Which of the following best describes the electrical conduction system of the heart?

An arrhythmia can be best described as: 

Which of the following tests requires injecting the patient with a radioactive substance?

When an astronomer involved in a research project says he is going to look at the observatory’s plate collection, what will he be looking at?

An astronomer is studying galaxies of stars that are very far away and thus look extremely faint (using visible light). What kind of telescope should she and her graduate students apply for time on?

Astronomical observatories have been available since ancient times, and many cultures set aside special sites for astronomical observations. The thing modern observatories have that was missing from these older observatories until about 1610 was:

In a radio telescope, the role that the mirror plays in visible-light telescopes is played by:

The first person who regularly turned a telescope to astronomical observations (and published his observations) was:

The first x-ray telescope capable of forming images of x-ray source was called:

The earliest telescopes used by astronomers were: