When we think of cosmic space, we tend to think of it as a dark, silent vacuum of nothing. Actually, that's not quite right. The vast expanse of space between the stars may be silent, but far from being a true "vacuum," it is filled with a wide variety of matter. These substances include interstellar gas, dust, and particle streams, which are called "interstellar matter".
Interstellar matter is closely related to the evolution of celestial bodies. Observations have confirmed that interstellar gas is mainly composed of two elements, hydrogen and helium, which are the same as the composition of stars. It has even been hypothesized that stars are formed by the "condensation" of interstellar gas. Interstellar dust is very small, solid material, including carbon compounds, oxides, and so on.
Interstellar matter is not evenly distributed in cosmic space. Under the effect of gravity, the gas and dust in some places may be attracted to each other and become dense, forming clouds. People figuratively call them "nebulae". According to their morphology, nebulae in the Milky Way can be categorized as diffuse nebulae, planetary nebulae, and so on.
Diffuse nebulae, as their name implies, do not have distinct boundaries and are often irregularly shaped. They have a diameter of a few tens of light-years and a density averaging 10-100 atoms per cubic centimeter (in fact this is much lower than the vacuum obtained in the laboratory). They are mainly found in the neighborhood of the silver channel plane (HOTKEY). Some of the better known diffuse nebulae are the Great Nebula in Orion and the Horsehead Nebula.
Planetary nebulae look a bit like spit smoke rings, with an empty center and often a very bright star. The star is constantly ejecting material outward, forming the nebula. It can be seen that planetary nebulae are the result of the evolution of stars in their later years. Some of the better known ones are the Aquarius Trunnion Nebula and the Ring Nebula in Lyra.
Below are a few types of nebulae
1. Faint Nebulae
Bright diffuse nebulae are bright because they are illuminated by one or more bright stars. If there were no bright stars near a gas-dust nebula, the nebula would be dark, i.e., a faint nebula.
The dark nebula, because it neither glows nor has light for it to reflect, but will absorb and scatter light from behind it, can be found in the star-dense Milky Way as well as against the backdrop of bright diffuse nebulae.
2. Supernova Remnants
Supernova Remnants are also a class of nebulae quite different in nature from Diffuse Nebulae, formed from the gas thrown out by supernova outbursts. Like planetary nebulae, these nebulae expand in size and eventually tend to dissipate.
The most famous supernova remnant is the Crab Nebula in the constellation Taurus. It is the remnant of an intragalactic supernova that exploded in 1054. A neutron star has been found in the center of this nebula, but it cannot be seen with optical telescopes because it is very small. It was discovered because it had pulsating radio wave radiation and was identified theoretically as a neutron star.
3. Diffuse Nebulae
Diffuse nebulae are bright nebulae caused by the concentration of the interstellar medium around one or more bright stars, all of which are young and recently formed. Diffuse nebulae are irregularly shaped, like clouds in the sky, but they can usually only be seen with telescopes, and many can only be revealed by long exposures with an astrocamera.
4. Planetary Nebulae
Planetary nebulae are round, oblate, or toroidal in shape, and some resemble large planets, hence their name. These nebulae are quite different in nature from diffuse nebulae, which are the products of the late evolution of stars of about the mass of the Sun, when nuclear reactions have ceased and they are dying. These nebulae are expanding in size and eventually tend to dissipate. At the center of a planetary nebula is a hot star called the central star of the planetary nebula. This is the star that is evolving into a white dwarf.
Nebulae
Nebulae are formed by the dust and gas of the Universe, and their main component is hydrogen. Briefly, they can be divided into four categories: emission nebulae, reflection nebulae, dark nebulae, and planetary nebulae.
Emission Nebulae
Emission nebulae are excited by nearby, hot, luminous stars that emit ultraviolet light that ionizes the hydrogen gas (Hii regions) in the nebula, causing them to glow. There are many familiar emission nebulae in the sky, such as the Great Nebula in Orion M42, which is visible to the naked eye at visual magnitude 4. It is 1600 light-years away and 30 light-years in diameter. Gaseous conditions can be easily observed with small aperture telescopes, as well as the quartet of stars at its center (six can be seen with large aperture telescopes), which formed at the center of the Great Nebula in Orion.
Reflection Nebulae
Reflection nebulae, unlike reddish emission nebulae, shine blue by reflecting the light of nearby stars. Reflection nebulae are fainter and easier to observe than their red counterparts, such as those surrounding the Seven Sisters cluster of M45 Tauri, which can be seen with a telescope on a clear, moonless night as a pale blue nebula wrapping around the cluster.
Dark Nebulae
Dark nebulae do not emit light and do not contain stars, but are visible because they block the light of background nebulae or stars. Some famous dark nebulae are the Coal Sack Nebula in the southern sky and the Horsehead Nebula (B33) in Orion in the northern sky. The Horsehead Nebula is regarded by amateur astronomers as the ultimate in visual deep-sky astronomy. No more than a dozen astronomers in Hong Kong have observed the Horsehead Nebula because it can only be seen with the aid of a large aperture telescope.
Planetary Nebulae
Unlike the three types of nebulae mentioned earlier, planetary nebulae are the products of stars in their later years. When viewed through a telescope, most planetary nebulae appear as planet-like disks that have nothing to do with planets.
When a low-mass star reaches the end of its life, it expands into a red giant, and when it does, it collapses inward again, and as it does, some of the material continues to expand outward, forming a gas shell (the planetary nebulae we call planetary nebulae), with white dwarfs forming in the center. The "life" of planetary nebulae is generally very short, and these gas shells usually disappear within tens of thousands of years. Not all planetary nebulae are round, and some have very distinctive shapes, such as the M27 Dumbbell Nebula in Vulpecula and the M76 Little Dumbbell Nebula in Perseus.