Formation of nebulae:
When we talk about space, we often think that there is nothing there, but a dark and silent vacuum. Actually, this is not entirely right. The vast interstellar space may be silent, but it is far from real? Vacuum? But there are all kinds of substances. These substances include interstellar gas, dust and particle flow. People call it? Interstellar matter? .
Interstellar matter is closely related to the evolution of celestial bodies. Observations have confirmed that interstellar gas is mainly composed of hydrogen and helium, which is the same as that of stars. In fact, stars are made of interstellar gas? Coagulation? Made of. Interstellar dust is a small solid substance, which is composed of carbon compounds, oxides and so on.
The distribution of interstellar matter in space is not uniform. Under the action of gravity, gas and dust in some places may attract each other, become dense and form clouds. People call them figuratively? Nebula? . According to the morphology, the nebulae in the Milky Way can be divided into diffuse nebulae and planetary nebulae.
Compared with stars, nebulae have the characteristics of large mass, large volume and low density. The mass of an ordinary nebula is at least equivalent to thousands of suns, and the radius is about 10 light-year.
Nebulae are usually named according to their positions or shapes, such as Orion Nebula and Lyra Nebula.
Classification of nebulae:
Divided by luminous characteristics
Can be divided into:
emission nebula
reflection nebula
dark nebula
emission nebula
The emission nebula is excited by nearby hot stars, and the ultraviolet rays emitted by these stars will ionize the hydrogen in the nebula (Hⅱ region) and make it glow. The emission nebula is a mass of free gas (that is, plasma) that can emit different colors of light. The reason for dissociation is usually high-energy photons emitted by nearby stars. Some types of these different emission nebulae are hydrogen ⅱ region, where young stars are born, and photons of massive stars are ionization sources; Planetary nebulae are shells thrown by dying stars and heated by exposed hot cores.
The color of the nebula depends on the chemical composition and ionization degree. Because most interstellar gas is hydrogen, which can be ionized at relatively low energy, many emission nebulae are red. If higher energy can cause other elements to dissociate, then both green and blue clouds may appear. By studying the spectra of nebulae, astronomers can infer the chemical elements of nebulae. Most emission nebulae contain 90% hydrogen, and the rest are helium, oxygen, nitrogen and other elements.
In the northern hemisphere, the most famous emission nebulae are the North American Nebula (NGC7000) and the Netted Nebula (NGC6960/6992) in Cygnus. In the southern hemisphere, the best are the lagoon nebula M8/NGC6523 in Sagittarius and the Orion nebula (M42) in Orion. Further south in the southern hemisphere is the bright Callina Nebula (NGC3372).
Black spots often appear in emission nebulae because the dust in the cloud gas blocks the light. The combination of emission nebula and dust often produces some interesting-looking celestial bodies, and many of these celestial bodies have vivid or vivid names, such as North American Nebula and Cone Nebula.
Some nebulae are composed of reflection nebulae and emission nebulae, such as trifid nebula.
reflection nebula
Reflective nebulae emit light by reflecting light from nearby stars, which is blue.
[Since scattering is more effective for blue light than red light (this is the same process as the sky appears blue and the sunset appears red), reflection nebulae are usually blue]
From the astronomical point of view, the reflection nebula is only composed of dust, and it simply reflects the light from nearby stars or clusters. These nearby stars don't have enough heat to make clouds glow like emission nebulae, but they have enough brightness to make dust particles visible because of scattered light. Therefore, the reflection nebula shows a spectrum similar to that of the star that illuminates it.
dark nebula
If there are no bright stars near the gas dust nebula, then the nebula will be dark, that is, a dark nebula. Dark nebulae can be found in galaxies with dense stars and bright diffuse nebulae, because they neither emit light nor reflect light, but absorb and scatter light from behind.
The density of dark nebula is enough to block the light emitted by emission nebula or reflection nebula (such as horsehead nebula) or stars in the background. Astronomical extinction usually comes from interstellar dust particles in the part with the lowest temperature and the highest density of the large molecular cloud. Large and complex dark nebula polymers are often associated with huge molecular clouds, and small and lonely dark nebulae are called buckyball.
The formation of these dark nebulae is usually irregular: they have no clearly defined shapes and boundaries, and sometimes they form complex dragonflies. The naked eye can see huge dark nebulae, showing dark patches in bright galaxies. The interior of the dark nebula is where important events occur, such as the formation of stars.
Divide by form
Can be divided into:
diffuse nebula
planetary nebulae
Supernova remnant
Bipolar nebula
diffuse nebula
Diffuse nebulae, like their names, have no obvious boundaries and often show irregular shapes, just like clouds in the sky, but they are usually observed through telescopes, and many of them can only show their beauty through long-term exposure of celestial cameras. They are about several dozen light-years in diameter and have an average density of atoms per cubic centimeter 10- 100 (which is actually much lower than the vacuum obtained in the laboratory). They are mainly distributed near galactic plane. The more famous diffuse nebulae are Orion Nebula and Horsehead Nebula. Diffuse nebulae are bright nebulae gathered by interstellar medium around one or several bright stars, all of which are young stars that have not been formed for a long time.
planetary nebulae
Planetary nebulae are round, oblate or annular, and some of them are very similar to big planets, hence the name, but they have nothing to do with planets. Not all planetary nebulae are round. Some planetary nebulae are very unique in shape, such as M27 dumbbell nebula in Fox and M76 dumbbell nebula in Perseus.
It looks a bit like a smoke ring with an empty center. There is often a bright star in the center of the planetary nebula, called the central star of the planetary nebula, which is evolving into a white dwarf. The central star constantly throws matter outward, forming nebulae. It can be seen that the planetary nebula is the result of the evolution of stars in their later years. They are the products of the late evolution of solar-mass stars, and they die after the nuclear reaction stops. The famous spiral nebula in Aquarius and the ring nebula in Lyra are completely different from the diffuse nebula in nature. The volume of these nebulae keeps expanding and finally tends to dissipate. Planetary nebula? Life? It is very short-lived, and usually these gas shells will gradually disappear within tens of thousands of years.
Supernova remnant
Supernova remnant is also a kind of nebula completely different from diffuse nebula, which is formed by the gas thrown out after supernova explosion. Like planetary nebulae, the volume of such nebulae keeps expanding and finally tends to dissipate.
The most famous supernova remnant is that crab nebula in the golden constellation. It is the remnant of a supernova in the Milky Way that broke out in 1054. A neutron star was found in the center of this nebula, but it was too small to be seen with an optical telescope. It was discovered because of its pulsed radio wave radiation, and it was theoretically determined that it was a neutron star.
Bipolar nebula
Bipolar nebulae are characterized by unique lobes forming axisymmetric nebulae.
Many, but not all, planetary nebulae show binary star structures in observations. These may be two types of nebulae that are directly related. In the development of nebulae, one is in front of the other, or will replace the other.
Although the exact cause of the nebula is not clear, it may be a physical process called bipolar escape, in which stars throw high-energy particles as streams flowing from the poles. One theory is that these effluents will collide with the material around the star (possibly interstellar dust or the surrounding shell in early supernova events).
Other classifications
Some nebulae are the birthplaces of stars, and their dust gradually shrinks into new stars under the action of gravity, such as the M42 nebula in Orion, while others are the remains of old stars after explosion, such as the reticular nebula in Cygnus.