An article published in “The Astrophysical Journal” describes a research about the Boomerang Nebula, described as the coldest spot in the universe. A team led by Raghvendra Sahai of NASA’s Jet Propulsion Laboratory used the ALMA radio telescope to understand why the outflow of materials from the star at the center of the nebula is expanding very quickly. According to the astronomers there’s a companion and the gravitational interactions between the two stars accelerated that outflow.
The Boomerang Nebula is about 5,000 light years from Earth and at its center has a giant red star in its final agony that will lead to its death. When it was discovered in 1995, astronomers were puzzled because the observations indicated that it absorbed the light of the cosmic microwave background radiation, which in very simple terms is a kind of echo of the Big Bang. That radiation also determines the background temperature of space at almost 2.8 Kelvin and the fact that the Boomerang Nebula absorbs it means that it’s even colder.
The star agony stage is called a protoplanetary or preplanetary nebula. The outflow of stellar material that gives the nebula its shape is expanding very quickly and that’s why it’s so cold, less than half a Kelvin, a temperature close to absolute zero. This is a very abnormal phenomenon that has been puzzling astronomers for two decades but now this new research offers an explanation.
A few years ago, the Boomerang Nebula was already studied using the ALMA radio telescope but at that time the array of antennas that determined its extraordinary sensitivity hadn’t been completed yet. This time, ALMA (Atacama Large Millimeter/submillimeter Array), inaugurated in March 2013, was used at full power, allowing to calculate the nebula’s extent, age, mass and kinetic energy.
The collected information allowed the astronomers to estimate that the hourglass-shaped nebula extends over three trillion kilometers. The super-cold outflow from the red giant at its center is up to 10 times larger and travels at a speed of more than 150 kilometers per second. The expansion proceeds at a speed about 10 times that a star could produce.
The astronomers concluded that the only way to eject so much mass at such high speeds comes from the gravitational energy of two stars that are interacting. The red giant’s companion could be a small star that ended up at the heart of the other creating the super-cold outflow of abnormal properties that stirred astronomers’ perplexity for twenty years.
The article that describes this research describes the Boomerang Nebula as the coldest place in the universe but astronomer Lars-Åke Nyman, one of the authors, stated that it could actually be a common situation in the universe. Binary systems are common but planetary nebulae generally have a very short duration in astronomical terms and the ones like the Boomerang Nebula may last even less. In essence, it’s possible that we were lucky to find it during the millennia when its temperature is extreme.