# Tag Info

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Generally speaking they refer to the distances from us when the light as emitted. No correction is usually made to say how far away the object is from us now, because this correction would be very small and inconsequential compared to the uncertainty in the original distance measurement. For instance, taking the Andromeda M31 galaxy as an example. Riess et ...

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A recent overview can be found in Pireaux et al (2001). I quote the authors on page 3: However, the perihelion shift of planets, and hence Mercury, can not be measured directly because the perihelion is a Keplerian element whereas the motions of the planets are not exactly Keplerian due to mutual gravitational interactions and figure effects. So, only ...

3

An alternative method to John's answer is to look at the total number of atoms in the observable universe. Thanks to measurements of the cosmic microwave background, we have a fairly precise estimate of this number. Indeed, we know that ordinary matter makes up about 4.9% of the energy content of the universe. In this previous post, I calculated that this ...

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"Only about 10 percent of the total baryonic matter is sufficiently condensed by gravity to form stars and galaxies. More than 90 percent was left between the galaxies." http://hubblesite.org/hubble_discoveries/science_year_in_review/pdf/2008/searching_for_baryonic_matter_in_intergalactic_space.pdf 6% of baryonic matter is within stars according to the ...

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Many planets have been found where their orbital axes do not align with the rotation axis of their star. This is achieved using measurements of the Rossiter-McLaughlin effect in transiting systems or by observing planets transit over spotted features on a star's surface. As the stellar rotation axis is highly likely to coincide with its protoplanetary disk ...

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Richard Fitzpatrick's free e-book A Modern Almagest: An Updated Version of Ptolemy’s Model of the Solar System presents a modernized version of Ptolemy's Almagest. (I mention it here.)

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I think the plot you show is the estimated abundance of the interstellar medium from which the Sun has formed. The chemical abundances of the interstellar medium change with time, so you have to define some point in time at which to estimate them. As the initial chemical abundance in the Universe is basically H, He, with traces of D, Li and Be, then it ...

2

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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Lets make it 10 minutes of travel, because I'm lazy. At 100x light speed you travel 1000 light minutes from Earth, so looking at earth you see images from 10:10 minus 1000 minutes which is 10:10 minus 16.6 hours. Which I am also too lazy to work out, but you get the idea?

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This paper reports modelling of NGC 1097, and they use a figure of around $10^{10}$ solar masses. They cite a paper that isn't available online. I have to confess this seems small to me. It's only 1% of the mass of the Milky Way.

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Astronomy -- especially exoplanet science -- has gotten very good at detecting impossibly faint signals. In this case a very recent Nature article, Two families of exocomets in the β Pictoris system, claims to see thousands of exocomet signatures in the not-too-distant β Pictoris system. This is a very young system with an edge-on debris disk -- essentially ...

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I see this type of questions/answers everywhere, but I have a bit different view on it. It's like asking "should I buy a bike with training wheels". Of course all experienced guys out there would say no, it's a waste of money and you'll throw them away after two weeks. But I believe that some lessons are worth 50 dollars. The point is, if you're just a ...

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