Compared to those particles, Hua Feng's interest in stars was obviously higher, and he didn't feel bored for a week.

Among them, physical variable stars, according to the physical mechanism of light variation, are mainly divided into two categories: pulsating variable stars and explosive variable stars. The reason for the change of light in pulsating variable stars is that the atmosphere of a star expands and contracts periodically or non-periodically after a long period of main sequence (see Hérault diagram), which causes pulsating changes in luminosity.

Theoretical calculations show that the pulsating period is inversely proportional to the square root of the stellar density. Therefore, those late irregular variables, semi-regular variables and long-period variables with a repeating period of hundreds or even thousands of days are the classical Cepheid variables with a period of about 1~50 days and RR Lyraids (also known as cluster variables) with a period of about 0.05~1.5 days are the two most important pulsating variables.

Observations show that the absolute magnitude of the former decreases with the increase of the period (which is adapted to the relationship between density and period), so that the distance between themselves and the group of stars in which they are located can be deduced by accurately measuring their photoperiod, so Cepheids are also called "lighthouses" or "measuring scales" in the universe. RR Lyrai variable stars also serve as a measuring scale.

There are also pulsating variable stars with periods shorter than 0.3 days (including '" class=li

k>Shieldids, Galusids, and Galus'" class=li

k>;Variables of the Cepheid type), their atmosphere is divided into several layers, each layer pulsates in different periods and forms, therefore, its luminosity change law is the superposition of several periodic changes, the shape of the light variation curve varies greatly, and the relationship between the light variation and the radial velocity curve is also different.

δ Shields and AI Galis may be less massive and denser stars, while β Ceplinids belong to the category of high-temperature giants or subgiants.

Erupting variables can be divided into supernovae, novae, dwarf novae, novae, and flare according to the size of the eruption. Supernovae can increase in brightness hundreds of millions of times in a short period of time, and then become very faint within a few months to a year or two. For the time being, most people think that this is a phenomenon in the late evolution of stars.

The outer shell of a supernova expands outward at a rate of thousands or even tens of thousands of kilometers per second, forming a gradually expanding and thin nebula, while the inner shell is extremely compressed to form objects such as very dense neutron stars. The most famous galactic supernova is the "Tianguan Gueststar" discovered in the constellation Taurus during the Song Dynasty (1054 AD) in China. Pulsar. Pulsars are considered to be neutron stars that rotate rapidly.

The luminosity of a nova in the visible band suddenly increases by about 9 magnitudes or more over a period of several days, and then gradually returns to its original state over several years. The Nova discovered in the constellation Cygnus in August 1975 was the largest known light variation. Spectral observations show that the gas shell of the nova expands outward at a speed of 500~2,000 kilometers per second.

It is generally believed that a nova explosion is only an explosion of the shell, and the mass loss is only about one thousandth of the total mass, so it is not enough to cause a qualitative change in the star. Some eruptive variable stars will make another explosion of considerable size, called a new nova.

The luminosity of dwarf nova and nova-like variable stars is similar to that of novae, but the amplitude change is only 2~6 magnitudes, and the luminosity period is much shorter. Most of them are substars of the binary star, so many people tend to believe that the explosion of this type of variable star is caused by the accretion process of some kind of material in the binary star.

Flares are very irregular fast-changing stars whose luminosity suddenly brightens up in a matter of seconds to minutes and then quickly returns to their original shape. They are considered to be some of the main order prestars of low temperatures.

There is also a type of Corona R variable star, which is the opposite of novae, and quickly and suddenly dims by a few magnitudes and then slowly rises to its original brightness. Observations have shown that they are some carbon-rich stars. The sudden increase in the number of carbon dust particles in the atmosphere causes their luminosity to suddenly dim, which is why some people call them carbon-burst variable stars.

With the development of observation technology and the expansion of observation bands, radio variable stars with varying radio bands and X-ray variable stars with X-ray radiation flux have also been discovered.

In addition to individual stars, a binary system can be two or more stars orbiting each other due to gravity, the most common binary system is a binary star, but systems with three or more stars have also been discovered.

And because the orbits need to be stable, these binary systems often form a class system of common orbital binars. There are also larger groups known as star clusters: ranging from constellations with only a few stars, to the largest groups with hundreds of thousands of stars, called globular clusters.

A binary star system is a group of stars that have been constrained by a specific gravitational field for a long time, usually composed of huge O and B type stars, and 80% of the stars are multi-star systems. However, the star-only portion of the star has increased due to the discovery of smaller objects, with only 25% of red dwarfs found to have companion stars. Because 85% of stars are red dwarfs, most stars in the Milky Way are separate.

Stars are unevenly distributed in the universe and are usually present in galaxies along with interstellar gas and dust. A typical galaxy has hundreds of billions of stars, and the number of galaxies in the observable universe is more than 100 billion. In the past, it was believed that stars existed only in residual galaxies, but stars have also been found in intergalactic space. Astronomers estimate that the universe has at least 700 stars.

Aside from the Sun, the closest star to Earth is Proxima Centauri at a distance of 39.9 megakilometers, or 4.2 light-years. It takes 4.2 years for light to reach Earth from Proxima Centauri in the constellation Centauri. A space shuttle orbiting the Earth would travel at about 8 km/s (about 30,000 km/h) and would take 150,000 years to get there.

Distances like this, including regions adjacent to the solar system, are typical in galactic disks. In the center of galaxies and within globular clusters, the stars will be closer together and farther away in the halo.

Since the distance between the stars is very wide relative to the center of the galaxy, the collision of stars with each other is very rare. But in globular clusters or galaxies, stellar collisions are common. Such collisions result in the formation of blue-out stars, which have higher surface temperatures than main-sequence band stars with the same luminosity in the same cluster.

The distances between stars are very long, and astronomy generally measures the distance between stars in light years. The distance can be measured by the annual parallax method, the cluster parallax method, the mechanical parallax method, the Cepheid variable star method, etc.

Everything in the world is in motion [8] and although the star seems to be constant in the sky, it actually has its own motion. Because different stars move at different speeds and directions, their relative positions in the sky change from one to another, and this change is called the star's self.

Among the stars of the day, including those faint stars that are invisible to the naked eye, Barnard's star is the fastest on its own, reaching 10.31 arc seconds per year (1 arc second is 1/3600th of a degree on the circumference). Stars in general, on their own, are much smaller, the vast majority of which are less than 1 arc second.

The size of the star on its own does not reflect the magnitude of the star's true velocity. At the same speed, a long distance will seem slow, while a close distance will seem fast. Because Barnard is so close to us, less than 6 light-years, the real speed of movement is only 88 km/s.

The self-possession of a star only reflects the movement of the star perpendicular to the direction of our line of sight, which is called the tangential velocity. Stars are also moving in the direction of our line of sight, and this speed of motion is called radial velocity. The radial velocity of Barnard is - 108 km/s (a negative radial velocity indicates a nearing us, while a positive radial velocity indicates a departure from us).

The velocity of a star in space should be the combined velocity of tangential velocity and radial velocity, and for Barnard's star, its velocity is 139 km/s.

The spatial motion of the aforementioned stars consists of three parts. The first is the circular motion of the star around the center of the Milky Way, which is a reflection of the Milky Way's rotation. The second is the reflection of the Sun's participation in the rotational motion of the Milky Way. After deducting the reflection of these two motions, it is really the motion of the star itself, which is called the intrinsic motion of the star.

The η of the constellation is one of the most massive stars known, about 100–150 times that of the Sun, so it has a very short lifespan, at most four million years. According to the Arch Cluster (A

ches cluste

) believed that there should be massive stars in the universe that are 150 times more massive than the Sun, but in fact they have not been found. While the reason for this limit is still unclear, Eddington's luminosity gives part of the answer, as it defines the maximum luminosity that a star can emit into space without ejecting the outer atmosphere.

The first stars born after the Big Bang must have been very massive, perhaps 300 times or more than the Sun, and since there were no elements heavier than lithium in their composition, this generation of supermassive stars should have been extinct, and the third family of stars existed only in theory for the time being.

AB C in Swordfish, the companion star of AB Swordfish A, is only 93 times the mass of Jupiter and is the least massive star known to have a core capable of nuclear fusion. Stars with a similar amount of metal to the Sun can theoretically still carry out fusion reactions at a minimum estimated mass of about 75 times the mass of Jupiter.

When the amount of metal is low, current studies of the faintest stars have found that the smallest star appears to have only 8.3% of the mass of the Sun, or 87 times the mass of Jupiter. The smallest stars are gray areas between stars and gas giants, brown dwarfs with no clear definition.

The gravitational pull on the surface of a giant star is much lower than that of a main-sequence star, and much stronger than in a degenerate state, such as a white dwarf. Surface gravity also affects the spectrum of stars, with higher gravitational pulls resulting in a more pronounced broadening of the absorption lines.

As early as 2010, scientists at the University of Sheffield in the United Kingdom discovered the most massive star to date, RMC 136a1, which may have a mass of up to 320 times the mass of the Sun in the early stage of formation, a brightness close to 10 million times that of the Sun, and a surface temperature of more than 49,000 degrees Celsius.

Stellar classification is a binary classification based on spectral and luminosity. In the popular simplified classification, the former can be divided by the appearance of the star, and the latter is roughly divided into "giant star" and "dwarf star", for example, the sun is a "yellow dwarf", and the common names are "blue giant" and "red giant".