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4

You are looking for the initial mass function (IMF). This tells the probability of finding a star of mass $m$ (in solar mass units). The prototypical IMF is the Salpeter IMF, $$ \phi(m)\sim m^{-2.35} $$ This gives a decent and quick approximation, though a multi-power-law fit seems to be better; thus we have the Kroupa 2001 model: $$ \phi(m)\sim ...


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Have a look at this article. It gives the number as $10^{24}$ rather than $10^{23}$, but it's such a vague estimate that a factor of ten is within the expected error. The number is the number of stars in the observable universe i.e. within 13.7 billion light years of Earth at the time the light we see today was emitted. Note that visible means visible to a ...


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The first definition of $\mu=GM$ is the standard definition of the SGP. The second one comes from the velocity of a circular orbit. If you have an object in a circular orbit of radius $r$ and velocity $v$ around a body of mass $M$, then the velocity is given by $$v=\sqrt{\frac{GM}{r}}$$ From this you can see that $rv^2=GM$ for circularly orbiting objects. ...


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I have taken the revised Hipparcos parallax catalogue, produced by van Leeuwen (2007, Astronomy & Astrophysics, 474, 653) and taken a subset of stars with Hpmag <6 (i.e. roughly the naked eye limit) and accepted only those objects with a parallax/error in parallax > 2.5. Anything with a larger fractional error in parallax really can't be trusted. If ...


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If we start with the equation you quote: $$ \rho_c = \frac{3H^2}{8\pi G} $$ and rewrite it as: $$ \rho_c = \frac{3}{8\pi G} H^2 $$ then it's the same as your equation: $$ \rho_c = E H^2 $$ because all you've done is to replace the constant factor of $3/8\pi G$ with the symbol $E$. This is of couse a perfectly reasonable thing to do, but it isn't new in ...


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The ejecta of a supernova does indeed move at a fraction of the speed of light (somewhere around the 10% mark). However, it does not remain at this speed forever. As the supernova ejecta expands outwards, it creates a shell of material that is actually gathering up particles in the ambient medium (typical interstellar densities are around 1 particle per ...


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This is actually more subtle than you would think it would be. First, remember that a fourier transform is defined, for some time-dependent signal $F(t)$, as${}^{1}$: $$F(\omega) = \frac{1}{\sqrt{2\pi}}\int_{-\infty}^{+\infty} dt\, e^{i\omega\,t} F(t)$$ Well, this is great in special relativity, but in general relativity, what time do we actually use? ...


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The radial velocity $V_r$ is the velocity resolved along the dashed line (the line of sight from the Sun to the star) with respect to the Sun. If $V_c$ is the velocity of the star, then the component along the dashed line is $V_c \cos (\alpha)$. We then have to subtract the velocity of the Sun resolved in the same direction. This is $V_{c,0} \sin (l)$. Hence ...


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The cosmic microwave background gives us a convenient rest frame in that it represents the average distribution of matter in the universe. So if the CMB looks isotropic, i.e. there is no Doppler shift in different directions then this is a plausible definition of stationary. A velocity relative to the CMB would then indicate a peculiar velocity. The Milky ...


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NASA still images; audio files; video; and computer files used in the rendition of 3-dimensional models, such as texture maps and polygon data in any format, generally are not copyrighted. You may use NASA imagery, video, audio, and data files used for the rendition of 3-dimensional models for educational or informational purposes, including photo ...


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The basis for a stellar engine (according to the Wikipedia link) is that the radiation pressure from the star is used to create thrust, so the question reduces to "Do brown dwarfs emit enough radiation"? Stars like the Sun do emit radiation, and lots of it - in the Sun's case, $3.846 \times 10^{26}$ watts. That's a lot of power! Brown dwarfs can't undergo ...


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This is a very late response, but there is no accepted answer as of yet, and none of the answer quite hit the mark. Regarding the magical collision hypothesis, that smacks of being rather non-scientific. Scientists as well as Missourians are wont to say, "Show me!" Other than the fact that Venus's rotation is anomalous, what, exactly, is the evidence for ...


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Here are the systems I found: 6: ADS 9731 Beta Tucanae Gamma Velorum Kappa Tauri Mu Sagittarii 7: AR Cassiopeiae Nu Scorpii ... no physical multiple stars of greater multiplicity yet found.


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The Hunt for Exomoons with Kepler Project has so far failed to find any exomoons. This is a negative finding (so far). Negative findings are always a bit trickier to explain than are positive findings. This negative finding might mean something very significant, or it might have very little significance: Maybe exoplanets are much less likely to have ...



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