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There can be no closer white dwarf. The coolest, oldest white dwarfs (3000K), would be rare, but are still luminous enough $6\times10^{-6} L_{\odot}$ to have been easily detected at distances closer than Sirius. At the distance of Sirius, such an object would have a visual magnitude of around 12-13 and would be brighter at near infrared wavelengths where all ...

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Yes, there probably are more. There is a minor problem of definition. Unlike the case for "planet", the IAU has not produced a definition of "moon". There is no requirement to be round, or have cleared an orbit. So any chunk of rock or ice that we can see orbiting a planet can reasonably be called a moon, and Jupiter has, over the years, picked up a lot of ...

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Yes, isotopic ratios can be readily established by spectroscopy. Molecules where the constituent atoms differ by one mass unit have different vibrational and rotational energy levels and the gaps between them depend on the reduced mass of the molecule. For example, in a diatomic molecule $$\mu = \frac{m_1 m_2}{m_1 + m_2}$$ and the vibrational frequencies ...

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Yes, this is the correct approach. The $h$ in the equation is the altitude above the horizon of the object at which you consider it to be rising or setting. This is typically non-zero, because of atmospheric refraction, and, in the case of the Sun or the Moon, because of their finite diameters. In your case, the object 'rises' when it climbes above \$h=30^\...

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Yes, the neutron star RX J1856.5-3754 has an observed* radius of 17 km. After accounting for general relativity, its actual radius is calculated as 14 km. It is not the only neutron star whose radius is known. * It's worth mentioning how astronomers arrived at this radius. Technically, fitting the X-ray data to a blackbody arrives at a calculation of a 5 ...

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Think of when the planet is at the "side" - there's a little bit of light from the planet (ie, reflecting off the planet) shining towards us. Could it be due to that little bit of light - when the planet is behind the star, it no longer reflects towards us? Maybe that's the effect you have in mind?

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If they knew exactly where to point Mr. Hubble, then yes, it should be easily visible, though at that distance, it would still be blurry, not a clean image. The estimated apparent magnitude of Planet Nine varies based on which website you believe, Google provides ranges from 20 to 25 with Wikipedia saying >22. Higher numbers mean less visible. Hubble can ...

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