This article is from WeChat official account:Science University (ID: kexuedayuan), author: Wang Zheng, head Figure from: vision China
The biggest feature of a comet is its long tail. Therefore, it was also called a “broom star” in ancient times.
The curious thing is, why does the comet drag a long tail? How is this long tail formed?
To answer this question, you must first understand the structure of comets.
The trilogy of comet structure: comet head, comet hair and comet tail
In simple terms, comets are divided into three parts: “comet head”, “coma hair” and “comet tail”. The comet nucleus at the head of a comet is its core and is made of ice material because the place where it was born is very cold. When a comet approaches the sun, the material in the comet nucleus sublimates, forming a hazy mass around the comet nucleus on the head, called coma. The comet’s volatile matter drags into a thin stream of tail, which is shaped like a broom, called a comet tail.
Photo of ISON Comet (Source: http://i3.sinaimg.cn/IT/2013/1205/U5385P2DT20131205094233.jpg)
The core of the comet head is the comet nucleus, which is also the core of the comet and the source of volatile matter. Generally, we think that the comet nucleus is a solid, composed of rocks, iron, dust, ammonia, methane, and ice. This is why we sometimes call comets “dirty snowballs”: “snow” refers to water frozen into ice, and “dirty” refers to mixed impurities. The diameter of the comet nucleus is very small, ranging from a few kilometers to a dozen kilometers, and the smallest is even a few hundred meters. The average density of the comet nucleus is 1 gram per cubic centimeter, and it alone accounts for more than 95% of the comet’s mass. It can also be seen how thin and “elusive” the other parts to be discussed next are.
Coma is a spherical mist composed of gas and dust around the comet nucleus, which wraps the comet nucleus like an outer membrane, or like the “atmosphere” around the comet nucleus. However, the coma is much larger than the nucleus, with a radius of several hundred thousand kilometers, and the average density is less than one billionth of the density of the earth’s atmosphere. A coma is the gas volatilized from the nucleus of a comet, and the average velocity flowing out from the inside out is 1 to 3 kilometers per second. The main components of gas in coma are neutral molecules and atoms, including hydrogen, hydroxyl, oxygen, sulfur, carbon, carbon monoxide, amino, cyanide, sodium and so on.
In actual observations, scientists discovered that some comets are surrounded by a cloud of hydrogen atoms outside the coma, which can reach 1 to 10 million kilometers in diameter. This material is called “comet cloud” or “hydrogen cloud”, and can also be counted as a component of comet head. But this is only part of comets.
The general structure of the comet (Source: http://5b0988e595225.cdn.sohucs.com/images/20171211/00c06b232b7d4949b453a707f98539c3.jpeg)
We watched airplanes flying in the sky, sometimes leaving such a white mark. So, comets have such a thin gas, when flying at high speed, “tail”Is it the trajectory behind it? In fact, the reason for the formation of the comet tail is not so simple.
What conjectures have scientists made to explore the comet tail?
Scientists have discovered through constant observation that when a comet moves near the sun, the ionized tail always faces away from the sun, and the closer it is to the sun, the longer the tail. In other words, if it approaches the sun and then flies away, its flight direction and the direction of the comet tail are both away from the sun.
About 1950, Hoffmeister(Hoffmeister) and Bierman(Biermann) proposes an explanation that this phenomenon can be explained by the continuous outflow of matter emitted from the sun transferring momentum to the comet gas, and the estimated flow of solar particles from this (solar wind) speed is about 400km/s. This shows that the flow of solar particles (solar wind) is a daily phenomenon, and it fills the entire space.
However, this is not the beginning of speculation. As early as 1858, astronomical observers discovered that magnetic storms would reappear every 27 days. At the beginning of the 20th century, observations and studies of the northern lights and geomagnetic activity found that both aurora and magnetic storms have 27-day reproducibility, which happens to be the cycle when we stand on the earth and watch the sun make a full circle.
Speaking of which, something must have reached the earth after being emitted from a certain position on the surface of the sun. Therefore, after 27 days, the sun will turn around and it will visit us again. Of course we have not been able to calculate its speed, maybe what it emits is just radiation?
In 1859, astronomer Carrington (Carrington) and others first observed a white light flare. About two days later, the earth appeared very strong Geomagnetic storm. This is very strange because the flare radiation is electromagnetic waves, which can reach the earth in 8 minutes. But this kind of thing similar to flare radiation took about two days to reach the earth, which caused the disturbance of the earth’s magnetic field. This phenomenon is the beginning of the idea that “the sun emits matter outward”.
Art Picture: The Sun Releases Substances (Source: http://n.sinaimg.cn/sinacn20114/232/w659h373/20190729/3618-iakuryx6953792 .png)
In the 1930s and 1940s, the famous British scientist Chapman (Chapman) and Ferraro (Ferraro) has done a lot of research on magnetic storms and aurora, and believes that these geophysical phenomena originate from the stream of solar particles that shoot towards the earth.
Chapman proposed an idea: When the stream of charged particles emitted from the sun rushes near the earth, it will form an additional current loop at high altitude, which disturbs the geomagnetic sound and produces a magnetic storm. According to this assumption, there should be an area on the sun that constantly emits particle streams, because the sun is rotating, and this area should also rotate with the sun. This area is gradually called M area-M is the “mysterious” (Mystery) and the “magnetic storm” (Magneticstorm) The first English letter of two words.
In 1932, Barteles proposed that it was the particle flow in the M area that caused some geomagnetic storms that were not related to flares. Now we call the M area a coronal hole, and it is the high-speed solar wind that flows out.
Photo of Crown Cave (Source: http://www.people.com.cn/mediafile/pic/GQ/20151015/96/14975459038735175628.jpg)
There is a debate about the solar wind, which determines the formation of the comet tail
People are so close to the “solar wind” imagination, but we are not sure that the solar wind is a daily and continuous phenomenon.
In the 1950s, the scientific authority Chapman proposed that the atmosphere around the sun (that is, the corona) is in static mode, which is an analogy The atmosphere of the earth. The sun’s corona is very hot, and the particles in it are fully ionized ions (mainly hydrogen ions, that is, protons) and electrons.
Chapman assumes that the corona and its outwardly extending gas are in a static state(that is, static particles fill the space of the universe), Only heat conduction transfers energy, the sun’s huge gravitational force sucks the gas inward, but the pressure caused by the movement of the gas at very high temperature pushes the gas outward, and the two sides reach equilibrium. According to the calculated solar coronal temperature and calculated in accordance with the corresponding laws of physics, the sun’s gravitational force cannot hold the gas at such a fast temperature from running out, so the theory is in contradiction.
In 1958, the scientist Parker (Parker)The theory of supersonic expansion of the corona is put forward, which explains this problem well. He believes that due to the high temperature at the bottom of the corona, there is a relatively large pressure gradient force in the corona. Under this action, the particles of the solar atmosphere escape the shackles of the sun’s gravity and flow out, and they are gradually accelerated and eventually reach supersonic speed, forming the solar wind. .
However, on the one hand, Chapman is an authority in academia and there are many people who believe in it. Parker’s thesis has been suppressed. On the other hand, some people have also proposed that the solar wind calculated by the equation has never reached supersonic speed. It seems to make sense. It is also called “solar breeze”, but in fact this theory also encounters the problem of wind pressure at infinity.
Practice is the only criterion for testing truth. After the human spacecraft went to the sky, they began to work hard to find the solar wind and its matching characteristics. The spacecraft represented by Mariner II detected a long-term, continuous flow of supersonic solar wind particles, which confirmed the existence of the solar wind and ended the dispute between Chapman and Parker about the solar wind.
Diagram: Solar wind from the sun to the earth (Source: http://n.sinaimg.cn/sinacn20114/270/w1080h790/20190729/c331-iakuryx6954126. jpg)
How is the comet tail formed?
Let us return to the problem of the comet tail. After confirming the existence and characteristics of the solar wind, this problem is easily solved. The comet tail is the volatile matter “blowed” by the solar wind. Under the sunlight, we The bright structure can be seen.
The size of the comet is actually very small when it is far away from the sun, but when it is close to the sun, due to the increase in temperature and the intensified volatilization, the coma becomes larger and larger, and the tail also becomes longer. The volume of the entire comet becomes very huge. The longest comet tail can be hundreds of millions of kilometers. For example, the tail of comet 1842 I can be as long as 320 million kilometers.The sun extends to the orbit of Mars.
The comet tail is so huge that the earth can pass through it. People were quite frightened when they predicted such a phenomenon hundreds of years ago. There are rumors that humans will be poisoned to death by the gas from the comet tail. But now we know that humans have no sense of this phenomenon.
According to the shape, comet tails are mainly divided into two categories: One type is “ionic comet tail”, which is composed of ionic gas, such as carbon monoxide, hydrogen, carbon dioxide, carbon, hydrogen radicals and other ionized molecules. This type of comet tail is relatively straight, thin and long, so it is also called “gas tail” or type I comet tail.
The other type is the “dust comet tail”, which is composed of fine dust and is yellow in color. It is formed by repelling fine dust under the radiation pressure of solar photons. This kind of comet tail is curved, and the wider one is also called type Ⅱ comet tail, and the very curved, short and wide comet tail is also called type Ⅲ comet tail.
There is also an “abnormal comet”, which is fan-shaped or spike-shaped extending toward the solar system. Generally, a comet has more than two different types of tails.
The comet tail facing away from the sun (Source: http://5b0988e595225.cdn.sohucs.com/images/20171211/01a87b1459ce424fa7c94c6d605c807a.jpeg)
This article is from WeChat official account: Science compound (ID: kexuedayuan) , Author: Reno < br>