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3

Absurdly cheap and (potentially) very fun! The absolute cheapest telescope to make would be extremely hard to make, you'd be doing something for the first time, but it would at least be fun! Make your objective lens out of ice, and find a way to polish it smooth. Don't make it as big as these though. You can then use an eyepiece out of a pair of old ...


30

Let's say we have a spherical shell of some material emitting light, much bigger than the star it surrounds. If we look right down the center of the shell, our line of sight takes us through only a small amount of gas - not enough for there to be significant emission. On the other hand, if we look at the nebula near the outer edge, our line of sight takes us ...


19

Short Answer Thin gas in a nebula only absorbs some portion of star light corresponding to a small subset of the overall visible spectrum, according to the corresponding molecular composition of the nebula. For even these limited spectral ranges, the star is still often visible in a nebula since the gas can be too thin to absorb all the energy emitted by the ...


8

The answer is quite simple: you can also see the other side of the room (or maybe through the fume or haze above your oven) when you create a tasty meal) - even when the space between you and the wall is not empty: it is filled with air. But air is thin enough that you can see through even when it might be thick enough to both, absorbe a bit light and emmit ...


0

The reddening line is the locus that a star would move along in a colour-colour diagram as the interstellar reddening towards it increases. i.e. It is an equation on the colour-colour plot of the form (for this colour-colour plot) $$E(U-B) = f(E(B-V)) + c\ ,$$ where $E(U-B)$ is the colour excess in $U-B$ due to interstellar reddening, $E(B-V)$ is the colour ...


8

Just a short answer, and likely others will fill in more details. If there is ionization of some atoms, then generally there is recombination as well - you will have both processes going on, roughly in balance with each other. Typically when an electron recombines with an atom, it does so into some excited state. Then as it drops from that excited state ...


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