

Orion annotated Derrick Lim .png
Credit: Derrick Lim

Note in Stanislav Volskiy's image that Saiph does not seem to be involved much (or at all) with Barnard's Loop, because the red nebulosity of this large loop does not change its appearance in the vicinity of Saiph. And Bellatrix, the "other shoulder" of Orion, is a foreground star, and should have nothing to do with Barnard's Loop.

But Rigel is affecting and interacting with a small part of Barnard's Loop:

Rigel detail Volskiy.png

Note how the blue light of Rigel turns the red nebulosity of Barnard's Loop into a magenta hue. But also note how Rigel seems to have "cleared a hole" in the red nebula around itself. This is very obvious in the APOD.

In my opinion, it is the strong wind from Rigel that has blown this hole in the red nebula. Because we expect almost all supergiant stars to blow a strong wind.

The strong wind of Rigel has not only blown a hole in the red nebula around it, but it has sent red cloudlets of glowing hydrogen flying in all directions around itself, including above and below the supergiant star. So blue star Rigel attacks the red clouds that are coming its way.

Ann
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Re: APOD: Rigel Wide (2023 Apr 07)

Post by wolfie138 » Fri Apr 07, 2023 7:46 am

there's a big white lizard crawling across it.

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Re: APOD: Rigel Wide (2023 Apr 07)

Post by mountainjim62 » Fri Apr 07, 2023 1:41 pm

I see the lizard! His tongue is hanging out of the left side of his mouth. Looks like a HE to me but I'm not woke, so who knows.

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Re: APOD: Rigel Wide (2023 Apr 07)

Post by orin stepanek » Fri Apr 07, 2023 4:38 pm

Rigel_wide1200.jpg
Rigel; And Lizard? Looks like Sorta',---OK Sounds good to me! :mrgreen:
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Re: APOD: Rigel Wide (2023 Apr 07)

Post by johnnydeep » Fri Apr 07, 2023 8:53 pm

orin stepanek wrote: Fri Apr 07, 2023 4:38 pm Rigel_wide1200.jpg
Rigel; And Lizard? Looks like Sorta',---OK Sounds good to me! :mrgreen:
Lizard? Do you mean the elongated gray dust to the right of Rigel, also known as The Witch Head Nebula?
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Re: APOD: Rigel Wide (2023 Apr 07)

Post by johnnydeep » Fri Apr 07, 2023 8:54 pm

So, Ann, if Rigel is not responsible for the red in the part of Bernard's Loop that it is near, what is?
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Re: APOD: Rigel Wide (2023 Apr 07)

Post by johnnydeep » Fri Apr 07, 2023 8:55 pm

How is it determined (or estimated I suppose) that Rigel is 74 times the radius of the Sun?
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Re: APOD: Rigel Wide (2023 Apr 07)

Post by sc02492 » Sat Apr 08, 2023 12:15 am

johnnydeep wrote: Fri Apr 07, 2023 8:55 pm How is it determined (or estimated I suppose) that Rigel is 74 times the radius of the Sun?
By using the Stefan-Boltzmann Law, which states that a star's luminosity L is proportional to its radius^2 and temperature^4. It's been a (long) while since I crunched these numbers myself, but you can determine a star's luminosity (by knowing its relative brightness and distance from Earth) and its temperature (based on its spectral class). Then you would rearrange the SB Law for radius (i.e., R is proportional to sqrt (L)/T^2), and then form a ratio with the equivalent relationship for our own Sun (i.e., divide both equations by one another, so you get the ratio of R(Rigel)/R(Sun). This would give you a rough estimate of the relative size of Rigel's radius compared to that of our Sun's.

Sorry to be so vague, but I think you get the idea. Tomorrow if I have time I can try to put numbers to this....

Steve

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Re: APOD: Rigel Wide (2023 Apr 07)

Post by alter-ego » Sat Apr 08, 2023 1:38 am

sc02492 wrote: Sat Apr 08, 2023 12:15 am
johnnydeep wrote: Fri Apr 07, 2023 8:55 pm How is it determined (or estimated I suppose) that Rigel is 74 times the radius of the Sun?
By using the Stefan-Boltzmann Law, which states that a star's luminosity L is proportional to its radius^2 and temperature^4. It's been a (long) while since I crunched these numbers myself, but you can determine a star's luminosity (by knowing its relative brightness and distance from Earth) and its temperature (based on its spectral class). Then you would rearrange the SB Law for radius (i.e., R is proportional to sqrt (L)/T^2), and then form a ratio with the equivalent relationship for our own Sun (i.e., divide both equations by one another, so you get the ratio of R(Rigel)/R(Sun). This would give you a rough estimate of the relative size of Rigel's radius compared to that of our Sun's.

Sorry to be so vague, but I think you get the idea. Tomorrow if I have time I can try to put numbers to this....

Steve

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More directly, recent 2018 measurements using the Navy precision optical interferometer yields an angular diameter of 2.606±0.009 mas.
https://en.wikipedia.org/wiki/Rigel wrote: A 2018 study using the Navy Precision Optical Interferometer measured the angular diameter as 2.526 mas. After correcting for limb darkening, the angular diameter is found to be 2.606±0.009 mas, yielding a radius of 74.1+6.1/−7.3 R☉.[70] An older measurement of the angular diameter gives 2.75±0.01 mas,[71] equivalent to a radius of 78.9 R☉ at 264 pc.[18] These radii are calculated assuming the Hipparcos distance of 264 pc; adopting a distance of 360 pc leads to a significantly larger size.[54]
Gaia now lists Rigel having a parallax = 3.24mas ± 0.055mas → 310 pc. Assuming the interferometer gives a good angular size, then Rigel's diameter probably lands somewhere around ~70 R☉ to ~85 R☉.

Steve, it would still be interesting to know what you estimate.
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Re: APOD: Rigel Wide (2023 Apr 07)

Post by Ann » Sat Apr 08, 2023 5:12 am

johnnydeep wrote: Fri Apr 07, 2023 8:54 pm So, Ann, if Rigel is not responsible for the red in the part of Bernard's Loop that it is near, what is?
Britannica wrote:
H II region, also called diffuse nebula or emission nebula, interstellar matter consisting of ionized hydrogen atoms. The energy that is responsible for ionizing and heating the hydrogen in an emission nebula comes from a central star that has a surface temperature in excess of 20,000 K. The density of these clouds normally ranges from 10 to 100,000 particles per cubic cm; their temperature is about 8,000 K.
Wikipedia wrote:
Usually, a young star will ionize part of the same cloud from which it was born, although only massive, hot stars can release sufficient energy to ionize a significant part of a cloud. In many emission nebulae, an entire cluster of young stars is contributing energy.

Stars that are hotter than 25,000 K generally emit enough ionizing ultraviolet radiation (wavelength shorter than 91.2 nm) to cause the emission nebulae around them to be brighter than the reflection nebulae. The radiation emitted by cooler stars is generally not energetic enough to ionize hydrogen, which results in the reflection nebulae around these stars giving off more light than the emission nebulae.
Since the surface temperature of Rigel is ~ 12,000 K, Rigel's ability to create a (blue) reflection nebula around itself is much greater than its ability to ionize a (red) emission nebula. But the strong wind of Rigel can scatter and spread out the glowing red clouds of hydrogen that are a part of Barnard's Loop, and which have been ionized by other means.

Rigel detail Volskiy.png
Nebulosity around Rigel. Credit: Stanislav Volskiy.

So what caused Barnard's Loop, then?
Wikipedia wrote about Barnard's Loop:

The loop extends over about 600 arcminutes as seen from Earth, covering much of Orion. It is well seen in long-exposure photographs, although observers under very dark skies may be able to see it with the naked eye.

Recent estimates place it at a distance of either 159 pc (518 light years)[1] or 440 pc (1434 ly) giving it dimensions of either about 100 or 300 ly across respectively. It is thought to have originated in a supernova explosion about 2 million years ago, which may have also created several known runaway stars, including AE Aurigae, Mu Columbae and 53 Arietis, which are believed to have been part of a multiple star system in which one component exploded as a supernova.

The Wikipedia info on AE Aurigae, Mu Columbae and 53 Arietis doesn't quite corroborate the suggestion that there was a supernova in Orion some 2 million years ago... but, you know. Estimating exactly when something happened out there in space is a tricky business!

We may also ask why Barnard's Loop, if it is a supernova remnant, is so "one-sided". Shouldn't it look like a "full circle"?

Not necessarily. Other supernova remnants are also asymmetrical, like, for example, RCW 86.


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Re: APOD: Rigel Wide (2023 Apr 07)

Post by sc02492 » Sat Apr 08, 2023 10:50 am

alter-ego wrote: Sat Apr 08, 2023 1:38 am Steve, it would still be interesting to know what you estimate.
Here is what I obtained-

From my first post:
Radius(rigel)/Radius(sun) = [sqrt(LUM rigel)/Temp^2 rigel] / [sqrt(LUM sun)/Temp^2 sun]

Values for the above variables obtained from Wikipedia:
LUM(rigel) = 1.2x10^5 * LUM(sun)
Temp rigel (photosphere) = 12.1*10^3 K
Temp sun (photosphere)= 5.8 *10^3 K

Radius(rigel)/Radius(sun) = [sqrt(LUM rigel)/Temp^2 rigel] / [sqrt(LUM sun)/Temp^2 sun]:

= [sqrt(1.2*10^5)/(12.1*10^3)^2 ] / [sqrt(1)/(5.8*10^3)^2] (note I used value of 1 for the sun's LUM since the LUM of Rigel is expressed relative to the sun's LUM);

= 346/(12.1*10^3)^2 / [1/(5.8*10^3)^2]

= 346/(1.46*10^8) / 1/(3.36*10^7) = 346 *3.36*10^7 / 1.46*10^8

= 346 *(0.23)

= 79.6 times the radius of the sun.

Good agreement with the 2018 study that you mentioned (and I'm assuming that the values of LUM and TEMP that I obtained from Wiki were derived independently from other methods of calculating the radius of Rigel).

Steve

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Re: APOD: Rigel Wide (2023 Apr 07)

Post by orin stepanek » Sat Apr 08, 2023 9:13 pm

johnnydeep wrote: Fri Apr 07, 2023 8:53 pm
orin stepanek wrote: Fri Apr 07, 2023 4:38 pm Rigel_wide1200.jpg
Rigel; And Lizard? Looks like Sorta',---OK Sounds good to me! :mrgreen:
Lizard? Do you mean the elongated gray dust to the right of Rigel, also known as The Witch Head Nebula?
+1 :mrgreen:
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Re: APOD: Rigel Wide (2023 Apr 07)

Post by wolfie138 » Sun Apr 09, 2023 8:21 am

Image