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, with a zenith hourly occurrence rate (ZHR) of 120. The meteors blooming in the sky can be described as a romantic blessing for the New Year. However, it is regrettable that 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 second quarter moon.
Although I regret it, "On this day next year, Qingyun will go, but 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
01
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 fire meteors in the report can cover the entire visible light 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.
〇 Single-shot with the camera
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 from right to left? Don't worry, the answer is hidden in the color of this kind of meteor, 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
〇 Direct recording of camera video
Some high-sensitivity cameras can shoot directly for video. As you can see from the GIF below, there are two shades of green before and after the meteor.
Direct Recording Meteor丨Photography: Zhang Chao
〇 Meteor spectrum
Splitting meteors with a spectrometer allows us to better distinguish the colors of meteors. The image below shows a group of meteors in the Perseid meteor shower, with a single-line of colors in the background in the near horizontal direction being stars and a shooting star 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
02
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, the atoms such as oxygen and nitrogen in the atmosphere on its path also emit a specific wavelength of light due to heat, adding a special color to the meteor. For example, the topmost line in the image above with a yellowish-green wavelength of 557.7 nm mainly comes from oxygen atoms in the atmosphere, so it is not the color of the meteor itself.
But not all meteors have this green line of 557.7 nm oxygen atoms. The average velocity of the Perseid meteor is about 62 km/s, and the greater the 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, causing the end of the meteor to be red, 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 difference between the outermost electron of the oxygen atom is larger, and the transition from S to D produces a green spectral line with a higher frequency and a shorter wavelength (557.7 nm), while the energy difference between the D transition to P and P is smaller, so it emits red light with a lower frequency and longer wavelength (636.4 nm, 630.0 nm).
快速流星的光谱，可见一条很强的557.7nm氧线丨图源：Society for Popular Astronomy
03
Spectra of different meteor showers
There is a close relationship between the speed and composition of meteors and the spectrum. Meteor spectra can be divided into four types: X, Y, Z, and W according to the characteristics of their emission lines, among which the X type has strong calcium (Ca) HK lines, 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 the type of other type of product that does not exhibit type X, Y, or Z features. The velocity of these four types corresponds to 15-20 km/s for X, 30 km/s for Z, and 60 km/s for Y, while W is an 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.
不同流星雨的光谱比较丨改自：Society for Popular Astronomy
As can be seen in the figure, since the Geminids are asteroids and 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 at the red/near-infrared end (right) also increases, and the proportion of metals (Ca, Cr, Si and Fe) at the blue terminal decreases on the left.
In addition, the oxygen line of the Geminid meteor at 557.7 nm is relatively weak. 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. The 557.7 nm and 777.7 nm rays (infrared, invisible) are the first oxygen lines of the fast meteor spectrum.
04
Meteor spectrum photography
Having said all this, are you also interested in the meteor spectrum and ready to try it in the next Quadrantids?
The principle of prismatic spectroscopy (which is also the principle of rainbow formation) is actually the dispersion of light. When light passes through different mediums, its direction of propagation is deflected, 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 separated 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
05
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 meteor to have a red color. So the direction of the meteor is drawn from right (green) to left (red). Did you get it right? 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.
Resources:
[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
Master student of the Astrochemistry Group of the Purple Mountain Astronomical Observatory, Chinese Academy of Sciences.
Rotating Editor-in-Chief: Chen Xuepeng
Editor: Wang Kechao
The reprinted 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 to reprint, please contact the original official account
Source: Purple Mountain Observatory, Chinese Academy of Sciences
Edit: Snow Shadow