COVID-19 has a little knowledge.

The knowledge of stars Stars are composed of hot gases. They are spherical or quasi-spherical objects that can emit light by themselves.

Because the stars are too far away from us, it is difficult to find their position changes in the sky without special tools and methods, so the ancients regarded them as fixed stars. The sun, the main star of our solar system, is a star.

1. 1 evolution of stars. Stars are gas planets. On a clear moonless night, in areas without light pollution, most people can see more than 6,000 stars with the naked eye.

With the help of a telescope, you can see hundreds of thousands or even millions. It is estimated that there are about150-200 billion stars in the Milky Way.

Two important features of a star are temperature and absolute magnitude. About 100 years ago, Einar hertzsprung of Denmark and henry norris russell of the United States drew a chart to find out whether there is a relationship between temperature and brightness, which is called Herro chart or H-R chart.

In the H-R diagram, most stars form a diagonal region, which is called the main star sequence in astronomy. In the main sequence, when the absolute magnitude of the star increases, the surface temperature of the star evolution also increases.

More than 90% of the stars belong to the main sequence, and the sun is also one of these main sequences. Superstar and Supergiant star are on the right of the H-R diagram.

Although the surface temperature of the white dwarf is high, its brightness is not great, so it is only in the middle and lower part of the figure. 1.2 Continuous changes of stars in their lifetime (luminous and heating period).

Life span varies with the size of a star. The evolution of a single star cannot be completely observed, because these processes may be too slow to be detected.

Therefore, astronomers observe many stars in different life stages and use computer models to simulate the evolution of stars. Astronomer hertzsprung and philosopher Russell first put forward the relationship between star classification and color and luminosity.

The star-Herodotus system established a star evolution relationship called "Herodotus", which revealed the secret of star evolution. In "Herro-Roto", from the high-temperature strong light area at the upper left to the low-temperature weak light area at the lower right, it is a narrow star-intensive area, and our sun is also in it; This sequence is called the main sequence, and more than 90% stars are concentrated in the main sequence.

Above the main sequence area are the giant star and Supergiant star area; On the lower left is the white dwarf region. 1.3 stars formed when the universe develops to a certain period. The universe is full of uniform neutral atomic gas clouds, and the massive gas clouds collapse due to their own gravity.

In this way, the star has entered the formation stage. At the initial stage of collapse, the pressure inside the gas cloud is very small, and the matter accelerates to fall toward the center under the action of its own gravity.

When the linearity of matter is reduced by several orders of magnitude, the situation is different. On the one hand, the density of gas increases sharply. On the other hand, because the lost gravitational potential energy is partially converted into heat energy, the temperature of the gas is also greatly increased. The pressure of a gas is directly proportional to the product of its density and temperature, so the pressure increases faster in the process of collapse. In this way, a pressure field that is enough to compete with self-gravity is quickly formed inside the gas. The mechanical balance of the star blank is caused by the internal pressure gradient and its own gravity, but the existence of the pressure gradient depends on the inhomogeneity of the internal temperature (that is, the temperature in the center of the star blank is higher than that in the periphery), so it is an unbalanced system in terms of heat, and the heat will gradually flow out from the center.

This natural trend of heat balance plays a weakening role in mechanics. Therefore, the star blank must shrink slowly, and the decrease of its gravitational potential energy will increase the temperature, thus restoring the mechanical balance; At the same time, it also provides the energy needed for star blank radiation by reducing gravitational potential energy.

This is the main physical mechanism of stellar blank evolution. The latest observation found the star S 1020549. We use the classical theory of gravity to roughly discuss this process.

Considering the spherical gas cloud system with density ρ, temperature t and radius r, the gas thermal motion energy: ET= RT= T (1), the gas is regarded as a monatomic ideal gas, μ is the molar mass, and r is the universal constant of the gas. In order to get the gravitational energy Eg of the gas cloud ball, imagine that the mass of the warped ball moves to infinity bit by bit, and the work of the field force is equal to-eg.

When the mass of the ball is m and the radius is r, when dm is removed from the surface, the field force does work: dW=- =-G() 1/3m2/3dm(2), so: -Eg=- () 1/3m2/3dm= G( M5/3. So: Eg=- (2).

Total energy of gas cloud: E=ET+EG (3). The soul nebula will form a new planetary thermal motion to make the gas evenly distributed and gravity to concentrate the gas.

Now the two work together. At E>0 point, the thermal motion is dominant, the gas cloud is stable, and the small disturbance will not affect the gas cloud balance; When e

The critical radius of contraction is obtained from E≤0: (4) The critical mass of the corresponding gas cloud is: (5) The original gas cloud has a small density and a large critical mass. So few stars are produced alone, and most of them are produced by a group of stars forming a cluster together.

Globular clusters can contain 10 5 → 10 7 stars, which can be considered as simultaneous. We know that the mass of the sun: mθ = 2 * 10 33, and the radius r = 7 * 10 10. We bring in (2) and we can get the gravitational energy released by the sun shrinking to today's state.

The total luminosity of the sun l = 4 *10 33 erg. S- 1 If this luminosity is maintained by gravity as the energy source, the duration is as follows: Many proofs show that the sun has been in today's stable state for 5 * 10 9 years, so the star blank stage can only be a very short period before the sun forms such a stable state as today. This raises a new question, how does the gravitational contraction of the star blank stop? After that, what is the energy source of solar radiation? 1.4 The density of the main sequence star increases during the contraction. We know that ρ∝r-3 is given by formula (4), and rc ∝ R3/2, so rc drops faster than R, and some contracted gas clouds reach the critical point under new conditions. A small disturbance may cause a new local collapse.

In this way, under certain conditions, the large gas cloud shrinks into a condensate and becomes the original appearance.

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Popularization of food nutrition knowledge of Xiao Hongli: encyclopedia of Xiao Hongli's efficacy and taboo life. Xiaohong pear food has high nutritional value, and there are many delicacies and delicacies. Whether cooking, stewing, pickling or frying, Xiao Hongli has rich flavor and nutrition. Xiao Hongli also has a high nutritional value for dietotherapy. This paper introduces the nutritional value analysis of Xiaohong pear and the nutrition manual of food ingredients, so that you can eat healthily in your life.

Calories (kcal) 30 thiamine (mg) .07 calcium (mg) 1 1

Protein (g) .2 riboflavin (mg) .46 magnesium (mg) 5

Fat (g) 0 nicotinic acid (mg) .6 iron (mg) .4

Carbohydrate (g) 7.3 Vitamin C (mg) 4 Manganese (mg)

Dietary fiber (g) 3.2 vitamin E (mg) .46 zinc (mg) .04

Vitamin A (μ g) 2 Cholesterol (mg) 0 Copper (mg) .06

Carotene (microgram). 2 potassium (mg) 78 phosphorus (mg) 7

Retinol equivalent (μ g) 89. 1 sodium (mg) 3.4 selenium (mg) .2

As early as the Ming and Qing Dynasties, Xiao Hongli was already a royal product. According to Compendium of Materia Medica, "Xiao Li has the functions of treating wind and heat, moistening lung and cooling heart, resolving phlegm and diminishing inflammation, and detoxifying". Moreover, folk allusions are widely circulated. Xiao Hongli can cure all diseases, and it is a good fruit of dietotherapy for all ages in Xian Yi. Because of its special soil and water quality, Xiao Hongli has round fruit, big head, thick meat, sour taste and cool nature. It contains many trace elements and vitamins needed by human body, and has the effects of promoting fluid production to quench thirst, steaming, moistening lung and relieving cough. Located on the warm slope in front of the Yanshan Mountains, the unique sandy loam, strong temperature difference between day and night, and abundant sunshine endow the hammer peak brand "Xiao Hongli" produced in Dachengzi Town with better nutritional quality and fresher and pure Shan Ye flavor. According to historical records, the Yanshan Mountains are mostly gneiss, which is rich in wild plant resources, and there are many night resources around its branch and cone peaks, especially Hongxiaoli. Cultivation and utilization began in the Ming Dynasty. . The town has red pears 1 1,000 mu, with an annual output of 8.5 million kilograms of high-quality red pears and nearly 10,000 pear trees over 1 1,000 years old. It is the largest Xiaohong pear producing area in northern China. Fresh food to quench thirst, steamed food to moisten the lungs and relieve cough "and" sugar pear in the first month, pear in the second month "are Beijing people's praise and evaluation of pears. In particular, modern people eat very little coarse grains, which is not conducive to the stomach. Xiaohong pear contains more stone cells and trace element selenium, which is beneficial to the stomach and can prevent the occurrence of gastrointestinal cancer.

3. Little scientific knowledge

Small scientific knowledge

Why does sorbet get angry?

The reason why sorbet bubbles is because there is a lot of invisible water vapor in the outside air. When it comes to cold sorbet, it will liquefy into droplets when it is cold. It seems that the sorbet is bubbling.

Why do sunflowers always face the sun?

Sunflower stems contain a wonderful auxin. This auxin is very afraid of light. When it is illuminated, it will go to the backlight side, and the cells on the backlight side will multiply rapidly. Therefore, the backlit side will grow faster than the bright side, which will bend the sunflower to light.

Why does cicada shed its skin?

Cicada's shell (exoskeleton) is hard and can't expand with the growth of cicada. When the cicada grows to a certain stage, the cicada's exoskeleton restricts its growth, and the cicada loses its original exoskeleton, which is cicada slough.

How do bees make honey?

First, the bees spit the sweet juice of the flowers they picked into an empty hive. At night, they suck sweet juice into their honey stomachs to make it, and then spit it out and swallow it. It takes 100~240 times to make sweet honey.

4.50 scientific tips

1. Why do sunflowers always face the sun? Sunflower stems contain a wonderful auxin.

This auxin is very afraid of light. When it is illuminated, it will go to the backlight side, and the cells on the backlight side will multiply rapidly. Therefore, the backlit side will grow faster than the bright side, which will bend the sunflower to light.

2. Why do the stars twinkle? We see the stars shining, not because the luminosity of the stars has changed, but because of the occlusion of the atmosphere. The atmosphere is between us and the stars. When starlight passes through the atmosphere, it will be affected by the density and thickness of the atmosphere.

The atmosphere is not absolutely transparent, and its transparency will change according to its density. So when we look at the stars through it on the ground, we will see that the stars seem to be flashing.

3. Why do people yawn? When we feel tired, a lot of carbon dioxide has been produced in our bodies. When there is too much carbon dioxide, oxygen must be supplemented to balance the needs of the body.

Because these residual carbon dioxide will affect our body's functional activities, then the body will send out a protective reaction, so we yawn. Yawning is a deep breathing action, which will let us breathe in more oxygen and expel more carbon dioxide than usual, and it can also eliminate fatigue.

How high is the blue sky? "Blue sky" is actually the earth's atmosphere. The atmosphere surrounds the earth's air and is divided into five layers according to the air density, with a total thickness of 2000-3000 kilometers.

But most of the air is concentrated below 15 km from the ground, and the higher the air, the thinner it is. How thick the atmosphere is, how high the blue sky should be.

5. Why are there no stars in the daytime? Because part of the sunlight is scattered by the gas and dust in the atmosphere during the day, the sky is very bright and the light radiated by the sun is so strong that we can't see the stars. 6. What celestial bodies are there in the solar system? There are nine planets in the solar system.

They are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. In addition, there are many asteroids, comets and meteors in the solar system, and there are 2958 officially numbered asteroids.

The most famous comet is Halley's comet. 7. Why does it thunder? This is a natural phenomenon that negative electricity meets positive electricity.

When it rains, some clouds in the sky are positively charged and some are negatively charged. When two clouds meet, they will discharge, emit bright and bright lightning, and at the same time release a lot of heat, which will make the surrounding air heated, expand and make a loud noise. This is thunder. 8. Why can an airplane fly into the sky? This plane has two wings, like those of a bird. It has a propeller.

The wing can generate lift and lift the plane into the air; Propeller can produce the ability to push the plane forward. Therefore, the plane can fly into the sky like a bird.

33. For a quartz clock hanging on the wall, when the battery runs out and stops, its second hand often stops at the position of "9" on the dial. This is because the second hand is most hindered by the large torque at the "9" position.

9. Before the water boils, the bubbles in the water rise and vibrate up and down due to convection. Most of the bubbles burst under the pressure in the water, and their cracking and vibration sound together with the container produce a * * * sound, so the sound is very loud. After water boils, it is isothermal up and down, and the bubble volume increases. Under the action of buoyancy, it will explode when it reaches the water surface, so the noise is relatively small.

10. Fire-water incompatible substances must reach the ignition point when burning. Because of the large specific heat of water, water can absorb a lot of heat when it comes into contact with fire, thus reducing the temperature of ignition materials; At the same time, the vaporized water vapor surrounds the burning object, making it impossible for the object to contact with air. Without air, combustion cannot be carried out. 1 1. Sit on the ground and walk for 80,000 days. Because the radius of the earth is 6370 kilometers, every time the earth makes a circle, the distance that objects on its surface "walk" is about 40003.6 kilometers, about 80000 miles.

This is a poem sung by * *, which also scientifically reveals the relationship between motion and stillness-motion is absolute, and stillness is always relative to the reference object. 12. According to the principle of lever balance, if the power arm is a fraction of the resistance arm, then the power is several times the resistance.

If the weight of the arm is very large, it is entirely possible to "dial one or two kilograms." 13. When the distance between molecules is large (more than several hundred angstroms), the attraction between molecules is very small, almost zero, so it is difficult to reassemble a broken mirror.

14. If the directions of the components are the same, the resultant force is equal to the sum of the components. 15. Tofu can't be hung with hemp rope. When the pressure is constant, if the stress area is small, the pressure will be great.

16. True gold is not afraid of fire, and truth is not afraid of argument. Judging from the melting point of gold, although it is not the highest, it is 1068℃, while the temperature of general flame is about 800℃. Because the temperature of the flame is lower than the melting point of gold, gold cannot be melted. 17. With a long whistle, people in the valley should give a long whistle. After many reflections, the sound can form a loud echo, which lasts for a long time. As if the mountains were shouting and the valleys were echoing.

18. Looking at the sky from the well, you can see very little. Because light travels along a straight line, from the knowledge of geometric drawing, the frog's field of vision will be very small. 19. It can be seen from the pressure formula that when the pressure is constant, the smaller the stress area, the greater the pressure.

People will feel very uncomfortable sitting on such a blanket. 20. Good years are bright. Because snow is a bad conductor of heat, when it covers crops, it can well prevent heat conduction and air convection, so it can play a role in heat preservation.

2 1. It is cold before frost and cold after snow. In the late autumn night, the temperature near the ground suddenly turns cold (the temperature is below 0℃), and the water vapor in the air condenses into small ice crystals and adheres to the ground to form frost, so it feels "cold before frost". When snow melts, it needs to absorb heat to reduce the temperature of the air, so we have the feeling of "cold after snow".

22. Eggs touch stones-overreached eggs touch stones. Although the force is the same, the pressure that each object can bear is certain. If this limit is exceeded, the object may be damaged. Eggs can withstand less pressure, so they will break.

23. Before the jade is polished, the surface is uneven and the light is diffuse. After the jade is polished, the surface is smooth and the light is mirrored. 24. The fan is cool and suitable for summer.

5. Simple and understandable scientific knowledge

We brush our teeth, because there will be food residue left on our teeth, which will corrode our teeth, make them worse, cause toothache and tooth loss.

We wash our hands with soap because there are bacteria on our hands. The substances in soap can kill bacteria and protect our skin health.

Some children wear eyes because the concave lens has the function of diverging light, which enlarges the image produced by the object in front of our eyes and makes us see more clearly.