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A truly dark sky has a surface brightness of 2 × 10 −4 cd m −2 or 21.8 mag arcsec −2. [9] [clarification needed] The peak surface brightness of the central region of the Orion Nebula is about 17 Mag/arcsec 2 (about 14 milli nits) and the outer bluish glow has a peak surface brightness of 21.3 Mag/arcsec 2 (about 0.27 millinits). [10]
By the 2015 definition, 1 au of arc length subtends an angle of 1″ at the center of the circle of radius 1 pc. That is, 1 pc = 1 au/tan( 1″ ) ≈ 206,264.8 au by definition. [ 9 ] Converting from degree/minute/second units to radians ,
The largest star in the system, Gliese 667 A (GJ 667 A), is a K-type main-sequence star of stellar classification K3V. [2] It has about 73% [5] of the mass of the Sun and 76% [2] of the Sun's radius, but is radiating only around 12-13% of the luminosity of the Sun. [15] The concentration of elements other than hydrogen and helium, what astronomers term the star's metallicity, is much lower ...
If a total mass M is confined within a radius R, then the moment of inertia is roughly MR 2, and the left hand side of the virial theorem is MR 2 / τ 2 . The terms on the right hand side add up to about pR 3 , where p is the larger of the plasma pressure or the magnetic pressure.
Crumey obtained his formula as an approximation to one he derived in photometric units from his general model of human contrast threshold. [14] As an illustration, he calculated limiting magnitude as a function of sky brightness for a 100mm telescope at magnifications ranging from x25 to x200 (with other parameters given typical real-world values).
Vesta (radius 262.7 ± 0.1 km), the second-largest asteroid, appears to have a differentiated interior and therefore likely was once a dwarf planet, but it is no longer very round today. [74] Pallas (radius 255.5 ± 2 km ), the third-largest asteroid, appears never to have completed differentiation and likewise has an irregular shape.
This combination greatly simplifies the equations of general relativity into a form called the Friedmann equations. These equations specify the evolution of the scale factor the universe in terms of the pressure and density of a perfect fluid. The evolving density is composed of different kinds of energy and matter, each with its own role in ...
One study found that a not yet directly-observed, dust-modulating star or white dwarf of 1.17 ± 0.7 M ☉ at a distance of 8.60 ± 0.33 AU would be the most likely solution for Betelgeuse's 2170-day secondary periodicity, fluctuating radial velocity, moderate radius and low variation in effective temperature. [193]