The Orion Nebula is a familiar sight to amateur astronomers as one of the brightest gas clouds in the sky. Plus it is easy to find within one of the most prominent constellations, in the Sword of Orion.
The nebula, which appears as a swirling maelstrom through large telescopes, is a famous example of a cosmic nursery where new stars are being formed from gas and dust.
Now new research suggests that the nebula contains an intriguing extra component – a black hole with 200 times the mass of the Sun. A team of astronomers claims that this explains a mysterious force that binds a cluster of unruly, rapidly moving stars.
The nebula is the closest stellar birthplace to us at a distance of around 1,344 light-years. A black hole at such a close distance would offer a great opportunity to study such an object.
The cluster, whose four brightest stars are known as the Trapezium, has interested astronomers for a long time because the stars move at rapid speed as if they were flying apart.
And compared to the number of low-mass (light-weight) stars that can be seen in the cluster, the number of high-mass (heavy-weight) stars are too few and moving especially fast.
The discovery that a black hole is likely to be responsible for this erratic behaviour was made thanks to extensive computer modelling by the international team. The model, representing a tight cloud of inter-stellar gas containing the right combination of heavy and light stars, neatly explained what was being observed.
The study’s lead author, Dr Ladislav Subr, of Charles University in Prague, said: “In our model, we had to invent a new method of dealing with the gas and the way it is driven out from the cluster by the intensely radiating high-mass stars.
“Our scenario neatly accounts for virtually all observed properties of the Orion Nebula Cluster, that is, its low number of high-mass stars, and its rapidly-moving central stars, and suggests that the massive stars near the centre of this cluster are bound by a black hole.”
Team member Dr Holger Baumgardt, of Australia’s University of Queensland, said such dense star cluster models were a challenge to compute due to the large number of calculations that had to be made.
The computations showed that, as the gas was being driven outwards, the cluster began to expand, explaining why most stars move rapidly.
Many of the heavy stars were sling-shot out of the cluster, while some were driven into the centre of the cluster and collided with the most massive star there.
At some point, this massive star became unstable and imploded into a black hole, with a mass about 200 times larger than the sun.
The team said that the finding had dramatic implications for our understanding of how massive stars form and how such rich star clusters hatch from their gaseous cocoons.
“Having such a massive black hole at our doorstep would be a dramatic chance for intense studies of these enigmatic objects,” said co-author Professor Pavel Kroupa from the University of Bonn in Germany. The study appears in The Astrophysical Journal.
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