# Tag Info

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I've added this because I don't think the accepted answer is very clear. Estimating the number of stars in the Galaxy relies mostly on two things. We estimate the present day mass function (that is the number of stars that exist per unit mass per unit volume) in the solar neighbourhood. We construct a model for the overall density distribution of the ...

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In this link, the contradiction is "explained": The tremendous expansion greatly dilutes any initial curvature. Think, for example, of standing on a basketball. It would be obvious that you are standing on a (2-dimensional) curved surface. Now imagine expanding the basketball to the size of the Earth. As you stand on it now, it will appear to be flat ...

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Your method is correct. When the angle is "small" so that we can ignore curvature, then the rectangular solid angle is just the product of the two side angles. (This doesn't hold for "large" angles). I cannot find a perfect explanation, but one source of this confusion may be that the UDF was imaged by two separate instruments, one optical, one ...

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Both elliptical and spiral galaxies have a stellar mass profile which have a well-defined maximum. In addition, both types of galaxy have symmetries - axial in the case of spirals and either axial or triaxial in the case of ellipticals. Since, in general terms, the luminosity tends to follow the stellar mass, this means that their projected brightness on the ...

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The angular resolution of a telescope is given approximately by $1.22 \lambda/D$ in radians, where $\lambda$ is the wavelength of observation and $D$ is the diameter of a circular mirror. Say we study a star that is 10pc away with a telescope working in the optical band (you didn't specify) at 500nm, then the spatial resolution of a 100m telescope at the ...

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I think the following image, which comes from Tomczak et al. (2014) and the so-called ZFOURGE/CANDELS galaxy survey should do the trick. It shows how the galaxy stellar mass function (i.e. the number of galaxies per unit mass per cubic megaparsec that have a certain stellar mass) evolves as a function of redshift. As you might imagine this is not just a ...

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Very simply, some fraction (usually 50-80%) of photons impacting each pixel on the CCD will result in an electron being excited out of the photoactive silicon layer and adding to the charge on a capacitor associated with that pixel. As the number of photons increases, so does the charge on each capacitor, in an approximately linear way. When the CCD is ...

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There are several concepts and issues going on here that are getting mixed together. First the ADC; An ADC (Analog To Digital Convertor) takes signal values in a certain range and converts then to a digital number. Let's run some numbers: A N bit ADC will be able to represent 2^N states. So a 4 bit ADC can represent 16 distinct states, 8 bits => 256 ...

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Assuming you are talking about exoplanets, I'll offer this. To obtain a density you need a mass and radius. Masses come via two methods - either measuring the radial velocity variations of the star it orbits (the bigger the RV variations, the bigger the planet mass), or so-called transit timing variations. This latter works in multiple "transiting planet" ...

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The main mechanism for the scattering of light by the interstellar medium is Mie scattering. This mechanism dominates when the scattering bodies are comparable to the wavelength of the light. Scattering by bodies much smaller than the wavelength of the light is fairly simple to model, and is given by the Rayleigh formula. In this regime the scattering is ...

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A recent paper by Dan Whalen titled, Finding the First Cosmic Explosions. I. Pair-instability Supernovae discusses this very problem. The pair-instability supernova (PISNe) is a special case of massive stars, around 100 $M_\odot$, in which the thermal pressure inside the star is reduced via the production of electron-positron pairs. Runaway thermonuclear ...

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Its complicated. There wasn't just one Hubble Ultra Deep Field (UDF) observation, but several, taken at different times, with different instruments, but pointed in (almost) the same direction. The first image you refer to was taken with the WFC Infra red camera in 2009, in near infrared bands (1-1.6 microns). The area covered by this camera is $2.4$ ...

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Firstly, it is important to note that the old Big Bang cosmology is no longer the most widely accepted theory. We include inflation into the mix in current theories. That said, there is an ambiguity in the definition of the Big Bang (you can find information on that in my question here). If we take the definition of the Big Bang as coming before inflation, ...

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If I understand your problem correctly, the diagram looks like this: Where I am drawing on the unit sphere, so distances $a$ and $b$ scale from 0 to $2\pi$. The sine rule for spherical triangles is $$\frac{\sin{A}}{\sin{a}}=\frac{\sin{B}}{\sin{b}}$$ Now since $B=90°$, it follows that $$a = \sin^{-1}(\sin{A} \sin{b})$$ Please see whether that makes ...

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