What Is the Size of a Typical White Dwarf?

A white dwarf is the end of the line for a star of a certain size--a star like our Earth's life-giving Sun. For a star, a white dwarf may seem small, especially compared to some other stellar phenomenon, such as red giants. But its size is deceiving--a white dwarf weighs much, much more than similar objects in the solar system that are the same size.

Life Cycle

Stars have a natural life cycle, just like humans. Born out of a giant molecular cloud of dust and gas, stars are created when gravitational forces make the dust and gas collapse inward until nuclear fusion occurs. This fusion turns hydrogen into helium (as you read this, the Sun is undergoing this process). Eventually, the hydrogen fuel runs out, although it takes a while (in the Sun's case, about 10 billion years). Depending on the size of the star, it will then either start fusing helium and expand into a red giant or simply keep fusing until it explodes into a supernova. When red giant stars finally run out of fuel, they turn into white dwarfs.

Identification

A white dwarf star is a mid-sized star that's reached the end of its stellar life cycle. All its fuel has been burnt and the outer layers of the star have drifted off into space, forming a nebula. What's left is the hot inner core of the star--its "furnace"--which will cool down over untold numbers of years. The white dwarf sends out low-energy x-rays into the universe. Eventually the white dwarf should cool down enough to become a "black dwarf," but the universe is not old enough to have such fully cooled stars so the existence of the black dwarf is merely a theory.

Size

The average white dwarf is comparatively small--only a bit bigger than the Earth itself (the Earth's diameter is approximately 12,742 km). A better way of realizing just how small in size a white dwarf is to count the number of Earths that could "fit" into one Sun: 1 million. That means that about 1 million white dwarf stars could fit in the space that the Sun takes up. At some point in time, the Sun will itself shrink down to white dwarf size.

Mass

The truly impressive aspect of a white dwarf is its sheer mass. Even though a white dwarf may be the same size as the Earth, it is much more massive (or "heavy," in simple terms)--200,000 times more massive, that is. A white dwarf is so massive because gravity has made it so compact that the electrons in it are crammed together.

Appearance

The atmosphere of a white dwarf star is made up of a thin layer of hydrogen and helium. This layer is so thin that a tall Earth building could pierce through the top of the layer. Underneath this layer is a crust that's about 50 km thick, supported by a structure of carbon and oxygen atoms.