APOD: The Tails of Comet NEOWISE (2020 Jul 11)

[APOD Robot]

The Tails of Comet NEOWISE

Explanation: Comet NEOWISE (C/2020 F3) is now sweeping through northern skies. Its developing tails stretch some six degrees across this telescopic field of view, recorded from Brno, Czech Republic before daybreak on July 10. Pushed out by the pressure of sunlight itself, the comet's broad, yellowish dust tail is easiest to see. But the image also captures a fainter, more bluish tail too, separate from the reflective comet dust. The fainter tail is an ion tail, formed as ions from the cometary coma are dragged outward by magnetic fields in the solar wind and fluoresce in the sunlight. In this sharp portrait of our new visitor from the outer Solar System, the tails of comet NEOWISE are reminiscent of the even brighter tails of Hale Bopp, the Great Comet of 1997.

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[orin stepanek]

Explanation: Comet NEOWISE (C/2020 F3) is now sweeping through northern skies. Its developing tails stretch some six degrees across this telescopic field of view, recorded from Brno, Czech Republic before daybreak on July 10. Pushed out by the pressure of sunlight itself, the comet's broad, yellowish dust tail is easiest to see. But the image also captures a fainter, more bluish tail too, separate from the reflective comet dust. The fainter tail is an ion tail, formed as ions from the cometary coma are dragged outward by magnetic fields in the solar wind and fluoresce in the sunlight. In this sharp portrait of our new visitor from the outer Solar System, the tails of comet NEOWISE are reminiscent of the even brighter tails of Hale Bopp, the Great Comet of 1997.

The tale of the tails! gotta love it!

I love comets; they give a little extra to the already exciting universe!

[Tszabeau]

What is making the orange glow in lower left? Sun or Milky Way? If the picture was recorded on the morning of 7/10, why does it say 7/7 on the image frame?

[neufer]

I love comets; they give a little extra to the already exciting universe!

• Famous last words from an old dinosaur.

[orin stepanek]

I love comets; they give a little extra to the already exciting universe!

• Famous last words from an old dinosaur.

Hey Neufer! I better be careful; I am an old dinosaur!

[bystander]

... If the picture was recorded on the morning of 7/10, why does it say 7/7 on the image frame?

It doesn't. The date on the frame is 10/7/2020. In European format, (d/m/y), that is July 10th.

[Sa Ji Tario]

This comment is for people like me who are joining this science
The image clearly shows several tails of different natures. the upper part of the streams corresponds to the ion tail caused by the solar winds and is always oriented in opposition to the direction of the Sun, it is composed of the ions that fix the solar wind of the components of the sublimated material of the interior or surface of the comet, we can say that it is a stream of plasma. The lower one is the classic tail of dust curved by the orbitality of the comet, that is, the dust is left behind in its orbital path but continues to slow down in the same orbit of the nucleus and that reveals the curved path.-
Each comet has a finite life as such, the material that sublimates when approaching the Sun is not inexhaustible and after several passes and for the same reason it stops emitting tails and becomes a "cometary asteroid", an asteroid because it lacks a tail and cometary for the genesis of its orbit, provided they are "periodic comets" with elliptical orbits. Comets with parabolic or hyperbolic orbits pass once, waving and saying goodbye forever.

[Sa Ji Tario]

Tzabeau, it must be Earth's atmosphere

[johnnydeep]

The Tails of Comet NEOWISE

Explanation: Comet NEOWISE (C/2020 F3) is now sweeping through northern skies. Its developing tails stretch some six degrees across this telescopic field of view, recorded from Brno, Czech Republic before daybreak on July 10. Pushed out by the pressure of sunlight itself, the comet's broad, yellowish dust tail is easiest to see. But the image also captures a fainter, more bluish tail too, separate from the reflective comet dust. The fainter tail is an ion tail, formed as ions from the cometary coma are dragged outward by magnetic fields in the solar wind and fluoresce in the sunlight. In this sharp portrait of our new visitor from the outer Solar System, the tails of comet NEOWISE are reminiscent of the even brighter tails of Hale Bopp, the Great Comet of 1997.

So, I have to ask: what element(s) are causing the blue ion tail?

[Ann]

The Tails of Comet NEOWISE

Explanation: Comet NEOWISE (C/2020 F3) is now sweeping through northern skies. Its developing tails stretch some six degrees across this telescopic field of view, recorded from Brno, Czech Republic before daybreak on July 10. Pushed out by the pressure of sunlight itself, the comet's broad, yellowish dust tail is easiest to see. But the image also captures a fainter, more bluish tail too, separate from the reflective comet dust. The fainter tail is an ion tail, formed as ions from the cometary coma are dragged outward by magnetic fields in the solar wind and fluoresce in the sunlight. In this sharp portrait of our new visitor from the outer Solar System, the tails of comet NEOWISE are reminiscent of the even brighter tails of Hale Bopp, the Great Comet of 1997.

So, I have to ask: what element(s) are causing the blue ion tail?

Cosmos - The SAO Encyclopedia of Astronomy wrote:

While the dust tails of comets shine by reflected sunlight and are therefore yellow in colour, the gas tails shine through fluorescence. In particular, the dominant CO+ molecule absorbs sunlight which it then re-emits at a wavelength of 4,200 angstroms as it de-excites. For this reason, ion tails tend to be blue in color.

Ann

[johnnydeep]

The Tails of Comet NEOWISE

Explanation: Comet NEOWISE (C/2020 F3) is now sweeping through northern skies. Its developing tails stretch some six degrees across this telescopic field of view, recorded from Brno, Czech Republic before daybreak on July 10. Pushed out by the pressure of sunlight itself, the comet's broad, yellowish dust tail is easiest to see. But the image also captures a fainter, more bluish tail too, separate from the reflective comet dust. The fainter tail is an ion tail, formed as ions from the cometary coma are dragged outward by magnetic fields in the solar wind and fluoresce in the sunlight. In this sharp portrait of our new visitor from the outer Solar System, the tails of comet NEOWISE are reminiscent of the even brighter tails of Hale Bopp, the Great Comet of 1997.

So, I have to ask: what element(s) are causing the blue ion tail?

Cosmos - The SAO Encyclopedia of Astronomy wrote:

While the dust tails of comets shine by reflected sunlight and are therefore yellow in colour, the gas tails shine through fluorescence. In particular, the dominant CO+ molecule absorbs sunlight which it then re-emits at a wavelength of 4,200 angstroms as it de-excites. For this reason, ion tails tend to be blue in color.

Ann

Thanks - cool site! (and very appropriate emojis) And now I have to ask the obvious follow-up: is it always primarily carbon monoxide ions (and why would that be, unless all comets are made up of the same stuff, which I find hard to believe), or have there been other comets with different colored ion tails caused by different molecules?

[Ann]

So, I have to ask: what element(s) are causing the blue ion tail?

Cosmos - The SAO Encyclopedia of Astronomy wrote:

While the dust tails of comets shine by reflected sunlight and are therefore yellow in colour, the gas tails shine through fluorescence. In particular, the dominant CO+ molecule absorbs sunlight which it then re-emits at a wavelength of 4,200 angstroms as it de-excites. For this reason, ion tails tend to be blue in color.

Ann

Thanks - cool site! (and very appropriate emojis) And now I have to ask the obvious follow-up: is it always primarily carbon monoxide ions (and why would that be, unless all comets are made up of the same stuff, which I find hard to believe), or have there been other comets with different colored ion tails caused by different molecules?

Ethan Siegel of Forbes wrote:

Comets are made out of a mix of rocky components, similar to what makes up the Earth's mantle, dust, and ices. Ice doesn't just mean water-ice (H2O), but also volatile components like dry ice (solid CO2), methane (CH4), ammonia (NH3), and carbon monoxide (CO). The full suite of cometary ices was investigated by the Rosetta mission, but these are the big five. Under typical, cold conditions, the ices remain frozen, but as the comet nears the Sun, they start to heat up.

The first thing that happens to a comet, as it approaches the Sun, is that the amount of ultraviolet light striking it becomes great enough that it can start ionizing the weakest molecule there: carbon monoxide. This creates an abundance of the CO+ ion, which streams directly away from the Sun. This turns into a blue ion tail, and is the first comet-like feature to appear as a comet begins to heat up.
...
As the nucleus of the comet gets hot, more of the ices melt and diffuse away from the surface, creating a large, diffuse set of particles around the nucleus. This diffuse region is known as the coma of a comet, and is made of a mix of gas and dust.
...
But the coma is more than dust. There is also gas, created from the sublimated compounds that were part of the comet. There aren't merely simple ices and rocks on this body, but more complex molecules made out of these fundamental building blocks: mostly hydrogen, oxygen, carbon, and nitrogen. Two molecules that are of particular interest are cyanide/cyanogen (CN: a carbon-nitrogen bond) and diatomic carbon (C2: a carbon-carbon bond).
...
When you see that green color, it's an indicator of a combination of things:

that the coma contains large amounts of CN and C2 molecules,
that the comet is active (outgassing) and warm (close to the Sun), and
that the potential for a schism or eruption is at its highest.

So, to summarize: The ion tail is blue because ultraviolet photons from the Sun excite CO molecules and make them emit blue light of 420 nm. The comet coma is bluish green because ultraviolet photons from the Sun excite CN and C2 molecules.

Note that Comet NEOWISE doesn't seem to have a green coma. This is unusual. I suspect that it does have a green coma after all, because pretty much all comets do (I believe). Maybe Comet NEOWISE is illuminated in such a way that the sunlight that strikes it has passed through the Earth's atmosphere first? So that, from our point of view, the sunlight lighting up Comet NEOWISE has been reddened?

Or something?

Ann

[neufer]

Click to play embedded YouTube video.

[johnnydeep]

Ann

Thanks - cool site! (and very appropriate emojis) And now I have to ask the obvious follow-up: is it always primarily carbon monoxide ions (and why would that be, unless all comets are made up of the same stuff, which I find hard to believe), or have there been other comets with different colored ion tails caused by different molecules?

Ethan Siegel of Forbes wrote:

...
When you see that green color, it's an indicator of a combination of things:

that the coma contains large amounts of CN and C2 molecules,
that the comet is active (outgassing) and warm (close to the Sun), and
that the potential for a schism or eruption is at its highest.

So, to summarize: The ion tail is blue because ultraviolet photons from the Sun excite CO molecules and make them emit blue light of 420 nm. The comet coma is bluish green because ultraviolet photons from the Sun excite CN and C2 molecules.

Note that Comet NEOWISE doesn't seem to have a green coma. This is unusual. I suspect that it does have a green coma after all, because pretty much all comets do (I believe). Maybe Comet NEOWISE is illuminated in such a way that the sunlight that strikes it has passed through the Earth's atmosphere first? So that, from our point of view, the sunlight lighting up Comet NEOWISE has been reddened?

Or something?

Ann

Neat. I've been looking for a comet with a green ion tail - because why couldn't there be one? - but I haven't found any. Maybe there's a reason?

As for why we don't see a green coma with NEOWISE, I'm not sure. Maybe it needs to be closer to the sun? But having things line up so that sunlight hitting NEOWISE passes through earth's atmosphere first seems astronomically unlikely given the distances and size difference between earth and the comet, no?

[Ann]

Neat. I've been looking for a comet with a green ion tail - because why couldn't there be one? - but I haven't found any. Maybe there's a reason?

As for why we don't see a green coma with NEOWISE, I'm not sure. Maybe it needs to be closer to the sun? But having things line up so that sunlight hitting NEOWISE passes through earth's atmosphere first seems astronomically unlikely given the distances and size difference between earth and the comet, no?

Judging from all the color photos I have seen of comets, there don't seem to be any comets with obviously green ion tails. I suppose the reason for that is that CO is the ice that is most easily turned into a gas and becoming ionized by ultraviolet photons from the Sun.

It is my impression, however, that many comets have quite weak ion tails. Comet NEOWISE appears to be one of them. A possible reason could be that these comets have already lost much of their CO ice, perhaps because they have come close to the Sun during prior passages and then lost much of their CO reservoirs.

Take a look at this picture of Comet Machholz, however. I love how its passage right in front of the Pleiades gave us an excellent opportunity to judge the color of its green coma, its blue ion tail and its yellowish dust tail.

Read more about the picture here.

Ann

[Chris Peterson]

As for why we don't see a green coma with NEOWISE, I'm not sure.

Really bright comets (that is, naked eye) don't usually have obvious green comas, because the bright reflected sunlight from dust overwhelms the much dimmer green glow. All those pretty comets with their green comas are more gassy than dusty, often farther from the Sun, and generally photographic objects, not visible.

[johnnydeep]

Neat. I've been looking for a comet with a green ion tail - because why couldn't there be one? - but I haven't found any. Maybe there's a reason?

As for why we don't see a green coma with NEOWISE, I'm not sure. Maybe it needs to be closer to the sun? But having things line up so that sunlight hitting NEOWISE passes through earth's atmosphere first seems astronomically unlikely given the distances and size difference between earth and the comet, no?

Judging from all the color photos I have seen of comets, there don't seem to be any comets with obviously green ion tails. I suppose the reason for that is that CO is the ice that is most easily turned into a gas and becoming ionized by ultraviolet photons from the Sun.

It is my impression, however, that many comets have quite weak ion tails. Comet NEOWISE appears to be one of them. A possible reason could be that these comets have already lost much of their CO ice, perhaps because they have come close to the Sun during prior passages and then lost much of their CO reservoirs.

Comet Machholz and the Pleiades astrosurf.png

Take a look at this picture of Comet Machholz, however. I love how its passage right in front of the Pleiades gave us an excellent opportunity to judge the color of its green coma, its blue ion tail and its yellowish dust tail.

Read more about the picture here.

Ann

Very pretty comet. Red dust tail, Green coma, and Blue ion tail - I hereby christen it an "RGB" class comet

[johnnydeep]

As for why we don't see a green coma with NEOWISE, I'm not sure.

Really bright comets (that is, naked eye) don't usually have obvious green comas, because the bright reflected sunlight from dust overwhelms the much dimmer green glow. All those pretty comets with their green comas are more gassy than dusty, often farther from the Sun, and generally photographic objects, not visible.

Ok, thanks - that makes sense.

[Boomer12k]

Awesome... so nice, it's framed!!!

:---[===] *

[neufer]

C/2014 Q2 (Lovejoy)

---

<<C/2014 Q2 (Lovejoy) is a long-period comet discovered on 17 August 2014 by Terry Lovejoy using an 8 in Schmidt–Cassegrain telescope. Its blue-green glow is the result of organic molecules and water released by the comet fluorescing under the intense UV and optical light of the Sun as it passes through space.

C/2014 Q2 originated from the Oort cloud, but is not a dynamically new comet. Before entering the planetary region (epoch 1950), C/2014 Q2 had an orbital period of about 11,000 years, with an aphelion about 995 AU from the Sun. After leaving the planetary region (epoch 2050), it will have an orbital period of about 8,000 years, with aphelion of about 800 AU.

The comet was observed to release 21 different organic molecules in gas, including ethanol and glycolaldehyde, a simple sugar. The presence of organic molecules suggests that they are preserved materials synthesized in the outskirts of the solar nebula or at earlier stages of the Solar System formation. >>

[Ann]

C/2014 Q2 (Lovejoy)

---

<<C/2014 Q2 (Lovejoy) is a long-period comet discovered on 17 August 2014 by Terry Lovejoy using an 8 in Schmidt–Cassegrain telescope. Its blue-green glow is the result of organic molecules and water released by the comet fluorescing under the intense UV and optical light of the Sun as it passes through space.

C/2014 Q2 originated from the Oort cloud, but is not a dynamically new comet. Before entering the planetary region (epoch 1950), C/2014 Q2 had an orbital period of about 11,000 years, with an aphelion about 995 AU from the Sun. After leaving the planetary region (epoch 2050), it will have an orbital period of about 8,000 years, with aphelion of about 800 AU.

The comet was observed to release 21 different organic molecules in gas, including ethanol and glycolaldehyde, a simple sugar. The presence of organic molecules suggests that they are preserved materials synthesized in the outskirts of the solar nebula or at earlier stages of the Solar System formation. >>

Thanks for that excellent example of a gas-rich comet, Art.

Ann

[Ann]

As for why we don't see a green coma with NEOWISE, I'm not sure.

Really bright comets (that is, naked eye) don't usually have obvious green comas, because the bright reflected sunlight from dust overwhelms the much dimmer green glow. All those pretty comets with their green comas are more gassy than dusty, often farther from the Sun, and generally photographic objects, not visible.

Gas-rich bright comet Hyakutake with a green coma and a long quite bright blue ion tail. Hyakutake didn't have an obvious dust tail. Photo: Fred Espenak.

---

Gas-poor bright comet Hale-Bopp with a bright white coma and a bright but somewhat short yellowish dust tail. The blue ion tail of Hale-Bopp was rather faint. Photo: Michael K. Fairbanks.

---

Thanks, Chris, that makes perfect sense!

However, bright comet Hyakutake did have a green coma and a long blue ion tail. On the other hand, extremely famous comet Hale-Bopp seemed to lack a green coma. I have looked at many color pictures of Hale-Bopp, and I haven't seen even one picture of this comet that shows an obvious green coma. Hale-Bopp had a bright white coma and a bright but rather short white or yellow-white dust tail. Hale-Bopp also had a blue ion tail, but the ion tail was considerably fainter than the dust tail.

The most likely explanation as to why comet Hale-Boop didn't have a green coma is that the green light from CN and C2 was overwhelmed by the reflected sunlight of the extremely bright dusty coma.

I read somewhere that Comet NEOWISE resembles Hale-Bopp, except that it is fainter. So comet NEOWISE is one of those gas-poor comets with a bright dusty coma reflecting the sunlight.

Ann