Explainer: what is a hydrogen bomb? (And why it may not be what North Korea exploded)

Reports that North Korea has launched a fourth nuclear weapons test – backed by convincing seismic data – have caused widespread alarm. North Korean officials announced in advance that the test would involve "a totally different type of nuclear bomb" from those trialed in previous years. Following the test, North Korean state television lauded the first detonation of a "hydrogen bomb" as a "national epoch-making event".

Reports that North Korea has launched a fourth nuclear weapons test – backed by convincing seismic data – have caused widespread alarm. North Korean officials announced in advance that the test would involve "a totally different type of nuclear bomb" from those trialed in previous years. Following the test, North Korean state television lauded the first detonation of a "hydrogen bomb" as a "national epoch-making event".…