For a moment, Hua Feng felt that he was getting farther and farther away from his former life, as if he was born not to belong to that world.

He slowly learned that the temperature of a star can determine the rate at which different elements are ionized or activated, and the results are characterized by spectral absorption lines. The surface temperature of a star, along with its visual absolute magnitude and absorption characteristics, is used as the basis for stellar classification.

The surface temperature of a massive main-sequence star can be as high as 40,000 K, while the surface temperature of a smaller star like the Sun is only a few thousand degrees. Comparatively speaking, the surface of a red giant star has a low temperature of 3,600 K, but it has high brightness due to its huge surface area.

The temperature of the surface of a star is generally expressed in terms of effective temperature, which is equal to the temperature of an absolute black body with the same diameter and the same total radiation. The spectral energy distribution of a star is related to the effective temperature, from which it can be determined that stars of the same spectral type (also known as temperature type) such as W, O, B, A, F, G, K, M, etc., the larger the volume, the greater the total radiant flux (i.e., luminosity), and the smaller the absolute magnitude.

The luminosity of stars can be divided into I., II., III., IV., V., VI., VII., and are called in order: I. supergiants, II. bright giants, III. normal giants, IV. subgiants, V. dwarfs, VI. subdwarfs, and VII. white dwarfs. The spectral type of the sun is G2V, the color is yellowish, and the effective temperature is about 5,770 K. A0V stars have an average color index of zero and a temperature of about 10,000 K. The effective surface temperature of stars varies greatly from tens of thousands of degrees Celsius in the early O type to several thousand degrees in the late M type.

The closest star to Earth is the Sun. This is followed by Proxima Centauri, which emits light that takes 4.3 years to reach Earth.

The magnitude of stars varies greatly, and there is certainly a reason for the strength of the star itself, but the distance from us also plays a significant role. The most basic method for determining the distance of a star is the trigonometric parallax method, which is mainly used to measure the distance of the nearest star, the process is as follows, first measure the opening angle of the half-long diameter of the Earth's orbit at the star (called annual parallax), and then after a simple operation, the distance of the star can be calculated.

This is the most straightforward way to determine distance. After Copernicus published his heliocentric theory in the 16th century, many astronomers tried to determine the distances of stars, but they were unsuccessful due to their small values and the low accuracy of their observations at the time. It was not until the second half of the thirties of the XIX century that it was successful.

However, for most stars, this angle is too small to be accurate. Therefore, indirect methods are often used to determine stellar distances, such as spectroscopic parallax, cluster parallax, statistical parallax, and the determination of parallax by the circumferential relationship of Cepheid variable stars, etc. These indirect methods are based on the trigonometric parallax method. Since the twenties, many astronomers have carried out this work, and by the early nineties of the twentieth century, the distances of more than 8,000 stars have been measured by photographic methods. In the mid-nineties of the twentieth century, space astrometry carried out by the "Hipparcos" satellite was successful, and in about three years, the distances of 100,000 stars were determined with very high accuracy.

The distance of a star, expressed in kilometers, is too large and is usually measured in light years for ease of use. 1 light-year is the distance that light travels in a year. The speed of light in a vacuum is 300,000 kilometers per second, multiplied by the number of seconds in a year, and 1 light-year is equivalent to about 9.46 trillion kilometers.

The brightness of a star is often expressed in terms of magnitude. The brighter the star, the smaller the magnitude. The magnitude measured on the Earth is called the apparent magnitude, and the magnitude at 32.6 light-years from the Earth is called the absolute magnitude. The magnitude of the same star measured by detection elements that are sensitive to different wavelength bands is generally not equal. At present, one of the most common magnitude systems is the U (ultraviolet), B (blue), and V (yellow) trichromatic systems. B and V are close to photographic and visual magnitudes, respectively. The difference between the two is the commonly used color index. The Sun has a magnitude of V = -26.74, an absolute visual magnitude M = +4.83, a color index of B-V = 0.63, and U-B = 0.12. The color temperature can be determined by the color index.

The true diameter of a star can be calculated from the apparent diameter (angular diameter) and distance of the star. The commonly used interferometer or lunar occultation method can measure the angular diameter of a star as small as 0.01, and smaller stars are not easy to measure, plus the error of measuring distance, so the true diameter of a star is not very reliable.

Based on the orbital data of eclipse binary and spectroscopic binary stars, some star diameters can also be derived. For some stars, the true diameter can also be estimated from absolute magnitude and effective temperature. The diameters of different stars have been found in various ways, ranging from a few kilometers to more than 10 kilometers.

The size of the stars varies greatly, some are giants, some are dwarfs. The diameter of the Earth is about 12,900 kilometers, and the diameter of the Sun is 109 times that of the Earth. Giants are the largest in the star world, and they are tens to hundreds of times larger than the Sun. Supergiants are even larger, with a binary star called Pillar One, and a companion star 150 times the diameter of the Sun. The red supergiant Antares (α Scorpio) is 883 times the diameter of the Sun, and the Red Supergiant Betelgeuse (α Orion) is 1,200 times the diameter of the Sun.

They are not the largest, VV is a pair of binary stars, its host star A is 1600-1900 times the diameter of the Sun, and woh g62 is 2000 times the diameter of the Sun. The Canis Major VY can reach a diameter of 3.063 billion kilometers. These giants and supergiants are giants in the stellar world.

After looking at the giants in the stellar world, let's take a look at the gnomes among them. In the stellar world, the Sun is medium in size, and there are many stars smaller than the Sun, the most prominent of which are white dwarfs and neutron stars. White dwarfs are only a few thousand kilometers in diameter, about the same as Earth, and neutron stars are even smaller, they are only about 20 kilometers in diameter, and both white dwarfs and neutron stars are dwarfs in the stellar world.

We know that the volume of a sphere is proportional to the cube of the radius. If we compare the volume, the pillar one mentioned above is more than 80 billion times larger than the Sun, and the neutron star is hundreds of trillions times smaller than the Sun. This shows how different a giant is from a dwarf.

Scientists have found that the total number of stars in the universe may be three times our estimate, which means that there are 3×10^23 (to the power of 10) stars in the universe, which is more than the total number of sand grains in all the beaches and deserts on Earth, which greatly increases the likelihood of extraterrestrial life being found in other worlds other than Earth.

Scientists say the number of stars in the universe may have been grossly underestimated, and the true number of stars could be three times as high as the envisioned number. This underestimation mainly concerns dwarf stars in different galaxies that are cooler and less bright. If confirmed, it could potentially rewrite scientists' understanding of galaxy formation and evolution. Dwarf stars that exist in other galaxies are so faint that they have only one-third the mass of the Sun. ”

Therefore, the general method is to count the bright stars and estimate the number of invisible faint stars in proportion to the Milky Way. For every star that is as bright as the Sun is found, there should be about 100 invisible dwarf stars.

Due to the cooler temperature of dwarf stars, their radial colors and bands are different from those of other brighter stars. Therefore, by observing the intensity and characteristics of the radiation of the entire galaxy in this particular color or band, it is possible to deduce how many dwarf stars are needed to produce such radiation.

Based on this, they made observations and calculations of eight elliptical galaxies. The results show that in elliptical galaxies, the ratio of Sun-like main-sequence stars to invisible dwarf stars reaches 1000~2000:1, rather than about 100:1 in the Milky Way. Thus, a typical elliptical galaxy (generally thought to contain 300 billion stars) should actually contain 1 trillion or more stars. In the universe, elliptical galaxies account for about one-third of the total number of galaxies, so they concluded that the total number of stars in the universe is at least three times that of existing estimates.

In the same way that spectroscopic analysis is carried out in a terrestrial laboratory, the spectra of a star can be analyzed to determine the amount of elements in the star's atmosphere that form various spectral lines, although the situation is much more complex than that of a general spectral analysis on the ground.

Measurements over the years have shown that the chemical composition of a normal stellar atmosphere is similar to that of the Sun. In terms of mass, hydrogen is the most, followed by helium, and the rest are oxygen, carbon, nitrogen, neon, silicon, magnesium, iron, sulfur, etc. However, there are also some stellar atmospheres with different chemical composition from the solar atmosphere, such as Wolf-Layet, which is rich in carbon and nitrogen-rich (i.e., there is a distinction between carbon and nitrogen order). However, it is still a question whether this can be attributed to the high content of certain elements.

Theoretical analysis shows that in the process of evolution, the chemical composition of the star will gradually change with the change of thermonuclear reaction process, and the content of heavy elements will increase, but the chemical composition of the star atmosphere generally changes little.

In terms of mass, the ratio of stars to formation is about 70% hydrogen and 28% helium, with small amounts of other heavy elements. Because iron is a very common element and spectral lines are easy to measure, a typical heavy element measurement is based on the amount of iron in the star's atmosphere. Since the abundance of heavy elements in molecular clouds is stable and increases only by supernova explosions, measuring the chemical composition of a star can infer its age. The composition of heavy elements may also indicate the presence or absence of planetary systems.

The dwarf star HE1327-2326 has the lowest iron content of the measured star, and the iron ratio is only 1/20,000 that of the Sun. In contrast, the μ Leo has a higher amount of metal, which has twice as much iron as the Sun, while the other planet, Hercules14, is almost three times as rich as the Sun. There are also special stars with different chemical elements, which have absorption lines of certain elements, especially chromium and rare earth elements.

It has been observed that the physical properties of some stars, such as luminosity, spectra and magnetic fields, change periodically, semi-regularly or irregularly over time. Such stars are called variable stars. There are two types of variable stars: one is caused by changes in the geometric position of several celestial bodies or the special geometry of the star itself, and the other is caused by the physical processes inside the star itself.

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