When a star reaches its white dwarf stage, the star has consumed all the hydrogen it once used as nuclear field and nearly all of the material from the star is packed inside a radius one hundredth that of the original star – in this case it was believed to be about the size of the Earth but with 1.4 times the Sun’s mass.
And with that comes massive gravitational pull – 10,000 times the gravity at the surface of the Sun.
Astronomers, researchers using the European Space Agency’s INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) discovered a new X-ray source near the centre of the globular cluster NGC 6388. At first they thought the source of the X-rays was a supermassive black hole, but after further analysis ruled that out.
They then moved towards the idea that the rays were caused by hot gas swirling towards an intermediate-mass black hole but, in a follow-up X-ray observation, Chandra X-ray observatory helped determine that the X-rays were not coming from a black hole at the center of the cluster, but from a location slightly off to one side.
A composite image above shows NGC 6388 with X-rays detected by Chandra in pink and visible light from the Hubble Space Telescope in red, green, and blue, with many of the stars appearing to be orange or white.
Overlapping X-ray sources and stars near the center of the cluster also causes the image to appear white.
The X-ray source was then monitored with the X-ray telescope on board NASA’s Swift Gamma Ray Burst mission, during which time the source became dimmer. That tallied with theoretical models of a disruption of a planet by the gravitational tidal forces of a white dwarf.
In these models, a planet is first pulled away from its parent star by the gravity of the dense concentration of stars in a globular cluster. When such a planet passes too close to a white dwarf, it can be torn apart by the intense tidal forces of the white dwarf. The planetary debris is then heated and glows in X-rays as it falls onto the white dwarf. The observed amount of X-rays emitted at different energies agrees with expectations for a tidal disruption event.
Bill Condie is a science journalist based in Adelaide, Australia.
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