APOD: Thor's Helmet (2024 Jan 09)

[APOD Robot]

Thor's Helmet

Explanation: Thor not only has his own day (Thursday), but a helmet in the heavens. Popularly called Thor's Helmet, NGC 2359 is a hat-shaped cosmic cloud with wing-like appendages. Heroically sized even for a Norse god, Thor's Helmet is about 30 light-years across. In fact, the cosmic head-covering is more like an interstellar bubble, blown with a fast wind from the bright, massive star near the bubble's center. Known as a Wolf-Rayet star, the central star is an extremely hot giant thought to be in a brief, pre-supernova stage of evolution. NGC 2359 is located about 15,000 light-years away toward the constellation of the Great Overdog. This remarkably sharp image is a mixed cocktail of data from narrowband filters, capturing not only natural looking stars but details of the nebula's filamentary structures. The star in the center of Thor's Helmet is expected to explode in a spectacular supernova sometime within the next few thousand years.

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[Ann]

Thor's Helmet. Image Credit & Copyright: Ritesh Biswas

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I should not be the first person to comment on this image, because I'm the Color Commentator. I really really like RGB images, and I love all things blue. The APOD offers me neither.

Okay, but let's start with what is good about this APOD.

APOD Robot wrote:
This remarkably sharp image...

It is. And the colors are striking and captivating, too. An interesting aspect of the APOD is that it shows us three horns of Thor's Helmet, not just two, which is how Thor's Helmet is usually portrayed. The horns or legs look very impressive and sort of menacing. Beware of the angry Viking, or, more likely, beware of the insectoid or arachnid creature from space!!!

However, on a less fanciful note, let's compare a part of the APOD with an RGB image of Thor's Helmet:

Thor's Helmet in RGB. Credit: Cosmic Photo.

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You can see that the green color of Thor's "hat" is reasonably similar in both images. The green color comes from OIII at 501 nm, and it represents a high level of ionization typically found near very hot stars.

However, in the picture by Cosmic Photo, the green color is mostly limited to Thor's "hat", while in the APOD it extends much further. This may not be wrong, of course. Indeed, it may show us the full extent of the OIII emission.

You can see that the red "horns" of Thor's Helmet are clearly red in the RGB image, but they are orange-brown in the APOD. The red color comes from ionized hydrogen, hydrogen alpha, at 656 nm, and probably also from ionized nitrogen, NII, at 658 nm. I find the orange-brown color a bit strange, although it obviously has something to do with the dust content in the "horns".

And now for the blue stuff. Sigh.

You could say that the ionizing star, WR 7 or HD 56925, is surrounded by so much gas and dust that its light can't possibly look very blue as seen from the Earth. And you would be right, too. The light that reaches us from this star does not looking strikingly blue. Its apparent color is, in fact, very similar to the color of the star Altair.

The Summer Triangle. Altair is at lower left. Credit: John Chumack/SCIENCE PHOTO LIBRARY

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Altair looks bluish in this picture from John Chumack, but it doesn't look blue to the eye. It looks white to the eye, although it may indeed look bluish when seen through a relatively large telescope. I have seen it look blue-white myself through a 14 inch telescope.

Here's my point, though. Altair certainly doesn't look yellow to the eye. And it most definitely doesn't look orange.

Take a look at the APOD again. Where is the mighty ionizing star, WR 7? Perhaps we don't see it, because there are starless pictures of nebulas. But if we don't see it, then what is that small orange speck located just where the image from Cosmic Photo showed us that WR 7 is found? Isn't the orange speck WR 7?

Thor's Helmet in RGB. Credit: Cosmic Photo.

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Isn't the orange speck in the APOD the hot central star of Thor's Helmet, WR 7? If it is, the Color Commentator is not so happy.

Ann

[JimB]

If you were 60 light years away from Thor's Helmet, would you be able to see this amazing vista with your own eyes?

[joelsantiago]

I always have a wonder. How do astronomers capture images of space objects like Thor's Helmet? It's so greatgeometry dash subzero

[Chris Peterson]

If you were 60 light years away from Thor's Helmet, would you be able to see this amazing vista with your own eyes?

No. If you were 60 ly away it would look the same as if you viewed it from Earth with a telescope at 250 power. A faint gray, barely visible blob with little or no structure.

[Holger Nielsen]

If you were 60 light years away from Thor's Helmet, would you be able to see this amazing vista with your own eyes?

It would be invisible to the naked eye at any distance. If you move towards it, it also increases its extension in the sky.

[Chris Peterson]

If you were 60 light years away from Thor's Helmet, would you be able to see this amazing vista with your own eyes?

It would be invisible to the naked eye at any distance. If you move towards it, it also increases its extension in the sky.

It wouldn't be invisible. NGC 2359 is an accessible target for visual astronomers.

[Ann]

If you were 60 light years away from Thor's Helmet, would you be able to see this amazing vista with your own eyes?

Actually no, you wouldn't.

Okay, yes, you would! Chris says so, and if Chris says so, then you'd better believe him. But you wouldn't see it more clearly than we do now, thousands of light-years from the nebula.

Consider these images:

A bank of low clouds near Assateague Beach, Maryland, on June 6, 2017. Credit: Wayne and Amber Ayer/Facebook

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A bank of low clouds is seen from a road near Assateague Beach, Maryland, on June 6, 2017. (Wayne and Amber Ayer/Facebook)

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If you managed to get much closer to this particular cloud bank, would you see it more clearly? Would you see more details in it?

You wouldn't. Because even though the differences between an Earthly fog bank and an interstellar nebula are huge, they are both diffuse objects that may seem to dissolve before our eyes when we get closer. We often see their structures more clearly from a distance, because then we can see the contrast between the fog bank or the nebula and its background.

As for photographing nebulas, that is not easy, although many amateurs do an absolutely great job at photographing them.

Consider the Rosette Nebula. We are used to seeing the Rosette Nebula like this, flaunting its rosy red colors:

The Rosette Nebula. Credit: Andreas Fink/Wikimedia Commons.

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In reality though, the nebula is quite faint, whereas the stars are relatively bright. This is what the Rosette Nebula and the ionizing cluster, NGC 2244, looked like to a man who made a sketch of what he saw, when he looked at the Rosette Nebula through a telescope:

A sketch of NGC 2244, the central cluster of the Rosette Nebula, and some surrounding nebulosity. Credit: Michael Vlasov.

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Note the star colors. Michael Vlasov, who made the sketch, was indeed able to see color in at least some of the stars. But the Rosette Nebula itself was colorless and faint to him.

In order to really photograph a nebula, any nebula, and make it look colorful and bright, you need a camera and filters that are suitable for bringing out details and colors of nebulas. Also you have to do a lot of post-processing to bring out all the details you want to show the world. Remember that most nebulas are far fainter than the stars that ionize them, so you have to photograph the nebula so that you don't just bring out the stars.

And if you did come very much closer to Thor's Helmet, the star, WR 7, which is a point source, would get ever brighter and brighter. It really would.

Click to view full size image

Ann

[RandyM]

Thanks for APOD! Why is one star, located above the helmet at the 1 o'clock position and within the wings, so red?

[johnnydeep]

Thanks for APOD! Why is one star, located above the helmet at the 1 o'clock position and within the wings, so red?

I would assume for the same reason that the super bright star down in the lower right is also red. The colors we see are due both to the filters used to take the picture and the postprocessing that was done. I'm sure Ann - who has also mentioned the strikingly (and irksome) redness of the central W-R star in her post above - or Chris, can elucidate further.

Unfortunately, at the links in the Astrobin source image for this APOD, the search links for the individual filter frames used seem to be borked, or return to much too be useful, but perhaps I just don't know what I'm doing. E.g.:

https://www.astrobin.com/search/?q=Antl ... ha+1.25%22

[Chris Peterson]

Thanks for APOD! Why is one star, located above the helmet at the 1 o'clock position and within the wings, so red?

Well, that star, 2MASS J07181340-1315288 (Gaia DR3 3032940260440588544) appears to be very cool. No red band flux is available, but in the J band (1200 nm) it is about 16 times brighter than in the G band (464 nm). So the two red filters (H-a and S II) will pass a lot more light than the green filter (O III). I'm not sure what the final mapping used was, but looking at the histogram I'd guess that the blue and green channels are filled from the O III data and the red channel is filled from the H-a and S II data. Which would explain why the star is so red.

[johnnydeep]

Thanks for APOD! Why is one star, located above the helmet at the 1 o'clock position and within the wings, so red?

Well, that star, 2MASS J07181340-1315288 (Gaia DR3 3032940260440588544) appears to be very cool. No red band flux is available, but in the J band (1200 nm) it is about 16 times brighter than in the G band (464 nm). So the two red filters (H-a and S II) will pass a lot more light than the green filter (O III). I'm not sure what the final mapping used was, but looking at the histogram I'd guess that the blue and green channels are filled from the O III data and the red channel is filled from the H-a and S II data. Which would explain why the star is so red.

Would that also explain why the W-R star - which is not cool at all! - is so red, as well as the bright - foreground? - star in the lower right?

[Chris Peterson]

Thanks for APOD! Why is one star, located above the helmet at the 1 o'clock position and within the wings, so red?

Well, that star, 2MASS J07181340-1315288 (Gaia DR3 3032940260440588544) appears to be very cool. No red band flux is available, but in the J band (1200 nm) it is about 16 times brighter than in the G band (464 nm). So the two red filters (H-a and S II) will pass a lot more light than the green filter (O III). I'm not sure what the final mapping used was, but looking at the histogram I'd guess that the blue and green channels are filled from the O III data and the red channel is filled from the H-a and S II data. Which would explain why the star is so red.

Would that also explain why the W-R star - which is not cool at all! - is so red, as well as the bright - foreground? - star in the lower right?

The foreground star, 2MASS J07190568-1315389 is certainly a cool star that is much brighter at the longer wavelengths. So it would explain that.

The W-R star, 2MASS J07182912-1313015 is brighter in the IR than the visible, which may have to do with the effects of dust. (Thermal dust emission is common in W-R nebulas; the presence of dust could cause reddening in optical wavelengths.) Its R and G flux is similar, but maybe twice as much R signal is present because that's coming from two filters, while the green is coming from just one. So much depends on the processing, and the final colors we get from narrowband data can be quite variable.

[Ann]

Thanks for APOD! Why is one star, located above the helmet at the 1 o'clock position and within the wings, so red?

There is a star in this position, HD 57057, which is quite red. Its B-V index is almost +2.0, which is quite red for stars.

HD 57057 is apparently a giant, very cool, very luminous star. It's very distant too, at some 11,000 light-years away from us. Its V luminosity of 8.77 magnitudes means it's emitting some 3,000 times the light of the Sun in yellow-green light. In infrared light, at a wavelength described as J (I'll let someone else check exactly what wavelength that is), the star is much brighter, some 120,000 times the luminosity of the Sun.

Simbad Astronomical Database described HD 57057 as a long-period variable.

Wikipedia wrote:

Long period variables are pulsating cool giant, or supergiant, variable stars with periods from around a hundred days, or just a few days for OSARGs, to more than a thousand days. In some cases, the variations are too poorly defined to identify a period, although it is an open question whether they are truly non-periodic.[8]

LPVs have spectral class F and redwards, but most are spectral class M, S or C. Many of the reddest stars in the sky, such as Y CVn, V Aql, and VX Sgr are LPVs.

Most LPVs, including all Mira variables, are thermally-pulsing asymptotic giant branch stars with luminosities several thousand times the sun. Some semiregular and irregular variables are less luminous giant stars, while others are more luminous supergiants including some of the largest known stars such as VY CMa.

If many long-period variables are asymtotic giant branch stars, then they are on their last legs before they shed their atmospheres and become planetary nebulas and white dwarfs. These stars are typically both very red and really quite bright.

Globular cluster NGC 6397. Normally, the brightest, reddest stars of globular clusters are asymtotic giant branch stars, which are about to kick the bucket on their fusion lifeline and become planetary nebulas and white dwarfs. Credit: Credit: NASA, ESA, and T. Brown and S. Casertano (STScI) Acknowledgement: NASA, ESA, and J. Anderson (STScI)

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So we can probably say that HD 57057 is about to die, just like the famous Mira, but it's putting on one final bright red light show before its spectacular death.

Famous long-period soon-to-be-dead bright red star Mira. No one puts Mira in a corner, except perhaps DSS 2/ESO. Credit: DSS 2/ESO

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Ann

[johnnydeep]

Well, that star, 2MASS J07181340-1315288 (Gaia DR3 3032940260440588544) appears to be very cool. No red band flux is available, but in the J band (1200 nm) it is about 16 times brighter than in the G band (464 nm). So the two red filters (H-a and S II) will pass a lot more light than the green filter (O III). I'm not sure what the final mapping used was, but looking at the histogram I'd guess that the blue and green channels are filled from the O III data and the red channel is filled from the H-a and S II data. Which would explain why the star is so red.

Would that also explain why the W-R star - which is not cool at all! - is so red, as well as the bright - foreground? - star in the lower right?

The foreground star, 2MASS J07190568-1315389 is certainly a cool star that is much brighter at the longer wavelengths. So it would explain that.

The W-R star, 2MASS J07182912-1313015 is brighter in the IR than the visible, which may have to do with the effects of dust. (Thermal dust emission is common in W-R nebulas; the presence of dust could cause reddening in optical wavelengths.) Its R and G flux is similar, but maybe twice as much R signal is present because that's coming from two filters, while the green is coming from just one. So much depends on the processing, and the final colors we get from narrowband data can be quite variable.

Thanks! i think I even understood most of that.

[Holger Nielsen]

If you were 60 light years away from Thor's Helmet, would you be able to see this amazing vista with your own eyes?

It would be invisible to the naked eye at any distance. If you move towards it, it also increases its extension in the sky.

It wouldn't be invisible. NGC 2359 is an accessible target for visual astronomers.

I was talking about visibility to the naked eye. I dont't know the magnitude of NGC 2359, but was thinking of it as comparable to the Ring Nebula or the Crab Nebula, which are invisible to the naked eye - at any distance.

[JimB]

I always have a wonder. How do astronomers capture images of space objects like Thor's Helmet? It's so greatgeometry dash subzero

It's usually done with a digital camera attached to a telescope. The telescope doesn't have to be really big but it does have to be motor driven so that it tracks the target exactly across the sky as the earth rotates.
The night sky on it's own can be pretty impressive if you can find some place where there is no light pollution!

[Chris Peterson]

It would be invisible to the naked eye at any distance. If you move towards it, it also increases its extension in the sky.

It wouldn't be invisible. NGC 2359 is an accessible target for visual astronomers.

I was talking about visibility to the naked eye. I dont't know the magnitude of NGC 2359, but was thinking of it as comparable to the Ring Nebula or the Crab Nebula, which are invisible to the naked eye - at any distance.

This object is already visible to the naked eye, or we could not see it in a telescope. Telescopes do not make extended objects brighter, only larger.

At a distance of 60 ly this object to the naked eye would look exactly like it looks through an eyepiece at 250 power on Earth.

[johnnydeep]

It wouldn't be invisible. NGC 2359 is an accessible target for visual astronomers.

I was talking about visibility to the naked eye. I dont't know the magnitude of NGC 2359, but was thinking of it as comparable to the Ring Nebula or the Crab Nebula, which are invisible to the naked eye - at any distance.

This object is already visible to the naked eye, or we could not see it in a telescope. Telescopes do not make extended objects brighter, only larger.

At a distance of 60 ly this object to the naked eye would look exactly like it looks through an eyepiece at 250 power on Earth.

Wait, are you saying that every object that a ground based telescope is able to see is ALSO visible to the naked eye? That can't be right, since some deep sky objects simply don't emit enough photons for our eyes to detect. Yes, the photos enter our eye, but we don't register them due to our eyes' limited sensitivity.

[Chris Peterson]

I was talking about visibility to the naked eye. I dont't know the magnitude of NGC 2359, but was thinking of it as comparable to the Ring Nebula or the Crab Nebula, which are invisible to the naked eye - at any distance.

This object is already visible to the naked eye, or we could not see it in a telescope. Telescopes do not make extended objects brighter, only larger.

At a distance of 60 ly this object to the naked eye would look exactly like it looks through an eyepiece at 250 power on Earth.

Wait, are you saying that every object that a ground based telescope is able to see is ALSO visible to the naked eye? That can't be right, since some deep sky objects simply don't emit enough photons for our eyes to detect. Yes, the photos enter our eye, but we don't register them due to our eyes' limited sensitivity.

I'm saying that every (extended) object that is bright enough to see visually through a telescope eyepiece is bright enough to see with the naked eye. All a telescope can do is make it bigger, not brighter. Bigger can make it more visible because more of the retina is involved... but that is exactly what happens when you get closer, as well.

[johnnydeep]

This object is already visible to the naked eye, or we could not see it in a telescope. Telescopes do not make extended objects brighter, only larger.

At a distance of 60 ly this object to the naked eye would look exactly like it looks through an eyepiece at 250 power on Earth.

Wait, are you saying that every object that a ground based telescope is able to see is ALSO visible to the naked eye? That can't be right, since some deep sky objects simply don't emit enough photons for our eyes to detect. Yes, the photos enter our eye, but we don't register them due to our eyes' limited sensitivity.

I'm saying that every (extended) object that is bright enough to see visually through a telescope eyepiece is bright enough to see with the naked eye. All a telescope can do is make it bigger, not brighter. Bigger can make it more visible because more of the retina is involved... but that is exactly what happens when you get closer, as well.

Oh, I see where I misinterpreted you! I was thinking of a telescope recording an image with a camera, which allows more photons to be detected over time, which is a benefit not available to your eye when it look's though a telescope's eyepiece.

[Chris Peterson]

Wait, are you saying that every object that a ground based telescope is able to see is ALSO visible to the naked eye? That can't be right, since some deep sky objects simply don't emit enough photons for our eyes to detect. Yes, the photos enter our eye, but we don't register them due to our eyes' limited sensitivity.

I'm saying that every (extended) object that is bright enough to see visually through a telescope eyepiece is bright enough to see with the naked eye. All a telescope can do is make it bigger, not brighter. Bigger can make it more visible because more of the retina is involved... but that is exactly what happens when you get closer, as well.

Oh, I see where I misinterpreted you! I was thinking of a telescope recording an image with a camera, which allows more photons to be detected over time, which is a benefit not available to your eye when it look's though a telescope's eyepiece.

And if we're being very technical, that system with the camera isn't even using a telescope in the optical sense. A telescope requires an objective and an ocular, and is classified as an afocal system. One that has no focus. A camera on a "telescope" is just an objective, which brings an image to focus on the camera sensor.

[johnnydeep]

I'm saying that every (extended) object that is bright enough to see visually through a telescope eyepiece is bright enough to see with the naked eye. All a telescope can do is make it bigger, not brighter. Bigger can make it more visible because more of the retina is involved... but that is exactly what happens when you get closer, as well.

Oh, I see where I misinterpreted you! I was thinking of a telescope recording an image with a camera, which allows more photons to be detected over time, which is a benefit not available to your eye when it look's though a telescope's eyepiece.

And if we're being very technical, that system with the camera isn't even using a telescope in the optical sense. A telescope requires an objective and an ocular, and is classified as an afocal system. One that has no focus. A camera on a "telescope" is just an objective, which brings an image to focus on the camera sensor.

Hmm. So what would "using a telescope in the optical sense" mean? Does that only happen when you use your eye to look through the eyepiece? And if so, why? Just because your eye's lens is used to focus the incoming (already) parallel light rays?

[Chris Peterson]

Oh, I see where I misinterpreted you! I was thinking of a telescope recording an image with a camera, which allows more photons to be detected over time, which is a benefit not available to your eye when it look's though a telescope's eyepiece.

And if we're being very technical, that system with the camera isn't even using a telescope in the optical sense. A telescope requires an objective and an ocular, and is classified as an afocal system. One that has no focus. A camera on a "telescope" is just an objective, which brings an image to focus on the camera sensor.

Hmm. So what would "using a telescope in the optical sense" mean? Does that only happen when you use your eye to look through the eyepiece? And if so, why? Just because your eye's lens is used to focus the incoming (already) parallel light rays?

Exactly. At its most formal, a telescope is an afocal optical system. Your eye is a focal optical system, and that's what brings the image to focus. Of course, in practical usage "telescope" has come to mean "big tube with lenses and/or mirrors used to examine astronomical objects... even if it's just a single objective. (And I'm not arguing against that usage, just pointing out an interesting technicality.)

[JimB]

I'm saying that every (extended) object that is bright enough to see visually through a telescope eyepiece is bright enough to see with the naked eye. All a telescope can do is make it bigger, not brighter. Bigger can make it more visible because more of the retina is involved... but that is exactly what happens when you get closer, as well.

I really have to query this. The bigger the telescope aperture, the fainter the objects that you will be able to observe. An 8 inch diameter mirror will have almost twice the light gathering power as a 6 inch mirror, so if the same magnification is used on both then the image will be brighter with the 8 inch telescope.