APOD: Meteor in the Milky Way (2015 Apr 23)

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APOD: Meteor in the Milky Way (2015 Apr 23)

Post by APOD Robot » Thu Apr 23, 2015 4:06 am

Image Meteor in the Milky Way

Explanation: Earth's April showers include the Lyrid Meteor Shower, observed for more than 2,000 years when the planet makes its annual passage through the dust stream of long-period Comet Thatcher. A grain of that comet's dust, moving 48 kilometers per second at an altitude of 100 kilometers or so, is swept up in this night sky view from the early hours of April 21. Flashing toward the southeastern horizon, the meteor's brilliant streak crosses the central region of the rising Milky Way. Its trail points back toward the shower's radiant in the constellation Lyra, high in the northern springtime sky and off the top of the frame. The yellowish hue of giant star Antares shines to the right of the Milky Way's bulge. Higher still is bright planet Saturn, near the right edge. Seen from Istra, Croatia, the Lyrid meteor's greenish glow reflects in the waters of the Adriatic Sea.

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the old blind man

Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by the old blind man » Thu Apr 23, 2015 4:33 am

While 'meteor in the Milky way' is technically correct, which is the best kind of correct, to an Earth-based observer 'A meteor in front of Milky way' may have been a better description. :lol2:

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Boomer12k » Thu Apr 23, 2015 9:52 am

Ummmm.....Isn't that how "War of the Worlds", starts????? :shock:

Really awesome pic...
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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Boomer12k » Thu Apr 23, 2015 9:54 am

the old blind man wrote:While 'meteor in the Milky way' is technically correct, which is the best kind of correct, to an Earth-based observer 'A meteor in front of Milky way' may have been a better description. :lol2:
They do it for effect...

Like "Five Planets", and they show only four.....ahhhhh...but there is the EARTH LANDSCAPE in the shot.... so Five....HA, HA....

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Ann » Thu Apr 23, 2015 11:08 am

Well, I like the bright orange and bright blue (or turquoise) color of the artificial Earth lights, reflecting (and enhancing) the stars colors of yellow-orange and blue. And the meteor, quite bright, flashes almost OIII-green.

I'm kind of surprised that the Lyrids, supposedly emanating from the direction of the constellation Lyra, produce meteors so far south. No wonder I haven't seen any lyrids. But a couple of nights now I have seen Antares. Yay! Antares is invisible from even the southernmost Swedish latitudes for most of the year, but in April it can be seen two hours before sunrise from where I live. And when seen from Sweden, Antares is always low in the sky and sparkling like an orange-red firecracker. Even I am impressed! :D

Oh, and I really like the APOD, too!

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Wayne N.

Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Wayne N. » Thu Apr 23, 2015 1:37 pm

It's hard to believe that streak of light in the sky is from a "grain" from a comet. If you exploded a "grain" of gun powder, you probably wouldn't even see if from 100 km away

Guest

Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Guest » Thu Apr 23, 2015 1:44 pm

That distinct green hue in the comet trail: does that indicate copper, or is it an artifact of something else?

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Chris Peterson » Thu Apr 23, 2015 1:47 pm

APOD Robot wrote:A grain of that comet's dust, moving 48 kilometers per second at an altitude of 100 kilometers or so, is swept up in this night sky view from the early hours of April 21.
A fireball like this is probably produced by a body on the order of a centimeter or more in diameter (something larger than a "grain"), and has probably survived to well below 100 km by the end of its flight.
Chris

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Chris Peterson » Thu Apr 23, 2015 1:49 pm

Guest wrote:That distinct green hue in the comet trail: does that indicate copper, or is it an artifact of something else?
The green is from atmospheric oxygen. Spectroscopically we can observe emission lines from elements in the meteoroid, but most of the color we see visually comes from ionized atmospheric gases.
Chris

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Chris Peterson » Thu Apr 23, 2015 2:08 pm

Ann wrote:And the meteor, quite bright, flashes almost OIII-green.
Because the color most likely is [O III] emission.
I'm kind of surprised that the Lyrids, supposedly emanating from the direction of the constellation Lyra, produce meteors so far south.
The radiant is at declination +32, so quite far south. This means that Lyrids can be seen from the entire northern hemisphere, and most of the southern hemisphere (down to latitude 58° south). As a rule, the farther the meteor is from the radiant, the longer the trail will appear. The meteor in this image from Istria (not "Istra") is about 60° from the radiant, which is very typical for a nice shower fireball.

In southern Sweden you are very well placed to see Lyrid meteors over the entire sky. The reason you probably haven't seen any is because it's a low activity shower, producing only a dozen or so meteors per hour, most of which are not fireballs. So this is a shower that typically goes unnoticed except by dedicated observers.
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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by neufer » Thu Apr 23, 2015 4:15 pm

Chris Peterson wrote:
Ann wrote:
And the meteor, quite bright, flashes almost OIII-green.
Because the color most likely is [O III] emission.
Chris Peterson wrote:
Guest wrote:
That distinct green hue in the comet trail: does that indicate copper, or is it an artifact of something else?
The green is from atmospheric oxygen. Spectroscopically we can observe emission lines from elements in the meteoroid, but most of the color we see visually comes from ionized atmospheric gases.
  • Not copper at least:
http://en.wikipedia.org/wiki/Meteoroid#Meteor wrote:
The visible light produced by a meteor may take on various hues, depending on the chemical composition of the meteoroid, and the speed of its movement through the atmosphere. As layers of the meteoroid abrade and ionize, the color of the light emitted may change according to the layering of minerals. Colors of meteors depend on the relative influence of the metallic content of the meteoroid versus the superheated air plasma, which its passage engenders:
  • Orange-yellow (sodium)
    Yellow (iron)
    Blue-green (magnesium)
    Violet (calcium)
    Red (atmospheric nitrogen and oxygen)
http://www.theskyscrapers.org/meteor-showers/ wrote: ------------------------------------------------
Leonids: November 16-18 : Comet 55P/Tempel-Tuttle

Mostly blue or green in color (magnesium?), with many leaving persistent dust trains behind them upon disintegrating.
------------------------------------------------
Geminids: December 12-14 : Minor planet 3200 Phaethon

The Geminids are characterized by their multi-colored display--65% being white, 26% yellow (sodium), and the remaining 9% blue, red and green.
------------------------------------------------
Capricornids: July 29-30 : Comet 169P/NEAT

The Capricornids are characterized by their often yellow (sodium) coloration and their frequent brightness.
------------------------------------------------
Lyrids: April 21-23 : Comet Thatcher

The swift and bright Lyrid meteors disintegrate after hitting our atmosphere at a moderate speed of 29.8 miles per second. They often produce luminous trains of dust that can be observed for several seconds.
Art Neuendorffer

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Chris Peterson » Thu Apr 23, 2015 4:59 pm

Both sources are only marginally accurate. Yes, those elements produce those colors. But little of the color we see visually in meteors comes from them.
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Wayne N.

Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Wayne N. » Thu Apr 23, 2015 5:19 pm

What are the calculations you use to compute the size that the "grain" must be to contain enough energy to create a streak of light in the sky of that magnitude? I'm still having a problem with a "grain" of only 1 cm diameter containing enough energy to create enough light to be observed for 10s of miles away.

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Chris Peterson » Thu Apr 23, 2015 6:18 pm

Wayne N. wrote:What are the calculations you use to compute the size that the "grain" must be to contain enough energy to create a streak of light in the sky of that magnitude? I'm still having a problem with a "grain" of only 1 cm diameter containing enough energy to create enough light to be observed for 10s of miles away.
Various models are used to determine the mass of a meteoroid from its radar properties (ionization mass) and optical properties (photometric mass). For the latter, the simplest model has the total intensity proportional to the product of the time derivative of the mass and the velocity squared (dating from work by Ceplecha in the 1950s). In recent years this has been tweaked quite a bit to take into consideration bulk material properties and other factors, as well as corrections from photometric mass to absolute mass. Still, it's substantially derived from the simple equation for kinetic energy.

The constant of proportionality is called the luminous efficiency, and relates the amount of energy given up as visible light to the total kinetic energy. This can be taken as around 0.005 for cometary fireballs. So for a 1 cm particle (density = 1) moving at 48 km/s and burning up completely in one second, the KE is 1 MJ, the optical energy is 5000 J, which corresponds to a 5000 W source persisting for one second. Certainly, a 5000 W source is very bright against the night sky even when viewed from 100 km away. (In reality, the density is probably higher than one, and the luminous efficiency could be as high as a few percent, depending on the material properties of the meteoroid.)
Chris

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by neufer » Thu Apr 23, 2015 6:56 pm

Chris Peterson wrote:
Ann wrote:
And the meteor, quite bright, flashes almost OIII-green.
Because the color most likely is [O III] emission.
Chris Peterson wrote:
Both sources are only marginally accurate. Yes, those elements produce those colors.
But little of the color we see visually in meteors comes from them.
Well... the color is NOT doubly ionized [O III] 5007 Å emission.
Last edited by neufer on Thu Apr 23, 2015 7:03 pm, edited 1 time in total.
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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Chris Peterson » Thu Apr 23, 2015 7:00 pm

neufer wrote:Well... the color is NOT doubly ionized [O III] 5007 Å emission.
Actually, it probably is. You can't compare the color of a Lyrid to a Leonid, nor the spectrum.
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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by neufer » Thu Apr 23, 2015 7:08 pm

Chris Peterson wrote:
neufer wrote:
Well... the color is NOT doubly ionized [O III] 5007 Å emission.
Actually, it probably is. You can't compare the color of a Lyrid to a Leonid, nor the spectrum.
The oxygen & nitrogen lines should be about the same.

Find a bright doubly ionized [O III] 5007 Å emission line from any meteor you like then.
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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Chris Peterson » Thu Apr 23, 2015 7:47 pm

neufer wrote:
Chris Peterson wrote:
neufer wrote:
Well... the color is NOT doubly ionized [O III] 5007 Å emission.
Actually, it probably is. You can't compare the color of a Lyrid to a Leonid, nor the spectrum.
The oxygen & nitrogen lines should be about the same.

Find a bright doubly ionized [O III] 5007 Å emission line from any meteor you like then.
[O I] (558 nm) is common at the high altitude beginning of fast meteors. Both [O I] and [O III] (501 nm) are observed near the end of slower, brighter meteors. [O III] in particular is observed with bright Perseids.

While many meteors are reported as white, yellow, or blue, green is almost always associated with bright fireballs, not ordinary meteors.
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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by neufer » Thu Apr 23, 2015 9:29 pm

Chris Peterson wrote:
[O I] (558 nm) is common at the high altitude beginning of fast meteors. Both [O I] and [O III] (501 nm) are observed near the end of slower, brighter meteors. [O III] in particular is observed with bright Perseids. While many meteors are reported as white, yellow, or blue, green is almost always associated with bright fireballs, not ordinary meteors.
  • [O I] (558 nm) is common in aurora and might well be common for some meteors.

    [O III] (501 nm) is a forbidden line only common to deep space objects like planetary nebulae.
http://en.wikipedia.org/wiki/Doubly_ionized_oxygen wrote:
http://en.wikipedia.org/wiki/Doubly_ionized_oxygen wrote:
<<In astronomy and atomic physics, doubly ionized oxygen (also known as O III) is the ion O2+. Its emission forbidden lines in the visible spectrum, primarily at the wavelength 500.7 nm, and secondarily at 495.9 nm, are known in astronomical spectroscopy as [O III]. Before spectra of oxygen ions became known, these lines once led to a spurious identification of the substance as a new chemical element. Concentrated levels of O III are found in diffuse and planetary nebulae. Consequently, narrow band-pass filters that isolate the 501 nm and 496 nm wavelengths of light, that correspond to green-turquoise-cyan spectral colors, are useful in observing these objects, causing them to appear at higher contrast against the filtered and consequently blacker background of space (and possibly light-polluted terrestrial atmosphere) where the frequencies of [O III] are much less pronounced. These emission lines were first discovered in the spectra of planetary nebulae in the 1860s. At that time, they were thought to be due to a new element which was named nebulium. In 1927, Ira Sprague Bowen came up with the current explanation of them being due to doubly ionized oxygen.

Permitted lines of O III lie in the Middle Ultraviolet band and are hence inaccessible to terrestrial astronomy.>>
http://en.wikipedia.org/wiki/Aurora wrote:
<<Red aurora: At the highest altitudes, excited atomic oxygen emits at 630.0 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this color visible only under more intense solar activity. The low amount of oxygen atoms and their gradually diminishing concentration is responsible for the faint appearance of the top parts of the "curtains". Scarlet, crimson, and carmine are the most often-seen hues of red for the aurorae.

Green aurora: At lower altitudes the more frequent collisions suppress the 630.0 nm(red) mode: rather the 557.7 nm emission (green) dominates. Fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to high stability of the N2 molecule) plays its role here as well, as it can transfer energy by collision to an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mix together to produce pink or yellow hues.) The rapid decrease of concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the lower edges of the curtains.>>
Art Neuendorffer

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by bethel95 » Sat Apr 25, 2015 3:57 am

Is it some kind of parallax effect, or is there really something wrong with the green reflection on the ocean? Having observed many such reflections at sea during my naval service, it's been my observation that the reflection "line" always points directly toward the source illumination. In this photo the reflection points to the right of the endpoint of the meteor's path. Something is just not right with this.

Anyone have a plausible explanation for what we're seeing?

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Re: APOD: Meteor in the Milky Way (2015 Apr 23)

Post by Chris Peterson » Sat Apr 25, 2015 4:05 am

bethel95 wrote:Is it some kind of parallax effect, or is there really something wrong with the green reflection on the ocean? Having observed many such reflections at sea during my naval service, it's been my observation that the reflection "line" always points directly toward the source illumination. In this photo the reflection points to the right of the endpoint of the meteor's path. Something is just not right with this.

Anyone have a plausible explanation for what we're seeing?
Panoramic shots often produce various optical distortions, like pincushion effects. The slight shift of the reflection off of vertical alignment with the terminal explosion of the meteor seems pretty minor to me.

What looks more wrong to my eyes is the reflection extending to the horizon. That looks non-physical to me. But the horizon is also too sharp. I think all of this may be a consequence of the stacking required to place the Milky Way data onto the landscape data. It's hard to do that and get a perfectly natural look.
Chris

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