Hydrogen has the smaller, lighter, simpler nucleus.
Helium has the larger, heavier, more complex nucleus.
Hydrogen nuclei are fused to form helium nuclei.
When that happens, energy is released.
The answer to your question is Metal
Meters per second squared:

If you think about it, acceleration is about how fast speed changes. Speed is measured in meters per second:

So if you take that and just measure it over time, you get meters per second squared.