by Stacy Palen.

Until the result discussed in the article linked below, only two distinct neutrino sources were known: the Sun and Supernova 1987a. Now there is a third: a distant blazar.

This article complements material about active galactic nuclei, neutrinos, scientific instrumentation, and the process of science. Following are some questions that I thought of as I read the article. Share your own (with answers!) in the comments.

Article: https://www.eso.org/public/blog/pinpointing-the-source/

1. What is a blazar?
   
   Answer: A blazar is a particular kind of active galaxy in which the jet points at Earth.
   
   
2. A blazar is a little bit similar to a pulsar, but not exactly the same. Compare and contrast the two objects.
   
   Answer: A blazar is detected by the emission coming from its jet. In this way, it is something like a pulsar, which is observed when its jets point toward Earth. The pulsar, however, is much smaller and spins rapidly so that the jet points toward Earth only some of the time.
   
   
3. You have learned that there are many, many neutrinos passing through a human body in one second: 100 trillion, just from the Sun. How many neutrinos were detected from this blazar?
   
   Answer: Only one neutrino was detected! It is somewhat surprising that one neutrino out of so many can be important.
   
   
4. How many different regions of the electromagnetic spectrum were observed in this project? What are they?
   
   Answer: Five regions of the electromagnetic spectrum were observed: gamma ray, radio, infrared, optical, and X-ray.
   
   
5. This discovery is an example of what astronomers sometimes call "multi-messenger astronomy." What do they mean by that? If the neutrino had not been detected, would the discovery still be "multi-messenger?"
   
   Answer: Multi-messenger astronomy means that astronomers are getting information about an object from light (electromagnetic radiation) AND another source, like neutrinos or gravitational waves. If the neutrino had not been detected, this would not have been multi-messenger because all the other detections were made from observations of light.
   
   
6. Why has a blazar like this never been discovered before? Do you expect to see more discoveries in the future? Why or why not?
   
   Answer: Astronomers did not have equipment capable of discovering these neutrinos until IceCube became operational just a few years ago.
   
   
7. This discovery took many people working together, at many different facilities. The end of the article focuses on some of the difficulties and advantages of this approach. Describe one difficulty and one advantage of involving many scientists, particularly different kinds of scientists, in a scientific project.
   
   Answers will vary.