RX J1856.5-3754

This is a whole star's worth of stuff squeezed into a ball the size of a city — and it is the nearest one of its kind to us! It is not doing anything dramatic, just quietly cooling down after its star died.

🚀 Travel there

RX J1856.5-3754 — Drawn, not photographed.

What it is

Nearest known neutron star, glowing quietly from leftover heat with no radio pulses.

Neutron star: A city-sized ball so dense a teaspoon of it has the mass of a mountain. Every kind of thing there is →

Can you see it?

No. No telescope on Earth will show you this one; it is here to be understood, not seen. It is here because it is worth knowing about — and because leaving it out would make the sky look emptier than it is.

Worth knowing about RX J1856.5-3754

On the 100-meter-track scale model of the Milky Way, RX J1856.5-3754 would be about 40 centimetres from the Sun — roughly the length of a computer keyboard. Light takes 400 Earth years to get here from RX J1856.5-3754, so what you are looking at is RX J1856.5-3754 as it was 400 Earth years ago.

What it is made of: The collapsed core left behind after a massive star's supernova; 1.4-2 solar masses packed into a ~20 km sphere, so dense a teaspoon of it would have a mass of billions of tons.; How we know: X-ray telescopes detected its faint, extremely hot thermal glow with no radio pulses; parallax confirmed it's the nearest known object of its kind. Rung 5 of the cosmic ladder — the distance to the stars
What makes it worth knowing: Unlike more famous pulsars, has no visible beams sweeping past Earth.

How did anyone measure the Earth in the first place?

The numbers