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Jupiter is the fifth planet from the Sun and the most massive, equivalent to just under 320 Earths. The portion of the planet we can see — the cloud tops — is composed of 90% hydrogen and 10% helium. Being a gas giant, Jupiter's composition is more similar to the composition of stars and the universe in general, in contrast to rocky planets such as Earth, primarily composed of heavy elements such as oxygen, silicon, nickel, and iron.
Being the most massive planet, Jupiter's interior is highly pressurized, making it very hot. The Jovian interior is approximately 71% hydrogen, 24% helium and 5% other elements by mass. The core of Jupiter is thought to be primarily iron, the heaviest element found in significant quantities in the solar system.
If you were to travel to Jupiter's core, starting at the upper atmosphere, one of the first observations you might make are increasing levels of helium with depth. About 1,000 km (621 mi), the hydrogen making up the majority of Jupiter's atmosphere slowly gets more and more dense, eventually reaching a liquid phase. The boundary between the gaseous and liquid hydrogen in the Jovian atmosphere is thought to be gradual.
Even deeper, the liquid hydrogen becomes compressed enough to take on conductive qualities, entering into a phase known as metallic hydrogen. The core of Jupiter is surrounded by a layer of metallic hydrogen that extends outwards to as much as 78% of the radius of the planet. On Earth, metallic hydrogen has only been produced in a laboratory for about a microsecond, at pressures of over a million atmospheres (>100 GPa or gigapascals), and temperatures of thousands of kelvin. In Jupiter, metallic hydrogen is usually in a liquid form.
At the transition zone between normal and metallic hydrogen, the temperature is thought to be 10,000 K and the pressure is 200 GPa. These conditions are already more extreme than any found in the solar system outside of the gas giants and the Sun itself. Beneath an extremely thick layer of metallic hydrogen is the core of Jupiter itself, whose properties are not well known. The temperature at the core of Jupiter is estimated at 36,000 K and the pressure at roughly 3,000–4,500 GPa. Even though this seems like a lot, it isn't anywhere close to what is necessary to achieve stellar ignition and for the planet to become a star. To achieve these conditions, it is estimated the planet would need to be 75 times more massive than it is now.