The universe is full of amazing objects and some of the weirdest are the remnants of what is left over after stars die. The weirdest characteristic of these objects is how their masses relate to their sizes. Many of them are physically much smaller than our own planet, the Earth. We often just assume that our Earth is a puny little rock and these massive objects meander around the galaxy swallowing up unsuspecting planets. This is not the case, black holes can have massive mass but are only physically very small. Super massive black holes, on the other hand, are the scary giants of the Universe and they are truly massive. Closer to us are the White dwarfs which are planet sized and are all that remain of stars once like our own Sun. One of the most bizarre stellar objects, neutron stars, are tiny only being the size of a city.
Before we get into the crazy objects, lets have a look at the Earth. Our planet weighs 5.97237 x 1024 kg and has an average radius of 6371 km. This allows us to experience one g as we walk around on the surface, which is an acceleration due to gravity of 9.81 m/s2.
The first object to compare is the most numerous of the Universe, the red dwarf. These stars are the smallest, longest lived and the most common. The radii of the smallest red dwarfs are less than 10 times the radius of the Earth, so just a tiny bit smaller than Jupiter. The amazing attribute of these stars is that will outlive all of the other stars and survive for hundreds of billions of years. These are the smallest of the stars, the smallest of them still has 26,000 times more mass than the Earth.
The next are the first of the weird objects, the white dwarfs. These are the leftovers following the death of stars, not the most massive stars, we’ll deal with them soon. Our Sun is destined to become one of these objects. They are basically the core of a dead star, the outer shells have been cast off into space and all that remains in a super dense ball of helium or carbon. These remnants are hot, as much as 30,000 degrees on their surface. The strange thing about them is that the more massive they are the smaller in size they are. A white dwarf that is only half the mass of our Sun, is about 1.5 times larger than the Earth, in radius.
When larger stars die they leave an even stranger object than a white dwarf, a neutron star. This is a super dense ball of mainly neutrons all crushed together, one teaspoon of a neutron star would weigh about 1 billion tons. They are so dense that their mass, which is typically 1.5 to 2.0 times the mass of the Sun, is squashed into a ball about 10km in radius. They have enormous magnetic fields and some can spin at nearly 1000 times a second.
The last of the weird objects is the black hole. These are left over following the death of the most massive stars. They are so dense that there masses are crushed into a tiny distance. A black hole the mass of the Sun would be 11 km across. These weird objects occasionally crash into each other and merge causing large black holes to form. If we were to consider a black hole that was 1000 times the mass of the Sun it would only have a radius of about 1500 km.