The runaway star that left the Tarantula Nebula

There is a star in the Large Magellanic Cloud that should not be where it is.
It weighs about ninety times what the Sun does, and it is moving fast enough
to cover the Earth–Moon distance in roughly two hours — around 400,000
kilometres in the time it takes to watch a film. That speed is not itself
remarkable among stars. What is remarkable is the distance it has already put
between itself and the place it was born.

A star with no neighbours

Massive stars are not solitary by nature. They form in crowds, inside dense
clusters of gas and young stars, and they live short, bright lives close to
their siblings. Finding one alone, far from any cluster, is like finding a
single brick a hundred metres from a building site.

This one sits about 375 light years from R136, the fierce young cluster at
the heart of the Tarantula Nebula — a star-forming region in the Large
Magellanic Cloud, a satellite galaxy of our own. R136 is the obvious suspect.
It is the kind of place that makes stars this large, and nothing else nearby
is.

How you throw a star

A star that heavy does not drift. Something has to have thrown it, and there
are only two plausible mechanisms.

A supernova in a binary pair. Two stars orbit each other; one explodes;
the survivor is released at whatever speed it happened to be orbiting. This
works, and it happens — but it usually leaves evidence, and the speeds it
produces tend to be lower than this.

A gravitational slingshot. In the crowded centre of a cluster, stars pass
close enough to exchange momentum. A three- or four-body encounter can eject
one of the participants entirely. To throw something this heavy this fast, the
encounter has to have been violent, which means the cluster's core has to be
extraordinarily dense.

The second explanation is the one that fits, and it is the more interesting
of the two, because it is a measurement of something otherwise hard to see.

Why a single star tells you about a crowd

You cannot easily observe the inside of a cluster like R136. It is far away,
and the light of thousands of hot young stars blends together. But the things
a cluster throws out are measurable one at a time — their speed, their mass,
their direction.

A runaway of this mass and velocity is only possible if the core it came from
packs stars closely enough for near-collisions to be routine. In that sense
this one star is an instrument: it carries away information about a place we
cannot resolve directly, and it tells us that place is even more crowded than
models had assumed.

Where the picture comes from

The wide-field view of the Tarantula Nebula was taken with the 2.2-metre
telescope at the European Southern Observatory's La Silla site in Chile. The
runaway itself was identified in Hubble Space Telescope observations of
30 Doradus, the formal designation of the same region.