Sycamore Gap
© Owen Humphreys/PA
The objects have been seen close to the supermassive black hole at the centre of the Milky Way, which is seen here from Sycamore Gap in Northumberland. Photograph:
A number of bizarre shape-shifting objects have been discovered close to the supermassive black hole at the centre of the Milky Way.

The blobs are thought to be giant stars that spend part of their orbits so close to the black hole that they get stretched out like bubble gum before returning to a compact, roughly spherical form.

"These objects look like gas and behave like stars," said Andrea Ghez, a professor of astrophysics at the University of California Los Angeles and a co-author of the paper.

The observations raise the prospect of being able to track one of the objects as they drift across the point of no return and vanish over the black hole's event horizon into oblivion.

Ziri Younsi, an astrophysicist who researches black holes at University College London - who was not involved in the latest work - said: "The dream is that one of the orbits is eccentric enough that we get lucky and it will be swallowed. That will be really cool because we can get to see the black hole feeding in action."

The first of the unusual objects (later named G1) showed up on an astronomical survey in 2005 and was thought to be an astronomical anomaly until a second, similar object, G2, was spotted by German astronomers in 2012.

"We had seen it before, but it didn't look too peculiar until it got close to the black hole and became elongated, and much of its gas was torn apart," said Ghez. "It went from being a pretty innocuous object when it was far from the black hole to one that was really stretched out and distorted at its closest approach and lost its outer shell, and now it's getting more compact again."

Ghez and her research team believe that G2 is probably the product of a merger of two stars that had been orbiting the black hole in tandem before morphing into an extremely large star, cloaked in unusually thick gas and dust.

G2 is in an eccentric orbit about the Milky Way's black hole, Sagittarius A*, which has a mass equivalent to more than 4 million suns. When it reached its closest approach in 2014, observations revealed its outer shell being stripped off and swept down the cosmic sinkhole.

"When that happens, it might be able to produce an impressive fireworks show since the material eaten by the black hole will heat up and emit copious radiation before it disappears across the event horizon," said Mark Morris, a co-author and UCLA professor of physics and astronomy.

At the time, G2 was thought to be simply a large, diffuse cloud of gas, but astronomers were surprised to find that the object survived its close encounter with the black hole, suggesting that it must have a dense star-like interior.

The latest findings, based on 13 years of observations by the WM Keck Observatory in Hawaii, reveal a further four objects in similar orbits, all of which are within a few light months of the Milky Way's central black hole. By contrast, the Earth is 26,000 light years away.

"The Earth is in the suburbs compared to the centre of the galaxy," Ghez said. "The centre of our galaxy has a density of stars a billion times higher than our part of the galaxy. The gravitational pull is so much stronger. The magnetic fields are more extreme. The centre of the galaxy is where extreme astrophysics occurs."

The findings are published in the journal Nature.