A guide to cosmic fireworks, from stellar flares to black-hole beacons

TO HUMAN eyes, the night sky is serene, save for the moon and a few wandering planets. Peer into space with a telescope that can scan the entire sky in days, however, and it appears as a great cosmic fireworks display – a riot of bangs and flashes radiating across the electromagnetic spectrum, from radio waves to gamma rays.

Astronomers are eager to catch these fleeting astrophysical phenomena, known as transients, because they can reveal a great deal about the universe, from what matter does in the most extreme conditions to how the cosmos evolved. And we are about to see a lot more of them thanks to the Vera C. Rubin Observatory in Chile, which is expected to detect 10 million transients a night when it switches on next year.

The trouble is that, for the untrained observer, it can be difficult to make sense of transients. For starters, similar-looking outbursts can have different origins and vice versa, such that a single source can give rise to a variety of astrophysical signatures. That alone can make it tricky to keep track of what’s what.

Here, then, is a concise primer that also serves as a guide to the causes and consequences of these ephemeral illuminations – and how, in some cases, they challenge our understanding of the physics that governs the universe.

Stellar flares

A living star’s electromagnetic eruptions

Let’s start with an easy one. A stellar flare is an explosion in the atmosphere of a star that results in an intense flash of radiation across the electromagnetic spectrum. When our sun flares, for instance, we see a sudden burst of brightness before it quickly …

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