# Tag Info

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Spectroscopic parallax is the technique whereby you estimate the absolute magnitude (i.e. the brightness it would have if it were placed at 10 pc) by estimating what "type" of star it is using information fro a spectrum. It can be applied to any kind of star where (a) you have a reasonable chance of determining the type of star from its spectrum and (b) ...

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The positions of stars on the sky are defined against a co-ordinate reference frame that is ultimately defined by the positions of very distant radio sources (quasars at high redshift) that are assumed to be "stationary" in terms of their celestial grid co-ordinates (see International Celestial Reference Frame) All stars can have their positions precisely ...

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$\theta$ is half the difference in direction of 2 measurements of the position of the star taken 6 months apart.

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One piece of physics that you've missed is the most pulsars spin down due to the emission of magnetic dipole radiation. For instance, the crab pulsar has a period of 33.5028 (plus a few more sig figs) milliseconds, but slows down by 38 nanoseconds per day. Furthermore, the size of several more increasing order derivatives is known accurately. So in ...

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Sort of. This is demonstrated clearest in barred spiral galaxies, which make up about 1/2 to 2/3 of all spiral galaxies. A dramatic example is NGC 1365: Image courtesy of Wikipedia. Others, such as M95, have spiral arms that wrap even further around while still retaining the central bar: Original image courtesy of Wikipedia; color added by me in ...

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You are correct. If you worked through the same steps on the distance ladder in a universe with a different speed of light, you would find the answer in light years would be different. Your question on whether light travels one light year in one year, though trivial sounding, opens the door to difficulties that are inherent in the nature of space-time. Your ...

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See, first of all our eyes are not a good device to determine the main color of a group of photons. the main color that we see is actually the intensity of a specific range of frequency in the light wave. it means there are all kind of photons coming out of the sun, but the amount of "Yellow" photons are much much more. that's because we see the sun ...

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I would like to expand here on the mechanisms of scattering. Light is scattered by particles in the air which act like dipoles, and oscillate because of the electromagnetic frequency of light. Oscillating dipoles may emit a different frequency. There are two mechanisms of scattering. Rayleigh scattering: This occurs when particle sizes are smaller than ...

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No, there's not a map of all the black holes in the galaxy, remember they're quite hard to detect, but there's one of stars in our solar interstellar neighborhood.

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You have to distinguish between stellar mass and gravitating mass. The quoted Milky Way mass includes dark matter. Despite searching in the literature, I have yet to find reliable apparent magnitudes and good distance estimates for NGC 1097. The total Blue luminosity of this galaxy is near to Minus B-band absolute magnitude -21, but only ballpark ...

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The primary reason for the asymmetry of the Ly$\alpha$ line is bulk motion of neutral hydrogen, i.e. accreting gas (causing a blueshifted line) or galactic outflows (causing a redshifted line). Mechanism of the Ly$\alpha$ double peak In general, for Ly$\alpha$ photons produced in the center of a blob of neutral hydrogen (i.e. a galaxy), the photons must ...

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This answer is a summary, based on what I consider to be the most salient points made in the comments above. It is closely linked to the answer of my later question, Deflection of Earth directed NEOs using nuclear powered laser beams Below is the answer I received in relation to my second question: Project Excalibur  The idea of a nuclear pumped X-ray ...

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