# Tag Info

Accepted

### Is the Sun visible from Proxima Centauri to human eyes?

Well, there's two things we'll need for this: apparent magnitude (the brightness that an object appears to have) and absolute magnitude (the actual brightness an object has). Both of these scales are ...
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### Why are nearby stars like Proxima Centauri and Barnard's star not visible to the naked eye?

John's answer is correct. For a few more details: Stars brightness works out to roughly the 4th power of their relative mass. This falls off a bit for very large stars, but for smaller stars it's ...
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Accepted

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Accepted

### What is the "lunar irradiance" received by the Earth from the full Moon?

You can work in magnitudes, but the magnitude scale is logarithmic. Instead you can use luminosity. The moon has a luminosity that is 400,000 times less than the sun, so the maximum theoretical &...
• 93.4k

### How long does lunar opposition surge last? Are there measurements of the full Moon getting suddenly brighter?

The lunar opposition surge has been well studied, likely because we can study it in detail, we have surface samples, and so it serves as a baseline for other bodies in the solar system (as it does for ...
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### How many photons does it take to determine the existence of a distant object?

What matters is how many photons you collect versus how many you would expect to see if the object wasn't there. Photons would be present, without a source, for a variety of astrophysical (e.g. ...
• 120k
Accepted

### If Planet Nine indeed exists, how large a impact event there should be to be seen from Earth?

We can compare the brightness of a comet hitting planet 9 with the 1994 impact of Shoemaker-Levy 9 / SL9 on Jupiter. It was seen by Hubble, and Galileo which was orbiting Jupiter at the time. View ...
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### Why are nearby stars like Proxima Centauri and Barnard's star not visible to the naked eye?

Stars vary a lot in both size and brightness. The nearby stars you mentioned are less bright than many stars that are much further away. We see the bright stars but not the less bright ones. Its like ...
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