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what is a redshift nebula and what does it mean?
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Short answer: there’s no standard astronomical object called a “redshift nebula.” The phrase can mean one of two different things:
1) A nebula that appears red because of the light it emits or dust reddening — not because of Doppler/cosmological redshift.
- Emission nebulae (H II regions) glow red mainly from hydrogen alpha emission (Hα) at 656.3 nm (for example the Orion Nebula).
- Interstellar dust can also make a nebula look redder by scattering and absorbing blue light (extinction/reddening).
2) A nebula whose spectrum is redshifted — i.e., its spectral features are shifted toward longer wavelengths because the source is moving away or because of the expansion of the Universe. In that sense “a redshifted nebula” just means you can measure a redshift z for the nebula and use it to infer velocity or distance. Astronomers measure z from the amount spectral lines are shifted:
- z = (λobserved − λrest) / λrest.
- For small z, recession speed v ≈ c z (c = speed of light). Via Hubble’s law, v ≈ H0 × distance, so redshift gives distance for distant objects.
How to tell which meaning applies: look at the spectrum. If you see strong emission at Hα and other emission lines at their normal wavelengths, the nebula is intrinsically red. If you see those lines displaced to longer wavelengths by a common factor, that displacement is a redshift (motion/distance). If the continuum is reddened and lines are unchanged, dust extinction is probably the cause.
If you had a specific object or image in mind, tell me which and I can explain which of these applies.
1) A nebula that appears red because of the light it emits or dust reddening — not because of Doppler/cosmological redshift.
- Emission nebulae (H II regions) glow red mainly from hydrogen alpha emission (Hα) at 656.3 nm (for example the Orion Nebula).
- Interstellar dust can also make a nebula look redder by scattering and absorbing blue light (extinction/reddening).
2) A nebula whose spectrum is redshifted — i.e., its spectral features are shifted toward longer wavelengths because the source is moving away or because of the expansion of the Universe. In that sense “a redshifted nebula” just means you can measure a redshift z for the nebula and use it to infer velocity or distance. Astronomers measure z from the amount spectral lines are shifted:
- z = (λobserved − λrest) / λrest.
- For small z, recession speed v ≈ c z (c = speed of light). Via Hubble’s law, v ≈ H0 × distance, so redshift gives distance for distant objects.
How to tell which meaning applies: look at the spectrum. If you see strong emission at Hα and other emission lines at their normal wavelengths, the nebula is intrinsically red. If you see those lines displaced to longer wavelengths by a common factor, that displacement is a redshift (motion/distance). If the continuum is reddened and lines are unchanged, dust extinction is probably the cause.
If you had a specific object or image in mind, tell me which and I can explain which of these applies.
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