Particle physics studies the fundamental building blocks of matter and radiation, such as quarks and electrons. This includes famous experiments like the ones at the Large Hadron Collider looking for evidence of the Higgs boson. In theory, all of physics can be worked out from an understanding of these particles.
Astrophysics studies the physics of the universe beyond Earth, from the movement of planets and stars to the background radiation left from the Big Bang. This can be particularly challenging, because most of the universe is so far away. For example, most of our understanding of stars comes from studying the sun, simply because it is so much closer than any other star.
Theoretical physics uses maths to understand physics and makes predictions about things which haven’t yet been tested. For example, the Higgs boson was predicted by theoretical physicists in 1964, long before experiments began looking for it.
Medical physics isn’t a branch of the subject, but a way of applying it. It’s the use of physics in healthcare, from X-rays and CT scans to radiation therapy for cancer. Medical physicists are involved in the development and application of these techniques and in monitoring their safety. They also offer advice on dealing with radioactive materials safely.
Medical physicists need a relevant undergraduate degree followed by specific medical training, usually through the NHS Scientist Training Programme.