The distance from our sun to the next closest star, Alpha Centauri, is 25.6 trillion miles. While that may seem far, try to imagine 10,000 stars sitting between the two solar systems, meaning the stars would be spaced about the distance from Earth to Pluto. That’s exactly the scenario in M60-UCD1, a recently discovered ‘ultra-compact dwarf galaxy’ described in a study by a team of American and Australian astronomers in the latest edition of Astrophysical Journal Letters. “This galaxy is more massive than any ultra-compact dwarfs of comparable size,” said study author Jay Strader, an astronomer at Michigan State University, “and is arguably the densest galaxy known in the local universe.” “Traveling from one star to another would be a lot easier in M60-UCD1 than it is in our galaxy,” he added. “But it would still take hundreds of years using present technology.” About half of the galaxy’s mass is located within a radius of around 80 light years. This compactness would make the concentration of stars approximately 15,000 times greater in M60-UCD1 than what is found in Earth’s corner of the Milky Way. The team made their discovery using NASA’s Hubble Space Telescope, the space agency’s orbiting Chandra X-ray Observatory and the W. M. Keck Observatory on the summit of Mauna Kea in Hawaii. The 6.5-meter Multiple Mirror Telescope in Arizona was used to determine the amount of elements heavier than hydrogen and helium in the galaxy’s stars. The values were found to be close to those of our Sun. “The abundance of heavy elements in this galaxy makes it a fertile environment for planets and, potentially, life to form,” said co-author Anil Seth, an astronomer at the University of Utah. Study researchers also found that M60-UCD1 has a strong X-ray source in its center. One possible reason for this source could be a supermassive black hole weighing about 10 million times as much as our sun. Chandra scientists said they are currently trying to see if M60-UCD1 and other similar galaxies are either born that way or were once larger galaxies that had stars ripped away from their exterior. Large black holes are not typically seen in star clusters, so if there is a large, central black hole inside M60-UCD1, the galaxy was most likely produced through collisions between a bigger galaxy and one or more other galaxies. The weight of the galaxy and the Sun-like abundances of elements also indicate that the galaxy is leftover from a much bigger galaxy. “We think nearly all of the stars have been pulled away from the exterior of what once was a much bigger galaxy,” said co-author Duncan Forbes of Swinburne University in Australia. “This leaves behind just the very dense nucleus of the former galaxy, and an overly massive black hole.” If this stripping of stars did take place, then M60-UCD1 was originally 50 to 200 times bigger than it is now, which would make it more like the Milky Way and other typical galaxies. “Twenty years ago we couldn’t have done this,” Strader noted. “We didn’t have Hubble or Chandra. This is one of those projects where you bring together the full force of NASA’s great observatories, plus ground-based resources.”