26 votes
Accepted

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 ...
planetmaker's user avatar
  • 19.4k
23 votes
Accepted

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 ...
WarpPrime's user avatar
  • 6,673
16 votes

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 ...
ProfRob's user avatar
  • 152k
14 votes
Accepted

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 ...
James K's user avatar
  • 122k
14 votes

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 ...
James K's user avatar
  • 122k
13 votes
Accepted

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 ...
ProfRob's user avatar
  • 152k
13 votes

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 ...
Anders Sandberg's user avatar
12 votes
Accepted

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 ...
ProfRob's user avatar
  • 152k
10 votes

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 ...
ProfRob's user avatar
  • 152k
10 votes
Accepted

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....
ProfRob's user avatar
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9 votes

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 ...
Peter Erwin's user avatar
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9 votes

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 ...
Greg Miller's user avatar
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8 votes
Accepted

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 ...
James K's user avatar
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8 votes
Accepted

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 ...
ProfRob's user avatar
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8 votes

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, ...
David Hammen's user avatar
7 votes
Accepted

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 ...
ProfRob's user avatar
  • 152k
7 votes
Accepted

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 ...
ProfRob's user avatar
  • 152k
7 votes

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 ...
Mike G's user avatar
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6 votes
Accepted

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 ...
pela's user avatar
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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+...
Ruchi Pandey's user avatar
6 votes

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 ...
Prallax's user avatar
  • 4,431
6 votes
Accepted

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,...
ProfRob's user avatar
  • 152k
6 votes

What's the issue with Olbers' paradox?

Another version of Olber's Paradox: consider a cube with side length, say, 1kLY (1000 light years). This cube will have stars inside of it, and the light from those stars will be exiting the cube ...
Acccumulation's user avatar
6 votes
Accepted

Is there a specific constant name for the fifth root of 100?

The fifth root of 100 is occasionally referred to as "Pogson's Ratio" after Norman Pogson who systematized the magnitude system. I'd recommend not using this as it doesn't save any space or ...
James K's user avatar
  • 122k
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) ...
userLTK's user avatar
  • 24k
5 votes

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. ...
AtmosphericPrisonEscape's user avatar
5 votes

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 ...
Marco Leonardi's user avatar
5 votes
Accepted

What is meant by the notation $A^{\times{B}}_{\div{C}}$; where A,B,C are real numbers?

Small quibble to the (rightfully) accepted answer by James K that was too long for a comment: To be fair, $x=24^{+1}_{-3}$ doesn't mean that $21 \le x \le 25$, but that with a particular amount of ...
pela's user avatar
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