From the Earth we look down the barrel of jets from supermassive black holes, however these so-called "blazars" are at much safer distances of millions or billions of light years.

地球处于来自超大质量黑洞产生发射物的“枪口”方向，但是这些被叫做耀变体的天体处于一个对于我们来说非常安全的距离位置上，成万上亿光年。

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Markarian 421 was already in the known catalogue of blazars, being somewhat variable and having had something of a brightening, or flare, in 1996.

BBC: Blazar Markarian 421's flare-up is cosmic coincidence

Using almost every type of telescope known to humankind, Prof Marscher believes he has worked out where and how the jets - or blazars - are formed.

BBC: Black holes reveal more secrets

Blazars are a special case of "active galaxies" - those whose supermassive black holes spray out great quantities of light across the whole electromagnetic spectrum as they feed on surrounding matter.

BBC: Blazar Markarian 421's flare-up is cosmic coincidence