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Up to Half of the Stars in the Entire Universe Are Lonely Orphans

A sounding rocket discovered that about half of the universe’s stars are lost in intergalactic space. Can you hear us, Major Toms?
Stars don’t need a galaxy to shine. Image: Mellostorm

The nearest star to the Sun, Proxima Centauri, is four light years distant, but that still makes the red dwarf a very close neighbor in cosmic terms. The experience of almost half of the stars in the universe may be much less social, because according to data collected by CIBER sounding rockets, that's how many stars are lost in intergalactic space.

Though the rockets didn't image individual orphaned stars, they did pick up an eerie background glow of infrared light that betrays their presence. "We actually set out looking for the light from the first galaxies, and were surprised to find our measurements were extremely bright," lead author and Caltech/Jet Propulsion Laboratory astronomer Michael Zemcov told me.

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"So we compared them to all the models and found that intra halo light explained things the best," he said. "We certainly didn't set out expecting to see what we saw, and it took a long time to convince ourselves we were correct."

The discovery was completely serendipitous, and is a great example of how smaller rocket missions like CIBER can lead to big insights. "Astrophysical sounding rockets are scientifically nimble, and a (relatively) cheap way to get into space for a short amount of time," Zemcov told me.

"Because of that, NASA is happier to take bigger risks with them, which means that chance discoveries like ours are more likely," he continued. "On the time scale of three to five years, we can put a payload together, fly it, and analyze the data, which means we can turn around and do it again much more quickly than a full space telescope which takes a decade or more to plan and execute."

The JPL-Caltech findings come just one week after the publication of another study on the eerie "ghost light" emitted by orphaned stars, this time conducted by the the Instituto de Astrofísica de Canarias (IAC), in Tenerife, Spain. The IAC researchers were focused on a massive galaxy cluster called Abell 2744 located a dizzying four billion light years from Earth.

They deduced that over a slow six million years, five Milky-Way-sized galaxies were brutally dismembered at the hands of the mega-cluster's gravitational forces. About 200 billion stars were ejected from the tidal wounds of their host galaxies, destined to wander the intergalactic void.

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Hubble fact sheet for Abell 2744. Image: NASA/ESA/M. Montes (IAC)/J. Lotz/M. Mountain/A. Koekemoer/the HFF Team (STScI)

But despite their dramatic exile from galactic life, the IAC team estimated that the orphaned stars still account for a full tenth of the Abell 2744's total luminosity, which is impressive when you think about how scattered they must be. According to Mireia Montes, who led the IAC study, published last week in The Astrophysical Journal, most of those stars are still cosmic nomads today.

"The average properties of the intracluster light are similar to a galaxy like our own Milky Way," she told me. "The observed age of the stars are 6 billion years, on average, and the light of the cluster was emitted three billion years ago. So, nowadays, the stars are ten billion years old, still living peacefully inside the cluster."

Though these orphaned stars were forcefully jettisoned from their galaxies, this transition would not have seemed as dramatic on a local scale. In fact, many of the stars likely retain any planets they may have had during their younger galaxy-bound years.

"The distances among stars in a galaxy are so big that they're not affected by these encounters," Montes said. "I will not say in all the cases, because it may happen, but [many of] the planets around these drifting stars are untouched."

The idea that there are billions upon billions of these orphaned solar systems traversing the barren intergalactic space is equal parts mind-boggling and evocative. In the original Cosmos, Carl Sagan wistfully imagined experiencing "a galaxy-rise, a morning filled with 400 billion suns" (a quote that, incidentally, makes for fantastic autotune fodder).

For many of these stars and planets, galaxy rises wouldn't be the "more glorious dawn" that Sagan envisioned, simply because they have drifted so far away from their original gravitational moorings. Their home galaxy might be as distant and intangible as Andromeda is in our own night sky. But it's still comforting to think that even in the darkest, coldest, loneliest reaches of intergalactic space, stars still shine on.