# Tag Info

45

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 similar to a metal-rich star - i.e. about 0.075 times the mass of the Sun. Any less than this and the brown dwarf (for that is what we call a star that never gets ...

38

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 like that in our atmosphere. On the other hand, stellar atmospheres have strong magnetic fields which reconnect, and that causes strong voltage spikes that ...

30

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 brown dwarf, an object needs to be large enough to have undergone deuterium fusion in its core. This requires a mass of at least 13 Jupiter masses. The other is ...

14

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 spectral type. Any object that dim will be tough to find, regardless of spectral type or what sort of telescope you're using - you can have the largest, highest-...

13

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 were made using the Hubble and Spitzer space telescopes and were able to show changing light patterns and distinct layers of material at different altitudes in ...

10

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, but at least hundreds of billions of years, perhaps hundreds of trillions of years. The upshot is that that no black dwarfs exist in the universe today. The ...

10

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 universe. But on the other hand, estimates of the abundances of deuterium, helium, tritium and lithium produced in the big bang indicate that only 5% of the ...

9

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 starting point. Well, actually it is fluid motions at the magnetic loop footpoints that is the starting point. The B-field and partially ionised plasma are coupled and ...

9

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. This happens very early in the life of a contracting protostar and, because it is fully convective and thoroughly mixed, all the D is "burned" in less than a ...

8

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 been able to detect any. Objects down to brown dwarf size that are closer than Alpha Centauri have not been found by extensive surveys. Here is a list of close ...

8

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 stops. The conducting channel is an insulator again. This requires insulating (dielectric) medium and a means of creating an intense electric fields. Given ...

7

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 body could be enough to generate sufficient heat and pressure in the core for nuclear reactions to start. If both brown dwarfs had a mass of about 60 Jupiters, ...

7

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 other objects in that list, there is no reported trigonometric parallax measurement for this very faint T7.5 brown dwarf. In fact, if it turned out to be a ...

7

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 in metal-rich gas. $0.064 \pm 0.012 M_\odot$ (the third significant figure is superfluous) is within one error bar of that limit, which in itself is only a 68% ...

6

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 locked into stars is tiny and will always remain tiny). If two brown dwarves collide (extremely unlikely, but if you wait long enough, anything with some tiny ...

5

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 very cool brown dwarfs - e.g. a 250K brown dwarf only 6 light years away Luhman et al. 2014). Since WISE was an all-sky survey and there is great interest in ...

5

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 is at a very similar temperature to an M3.5 star. Brown dwarfs begin their lives as hot balls of gas and gradually cool with time. They start off as M-type ...

5

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 about 13 times Jupiter. The weakness of this that we think isolated brown dwarfs could condense from a gas cloud that are less massive than this; and young brown ...

5

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 most important variables. Helium abundance, in addition to metallicity, could play a role. Rotation could be a very second order effect. Very rapid rotation would ...

5

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 the core of the Sun is 15 million K). Their temperature range means that they don't fuse hydrogen in the same way that "true" stars do. They may fuse deuterium (...

5

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 to understand. The object has cooled beyond the point at which D burning has ceased. It has also pretty much contracted to a minimum sized configuration where ...

5

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 required dark matter halo is still the subject of debate and research. Whilst computer simulations of galaxy formation predict mildly non-spherical or triaxial ...

4

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 mass of the Sun, depending on the metallicity of the brown dwarfs. If one of the brown dwarfs is on the brink of fusion, the gases that it would accrete from ...

4

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 brief deuterium burning phase) and as a result their fate is to cool and fade. The plot below (from Burrows et al. 1997) shows how the luminosity behaves as a ...

4

These are spurious data points. They are likely genuine objects, but the parallax value is incorrect. There are a small number of these in the Gaia data set. There are 59 object with a parallax greater than Proxima Centuri. These don't represent genuine objects, instead they are when two sources are closely aligned (within about 0.2 arcsec). Most of these ...

4

Yes, they are difficult to measure. The only way to measure a brown dwarf mass is if it is in a binary system. The binary frequency of brown dwarfs is low - perhaps 10-15%. Having found your binary system, then it has to be wide enough to resolve with your telescope, but not so wide that Kepler's third law means it takes centuries to do an orbit. The low ...

4

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 the planet is 12% greater than the radius of the star. \begin{array}{lll} \hline \text{} & \text{Mass} & \text{Radius}\\ \hline \text{Planet} & 12....

3

Brown dwarves usually exhibit deuterium burning in their core, which is where this omnious limit of 13 Jupiter masses comes from. It is the limit at which the core is dense and hot enough (as others have pointed out) enough to make deuterium nuclei fuse together. Deuterium has a relative abundance compared with Hydrogen of $10^{-4}$, but is easier to fuse ...

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