dwarf star

[ǎi xīng]

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Original reference itself luminosity Weaker stars, now specifically Stellar spectral classification The star with luminosity level V in is equal to Main sequence star 。 Dwarfs with spectral types O, B and A are called Blue dwarf (such as Weaver 1 and Sirius). The dwarf stars with spectral types F and G are called Yellow dwarf (such as the sun), and dwarf stars with spectral type K and later are called red dwarfs (such as Alpha B star). but White dwarf 、 Sub dwarf 、“ Black dwarf ”It is not a dwarf star. A class of stars with weak luminosity whose matter is in the degenerate state“ Degenerate dwarf ”It is also not a dwarf star.

Chinese name: dwarf star
Foreign name: dwarf
Classification: fixed star

Discipline: astronomy
Contains: Brown dwarf, white dwarf, red dwarf, etc
Pronunciation: ǎi xīng

catalog

Introduction to stars

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dwarf star

Dwarf（ Dwarf star ）: Small as the sun Main sequence star , if it is White dwarf Is like the sun fixed star The legacy of. Brown dwarf There is not enough material for melt Reaction.

The Remaining Core of a Star White Dwarf

Originally, it refers to the star with weak luminosity, but now it refers to the star with luminosity level V in the stellar spectral classification, which is equivalent to the main sequence star. Dwarfs with spectral types O, B and A are called Blue dwarf (e.g Sirius ）Dwarfs with spectral types F and G are called yellow dwarfs (such as the sun), and those with spectral types K and later are called Red dwarf (e.g Alpha B star). But white dwarfs Sub dwarf ﹑“ Black dwarf ”It is not a dwarf star. A class of stars with weak luminosity whose matter is in the degenerate state“ Degenerate dwarf ”It is also not a dwarf star. "Black dwarfs" are objects that are estimated to exist in theory, meaning that the mass is roughly one Solar mass Or a celestial body evolved from a smaller star, which is in a cold degenerate state and no longer emits radiant energy; Others specifically refer to stars with insufficient mass (less than about 0.08 solar mass) and no nuclear reaction energy.

The category of galaxies with the weakest luminosity has an absolute magnitude M of - 8~- 16. yes , we have Dwarf galaxy They are elliptical galaxies, and some are type I Irregular galaxy 。 Both of these dwarf galaxies are small and usually have few members. The quality is only 10 six ～10 nine Solar mass. Irregular dwarf galaxies contain a large amount of neutral hydrogen, and contain stars of galaxy group I. Elliptical dwarf galaxies are Elliptical galaxy Small and medium mass galaxies. They are related to Globular cluster Similarly, the difference between the two is that the diameter of the former is about 10 times that of the latter. stay Local galaxy group Of the 40 galaxies, more than 20 are elliptical dwarf galaxies, which shows the number of them. Such galaxies are weak, so they cannot be seen beyond 49999 seconds.

Brown dwarf

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dwarf star

Brown dwarf is a kind of quasi stellar objects, with a mass between 5 and 90 Jupiter. Different from ordinary stars, brown dwarfs cannot become main sequence stars because their cores do not fuse hydrogen atoms to give off light and heat due to their insufficient mass. However, their interior and surface are in a convection state, and different chemicals do not exist in layers inside. At present, people are still studying whether a brown dwarf has ever experienced nuclear fusion at a certain location in the past. It is known that a brown dwarf with a mass greater than 13 Jupiter can fuse deuterium.

Brown dwarfs were formerly known as“ Black dwarf ”, representing the stellar like objects floating in the universe or the objects whose mass is insufficient for nuclear reaction. But the term "black dwarf" now refers to some white dwarfs that have stopped emitting light and have died.

The brown dwarf star in the constellation of the hare

Early star models pointed out that a celestial body must have more than 80 Jupiter masses to produce nuclear reactions in order to become a true star. The theory of "brown dwarf" was first put forward in the early 1960s, which means that its number may be more than that of real stars. It is difficult to find it because it fails to emit light. They will emit infrared rays, which can be detected by infrared detectors on the ground, but it has taken decades from being proposed to being confirmed.

Recent studies have pointed out that the luminescence and heating of stars depend not only on their mass, but also on their compounds. Some brown dwarfs with a mass of 90 Jupiter cannot ignite the hydrogen inside. Also, when a nebula collapses, in addition to producing stars, it will also produce non luminous brown dwarfs with a mass of less than 13 Jupiter.

The first brown dwarf was confirmed in 1995, and there have been more than 100 so far. It is generally believed that brown dwarfs are Galaxy One of the largest number of celestial bodies in the world, the brown dwarf near the earth is located in Indian Epsilon star, which has two brown dwarfs 12 light-years away from the sun.

White dwarf

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brief introduction

Dwarf galaxy nearest to the Milky Way

White dwarf (White Dwarf) is a star with low luminosity, high density and high temperature. Because of its white color and small size, it is named White Dwarf Star.

Others believe that the predecessor of white dwarfs may be planetary nebulae.

White dwarfs are stars that have evolved to their later ages. In the later stage of evolution, stars eject a large amount of material mass loss If the mass of the remaining nuclei is less than 1.44 Solar mass The star may evolve into a white dwarf. Some people also believe that the predecessor of white dwarfs may be Planetary nebula (It is a circular or disk shaped material composed of high-temperature gas and a small amount of dust in the universe, and there is usually a star with high temperature in its center - the central star). Its nuclear energy has been basically exhausted, and the whole star has begun to slowly cool, crystallize, and finally "die".

features

White dwarfs have such characteristics:

1. Small in size, its radius is close to the radius of the planet, with an average of less than 10 ^ 3 km.

2. The luminosity (the total energy radiated by the star per second, that is, the size of the luminous ability of the star) is very small, and the brightness is only 1/1000 of that of normal stars.

Leos 1 dwarf galaxy

3. The mass is less than 1.44 solar masses.

4. The density is up to 10 ^ 6~10 ^ 7 g/cm3 Gravitational acceleration It is about 10 to 104 times the acceleration of gravity on the earth's surface. If people can reach White Dwarf Catalogue Then he can't stand up, because the gravity on it is so strong that human bones have been crushed by his own weight.

5. The surface temperature of white dwarfs is very high, with an average of 10000 ℃.

6. The magnetic field of the white dwarf is as high as 10 ^ 5~10 ^ 7 gauss

At present, more than 1000 white dwarfs have been observed. The companion star of Sirius is the first white dwarf discovered by people, and also the brightest white dwarf (star of magnitude 8) observed. The white dwarf star list published in 1982 shows that, Galaxy There are 488 white dwarfs, all of which are near the sun. According to the statistics of observation data, about 3% of stars are white dwarfs, but theoretical analysis and calculation suggest that white dwarfs should account for about 10% of all stars.

White dwarf is a special kind of celestial body. It is small in size, low in brightness, but large in mass and high in density. For example, Sirius companion star (it is the first white dwarf star discovered) is not much larger than the earth in volume, but it has the same mass as the sun! In other words, its density is about 10 million tons/cubic meter.

According to the radius and mass of the white dwarf, we can calculate that its surface gravity is equal to earth 10 million to 1 billion times the surface. Under such high pressure, any object no longer exists, and even atoms are crushed: electrons break away from atomic orbits and become free electrons.

The white dwarf is a late fixed star 。 According to modern Stellar evolution In theory, white dwarfs form at the center of red giants.

External region of red giant star Rapid expansion When the helium core is subject to the reaction force, it shrinks strongly inward, and the compressed material continues to heat up. The final core temperature will exceed 100 million degrees, so the helium begins to condense into carbon.

After millions of years, the helium core has been burned out, and now the structure and composition of stars are not so simple: the shell is still a mixture of hydrogen; There is a helium layer under it, and a carbon ball is buried in the helium layer. The nuclear reaction process becomes more complex, and the temperature near the center continues to rise, eventually transforming carbon into other elements.

At the same time, unstable pulsating oscillations began to occur outside the red giant star: the radius of the star became larger and smaller, the stable star sequence became a huge unstable fireball, and the nuclear reaction inside the fireball became more and more unstable, sometimes strong, sometimes weak. At this time, the density of the inner core of the star has actually increased to about 10 tons per cubic centimeter. We can say that at this time, a white dwarf star has been born inside the red giant star.

As we know, an atom is composed of nucleus and electrons. Most of the atomic mass is concentrated in the nucleus, but the volume of the nucleus is very small. For example, the radius of a hydrogen atom is one billionth of a centimeter, while the radius of a hydrogen nucleus is only one billionth of a centimeter. If the nucleus were as big as a glass ball, the electron orbit would be two kilometers away.

Under great pressure, electrons will be separated Nucleus And become free electrons. This free electron gas will occupy the space between atomic nuclei as much as possible, so that the material contained in the unit space will be greatly increased and the density will be greatly increased. Figuratively, the nucleus is "immersed" in electrons at this time.

This state of matter is generally called "degeneracy". The pressure of degenerate electron gas and the strong gravitational balance of the white dwarf maintain the stability of the white dwarf. By the way, when the mass of the white dwarf star further increases, the pressure of degenerate electron gas may be unable to resist its own Gravitational contraction , the white dwarf will Collapse More dense objects: neutron star or black hole 。

dwarf star

White dwarfs are objects produced at the end of stellar evolution. These stars cannot maintain nuclear fusion reaction, so after the helium flash evolution to the red giant stage, they will throw out their shells to form Planetary nebula , leaving behind a high-density core produced by nuclear fusion, that is, a white dwarf star. Due to the lack of energy sources, white dwarfs will gradually release heat energy to glow and cool. Its core relies on the repulsive force of electrons against gravity, and its density can reach 10 tons per cubic centimeter. The electronic repulsion force is insufficient to support more than 1.4 times Solar mass The gravity of its shell will further collapse the star into neutron star perhaps black hole 。 This process is often accompanied by supernova explosions.

The release of energy will cause the star to gradually cool, the surface temperature will gradually decrease, and the color of the star will also change. After hundreds of billions of years, white dwarfs will become black dwarfs when they are too cold to emit light. However, it is generally believed that the age of the universe (15 billion years) is not enough to make any white dwarf evolve to this stage.

formation

White dwarfs are the end of the evolutionary path of low and medium mass stars. At the end of the red giant stage, the center of the star will stop generating energy due to insufficient temperature and pressure or nuclear fusion reaching the iron stage (producing elements heavier than iron cannot generate energy, but needs to absorb energy). The gravity of the stellar shell will compress the star to produce a high-density object.

A typical stable independent white dwarf has about half the mass of the sun, slightly larger than the Earth. This density is second only to neutron star And quark stars. If the mass of the white dwarf is more than 1.4 times Solar mass , then the charge repulsion force between atomic nuclei is not enough to resist gravity, and electrons will be pressed into atomic nuclei to form neutron stars.

The evolution of most stars includes the white dwarf stage. Because many stars will pass through nova or Supernova The explosion throws out the shell, and some stars with slightly larger mass may eventually evolve into white dwarfs.

In binary or multi star systems, due to the exchange of interstellar matter, the evolution process of stars may be different from that of individual stars. For example, Sirius' companion star is an old white dwarf with about one solar mass, but Sirius is a main sequence star with about 2.3 solar masses.

Brown dwarf

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Dwarf galaxy debris

Brown dwarfs are gaseous objects that constitute similar stars but are not large enough to ignite fusion reactions in the core. Its mass lies between stars and planets.

Brown dwarfs are objects between the smallest star and the largest planet. For this reason, brown dwarfs are very dim. It is very complicated to find them, so it is more complicated to determine their size. But recently astronomers have successfully discovered two stars Brown dwarf After determining the parameters of their movement around the common center of gravity, the weight and size of the two brown dwarfs were calculated.

It took astronomers 12 years to find these two brown dwarfs. They observed more than 300 nights and made 1600 measurements in total. As a result, they calculated all the necessary parameters of two relatively young brown dwarfs (less than 1 million years), which are located in the constellation Orion 1500 light-years away from Earth. binary star The diameter of the larger brown dwarf is 50 times larger than Jupiter, while the diameter of the smaller brown dwarf is 30 times larger than Jupiter, that is, their diameters are 70% and 50% of the diameter of the sun respectively. Although they don't look small at first, their masses are Solar mass 5.5% and 3.5%.

Astronomers also accidentally found that the surface temperature of lighter brown dwarfs is higher, although the situation of "ordinary" stars is the opposite: the larger the star's mass, the hotter it is. Perhaps the reason for this abnormal phenomenon lies in some physical process Stellar structure The theory does not consider this physical process (such as the strong magnetic field of stars). In addition, these two Brown dwarf They may not be formed at the same time or at the same place, but are combined due to some kind of catastrophe, so their surface temperatures are different, but all this is still a hypothesis for the time being.

For a long time, many astronomers have been committed to finding planets with suitable atmospheres that can support extraterrestrial life. nowadays University of Edinburgh Astronomers have found that in addition to planets, extraterrestrial life may exist in the cold brown dwarf clouds. Known as "failed stars", brown dwarfs are special objects between planets and stars, and their size is between giant planets and small stars^[1] 。

Red dwarf

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White dwarf

According to the Herot chart, among many stars in the main sequence stage, red dwarfs are relatively small and low in size and temperature, and belong to K or M type in spectral classification. They are abundant in stars. The diameter and mass of most red dwarfs are less than one-third of the sun, and the surface temperature is less than 3500 K. The light released is also much weaker than the sun, sometimes less than one thousandth of the sun's luminosity. And because of the slow rate of nuclear fusion of the internal hydrogen element, they also have a long life. The internal gravity of a red dwarf is not enough to polymerize the helium element, so it is impossible for a red dwarf to expand into a red giant and gradually shrink until the hydrogen is exhausted. Because a red dwarf can live for tens of billions of years, which is longer than the age of the universe, there are no dying red dwarfs.

People can infer the approximate age of a star cluster based on the long life span of red dwarfs. Because the same cluster The formation time of the stars inside is the same. An older star cluster has more stars separated from the main sequence star stage, and the mass of the remaining main sequence stars is also lower. However, people can not find any red dwarfs separated from the main sequence star stage, which indirectly proves that Cosmic age The exists of.

It is believed that among the stars in the universe, red dwarfs account for the majority, about 75%. For example, the star closest to the sun, Centaur Of Alpha Adjacent star Is a red dwarf star, whose spectrum is classified as M5, Apparent magnitude 11.0。

Planetary cradle

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dwarf star

The humble T dwarf is not a star, but it implies extraordinary "talent". New evidence shows that objects surrounded by dust clouds may become the birthplace of planets, which is very similar to the results of their orbits around stars. When the dust particles are moving around the young star, they will stick together and produce crystals. The formation of planets starts at this time. With the passage of time, these dust will also form a flat and thin disk. Crystallization requires very high temperatures, and it was once thought that this energy came from the radiation of stars. Under this premise, astronomers speculate that, Brown dwarf ——Its mass is about 1% to 9% of the sun, and the core cannot burn hydrogen like ordinary stars because it cannot reach enough heat - the temperature is too low to convert the surrounding dust into planets.

To test this hypothesis, the United States University of Arizona D á niel Apai, an astronomer of Spitzer The space telescope has studied the T-dwarf star cluster in a nearby star forming region. These stars are between 1 million and 3 million years old. The research team found the infrared radiation phenomenon representing silicate dust particles in 6 of the 8 research objectives. The researchers then observed that the dust was growing, crystallizing, and gradually settling into a flat disk. This research result means that, T dwarf Can form planet 。 The researchers reported this discovery in the American journal Science published on October 21. Apai said, "We think if they (dust clouds) can start, then they must have results."

However, Leonardo Testi, an astronomer at the di Arcetri Astronomical Observatory in Florence, Italy, believes that there are still many uncertainties in the process of planet formation, and not all dust particles can form planets. He pointed out that according to theoretical predictions, when these cobblestone Large and small objects will destroy each other after colliding with each other, instead of sticking together. In addition, it is difficult for astronomers to track such particles, because as they become larger and larger, it becomes increasingly difficult to detect the longer wavelengths they emit. But Testi said that T dwarf is planet The term "cradle" is still very attractive, especially when these celestial bodies become our nearest neighbors.

Related information

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Dwarf position

Of the 40 galaxies in the local galaxy group, more than 20 are elliptical dwarf galaxies, which shows that the number of dwarf galaxies is as large as. Such galaxies are very difficult to detect, because they are not as obvious and shiny as large galaxies, but they are more than large galaxies in number. In our Galaxy Adjacent to it are many dwarf galaxies, more than all other types of galaxies combined. In adjacent Galaxy cluster A large number of dwarf galaxies have been found in. Some of them have regular shapes, and most galaxies contain Star Family II Stars; Irregular dwarf galaxies generally contain very bright blue stars.

Recently, astronomers said that NASA In a very short time, many previously unknown dwarf galaxies have been observed in huge ancient galaxies. Although dwarf galaxies are smaller objects in the whole universe, they play a crucial role in the evolution of the universe. Astronomers say that perhaps dwarf galaxies were the first to form in the universe, and dwarf galaxies formed large galaxies. So far, dwarf galaxies are the largest number of galaxies in the universe, and celestial bodies are also the largest number in the universe. They constitute the most basic universe. The computer simulation of the evolution of the universe also shows the ultrahigh density of dwarf stars in the universe. Like the dwarf stars observed this time, the number of dwarf stars in ancient giant galaxies may be much more than astronomers expected.

Early predictions predicted that the number of dwarf stars left over from the previous universe was much higher than what we can observe now, about 120-200 of them were independently orbiting the Milky Way, but so far people have only found a total of 20, which is very difficult to find. But in the near future, people will find more dwarf galaxies.

Xinhua Los Angeles, August 27 (Reporter Guo Shuang) Y dwarf is a kind of brown dwarf with the lowest surface temperature, also known as "failed star". Astronomers have recently discovered six ultra-low temperature brown dwarfs within 40 light years of the sun. One of them broke the record of the coldest brown dwarf with its "temperature" less than 25 degrees Celsius and lower than that of human beings.

The research report on the "coldest" brown dwarf was published in the latest issue of Astrophysical Journal. Main author of the report, subordinate to NASA Jet Propulsion Laboratory Michael Cushing, an astronomer from the University of California, said that the discovery of ultra-low temperature Y dwarfs will help people understand the formation conditions of stars and planets.

Brown dwarfs are a kind of strange objects whose size is between ordinary stars and planets and whose formation process is similar to stars. It is "inherently weak" and too small in mass to generate enough pressure in the center to cause nuclear fusion, so it cannot emit strong light like ordinary stars. The brown dwarf with a long birth time is too dim to be observed.

This time astronomers were able to find the coldest brown dwarf stars, relying on excellent "star scouts" -“ Wide angle infrared measurement detector (WISE), which is currently the best tool for observing star forming regions in the infrared band. Although it is not as sensitive as the "golden eye", it has helped people successfully find 100 brown dwarfs.

Cushing said in a press release released by NASA that the discovery of a brown dwarf near the sun is like finding a hidden house in the neighborhood where you live, which is really "frightening".

^[2]

Latest findings

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Wandering stars or brown dwarfs 100 light-years away

On April 7, 2017, Beijing time, according to foreign media reports, astronomers were very confused about a mysterious celestial body wandering in space recently.

This lonely wandering object, CFBDSIR J214947.2-040308.9, was first discovered in 2012 and was initially classified as a Rogue planet This kind of planet does not revolve around a star like the planets in the usual sense, but revolves around the center of the galaxy. However, the latest research suggests that this object may not be a planet, and it is also more "isolated" than we previously thought.

In fact, it is very likely to be a brown dwarf, which is a special type of celestial body between stars and planets. Its mass is far higher than that of planets, but its mass as a star is far from enough. The reason why this object is particularly interesting is that it is very close to the solar system, only about 100 light-years away^[3] 。

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