How small can a star get?

Twenty kilometres is a distance you might cycle in an hour.

It is also the diameter of the neutron star used in the comparison further down this page: a collapsed stellar core containing about 1.4 times the mass of the Sun. The distance is familiar. Almost everything else about the object is difficult to imagine. [1]

But a neutron star comes with a catch. It is the remnant of a star, no longer powered by the steady hydrogen fusion that keeps our Sun shining. To answer how small a star can get, we need to follow two different stories: stars born small, and stars that leave something small behind.

Start with a smaller sun

Proxima Centauri, the nearest star to the Sun, is a red dwarf. These small stars fuse hydrogen into helium in their cores, just as the Sun does. [2]

JHB’s rounded model puts Proxima at about 214,000 kilometres across. The Sun is roughly 6.5 times wider. On a model where the Sun measured a metre across, Proxima would be about 15 centimetres wide. [3]

That is a sizeable reduction, though you would still struggle to find somewhere to put it. Proxima is small only because we have chosen a star as our measuring stick.

There are smaller hydrogen-burning stars. One well-studied example, EBLM J0555-57Ab, was reported in 2017 with a radius similar to Saturn’s. The research team estimated its mass at about 85 Jupiters. [4]

Saturn-sized, yet vastly more massive than Jupiter: a star can occupy roughly the same amount of space as a giant planet while packing much more matter inside.

That mass matters. Gravity compresses a star’s interior, helping create the conditions needed for sustained hydrogen fusion. Objects below the necessary mass cannot keep that process going; brown dwarfs sit below this boundary. EBLM J0555-57Ab lies close to it. [2] [5]

Proxima is our illustrated example of a small star here, not a claim to the smallest-star record.

A star can leave behind something Earth-sized

Now follow a star like the Sun towards the end of its life. After its giant phases and the loss of its outer layers, the remaining core becomes a white dwarf. [6]

JHB’s example is 12,000 kilometres across, close to Earth’s diameter, but has 60% of the Sun’s mass. From the outlines alone, you might think you were looking at two roughly comparable worlds. [7]

A white dwarf no longer sustains ordinary hydrogen fusion in its core. It shines as it loses stored heat. Its small size tells you something about what has happened to it. [2]

This is the sort of remnant our Sun is expected to become. The Sun does not have the mass needed to end its life as a neutron star. [6]

Then there is the city-sized remnant

Some much more massive stars end in supernova explosions, leaving a neutron star behind. The surviving core is compressed to extraordinary density. NASA describes these remnants as roughly city-sized. [8]

The JHB model is 20 kilometres across. Earth is about 637 times wider. If Earth were a globe one metre across, this neutron star would be only about 1.6 millimetres wide. [1]

Twenty kilometres is an illustrative diameter, rather than a measurement shared by every neutron star. Their sizes depend on their masses and on the behaviour of matter under conditions scientists are still investigating. [1]

The cycling comparison refers only to that distance. A neutron star has no remotely rideable surface, and its gravity would make the visit impossible.

Still, the change of scale is striking. We began with a smaller version of the Sun and ended with an object whose width could fit into an ordinary day’s travel. Switch the last comparison to Mass. The tiny object that almost vanished beside Earth is suddenly the heavyweight.