One star that has run out of hydrogen is Sirius B, the white dwarf companion to Sirius A (the brightest star in the night sky).

Here's why:

* Sirius B is a white dwarf, which is the final stage of a star like our Sun after it has exhausted its hydrogen fuel.

* White dwarfs are incredibly dense, about the size of the Earth but with the mass of our Sun.

* They are no longer fusing hydrogen, but they still glow faintly due to residual heat.

While there are many white dwarfs in our galaxy, Sirius B is a particularly well-studied example, as it is relatively close to Earth and its companion Sirius A allows us to study its properties.