19 votes
Accepted

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 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 ...
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  • 114k
13 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 ...
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12 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 ...
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12 votes
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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 ...
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11 votes
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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 ...
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  • 114k
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 ...
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10 votes
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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....
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8 votes
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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 ...
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8 votes
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What is the Fundamental plane for Elliptical Galaxies?

I am not an expert, but it seems that the fundamental plane is a relation among characteristic quantities of the galaxy, showing a correlation that is the analogous of the Tully-Fisher relation for ...
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8 votes

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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7 votes
Accepted

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 ...
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7 votes
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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 ...
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6 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 ...
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6 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 ...
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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 ...
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5 votes
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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 ...
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5 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 ...
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5 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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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 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. ...
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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 ...
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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 ...
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5 votes
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How is the difference of bolometric magnitudes not dependent on the stars' radii?

The R in that equation is the distance from the star to observer, not the star radius. The light emitted from the star is distributed uniformly on a sphere of radius R, and when the light arrives to ...
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  • 88
4 votes

Can solar luminosity & activity be predicted?

All of these effects are related to the 11 year solar cycle. And while we know there are times when the sun is "active" versus "inactive" we aren't necessarily predicting exactly where sunspots or ...
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  • 1,453
4 votes

Conversion of magnitudes to Jansky and MAGPHYS?

Caveat this with the fact that I am not familiar with this code. The input asks you for the flux through the filter, but a filter has a bandpass, so if you have a 10 nm bandpass filter and you ...
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4 votes
Accepted

Red dwarf variation in Luminosity

The creator seems to be referring to flare stars. Flares may be magnetic in origin, like various manifestations of the Sun's magnetic field. These flares can be quit luminous across the ...
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