Stars in a distant galaxy move at stunning speeds — greater than 1 million mph, astronomers have revealed.

These hyperactive stars move at about twice the speed of our sun through the Milky Way, because their host galaxy is very massive, yet strangely compact. The scene, which has theorists baffled, is 11 billion light-years away. It is the first time motions of individual stars have been measured in a galaxy so distant.

While the stars' swiftness is notable, stars in other galaxies have been observed to travel at similarly high speeds. In those situations, it was usually because they were interlopers from outside, or circling close to a black hole.

But in this case, the stars' high velocities help astronomers confirm that the galaxy they belong to really is as massive as earlier data suggested.

Bizarre, indeed

The compact nature of this and similar galaxies in the faraway early universe is puzzling to scientists, who don't yet understand why some young, massive galaxies are about five times smaller than their counterparts today.

"A lot of people were thinking we had overestimated these masses in the past," said Yale University astronomer Pieter van Dokkum, leader of the new study. "But this confirms they are extremely massive for their size. These galaxies are indeed as bizarre as we thought they were."

Scientists used the new velocity measurements, conducted with the Gemini South telescope in Chile and the Hubble Space Telescope, to test the mass of a galaxy identified as 1255-0. The same way that the sun's gravity determines the orbiting speed of the Earth, the galaxy's gravity, and thus its mass, determines the velocities of the stars inside it.

The researchers found that indeed, the galaxy is exceptionally dense.

Given its distance of 11 billion light-years, galaxy 1255-0 is seen as it existed 11 billion years ago, less than 3 billion years after the theoretical Big Bang. Among other galaxies we can observe from this time period, about 30 to 40 percent are compact like this one. But in the modern, nearby universe, astronomers don't find anything similar.

Something wrong?

Somehow, high-mass galaxies from the young universe grow in size but not in mass – they spread out but maintain their overall heft – to become the high-mass galaxies we see today.

"It's a bit of a puzzle," van Dokkum told SPACE.com. "We think these galaxies must grow through collisions with other galaxies. The weird thing is that these mergers must lead to galaxies that are larger in size but not much more massive. We need a mechanism that grows them in size but not in mass."

So far, such a mechanism is elusive, but astronomers have some ideas. Perhaps these galaxies expand their girth by merging with many small, low-mass galaxies. Or maybe these galaxies eventually become the dense central regions of even larger galaxies.

"It could also still be that we are doing something wrong," van Dokkum said. "But I think at the moment you could say that the ball is somewhat in the court of the theorists. Hopefully they can come up with some kind of explanation that we can test further."

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