Comet Xinzhi returns once in 6000, don't just watch the excitement

　　In the past few days, a rapidly rising "net celebrity" comet has overwhelmed the circle of friends. Astronomy enthusiasts all over the world raised their observation equipment, and beautiful pictures of comets appeared one after another. It decorates the night sky in the northwest and also decorates our cyberspace. It is this year's "dark horse" comet-NEOWISE.

　　When the comet Xinzhi was first discovered, it was at the base of the carina, passing through Canis Major and Lepus in mid-to-late April, passing Monoceros and Orion at the end of May, and moving from the south of the ecliptic to the north of the ecliptic at the end of June. In July, it will pass Auriga, Lynx, and Ursa Major, and in August, it will enter the spring starry sky and arrive in the constellations of Oma, Boves and Virgo. After that, we are getting farther and farther away, and the brightness will become darker and darker. It will not return until more than 6000 years later.

　　"Little Transparent" suddenly shines in the night sky

　　On March 27, 2020, the Wide Area Infrared Survey Explorer Space Telescope discovered this celestial body during the NEOWISE mission. At that time, its brightness was only about 18 degrees, about 300 million kilometers away from the sun, and about 250 million kilometers away from the earth. In April 2020, this comet was named after the project that discovered it "NEOWISE", the comet number is C/2020 F3, where C means it is a long-period or non-periodic comet, and 2020 F3 means that it is in 3 The third comet discovered in the second half of the month.

　　The now-famous Comet Xinzhi was actually a "little transparent" when it was first discovered. As it gets closer and closer to the sun, Comet Xinzhi is getting brighter, and it is close to magnitude 6 in early June. But when it traveled to the back of the sun, the comet disappeared from people's vision. It wasn't until half a month later that Comet Xinzhi once again appeared on the stage, at which time its brightness had reached 2.5 levels.

　　In just over half a month, Comet Xinzhi was invisible to zero and its brightness increased by about 100 times. Why is the brightness change of the same comet so huge? In fact, the main reason is the distance between the comet and the sun.

　　The comet can be seen by us because it reflects the sunlight. On the one hand, as the comet approaches the sun, the intensity of the sun's light continues to increase, so it looks brighter and brighter. On July 3, 2020, Comet Xinzhi reached perihelion, only about 43 million kilometers away from the sun, so it would shine.

　　On the other hand, comets continue to accelerate, causing dust tails to stretch longer. Therefore, when the comet gets closer to the sun, it looks brighter. The energy of the sun heats the comet, and gas and dust erupt from the surface of the comet, forming a huge dust tail, usually yellow. At the same time, the comet will produce a stream of charged particles under the action of the solar wind, forming an ion tail, which is generally blue.

　　Can provide clues to the birth of the solar system

　　In addition to admiring comets, scientists can also obtain important scientific information from them.

　　In July 1994, a comet-wood collision shocked the world. A comet named "Shoemaker-Levy 9" under Jupiter's strong gravitational force, the comet nucleus burst into 21, which "bombed" the surface of Jupiter in a few days. Such an impact has so far left people's lingering fears-if this event happened on the earth, it would be a devastating blow. Therefore, "early detection and early treatment" has become one of the important goals of scientists in studying comets.

　　In addition, by observing comets, astronomers can also obtain a lot of valuable information, such as the proportion of gas and dust, the size distribution of dust particles, the abundance of certain elements (such as hydrogen) isotope, and the visible light reflected by the comet nucleus. Spectrum, thermal infrared spectrum of comet itself, rotation state of comet nucleus, etc.

　　Astronomical observations are generally divided into two types, one is imaging photometry, and the other is obtaining spectral data. The same is true for comet observation. For astronomers, when a comet is very far away from the sun, it is difficult to directly determine whether it is a comet. This requires a large-aperture survey telescope with highly sensitive terminal equipment to obtain more information.

　　Comets formed outside the snow line of the early solar system. It contains a lot of original information about the solar system, such as the abundance of hydrogen isotopes mentioned above, which may be different in different locations in the early solar system. Although the shapes, masses, and orbits of different comets are quite different, they have a common feature: they all contain ice. The water on the earth probably originated from early comet collisions. By comparing the abundance of hydrogen isotopes in the water on the earth and the data of different comets, combined with the orbits of various comets, scientists can constrain the early evolution model of the solar system.

　　Three factors affect public observation

　　Comet Xinzhi returns once in 6000 years. How can the public grasp this excellent opportunity for observation?

　　Comet Xinzhi is a comet that is very bright in recent years and suitable for observation in the northern hemisphere. Choose a night with better weather, and you can observe or photograph it in the city. In mid-to-late July, the brightness of Comet Xinzhi basically dropped below third magnitude. If you want to appreciate this comet, you still need to consider the following questions.

　　Weather is particularly important for astronomical observations. Eastern my country is located in the monsoon climate zone. July is the most rainy season. You can use astronomical weather software to check the weather and plan the observation time in advance. Of course, if conditions permit, it is recommended to select areas with a higher probability of sunny weather for observation.

　　In addition, the height of Comet Xinzhi is around 20 degrees after sunset, and then it will slowly decrease. Therefore, the northwest of the observation area is relatively open. In cities, you can choose to observe in parks or higher floors. In the area where the comet is visible all night, it is relatively open from the northwest to the northeast, especially the direction due north.

　　The beauty of a comet lies in its tail, but even in places where the observation conditions are relatively good, it is difficult to see its tail directly with the naked eye. Some equipment is needed to assist in observation, such as binoculars. It is portable and affordable. It is recommended that you use 7X50mm or 10X50mm binoculars (the number before X is a multiple, and the number after X is the caliber), the price ranges from 500 yuan to 1,000 yuan. In addition, it is recommended to purchase a special stand for binoculars, so that you can install the binoculars on a tripod to slowly appreciate the comet, and avoid overworked hands.

　　The biggest difficulty in using binoculars to observe Comet Xinzhi is how to "target" the comet. You can use the electronic star chart in advance to determine the position of the comet on the day, and then use the star bridge method or use the surrounding bright stars to locate it. For example, at 21:00 on July 21, 2020, the location of Comet Xinzhi is directly below the star of the Big Dipper. You can use binoculars to find the star of the sky first, and then look down vertically. It is easy to find the comet.

　　(Su Chen author unit: Beijing Planetarium)