What are nebulas?. And why nebulas are big as galaxies.
Basically, a nebula a huge cloud of gas and dust. Some nebulae are the rest of the death of a giant star, like a supernova. Other nebulae are where new stars are forming. (more)
The nebulae are divided into some types:
- Emission Nebulae
Emission nebulae are gas clouds with high temperature. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation when they decay to lower energy states. Emission nebulae are usually red, because of hydrogen, the most common gas in the Universe and commonly emitting red light.
- Reflection nebulae
Reflection nebulae are dust clouds that simply reflect the light of a star or nearby stars. Reflection nebulae are usually blue because the blue light is spread more easily. Emission and reflection nebulae are usually seen together and are sometimes called diffuse nebulae.
- Dark nebulae
There are also dark nebulae, they are clouds of gas and dust that almost completely prevent the light from passing through them, are identified by the contrast with the sky around them, which is always more starry or bright. They may be associated with star formation regions.
- Planetary nebulae
Planetary nebulae were named after William Herschel because when they first appeared to the telescope, they resembled a planet, later it was discovered that they were caused by ejected material from a central star. This material is illuminated by the central star and shines, and an emission spectrum can be observed. The central star usually ends up as a white dwarf.
- Remnant of supernova
Remnant of supernova is a gas envelope, composed of the remains of a star that was destroyed by a violent explosion, supernova, marking the death of this.