Half Of All Stars Could Exist Outside of Galaxies

Antennae galaxies (NGC 4038 & 4039) Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration

Generally, stars and planets are regarded as existing nicely within galaxies. We know that every now and again there are events that fling a star outside of the galaxy, doomed to wander the universe alone, but it was previously assumed to be relatively rare. A new study has indicated that rogue stars could make up as many as half of all stars in the universe. The research was led by Michael Zemcov of Caltech, and the paper was published in Science.

The data was collected between 2009 to 2013 using a series of four suborbital rockets called Cosmic Infrared Background Experiment (CIBER). The project was overseen by principal investigator Jamie Bock of Jet Propulsion Laboratory. CIBER used three instruments to measure near infrared wavelengths in the cosmic infrared background. 

Ultimately, they discovered the black, empty region between galaxies wasn’t as dark or vacant as they thought. The area between galaxies is littered with rogue stars, whose total brightness adds up to about the same as what can be seen in galaxies. The stars were too distant to be observed individually, but appear to make a considerable impact when observed all together.

”Measuring such large fluctuations surprised us, but we carried out many tests to show the results are reliable," Zemcov said in a press release.

This study follows previous observations from NASA’s Spitzer Space Telescope, which revealed unusual patterns in the cosmic infrared background that were too large to be attributed to galaxies. Spitzer, which collects data via longer infrared wavelengths, was not able to resolve what they were. CIBER was developed as a means of collecting the relevant data more thoroughly than could be done by existing telescopes, like ESA’s Herschel. The team initially speculated that this light was left over from some of the oldest galaxies and black holes in the universe, but the light's redshift indicated it was too bright to be that old.

"CIBER tells us a couple key facts," Zemcov continued. "The fluctuations seem to be too bright to be coming from the first galaxies. You have to burn a large quantity of hydrogen into helium to get that much light, then you have to hide the evidence, because we don't see enough heavy elements made by stellar nucleosynthesis [fusion of heavy elements in the core of stars] which means these elements would have to disappear into black holes."

While the data seems to best support the idea of an intergalactic sea of rogue stars, it isn’t quite a slam dunk. The color of the light is not quite as blue as would be needed to best explain it using solitary stars. Still, the amount of light being produced between galaxies is too great to ignore. Further study will need to take place to determine if these findings are accurate. The team is currently planning CIBER2 to follow up on these observations. The second generation will expand the spectrum of IR light to be collected.

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