During my career, I studied different aspects of the category of celestial objects known as compact objects. Under this definition we refer to objects whose density is so high that the behavior of gravity in their proximity cannot be approximated through Newtonian physics, but needs a relativistic treatment.

Black holes are well known at a popular level as objects "so heavy and small that light cannot escape from them". That is, we cannot see beyond their surface, also known as event horizon. But we can study them efficiently through looking at the behavior of matter around them, and this is one of the main interests in my field of research.

Neutron stars are so dense that the effects they produce on their surroundings are very similar to those produced by black holes. For example, light is curved so much around them that we can observe much more than half their surface. They have a mass sligtly larger than that of the Sun, but concentrated in a "ball" smaller than a big city. Or to give another image, their density is more or less the density we would obtain concentrating all humans on Earth in a teaspoon. Nonetheless, they are not sufficiently dense to create an event horizon like in black holes, and therefore we can observe their surface.

When these objects are in binary systems with another star, they are often able to capture matter from it. The captured matter slowly spirals around the compact object until it falls on it. These process is called accretion. When matter reaches the proximity of the object, it is so hot that it emits x-rays.