14

Leconte et al. (2015) suggested that the presence of an atmosphere could prevent or at least slow tidal locking. The star should exert two separate torques: one on the atmosphere and one on the solid body of the planet: $$T_a=-\frac{3}{2}K_ab_a(2\omega-2n),\quad T_g=-\frac{3}{2}K_gb_g(2\omega-2n)$$ where $$K_a\equiv\frac{3M_*R_p^3}{5\bar{\rho}a^3},\quad K_g\...


10

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 dwarfs 4 or 5 to 1; see, for example, the review article by Chabrier et al. (2014). This is supported by extensive surveys done in the 2000s and 2010s, ...


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 ...


8

Red dwarfs, depending on your definition, can range from 2.5 to 150 times more dense than the Sun. What is the cause of this discrepancy? They give no calculations, so I can only guess. The article is from 1946 and we've gotten a lot better at science. It's 1946 and information exchange is limited. No internet, no TV, and long distance calls are expensive....


8

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 the mass functions for various clusters in this talk), and simulations in more or less good agreement with that (e.g. here or here), seem to indicate that this ...


8

Your question may ulitmately be about the physiology of the eye, which is off-topic here. The spectrum of the Sun seen low on the horizon is quite different to the spectrum of an M-type red dwarf. The reason that a red dwarf is red, is not just that it is cool, but that there are great chunks of the spectrum that are absorbed by molecules in the photosphere ...


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% ...


7

Yes: It has a companion planet or an excessively large moon, with the two bodies orbiting their common center of mass (much like the Earth and the Moon). They could be tidally-locked to each other, but they cannot be tidally-locked to their star.


5

No. The reason is that gas recycling only recycles only about 40-50% of the gas in a sun-like star, leaving the rest as a white dwarf that slowly cools off. Heavier stars are even less effective in converting their gas into a condensed core. So the process will tend to produce a fair fraction of heavier white dwarfs now in the early stages of the ...


5

The more likely case is actually a spin-orbit resonance that is not 1:1 but a half odd multiple, like the 3:2 case of our own Mercury. Having eccentricity in the orbit encourages this situation. I’ve been meaning to write this up on the Worldbuilding.SE but I have not re-found enough references. But see this video.


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 ...


4

earth like planet orbiting around a sun like star (365 days) red dwarf on an elliptical orbit around the star that passes close to the planet (1896.59 days, eccentricity 0.866) If we work in AU and years we can use $GM_{Sun} = 4 \pi^2 (\text{AU}^3 / \text{year}^2)$ for the standard gravitational parameter of the Sun. If we also assume the red dwarf's mass ...


4

I found one "hot Jupiter" in the Kepler data (Kepler 45b). The star is a M dwarf with an effective temperature of 3820K. The planet has an estimated mass of 160.5 M(Earth) and radius of 10.76 R(Earth). This gives a density of about 0.8 g/cm2 which is consistent. The planet is located at approximately 0.03 AU from the star with an orbital eccentricity of 0.11....


4

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 electromagnetic spectrum, including in the x-ray wavelengths. Some red dwarfs are flare stars, though flare activity may be largely restricted to a small part of a red dwarf'...


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

"Back of an envelope" calculation: Proxima Centauri has an apparent magnitude of about $11$. The faintest objects visible to the unaided eye have magnitudes about $6.5$. So we need to decrease Proxima Centauri's magnitude by about $5$ in round numbers, which corresponds to an increase in brightness of about $100$ times. To achieve this we would have to be ...


3

I emailed the authors of the paper, asking whether blue dwarf stars could "become hot enough to pass the thresholds for Type B or Type O" and one of them replied: "We use the term 'blue' to mean 'bluer', so that when stars become blue, they get hotter than their usual main sequence temperatures. ... The point of the paper is that smaller stars get ...


3

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 of two white dwarfs or at a specific point in the evolution of some red giants Temperatures range from 20,000 K to 40,000 K Brightness is between 10 - 100 times ...


3

If they are close enough to exchange mass, the Red dwarf will always lose mass to the white dwarf. That doesn't mean there won't be some exchange going back the other way, but the white dwarf will always win the exchange rate and it won't even be close. It makes no difference whether it's heavier or lighter. What matters primarily, is how tightly held ...


3

If we take 1 atmosphere of optical depth to mean looking though the Earth's atmosphere at zenith, then the optical depth to scattering is small - probably of order 0.3 for blue light and much smaller (according to $\lambda^{-4}$) for red light. That means that when the Sun is at zenith, most of the light reaches the ground but some blue light is scattered ...


3

This is a brief letter to Nature from 1946, containing no quantitative justification of the density estimate In 1946, whilst the radius of some of the nearest red dwarfs could be estimated from their luminosities and blackbody temperatures, there would be little information about masses. There is little else to say. Modern models and measurements of masses ...


3

According to the wikipedia article on sub-brown dwarfs the lowest mass object that can form from collapse of gas cloud is about 1 Jupiter mass. https://en.wikipedia.org/wiki/Sub-brown_dwarf


2

This mechanism, being actively emitting in the radio-wavelengths, is certainly negligible for the overall atmospheric energetics at Proxima b. One can conclude this by taking the band luminosities from the cited paper ($\rm 2.51\times10^{20} erg/s$, p.3, first paragraph) and compare them to the solar constant at the planets orbit, which should amount to $\...


2

Sun-like yellow dwarf stars are generally thought to be the best places for intelligent life to develop, if only because 100% of known life is in orbit about a yellow dwarf. Larger, brighter stars don't last long enough for intelligence to evolve and red dwarf stars were thought to be too prone to solar flares. However yellow dwarf stars are relatively rare,...


2

There are a couple different tests you could use to determine whether the object is a brown dwarf of a red dwarf. You could try to use the lithium test to determine whether the object in question is a brown dwarf or a low-mass red dwarf. Brown dwarfs generally don't reach temperatures high enough for lithium burning to occur at any significant rate, and so ...


2

There is quite simple formula that will give you the tidal-locking half-time $$T = C\, \frac{a^6 R \mu}{ M_s M_p^2}$$ $a$ - semi-major axis, or simply the radius of the planet circular orbital trajectory in meters $R$ - planet radius in meters $M_s$ - planet mass in kg $M_p$ - parent planet/star mass in kg $\mu$ - rigidity, approximately $3×10^{10}$ for ...


2

Not necessarily. In order for a gas to collapse it needs to reach Jeans instability, which is determined by parameters on the radius of the nebulae and on its mass. It is a strongly local condition, and it doesn't necessarily determine which kinds of stars can come out of that unstable nebula, that could be determined after the already collapsing nebula ...


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