system
Given the following post, respond as if you're commenting on Reddit:
human
Why does a hydrogen bomb need deuterium (aka heavy hydrogen) yet the sun does just fine with regular hydrogen?

If the neutron in the deuterium atom is needed to sustain the chain reaction in a hydrogen bomb explosion, why isn't it needed in the sun?
gpt
Fusion in the Sun is a multi-stage process. The initial step involves the fusion of 2 protons into a fusion product called helium-2 or a diproton. However, the diproton is very unstable and often decays back into 2 separate protons. In a small fraction of the cases, the diproton can undergo a process called beta-plus decay immediately after fusing, in which a positron and neutrino are emitted and a deuterium nucleus is left (one of the protons in the diproton decays to a neutron in this process). The deuterium is then used in further stages of the fusion process until helium-4 is created.

Since the rate of beta-plus decay of the diproton is so low, the average time needed for a proton to become part of a deuterium nucleus is very long, with estimates putting it at a billion years. During this time, the proton will undergo many unsuccessful fusion attempts that involve the creation and immediate breaking up of a diproton.

The rest of the fusion process is much faster. So when making bombs, where it's important to detonate as much of the payload as possible in the very brief time window where the temperature and pressure are high enough to achieve fusion, it would be infeasible to start with pure hydrogen. So in fusion bomb designs, this slow proton-proton fusion step is simply skipped by starting with deuterium or tritium.