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The night sky is full of objects that change in brightness or move through our Solar System, but if we're not looking in the right place at the right time we'll miss them—especially if they’re far away or don’t give off much light. Rubin Observatory was designed specifically to detect changes in the sky on a broad scale and to alert us to each one. Rubin automatically compares new images to older images to detect changes in an object's position or brightness, and it generates an alert for each change it observes. If a star explodes or an asteroid moves across the sky, Rubin will catch it in the act.

There are a couple of reasons Rubin Observatory is particularly good at detecting changes. First, the telescope moves so quickly that it revisits the same area of sky every few nights, which means it can frequently capture images of the same objects. Second, Rubin Observatory's light-collecting power and sensitive camera ensure that even faint or faraway objects that change will trigger an alert. Rubin's combination of speed and imaging power helps it detect a lot of action—about ten million alerts are generated every single night! This constant series of alerts is called the "alert stream."

But can you imagine getting ten million alerts on your computer or your phone? To deal with this potentially-overwhelming alert stream, Rubin routes alerts to "brokers," who have developed software to categorize the alerts. So imagine you’re a scientist studying supernovae—thanks to the work of the alert brokers, you can log into the Rubin Science Platform to access Rubin data and filter the millions of alerts to study only the ones that were categorized as supernovae.

These alerts, together with some basic contextual information that will accompany each one, can also guide follow-up observations. If Rubin detects the increasing brightness of a supernova, which might only last a few hours or a few days, the resulting alert will tell scientists exactly where to look for the supernova with other telescopes in order to observe it in detail. Meanwhile, Rubin keeps searching the sky for more action.