# Tag Info

## Hot answers tagged black-body

### Why does the CMB conform to black-body formula for 2.71 degrees, when it's the stretched emission at a far hotter temp?

The Planck spectrum of a blackbody is given by $$B(\nu, T) = \frac{2h\nu^3}{c^2} \left( \exp[h\nu/k_BT] - 1\right)^{-1} \ ,$$ where $\nu$ is the frequency, $T$ is the temperature and $h$ and $k_B$ ...
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### Why does the CMB conform to black-body formula for 2.71 degrees, when it's the stretched emission at a far hotter temp?

According to the Hubble law all radiation is progressively red-shifted with distance traveled, with the wavelength changing from $\lambda$ to $\lambda(1+z)$ (with $z>0$). Now the formula for the ...
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### What is the RGB curve for blackbodies?

Ok, here's my take on calculating the color of a blackbody, or any spectrum in fact: Disclaimer: I'm not a color theorist, and there may be more accurate methods. But the result, shown in the bottom, ...
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### At high temperatures, do planets glow like blackbodies?

At high temperatures, do planets glow like blackbodies? Yes, and at low temperatures too!1 1As @DavidHammen points out, since there's likely going to be a star nearby the planet, it will also be ...
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### Trouble with plotting a graph for the Planck function

Here's what I got for $\nu B_{\nu}(T)$ using Python, which is easy enough to read that maybe you can check against your Excel calculation. I also plotted just $B_{\nu}(T)$ for comparison. Neither ...
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### How to Distinguish between Temperature and Doppler effect using Black-body radiation?

If the object you're observing is something good and hot, like a star, it's pretty easy to identify a set of Ballmer lines by the relative spacing between the lines. Then compare the absolute ...
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### Back when the CMB was predominantly in the visible spectrum, would it have been visible to the naked eye?

It wouldn't be like staring into the Sun so much as being inside the Sun (its photosphere, anyway). What we really see is not distant objects, but light that enters our eyeballs. The CMB radiation ...
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### Back when the CMB was predominantly in the visible spectrum, would it have been visible to the naked eye?

The cosmic microwave background (CMB) was emitted as a blackbody at a temperature of about 3000 K. The CMB temperature then falls as the universe expands and is currently at a temperature of 2.73 K. ...
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### Does most of the Sun's energy and light come from its black-body temperature, due to its massive size alone? Constant crashing of particles?

If you calculate the total thermal energy of the Sun now - i.e. the sum of the thermal (kinetic energy) of its constituent particles - then it could be used to supply it's current luminosity for 40 ...
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### Does most of the Sun's energy and light come from its black-body temperature, due to its massive size alone? Constant crashing of particles?

A collapsing cloud of gas will generate heat from the release of gravitational potential energy, and it's core temperature will continue to rise. Indeed this is the mechanism that Kelvin and Helmholz ...
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### Strange behaviour of black holes

What you need to understand is that a black hole is a region of space defined by an event horizon. The key point about an event horizon is that if you're inside the region it surrounds you cannot get ...
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### Strange behaviour of black holes

Black hole don't allow anything to escape, if the matter/light is inside the event horizon (boundary which describes the point of no return). The emission radiation that is referred in the question is ...
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### Does photon capture in a black hole (or any other "very compact" object) reduce the luminosity/photon flux from its self isotropic emission?

Let's answer the question from the point of view of a stable neutron star. The luminosity is $$L_0 = 4\pi R_0^2 \sigma T_{0}^4$$ in the inertial frame of the neutron star surface (assume a non-...
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### In what manner is the spectrum of a black body really a continuum?

A blackbody spectrum is continuous. As Dean comments, there are no true blackbodies. But this is not really the issue here: It's true that most emitted photons from, say, a star, which is quite ...
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### Why is a black hole's Hawking radiation temperature inversely proportional to its mass/size?

Þe olde goode dimensional analysis can help. The Hawking radiation is essentially a quantum field effect of the same nature as the Unruh effect. Quantum fields “live” in a distorted space-time of the ...
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### Why is a black hole's Hawking radiation temperature inversely proportional to its mass/size?

In a sense, you're right. Vacuum energy over a surface area is proportional to the surface area so larger area means more of the particle-antiparticle occurrences and that's the source of Hawking ...
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### How to convert from Black body intensity to MJy/sr?

Part 1 On the one hand, ${\rm 1\, ergs\cdot cm^{-2}\cdot s^{-1} \cdot sr^{-1}\cdot Hz^{-1}} = 10^{-3} {\rm J}/({\rm m^2} \cdot {\rm sr})$ has basic dimensions \${\rm mass} \cdot {\rm time}^{-2} \cdot {\...
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### Where can we find astronomical real world data (spectral radiance and wavelength particularly) of a star?

The best way to do this is to pick a particular astronomical instrument or science mission and go to their website. Almost all astronomy projects are required to release their data to the public and ...
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