45 votes
Accepted

How large can a ball of water be without fusion starting?

You really need a full-blown stellar evolution model to answer this precisely and I'm not sure anyone would ever have done this with an oxygen-dominated star. To zeroth order the answer will be the ...
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  • 113k
38 votes

Can lightning occur in stars like the Sun?

Stars are not actually gaseous, they are plasmas, i.e., highly ionized. Hence, the entire star is highly conductive and does not easily develop the voltage difference via friction needed for lightning ...
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  • 2,802
30 votes

Why can't our Sun be a binary with Jupiter as a T or Y dwarf?

But why can't Jupiter be a Y Dwarf who is in the binary relationship with the Sun? There are two reasons: One is that Jupiter is too small to have ever undergone fusion of any sort. To qualify as a ...
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15 votes

Why is detecting brown dwarfs difficult?

Brown dwarfs have historically been difficult to detect (directly) simply because of how faint they are. Typical luminosities may range from $10^{-3}L_{\odot}$ to $10^{-5}L_{\odot}$ depending on ...
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14 votes
Accepted

Do brown dwarfs have stripes?

A number of brown dwarfs have had 'surface maps' created using the light from those stars. In 2013, observations of 2MASS J22282889–4310262, a brown dwarf 35 light years away, were published. These ...
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11 votes
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Why aren't brown dwarfs the dark matter?

Two reasons. We know from looking at galaxy rotation curves and the motion of galaxies in clusters and from gravitational lensing, that the amount of "dark matter" is some 30% of the density of the ...
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  • 113k
10 votes

Is there such a thing as a black dwarf

A black dwarf is a white dwarf that has cooled to the point that it is no longer emitting light. However it takes a long time for a white dwarf to cool down. The exact timescale is rather uncertain, ...
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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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  • 113k
10 votes

Are Brown and Sub-Brown Dwarfs secretly more common than stars?

The answer to your first question is (now) fairly simple: No, brown dwarfs are not more common than red dwarfs. A crude approximation is that stars (which are indeed mostly red dwarfs) outnumber brown ...
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9 votes
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Is the mechanism of solar flares on red dwarfs and brown dwarfs the same as that on the Sun?

Yes, the basic mechanism is thought to be the same on red dwarfs and at least the hotter brown dwarfs, but the details can be different. As you say, magnetic reconnection in the corona is the ...
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  • 113k
9 votes
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Can lightning occur in stars like the Sun?

An essential feature of the lightning is the electrical breakdown - an insulator (air) becomes a conductor for a while, relatively high current flows in the conducting channel for a short while, then ...
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  • 1,770
8 votes

What is the closest heavenly body to our solar system

The closest body we currently know to exist is Proxima Centauri which is part of the Alpha Centauri system at about 4.25 light years away. There may be rogue planets that are closer but we haven't ...
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7 votes

If two binary brown dwarfs began to exchange gas with each other, would they both ignite into stars?

It is possible for two brown dwarfs to form a contact binary. This would not, of itself, cause core temperatures in either to rise. If the two brown dwarfs were to merge, the mass of the resulting ...
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7 votes
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Why is WISE J0521+1025's distance so uncertain?

The distance is that reported by Bihain et al. (2013), which is based on a mean relationship between absolute magnitude and spectral type that has a lot of scatter. i.e. in contrast to most (all?) the ...
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  • 113k
7 votes
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How do we know that 2MASS J0523-1403 is a red dwarf?

The brown dwarf "limit" is about $0.072 M_{\odot}$ at solar metallicity (e.g. Chabrier et al. 2000) and is composition dependent. It gets a little higher in metal-poor gas and a little lower ...
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  • 113k
7 votes

Are Brown and Sub-Brown Dwarfs secretly more common than stars?

This is an important question to ask about the initial mass function of objects in the Galaxy - and the final answer hasn't been cast as it is a matter of research. Yet, observational data (e.g. see ...
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7 votes

Can dark matter be normal matter after all?

Firstly, there is a requirement for the dark matter to be non-baryonic. There are manifold reasons for this. The most important are the comparison between the universal energy density implied by ...
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6 votes
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Mechanism for Brown Dwarf Fusion

Your scenario is, of course, rather speculative and considers the very distant future. I don't like that article you're referencing, some stuff seems inaccurate (for example the fraction of baryons ...
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6 votes

Are we looking for brown dwarfs where the dark matter has to be?

The dark matter has to have a roughly spherical distribution and a smooth radial distribution if it is to account for the dynamics of gas and stars in our and other Galaxies. The exact shape of any ...
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5 votes

We are sure that there are no brown dwarfs closer than ______

I think the answer is roughly 3-5 light years. The recent WISE survey in the near-infrared should have been capable of detecting even a very cool nearby brown dwarf (and indeed it has detected some ...
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5 votes
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How do I understand a brown dwarf with a M-type spectrum?

The spectral type of an object is almost entirely determined by the temperature of its photosphere. ie Saying something is type M3.5 is just a measure of its surface temperature. An M3.5 brown dwarf ...
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5 votes
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Brown dwarfs and planets

Stanley, there really isn't a very clear definition and this is still a keenly argued point. Definitions include: Browns dwarfs burn deuterium. In models this happens if they are more massive than ...
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5 votes

Can a Y dwarf star undergo a supernova?

Stars of spectral class Y are dim brown dwarfs, with temperatures of about 300-400 K (For comparison, Earth's average temperature is around 288 K, give or take, the surface of the Sun is 5,800 K, and ...
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5 votes

Using the "Lithium test" to distinguish low-mass stars and brown dwarfs

The "lithium test" for a brown dwarf involves measuring two things - the lithium content and the spectral type (a proxy for surface temperature) or luminosity. But you may also need to know (...
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5 votes
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"Turn on point" for fusion?

I don't have detailed model calculations to hand and I'm not sure they have been done for the small range of metallicity you mention. However, in handwaving terms. (1) Mass and composition are the ...
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5 votes

How do we calculate heat flow or cooling rate for no fusion brown dwarf star?

Whether or not the object is just above or below the threshold (about 13 Jupiter masses) at which the core will become hot enough to burn deuterium is not really relevant to the calculation you wish ...
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4 votes
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If two binary brown dwarfs began to exchange gas with each other, would they both ignite into stars?

If two brown dwarfs got close enough to exchange gas with each other, could it trigger nuclear fusion within their cores? Very roughly, a star can undergo fusion once it is approximately 7.5-8.3% the ...
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4 votes
Accepted

How close to a brown dwarf would a planet need to be to receive as much light as Earth?

Brown dwarfs are born moderately hot and luminous and then they contract and cool. Thanks to electron degeneracy in their cores, they never become hot enough to ignite hydrogen (though there is a ...
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