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Can we see emission lines in stars? I read that stars show both absorption and emission lines. While how we see absorption lines is clear to me, I don't understand how (and when) we see the emission lines. Based on what I read in several of your threads such as  we should not be seeing emission lines at all, since all the emissions would come from the hotter interior but those can't reach the outside (unless I misunderstood this point), and the only emission we see is blackbody radiation. But then how do we see the few emission lines such as in the spectrum shown ? How to prove that limit doesn't exist using epsilon-delta definition? It is easy to prove the limit exists, all we have to show is there exists a relationship between $\delta$ and $\epsilon$. But how are we supposed to prove limit doesn't exists? The problem is when we are proving for a limit we already know what the limit is and with that, algebra is all that's needed.   Please show through an example (you may show that $\lim_{x\rightarrow0} \frac{1}{x}$ doesn't exist)  If possible please use the explanation scheme that is used by this answer Show bounded and convex function on $\mathbb R$ is constant The cardinality of $\{f\subseteq\mathbb R^2\mid f\colon\mathbb{R\to R}\text{ is a function}\}$ is strictly greater than the cardinality of $\mathbb R$ Difference between staticmethod and classmethod Proper use of the IDisposable interface I know from reading  that the "primary" use of the IDisposable interface is to clean up unmanaged resources.  To me, "unmanaged" means things like database connections, sockets, window handles, etc.  But, I've seen code where the Dispose() method is implemented to free managed resources, which seems redundant to me, since the garbage collector should take care of that for you.  For example:  public class MyCollection : IDisposable {     private List&lt;String&gt; _theList = new List&lt;String&gt;();     private Dictionary&lt;String, Point&gt; _theDict = new Dictionary&lt;String, Point&gt;();      // Die, clear it up! (free unmanaged resources)     public void Dispose()     {         _theList.clear();         _theDict.clear();         _theList = null;         _theDict = null;     }   My question is, does this make the garbage collector free memory used by MyCollection any faster than it normally would?  edit:  So far people have posted some good examples of using IDisposable to clean up unmanaged resources such as database connections and bitmaps.  But suppose that _theList in the above code contained a million strings, and you wanted to free that memory now, rather than waiting for the garbage collector.  Would the above code accomplish that? Can I use Minecraft PC textures and worlds in Pocket Edition? Now that I've discovered that I can host a , I'd like to know if is it possible to use the textures and maps of the PC version for the pocket version.  I'm planning to visit Peru and Machu Picchu. It would be really cool to entice them for the voyage by letting them play a  in their favourite video game. Unfortunately, this God of Deceit world looks really sophisticated. I don't believe the Pocket Edition would be able to play it. How does a phonon cause two electrons to attract each other and form a cooper pair? We know that like charges repel each other. But my professor claimed that two electrons can attract each other as well. What he said was that due to screening an electron travelling at some speed won't repel another electron, but that they will, in some cases, attract each other due to weak phonon exchange. What does that mean? What is phonon exchange? Do two electrons really attract each other? Charging lead-acid batteries? I'm studying for a midterm and need some help with proving summation     $$\sum\limits_{k=0}^n\binom{n}{k}2^k = 3^n$$   using the binomial theorem.  This is what I've been thinking so far:   In the binomial theorem, we set $x = 0$ and $y = 2$, so:  $3^n = (x+y)^n = \sum\limits_{k=0}^n\binom{n}{k}y^k$  $ = \sum\limits_{k=0}^n\binom{n}{k}2^k$  Am I getting this correct so far or completely wrong? Any help would be appreciated. Thanks. Make a computer act as a virtual USB device for other equipments Intuitive explanation of why $\dim\operatorname{Im} T + \dim\operatorname{Ker} T = \dim V$ I'm having a hard time truly understanding the meaning of $\dim\operatorname{Im} T + \dim\operatorname{Ker} T = \dim V$ where $V$ is the domain of a linear transformation $T:V\to W$. I've used this equation several times in many problems, and I've gone over the proof and I believe that I fully understand it, but I don't understand the intuitive reasoning behind it. I'd appreciate an intuitive explanation of it.  Just to be clear, I do understand the equation itself, I am able to use it, and I know how to prove it; my question is what is the meaning of this equation from a linear algebra perspective. Can we deploy PROFILES alone from one sandbox to another? How can I take screenshots of YouTube videos in full screen? I am trying to take a screenshot of a YouTube video in fullscreen.  When I pause the video and wait a moment, only the bottom UI bar (play/pause/seek) disappears. The top UI bar (Like/Share/More Info) stays there.  So when I press PrintScreen on the keyboard, the screenshot is polluted by this top UI bar. Is there any way to remove it while the video is on pause? Where does the damage of relocated dashes land? The end location of a dash or jump can be changed by an ability that pushes () or pulls () you. So for example I try to  on the adc, but a Blitzcrank pulls me, preventing me from landing on my chosen spot.  Will I deal my AoE damage around the adc, where I tried to land, or around the Blitzcrank, where I ended up landing? Does a photon having superimposed frequencies exist?  (wrt frequency detected by prism or other detectors, not wrt human eye as only rod-cells can detect one photon falling in the detectable (visible) frequency band and the single photon is perceived by eye+brain as grey irrespective of frequency).  Also, with white here, I do not wish to restrict the question implying all VIBGYOR frequencies or only RGB as each color is also a range of frequencies and white to trichromatic-sensor human eye is just output from three frequencies RGB.  Or can it be that materials which slow light down cause a photon of some single frequency and higher energy to split into VIBGYOR? || and the entire spectrum of human eye's detection range is VIBGYOR+W (as monochromatic light of high intensity can also be perceived white by human eye and it is not related to individual color sensors of human eye at all but that above a certain energy per unit volume (referring to detector area of eye cells), we perceive it white, (somewhat but not exactly) similar to combination of red and green which human eye+brain calls yellow.  I have added the additional description to make the question more precise. If my description causes confusion, please stay with the original question and use description just to get an idea of the intent of the question. Let $ G $ be an abelian group of order 6.  Then $ G $ has one element of order 6.  And so, $ G $ is cyclic and isomorphic to $ \mathbb Z _6 $.  In general, It is not true that an abelian group of order $n$ has an element of order $n$.  However in the case of $n=6$, it is true. I'm wondering there is any rule of the case (for example, a rule of $n$ that makes an abelian group cyclic). Is this noun used as an adjective? I read this recently in :     At the end of the summit, the French and European officials had   claimed a points victory over the Germans by getting them to agree   more firmly to a target date of January 1st next year to entrust the   European Central Bank (ECB) with the ultimate authority to supervise   the euro zone’s 6,000-odd banks.   "Points" is a plural, and I suppose it doesn't function as a noun in this sentence since it's following the article "a", so is it an adjective adorning "victory"? Do lens hoods need to be matched to focal length? Is it true that not all generic lens hoods work on all lenses (provided the filter diameter matches, of course)? I read that the focal length plays a role in it - but I cannot find such readings on any lens hoods I see available online. I am a permanent resident (U.S) and I'm crossing the border into Mexico and taking a flight to Mexico city. The passport from my home nation is expired and I'm trying to avoid the last minute 500 mile+ trip to the nearest embassy. My question is, can I get on the plane with my green card or state license? or my expired passport and birth certificate? also, can i get the FMM/tourist card with my drivers license? all the official information says i need a valid passport, but other travelers have had different experiences, does it just depend on the officer? I already purchased my flight!