Google has some things called public service ads that display on a webpage when no ads are available. These ads are free and means that google thinks your site is very useful and you are not in it just for the money. astarmathsandphysics.com is now the beneficiary of some of these ads
Public service ad (PSA)
Print
PSAs are non-profit organization ads that are served to pages when targeted ads are unavailable, or when Google is unable to gather content from the page. Publishers do not receive earnings for clicks made on PSAs.
Here are some examples on a website called scienceniche
http://www.scienceniche.com/directory/website/allcategories/28/site-listings/page3.html
