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49 votes
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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 ...
ProfRob's user avatar
  • 155k
39 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 ...
eshaya's user avatar
  • 4,009
31 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 ...
David Hammen's user avatar
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21 votes
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What is going on with this (sub?) brown dwarf WISEPA J174124.26+255319.5?

I had a look at the original source of the measurements quoted on the Wikipedia page (Zhang et al. 2021) and the problem is simply systematic errors in the models that are being used to infer the ...
ProfRob's user avatar
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16 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 ...
HDE 226868's user avatar
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16 votes
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How can a brown dwarf be more massive than a star?

Yes, it has to do something with metallicity. Brown dwarf SDSS J0104+1535 $\to 0.086\rm\, M_\odot\to[Fe/H]=-2.4$ Red dwarf EBLM J0555-57Ab $\to 0.081\rm\, M_\odot\to [Fe/H]=-0.24$ There is an article ...
User123's user avatar
  • 2,879
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 ...
Dave Gremlin's user avatar
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12 votes
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What is it like to see a brown dwarf turn into a star?

Exactly what this would look like would depend a great deal on how (fast) the mass is accreted and whether there is a significant amount of energy accreted along with the mass. There are two ...
ProfRob's user avatar
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11 votes
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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 ...
Peter Erwin's user avatar
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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, ...
James K's user avatar
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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....
ProfRob's user avatar
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9 votes
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Existence of planets larger than their host star?

The answer to the question depends on the exact definition of planet that is used. A possible example is the L dwarf 2M 0746+20 (2MASS J07464256+2000321) and its planet 2M 0746+20 b. The radius of ...
aventurin's user avatar
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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 ...
fraxinus's user avatar
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9 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 universe's matter density implied by ...
ProfRob's user avatar
  • 155k
9 votes

Can a brown dwarf accrete enough mass from red giant to become a star?

One scenario that can work is if the wind from the red giant is accreted by the brown dwarf. The brown dwarf can be in quite a wide orbit and still accrete mass because the wind from the red giant, ...
ProfRob's user avatar
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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 ...
A. C. A. C.'s user avatar
8 votes
Accepted

Is stellar ignition all-or-nothing?

No fusion isn't all or nothing. Given the same chemical composition of constituents then there will be a smooth ramping down of the nuclear fusion rate as the mass decreases. The lower mass objects ...
ProfRob's user avatar
  • 155k
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 ...
James K's user avatar
  • 124k
7 votes
Accepted

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 ...
ProfRob's user avatar
  • 155k
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 ...
ProfRob's user avatar
  • 155k
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 ...
planetmaker's user avatar
  • 19.9k
6 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 (...
ProfRob's user avatar
  • 155k
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 ...
ProfRob's user avatar
  • 155k
6 votes
Accepted

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 ...
ProfRob's user avatar
  • 155k
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 ...
HDE 226868's user avatar
  • 36.6k
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 ...
ProfRob's user avatar
  • 155k
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 ...
ProfRob's user avatar
  • 155k
4 votes

Existence of planets larger than their host star?

Beyond red dwarfs, another possibility is that of a planet orbiting a type B subdwarf star. Some features of such stars: Composed almost entirely of helium Thought to be formed through the merger ...
Ingolifs's user avatar
  • 4,155

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