23
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How bright will Alpha Centauri A be from Earth when it becomes a red giant?
First of all, by the time Alpha Centauri A becomes a red giant, it will no longer be this close to the Sun due to the orbit of the stars around the galaxy so it probably wouldn't be visible. But let's ...
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What's the issue with Olbers' paradox?
Olber's paradox is - as you state - the phenomenon, that the night sky is dark, but would have to be bright as the sun if the universe was infinite and infinitely old. The unspoken assumption here is ...
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19
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Is the Sun really a medium size star?
It is true that a surprisingly large number of stars are smaller (and thus less massive) than the Sun. However, the stars that are bigger than the Sun are often much bigger.
Look at this chart:
...
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17
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Spectral class and luminosity of hydrogen bomb explosions?
Hydrogen bombs aren't like little stars. The process of fusion in stars is slow, releasing very little energy per cubic metre. As a result of this, and their large scale, stars are close to ...
- 146k
14
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Visible Stars in Andromeda Galaxy
You are right that most of the dots of light that you see in the at image of Andromeda are stars in our galaxy that happen to be on the same alignment.
However we can resolve stars in the Andromeda ...
- 116k
13
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What is the most appropriate way to estimate the helium composition of a star?
Nearly all the helium in the photosphere of the sun comes from the helium in the interstellar gas that collapsed to form the sun. That helium was produced shortly after the Big Bang (in about the ...
- 116k
13
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Visible Stars in Andromeda Galaxy
There is some confusion here about the word "resolve". In astronomy, to resolve an object means either to establish details of its structure and physical extent, rather than seeing it as a point ...
- 146k
13
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Confusion over habitable zone
The reason you get a too cold temperature for Earth is of course because you ignored the greenhouse effect. The atmospheric composition strongly affects the actual temperature of planets, which ...
- 14k
12
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Increase in Luminosity of the star
The luminosity mass relation is inexact. The luminosity also depends on the composition of the star, particularly in and around the nuclear burning regions.
The composition changes during the main ...
- 146k
10
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What is the radiant intensity of the celestial sphere?
The electromagnetic energy density is dominated by the cosmic microwave background and the optical/IR background. This Physics SE answer, contains the plot below, showing the contribution of energy ...
- 146k
10
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Is there a relation between mass, radius, and luminosity in deuterium-burning brown dwarfs?
Yes, there are monotonic relationships between mass and luminosity and radius on the "deuterium burning main sequence".
Deuterium "burning" begins when the core temperature exceeds just over $10^6$ K....
- 146k
9
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Visible Stars in Andromeda Galaxy
I have to imagine that the glow from the galaxy comes from the scattering and reflecting of all the collective starlight inside the galaxy off the dust and gas and whatnot contained therein, and that ...
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8
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Color of planets
Here are some values found by taking the hue from images, and adjusting the brightness to fit the albedo:
Mercury #1a1a1a Yes it is really that dark
Venus #e6e6e6 or perhaps a bit darker
Earth ...
- 116k
8
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Why does the Hertzsprung–Russell diagram's x-axis go from large temperatures to lower?
The original Hertzsprung-Russell diagrams constructed by Henry Russell and Eijnar Hertzsprung consisted of absolute magnitude on the y-axis and a spectral type or an indicator of spectral type on the ...
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8
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How can I calculate the luminosity and mass of a star only knowing it's peak wavelength and it's subtended angle?
The only star subtending an angle of 32 arcminutes at the Earth is the Sun!
The angular size combined with the orbital separation of the Earth from the Sun gives its radius. The peak wavelength gives ...
- 146k
8
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What's the issue with Olbers' paradox?
It's an easy integral assuming four things:
The universe is infinite in extent,
Stars are uniformly distributed in this infinite universe,
The universe is infinitely old,
Newtonian mechanics applies, ...
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7
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Why does the Hertzsprung–Russell diagram's x-axis go from large temperatures to lower?
Originally, what was plotted was luminosity against colour, and by colour I mean the wavelength of the peak intensity. Short wavelengths on the left, and long on the right, as you would expect.
Now ...
- 116k
7
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Solar luminosity proportional to $G^7$?
I think the argument is the following. The central temperature can be estimated from a form of the virial theorem. At least dimensionally speaking, total thermal energy $MkT/\mu$ (where $\mu$ is the ...
- 146k
7
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Visible Stars in Andromeda Galaxy
The question comes near the centennial of a famous debate about the nature of so-called spiral nebulae.
In 1920, Shapley argued that that they were clouds within our own galaxy, and Curtis argued that ...
- 18.2k
7
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What's the issue with Olbers' paradox?
A couple of good answers already, but this might be an easier way of thinking about it.
The light you see from a star comes only from the photons emitted at the tiny specific angle just right enough ...
- 5,562
6
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What is the frequency distribution for luminosity classes in the Milky Way Galaxy?
Here is how you do it "properly" for the Hipparcos data. As Warrick correctly points out, what you have done in your question is massively biased towards giant and supergiant stars, which actually ...
- 146k
6
votes
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How can I convolve a template spectrum with a photometric filter response spectrum?
Your feeling is right: You shouldn't convolve the spectrum and the filter, you should only multiply so that flux outside the bandpass is suppressed. Subsequently you integrate the resulting function ...
- 37k
6
votes
What does Eddington Limit (Eddington Luminosity) Show?
The Eddington limit represents the maximum luminosity that can be achieved by a body (such as the star) when there is hydrostatic equilibrium (http://astronomy.swin.edu.au/cosmos/H/Hydrostatic+...
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6
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Could a black hole's accretion disk be bright through natural means?
As I understand the quote, the "artificial" thing about your accretion disk is not that it is bright, but that it doesn't emit X rays.
In real life, SMBH accretion disks are usually ...
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What is the infrared self-luminosity value of Jupiter and Saturn?
According to Li et al. (2018), the internal heat coming from Jupiter (and emerging almost entirely as infrared radiation) is $7.485 \pm 0.160$ W/m$^2$.
If we take an average radius for Jupiter of 70,...
- 146k
5
votes
Accepted
How bright would it be on the "face" of Titan
It's not hard to calculated brightness to distance, but that doesn't take into account cloud cover, which is important for Titan.
Just looking at distance first, Titan (based on Saturn's distance) ...
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5
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Is it possible that quasars are not powered by black holes but some other kind of compact, massive object?
The solution to your question is surprisingly simple, I think:
A quasar that puts out energy around Eddington luminosity or higher, must accrete at a certain rate, corresponding to the energy output. ...
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5
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How to calculate the distance to galaxies with standard candles?
I can show you an ESA Series exercise I did a few months ago for an astronomy class.
Given the light curve of 12 cepheid variable stars in the galaxy M100 (which are very nice standard candles to ...
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