Shulou(Shulou.com)11/24 Report--

On March 14, 2023, Weber released a rare image of a star, a combination of near-infrared (NIRCam) and mid-infrared (MIRI) images, showing a massive star, WR 124, about to explode into a supernova at the end of its evolution. Since it hasn't exploded yet, what's going on with this nebula?

WR 124 and surrounding Nebulae in 1938, astronomers discovered a jet-like nebula (M1-67) in the direction of Sagittarius. Half a century later, the nebula was finally seen tens of thousands of light-years away through pictures taken by the Hubble Space Telescope. As can be seen from the form of diffusion, there seems to be some kind of explosion here.

In 2010, in a study of the nebula, researchers calculated its expansion rate from two sets of images 11 years apart, speculating that the explosion occurred about 10,000 years ago.

Obviously, the explosion should have originated from the star in the nebula. Yes, it is still a star and has not yet collapsed into a neutron star or black hole. That is to say, the previous explosion was not a supernova explosion. So what happened to this star?

To understand a star, it is inevitable to study its spectrum, and the spectrum of this star is really something special.

The spectrum of a star is mainly emitted by the photosphere in the star's atmosphere. Generally speaking, the spectra of most stars are more hydrogen and less heavy elements. Because heavy elements are generally concentrated in the core of stars, the outer layer is mainly hydrogen. Because the outer temperature of the star is much lower than the core temperature (for example, although the sun's interior can reach 1500 million degrees, but its surface is only a few thousand degrees), matter is more inclined to absorb light in this "low temperature" situation. The reaction in the spectrum is that there are some dark line gaps in the continuous spectrum, which corresponds to what element the light is absorbed by. This spectrum is also known as the absorption spectrum. So, from the absorption spectrum of the star, we can infer what elements are in the star's atmosphere.

The usual star spectrum is like deducting a few lines from the spectrum, but the star's spectrum seems to have been affixed with a few lines. Yeah, it's like a mask, it's the other way around. The previous absorption spectrum is because the element absorbs light, so this kind of spectrum is obviously that the element actively emits light, so it is also called emission spectrum.

Continuous spectrum vs emission spectrum vs absorption spectrum for matter to produce emission spectrum, it usually requires a very high temperature, and this is one of the special features of this star-the unusually high surface temperature.

Generally speaking, the greater the mass of a star, the higher the temperature. About 99% of stars have surface temperatures of thousands of degrees, and only those large (B or O stars) with more than several times the mass of the sun are likely to reach tens of thousands of degrees. This star is such a massive star with 20 times the mass of the sun, with a surface temperature of more than 40000 degrees! Seven times as much as the sun!

And from the emission line, the star's atmosphere seems to lack hydrogen, replaced by helium, nitrogen and even carbon, oxygen and other heavy elements. But these heavy elements rely on nuclear fusion and can only be produced in the interior of stars and should not appear in the outer atmosphere. This is another special feature of this star.

This massive star with ultra-high surface temperature and emission lines such as helium and nitrogen is the famous WR star.

In fact, Wolf-Layette was first discovered by Charles Wolf and George Laye in 1867, but because of the particularity of its spectrum, the mystery was not solved until decades later, and the name "Wolf-Layette" was established.

Since Wolf-Layette is big and hot, it is not far from death. Why? Because it can blow so well! Yes, it means "energy blowing" in the physical sense.

Wolf-Rayet stars are usually massive O-stars at a special stage at the end of their evolution. With the heavy nuclear fusion of the core, the temperature inside the star is getting higher and higher, which leads to the frenzied fusion of the hydrogen in the outer layer. The radiation pressure generated in the process pushes the outer material outward, causing the star to expand. As the matter expands outward, the gravity weakens gradually, so the surface material is easily blown away. The end result is that the internal heavy elements are exposed, which is why its spectrum lacks hydrogen and is rich in heavy elements.

The stellar wind is like a hair dryer, it can continuously and steadily disperse the outer material. However, although this method is stable, it is not efficient, and only low-mass stars such as the sun will mainly use it.

If the star were larger, its radiation pressure would be stronger, and its temperature and luminosity would be higher. When the luminosity exceeds the Eddington limit, the hydrostatic equilibrium of the star begins to be broken, and fluid instability occurs in the outer atmosphere. The result is that the outer material can be freed from the gravitational bondage by relying solely on radiation pressure instead of stellar wind. As a result, part of the star's shell begins to fall off in blocks, triggering a series of violent ejections of surface matter.

Remember when we said Betelgeuse darkened? Although Betelgeuse has not yet reached the stage of Wolf-Layette, it has become a red supergiant and is buried in the neck. The last time it suddenly darkened was because its photosphere lost a large chunk of material, which ejected directly out of the star's light through the torn chromosphere.

The reason Betelgeuse darkens is similar to the Wolf-Layette star mentioned today. The massive nebula around it comes from the outer layer of material ejected by it, which has the mass of the top 10 suns. The loss of so much mass in a short period of time shows how intense the ejection of matter here is.

In addition to the higher radiation pressure, for stars that rotate at high speed, they can throw themselves into an oval ball with flat poles and wide equator. In this case, the material at the equator is easily thrown out, which is one of the reasons why the outer layer of the star is ejected.

In fact, in addition to the star itself, theoretically, if it is in a binary system, then the accretion and stripping of it by the companion star mentioned earlier may also be a way to produce the Wolf-Layette star.

Either way, the star will eventually lose almost all the hydrogen in the outer layer, leaving only the core. If it were a star with a small mass like the sun, the core would be so small that no fusion reaction could occur, which is what we call a white dwarf.

But for a massive star, even if there is only a core, it is still so massive that heavy nuclear fusion such as carbon and oxygen can still take place. Coupled with the fact that part of the fusion reaction is already very close to the upper layer, the surface temperature of the star becomes very high.

How high is it? The hottest Wolf-Layette found so far is WR 102, also located in the constellation Sagittarius, with a surface temperature as high as 210000 degrees! It can be called the ceiling of the main sequence star boundary.

The nebula around WR 102 naturally has no indication of its brightness for such a hot star. According to the model, the luminosity of WR 102 is about 280000 times that of the sun. However, because the radiation peak of this star has reached the ultraviolet region beyond visible light, it is theoretically not that bright to the naked eye.

But as a "star", once "red", then the danger is not far away from it.

For stars that have exposed their cores a little bit, they are already very dense and are compact stars. WR 102, for example, is only half the radius of the sun, but its mass is 16 times that of the sun, and its average density is nearly 100 times that of the sun, indicating that it is already extremely compressed. According to astronomers' calculations, it will become a supernova in the next 1500 years, ending its stellar career as a neutron star or black hole.

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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