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Diffuse nebulae can be divided into emission nebulae, reflection nebulae and dark nebulae. Visible light nebulae may be divided into emission nebulae, which emit spectral line radiation from excited or ionized gas (mostly ionized hydrogen ); [ 25 ] they are often called H II regions , H II referring to ionized hydrogen), and reflection nebulae ...
The nebula measures around two light years long from end to end, and contains a central white dwarf with an apparent magnitude of 11. The inner layers of the nebula were formed some 1,500 years ago. [4] The two ends of the nebula are marked by FLIERs, lobes of fast moving gas often tinted red in false-color pictures. [5]
The closest and largest dark nebulae are visible to the naked eye, since they are the least obscured by stars in between Earth and the nebula, and because they have the largest angular size, appearing as dark patches against the brighter background of the Milky Way like the Coalsack Nebula and the Great Rift.
Glowing Eye Nebula or Dandelion Puffball Nebula: NGC 6751: 1863 6.5 11.9 Aquila: Turtle Nebula: NGC 6210: 1825 4.7 9.3 Hercules: Bow-Tie Nebula: NGC 40: Caldwell 2 ...
The central star of the nebula has a similar spectrum to a Wolf–Rayet star (spectral type [WC4]), [1] and has an effective temperature of about 140,000 K and a radius of about 0.13 R ☉. It is losing mass at a rate of 1 × 10 −6 M ☉ per year, and its surface composition is mostly helium and carbon .
Reflection nebula are usually blue because the scattering is more efficient for blue light than red (this is the same scattering process that gives us blue skies and red sunsets). Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae. Some 500 reflection nebulae are known.
The Trifid Nebula was the subject of an investigation by astronomers using the Hubble Space Telescope in 1997, using filters that isolate emission from hydrogen atoms, ionized sulfur atoms, and doubly ionized oxygen atoms. The images were combined into a false-color composite picture to suggest how the nebula might look to the eye.
The central star of the planetary nebula has a spectral type of [WC4], similar to that of a carbon-rich Wolf–Rayet star. [6] It is a pulsating star, meaning that its brightness varies regularly and periodically. In the case of NGC 1501's progenitor star, this is incredibly fast, with the star's brightness changing significantly in just half ...