Why don't stars have emission lines? I understand why stars have absorption lines, but I am not entirely sure why they don't have emission lines. Wouldn't the gas on the surface of the stars be hot enough to at least give off some emission?
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 ?
Find total number of relations that are equivalence as well as partial order set. Assume set contains total $n$ elements.     My attempt:  As equivalence relation has property reflexive, symmetric and transitive and POSET has property reflexive, antisymmetric and transitive.  Symmetric and antisymmetric will cancle each other except reflexive relation and transitive relation can case antisymmetric, so it's also absent on resultant relation.  Therefore, all diagonal element should be present to satisfy conditions of equivalence as well as POSET.  So, it's independent of number of total number of element of set and answer will be $1$.     Can you explain in formal way, please? Why Smart's attack doesn't work on this ECDLP? "SELECT *" why is it an antipattern On multiple questions here and on stack overflow i saw people saying in the comments and answers that select * from table is almost always an antipattern without any explaination why. Althrough I can sort of deduce why it is an antipattern. I may be looking over a detail that someone else with better understanding of the issue noticed.  So here is my question, why people say that select * is an antipattern. Can I visit Schengen countries on a Type-D Schengen Visa? When I go to Settings &gt; Applications &gt; Manage Applications &gt; Downloaded I see an application list. If I select some applications I see button "Force stop" active, but some applications have this button dimmed.  What does it mean? What happens if I press this button?  This button can't mean application running, since the applications with this button are not listed in Running section.  Can I list just applications which have the Force stop button active and uninstall them?    Android version: 3.2 How are Dementors created/born? This is inspired by some discussions in .  In the Harry Potter universe, how are Dementors born?  Or are they artificially created by some magical means? I'm a teacher and will ask my students to either bring their laptops with Ubuntu installed with an specific list of packages preinstalled or bring a USB Drive or External Hard Drive with Ubuntu and the packages preinstalled.  For those who are going to bring the USB are two posibilities:   Install with the Ubuntu's "Startup disk creator" program, and telling it to reserve some space for persistence. Boot with a LiveCD and Install Ubuntu in the USB drive.   Then, in either case, boot with that media and install the packages.  So, for my purpose or any other purpose:   What is the best choice and why? Do we have another option? full explanation please. MathJax isn't currently working on the Android app, which makes it totally useless for the 'technical' sites like  or .  For example, in the Android app:    Same post in a (mobile) browser:     Please add this functionality! What's the canonical way to check for type in Python? What is the best way to check whether a given object is of a given type? How about checking whether the object inherits from a given type?  Let's say I have an object o. How do I check whether it's a str? How to quickly and safely defrost chicken? The tilde operator in Python On sites with new responsive theme, word wrap on long titles breaks in the middle of a word Before upgrading to macOS Sierra, I really liked the Character Accent Menu by doing a long press on a key (for example, the character e) and being able to add an accent on it.   However, this functionality seems to not work anymore, and so far, I can't find out why?   Also, before, doing a long press on the delete button, would repeatly delete characters. Now, it is doing only one character deletion disregarding how long I'm pressing the key. So if I want to erase 10 characters, I need to press the delete button 10 times...  Also, I am not able to log off or shut down. When doing it, all applications are closed, status bar disappears but nothing happens. The dock remains here. I have no way to shut down or log out without doing a long press on the power button.  Is someone facing the same issue?  EDIT : Logging with a new user account or guest account fix all the problems. But I still would like to know what, in my user account, became corrupted during the upgrade. How to interpret a quantum circuit as a matrix? Nvidia graphics card no longer works in Windows 10 (error: Code 43) but works in Ubuntu I am a physics Master's student.  I have good grades, with CGPA above 9(/10)(However, all courses in my university are graded on a curve so the actual marks are significantly lower than 90%). This has been my marks throughout my undergraduate and graduate courses and I was happy because this was above average. Also, my summer projects in UG were usually computational and any difficulty I had in theoretical aspects I attributed it to my inexperience in advanced Physics.   However, recently I have started to work on pure theoretical project and while reading for the project I realized that I consistently miss some details from each topic. I understand the general idea, can re-derive most of the equations(with some effort) however when I discuss things with my professor or when I read further topics I realize that I have missed certain key-details. This is exactly similar to the case of me missing a few key questions in my exams and getting that 9 grade point instead of 10(or at-least I think so). I was not worried about this before but now I realize that there is some flaw in the way I study (but I do not know what).   Since I want to apply for Physics PhD I realized that I will need to have solid concepts and it would be best to correct my mistakes but I do not know how or even what could be the reason for my imperfect understanding.  Any direct suggestion would help or even certain general guidelines on what I can do.    Things I've tried recently -   1) I've started to written down almost everything I read. This is slightly laborious but is certainly helping. It also helps me to gloss over the things I read previously when I come back the next day.  2) I completely avoid smartphone when I'm library/study room so that I won't get distracted. Not sure if it helps me solve the above problem but certainly helps me study better in general.  3) I've tried to read the topics from several sources. The books I am reading is somewhat like a collection of research papers and hence does not have a exercises. So I though reading from various sources might help get better ideas but it's not really helping because most other sources are too high level as an introductory text. But this will probably help me in cases where it's practical to do this.  4) Somewhat similar to point (1)- I am trying reread, whenever I can, what I've done until that point. But not sure how much this will help me in filling the gaps. How did the ending happen from Neil's perspective? I am trying to figure out the scene near the finale of Tenet where Neil unlocks the door and gets shot in the face. I can't really see the flow where he opens and holds the door and the protagonist walks in and fights the bad guy. Can you tell the series of events as Neil sees them happen till he gets shot? UPDATE If I am not wrong, I see it like this: Neil was running down towards the door, both the protagonist and his mate are inside with the door locked fighting the bad guy, then, he unlocks the door and walks in and stands behind the door. The protagonist's mate comes out of the room, then the protagonist, then he closes the room, then he look back to get shot in the face. But this doesn't make sense, if he wanted to unlock the door when he was running down, they were already in fighting the bad guy!!!!! When is the vector space of continuous functions on a compact Hausdorff space finite dimensional? I know that the vector space of all real valued continuous functions on a compact Hausdorff space can be infinite dimensional.  When will it be finite dimensional?  And how will I identify that vector space with $\mathbb{R}^n$ for some $n$? Does the Lunatic Cultist spawn on iPad? I was searching for more answers on the internet. I kept searching, and I found one thing that help me a little. I went back to Terraria and tried it out, but the Lunatic Cultist didn’t spawn.  Does the Lunatic Cultist spawn on an iPad? Synonym for "Fellow Sufferers"?