North Korea has announced that it has tested a hydrogen bomb for the first time. It has run nuclear tests in the past — but the international reaction has been much more panicked this time than before.

Both atomic bombs and hydrogen bombs are kinds of nuclear bombs, meaning that the energy comes from nuclear reactions. But the way that they actually release that huge amount of energy is different — and leads to massive differences in the power that is unleashed when they are set off.

The big difference is that atomic bombs use nuclear fission, which splits a bigger atom into two smaller ones, to create their energy. Hydrogen bombs use fusion, which is done by fusing two or more atoms into a bigger one.

Hydrogen bombs have a fission weapon inside too, which is used to trigger the energy release from the fusion part of the bomb. That is needed because fusion weapons need to be triggered by a huge amount of energy — which can only be unleashed by an atomic bomb.

They get their name from the hydrogen that is used in the fusion.

North Korea allegedly test hydrogen bomb

Because of the way that they are made, hydrogen bombs are usually much more powerful than their atomic counterparts.

That is why people are so worried about North Korea’s announcement — it has already launched atomic bombs, but this is the first time that it has tested a hydrogen one.

North Korea did its first known nuclear test in 2006, and has done two more since.

In pictures: North Korea hydrogen bomb test Show all 15 1 /15 In pictures: North Korea hydrogen bomb test In pictures: North Korea hydrogen bomb test In pictures: North Korea hydrogen bomb test In pictures: North Korea hydrogen bomb test A lab employee from the Korea Institute of Nuclear Safety's regional office in Gangneung, east of Seoul, checks for radioactive traces in the air, in Gangneung, soon after North Korea announced it successfully conducted a hydrogen bomb test. The office in Gangneung is the closest one to the site of the North's claimed test. Officials said it will take three to four days to analyze air samples in detail for any traces of radioactivity, the Yonhap news agency reported EPA In pictures: North Korea hydrogen bomb test In pictures: North Korea hydrogen bomb test In pictures: North Korea hydrogen bomb test North Korean leader Kim Jong-Un signing a document of a hydrogen bomb test in Pyongyang In pictures: North Korea hydrogen bomb test People watch a TV news program showing North Korea's special announcement at the Seoul Railway Station in Seoul, South Korea AP In pictures: North Korea hydrogen bomb test Kuo Kai-wen, director of Taiwan's Seismology Center, explains the locations from a monitor showing North Korea's first hydrogen bomb test site, in Taipei Getty Images In pictures: North Korea hydrogen bomb test Kuo Kai-wen, director of Taiwan's Seismology Center, points at the curves chart received from Taiwan showing North Korea's first hydrogen bomb test, in Taipei Getty Images In pictures: North Korea hydrogen bomb test In pictures: North Korea hydrogen bomb test In pictures: North Korea hydrogen bomb test South Korean people watch TV news at Seoul station EPA In pictures: North Korea hydrogen bomb test Japan's meteorological agency officer Yohei Hasegawa displays a chart showing seismic activity, after a North Korean nuclear test, at the agency in Tokyo Getty Images In pictures: North Korea hydrogen bomb test Ko Yun-Hwa, administrator of Korea Meteorological Administration, briefs reporters showing seismic waves from the site of North Korea's hydrogen bomb test, at his office in Seoul Getty Images In pictures: North Korea hydrogen bomb test North Korea's border county of Kaepoong is seen from a South Korean observation post in Paju near the Demilitarized zone dividing the two Koreas as North Korea announced it had successfully carried out its first hydrogen bomb test Getty Images

The massive power of a hydrogen bomb is also one of the reasons that some analysts are sceptical of the concerns. Detonating such a weapon underground should be enough to cause a huge quake — but the impact was on a scale that could have been generated by an atomic bomb, and may have been less powerful than the one dropped in Hiroshima.