As the "canopy drama" at the beginning of 2024, the first large-scale meteor shower, the Quadrantids, reached its maximum at 5 p.m. Beijing time on January 4, and the zenith hourly occurrence rate (ZHR) could reach 120. The meteors blooming in the sky can be described as a romantic blessing for the New Year. Unfortunately, for most parts of the country, the meteor shower radiant does not rise until after midnight, making it difficult to observe the maximum period, and the observation conditions are not ideal due to the interference of the lower quarter moon.
Although I regret it, "next year, when Qingyun goes, I will laugh at the busy world." "Let's always look forward to the next meteor shower of our lives. In order to "learn something new every time", let's let you know the color of meteors this time.
Geminid Meteor Spectrum丨Photo: Zhang Chao.
The color of a shooting star.
Usually, fire meteor observers report the color of the meteor in detail because the fire meteor is bright enough that human color vision can perceive it. The range of colors of the meteors in the report can cover the entire visible spectrum, from red to blue, and very little violet. However, it's not just fire meteors that have a color, each meteor has its own color. Although it is impossible to distinguish with the naked eye, we can still get the color of this meteor by taking pictures with a digital camera or spectroscopy.
The camera takes a single shot.
The picture below shows the meteors in the Perseid group captured by the author at the Qinghai Observatory of the Purple Mountain Observatory in 2023, and it can be clearly seen that the left side of the meteor is red and the right side is green. Here's a small question, is the direction of this meteor from left to right, or right to left? Don't worry, the answer is hidden in the color of this kind of meteor, and I believe that after reading this article, you will be able to get the answer.
Perseid meteor at the Qinghai Observatory of the Purple Mountain Observatory丨Photo: Shan Jiahui.
Camera** direct recording.
There are some high-sensitivity cameras that shoot directly. As you can see from the following shot of the meteor falling **, the meteor shows two shades of green with a slight difference in hue before and after the meteor.
Direct Recording Meteor丨Photography: Zhang Chao.
Meteor spectrum. Splitting meteors with a spectrometer allows us to better distinguish the colors of meteors. The image below shows a group of meteors from the Perseid meteor shower, with a single-colored line in the background near horizontal and a meteor in the vertical direction. Through the prism we can get the color of the different phases and components of this meteor.
Perseid Meteor Spectrum丨Photo: Zhang Chao.
The principle of luminescence of meteors.
When a meteoroid rapidly breaks into the Earth's atmosphere, friction with the atmosphere causes it to heat up and emit light. The chemical composition of a meteor is the main factor that determines its color. Some elements emit a specific wavelength of light when burned at high temperatures, thus revealing their characteristic colors. For example, in the blue-green image above, the wavelength of magnesium wire is 518nm, and the yellow color is the wavelength of 589nm sodium wire. In addition, when a meteor passes through the atmosphere, atoms such as oxygen and nitrogen in the atmosphere on its path also emit a specific wavelength of light due to heating, adding a special color to the meteor. For example, the topmost wavelength in the figure above is 557The yellowish-green line at 7nm comes mainly from oxygen atoms in the atmosphere, so it is not the color of the meteor itself.
But not all meteors have this 557The green line of the oxygen atom at 7 nm. The average velocity of the Perseid meteor is about 62 kms, and the more kinetic energy it carries, the more it will start to glow high in the thin atmosphere. If the meteoroid is large enough to reach the lower part of the atmosphere, the atmosphere is no longer thinner and more oxygen and nitrogen are excited, emitting a wide red line, resulting in a red end of the meteor, which is the red line on the left in the image above.
In general, the color of a meteor is related to its chemical composition, size, velocity, and the height at which it is vaporized. So color is another key to understanding meteors, and although complex, it's also a lovely place to study meteor spectrum.
Interspersed here is a trivia of atomic physics: the outer electrons of atoms absorb energy at high temperatures and jump to higher energy levels, and when they return to lower energy levels, they produce photons of a specific wavelength. Due to the different energy differences between the different energy levels, the wavelengths of the photons produced by the transitions are also different.
Schematic diagram of oxygen atomic energy level丨Source: Sun Rong.
For example, the energy gap between the outermost electron S and D levels of an oxygen atom is even greater, and the transition from S to D produces green spectral lines with higher frequencies and shorter wavelengths (5577nm）；However, the energy difference between d and p transitions to p and p is smaller, so red light with a lower frequency and longer wavelength is emitted (6364nm，630.0nm）。
The spectrum of a fast meteor shows a strong 5577nm oxygen line丨Source: Society for Popular Astronomy
Spectra of different meteor showers.
There is a close relationship between the velocity and composition of meteors and the spectrum. Meteor spectra can be divided into four types according to their emission line characteristics: X, Y, Z, and W, among which the X type has strong calcium (Ca) H**, the Y type has strong sodium (Na) and magnesium (Mg) lines, and the Z type has abundant iron (Fe) or chromium (Cr) lines. Type W is a class of other types that do not exhibit type X, Y, or Z features. The velocity of these four types corresponds to meteoroids of about 15-20 km s, z about 30 km s, and y about 60 km s, while w belongs to meteoroids of unusual composition.
Some scholars have analyzed the spectra of the Geminids, Lyrids, Perseids, and Leonids meteor showers in 2017 in order of speed. Considering that no observation conditions and magnitudes are exactly the same, a representative spectrum with as close a magnitude and wavelength range as possible is selected for each meteor shower.
Spectral comparison of different meteor showers丨Adapted from: Society for Popular Astronomy
As can be seen in the figure, since the parent body of the Geminids is an asteroid, while the other meteor showers are all from comets, there are obvious differences in composition, so the metal line on the left side can be seen to be significantly stronger than other meteor showers. In addition to the Geminid meteor shower, we can also see that as the velocity increases, the energy carried by the meteoroid also increases, the proportion of oxygen and nitrogen in the atmosphere in the red near-infrared end (right) also increases, and the proportion of metals (Ca, Cr, Si and Fe) in the blue end decreases on the left.
In addition, the Geminid meteor 557The oxygen line at 7nm is weaker. Because this oxygen line only occurs on fast meteors that begin to interact with the atmosphere above 110 kilometers, the Geminid meteor shower is usually white in color. 557.7nm and 777The 7nm ray (infrared, invisible) is the oxygen line at the very beginning of the fast meteor spectrum.
Meteor spectrum photography.
With all that said, are you also interested in the meteor spectrum and ready to try it in the next Quadrantids? The following is an introduction to the method of using an object prism to shoot the meteor spectrum**.
The principle of prism spectroscopy (which is also the principle of rainbow formation) is actually the dispersion of light. When light passes through different mediums, it will deflect in the direction of propagation, a phenomenon known as refraction of light, and the magnitude of the light deflection is related to the frequency of the light and the refractive index of the medium. Due to the different frequencies of light of different colors, when they enter the prism from the air, the direction of propagation will be deflected by different amplitudes, so that we can distinguish the light mixed together, which is the dispersion of light.
Schematic diagram of prism spectroscopy丨Source: network.
According to this principle, the easiest way to get the spectrum of a meteor is to insert such an angled prism into the lens, regardless of the combination of prisms.
An object-end prism design scheme.
Prism spectroscopy is the easiest and cheapest way, and the object-end prism scheme can also capture comet spectra, stellar spectra, solar eclipse spectra, etc. The disadvantage is that it can only be used to detect narrow spectra, and the split spectra are not linear, and they have to be calibrated by stars with known spectra.
In addition to prism spectroscopy, we can also use gratings to obtain spectra, which will not be introduced in this article.
Spectrum of Perseid meteors captured using an object prism. The prism breaks up the background star along with the light of the meteor, and after several images are overlayed, some of the star's main absorption lines become more apparent. 丨Photography: Shan Jiahui.
Epilogue. There are still very few spectral shots of meteors in China, and I hope this article can help everyone's shooting and become a "brick" that attracts thousands of "jade".
Finally, to answer the initial question, because Perseid meteors are relatively fast, they begin to glow green in the thinner parts of the atmosphere, and then to the lower parts of the atmosphere, where a wide red band from oxygen and nitrogen dominates, causing the end of the meteor to be red. So the direction of the meteor is drawn from right (green) to left (red). Did you get it right? Let's throw another small question, what is the reason for the green color of the aurora?
Thanks to Mr. Zhang Chao and Mr. Ye Quanzhi for their correction of the article.
References: 1] IMO Photographic Handbook - Part 3: Meteor Spectra
2] cheng, s. and cheng, s. wgn, journal of the imo, 2011,39:2
About the Author. Shan Jiahui
Ph.D. candidate of the Solar High Energy Group of the Purple Mountain Observatory, Chinese Academy of Sciences.
Wang Yuting is a master's student in the Astrochemistry Group of the Purple Mountain Observatory of the Chinese Academy of Sciences.
Rotating Editor-in-Chief: Chen Xuepeng.
Editor: Wang Kechao.
*The content represents the author's views only.
It does not represent the position of the Institute of Physics of the Chinese Academy of Sciences.
If you need it, please contact the original***
*: Purple Mountain Astronomical Observatory, Chinese Academy of Sciences.
Edit: Snow Shadow.