# Tag Info

### How can comets have tails if there's no air resistance in space?

There are two forces that can cause the formation of a tail: the solar wind and radiation pressure. The first misconception in your question is "the dust [travels] slower than the nucleus". The tail ...
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### How did the Milky Way quasar not disrupt life on Earth?

An active galactic nucleus doesn't emit energy equally in all directions. It may form "jets", and if you are looking into the AGN at the right angle, and then nucleus is active enough, then ...
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### How can comets have tails if there's no air resistance in space?

First, there is not just one tail, it is several, but when traveling far from a star, they are "aligned". When it gets closer the different materials behave differently, both depending on the ...

### 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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Accepted

### Is Earth's Atmospheric Thickness Related to Cosmic Ray Radiation?

It won't be (and isn't) zero. To see that, you want the plot on a logarithmic scale. Thus there isn't really a question to answer. The attenuation of cosmic rays will follow some exponential decay ...
• 155k
Accepted

### Problem regarding the absorption lines of the Sun

Possibly you are under the misapprehension that the number of photons is a conserved quantity? That isn't true, there are more photons at any given wavelength when you are deeper into the star because ...
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### How can redshifted light be detected?

In a redshift (whether that be caused by relative motion, gravitation or cosmological expansion), all wavelengths are increased by the same factor. Redshift is determined by identifying features in a ...
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### How did the Milky Way quasar not disrupt life on Earth?

How much dangerous a quasar at Sag A* could be? A Fermi-inspired estimation: Assume it is 1000x the luminosity of the Milky way. This is rather at the high end (as per Wiki) and rounding out its ...
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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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### How did the Milky Way quasar not disrupt life on Earth?

I think the source of confusion here is that the two sources you mention implicitly have different definitions of what quasar is. Firstly, to our current understanding, quasars are not a well-defined ...
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Accepted

### What is the apparent size of Earth from the Sun?

The calculation of the apparent size of Earth as seen from the Sun is pretty straight forward. Just consider the triangle: from your observation point the distance to the center of the object, to the ...
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