Home > Professional > Fast Stars, Dark Matter: A Narrative Journalism Article by Eric Hal Schwartz

Fast Stars, Dark Matter: A Narrative Journalism Article by Eric Hal Schwartz

As part of my last semester of graduate school at Boston University, I had to write a long-form piece of narrative journalism. This was extraordinarily difficult at almost every step, but the final product is something I’m very proud of. I’ve decided to publish it (under the cut so you don’t have to scroll past it if you don’t want to read it) for people to see. I hope you read it and enjoy it, and I welcome any thoughts or comments you may have to share with me about it. The article is titled: Fast Stars, Dark Matter.

Fast Stars, Dark Matter

In the center of the galaxy there is no night.  25,000 light-years from Earth, there’s no silver glitter scattered across the heavens like on Earth.  Instead there’s an endless blaze of light, pouring from the millions of nearby suns.  Many of them are binary stars, two stars locked in orbit around each other, like dancers standing at arm’s length.  They whirl each other about, even as they dance around and through their stellar neighbors in three-dimensional maneuvers that would make a Broadway chorographer’s eyes pop.  And right at the very core of the galaxy sits a vast and greedy black hole, like a dark pit at the center of the ballroom.  Only this pit extends over 27 million miles, and contains over four million times the mass of the Sun, dwarfing other black holes as the Grand Canyon dwarfs a crack in the sidewalk.  And anything falling into the black hole, from a scrap of light to a star itself, will never be seen again.

Binary stars can sometimes get caught by the black hole’s intense gravity, slipping deeper and deeper into the gravity well even as they orbit each other. The intricate gravity web keeps them rotating even as they approach the event horizon, the point of no return for a black hole.  With shocking suddenness, one of the stars dips below, vanishing from the visible universe and abruptly breaking its connection to the other star.  Like God’s own slingshot, the remaining star is propelled directly away from the black hole with so much energy that it leaves not only the galactic center, but begins a journey that will take it out of the Milky Way entirely.

To astronomers, the universe is a jigsaw puzzle with a billion pieces and no picture on the box.  Astronomical research is built from the few hardy particles of light that manage to travel thousands of years through space and impact at just the right time and place for a telescope camera to catch.  Astronomers therefore have to be very good at squeezing every last bit of information from this light, not to mention turning those numbers into a coherent picture of the universe.  In the past, this meant squinting into a telescope for hours and hastily writing down observations, hoping to see enough useful sights before the sun rises.  Advances in computers helped sort the captured light faster and with greater accuracy than any human, but even computers are limited to what light actually get captured, and the results can be the subject of endless debate.

In 1988 on Earth, 80 million years after the star started to hurry away from the galactic core, debate raged in astronomical circles over what lies in the center of the galaxy, how the galaxy formed, even what keeps the galaxy together.  In particular there was debate over whether there was just one enormous black hole in the galactic core, or two or even more smaller black holes.  John Hills at the Los Alamos National Laboratory wondered if there was some way to settle that debate, even if only in theory.  Using math and the limited amount known about what the galactic core, he sketched out a thought experiment describing how, if there were just one massive black hole, stars might be thrown from the core with such force that they would leave the galaxy.  He named these theoretical objects hypervelocity stars for their enormous speed.  If instead there were two or more smaller black holes, hypervelocity stars would be impossible. “The discovery of even one such hyper-velocity star coming from the Galactic centre would be nearly definitive evidence of its having a massive black hole,” Hills wrote in the article, published in a letter to Nature that year.  Like most such papers, it quickly faded into obscurity since without proof it was only an interesting abstract idea.

The hypervelocity star Hills didn’t know he had described rushed on at 530 miles per second, nearly five times the speed of the Sun and similar stars.  The galactic plane now lay below the stars’ hurried flight.  The 400 billion or so stars rotated on in their stately spiral, the painfully bright core hiding the dark maw of the black hole.  The star now moved among stars that were old when its journey began, the possible remnants of ancient, small galaxies that once collided to form the Milky Way below, and the objects of avid attention from a vanishingly small blue dot lost in the blaze of light rising from the galaxy underneath.

In early 2004, Warren Brown spent most of his time staring at those remnants and dissecting their light.  Sitting in a patch of sunlight let in from the high windows in his office at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Brown would spend his hours examining pictures taken by the Multiple Mirror Telescope near Tucson, Arizona over the course of the previous few months.  At the time a post-doc, Brown’s job was to carefully note possible old, blue stars at the edge of the galaxy.  After each such find he would catalogue everything from their possible age and composition to their movement and how they interacted with their neighbors.  He and the rest of the team working on the project wanted to use the data they collected to understand how the Milky Way formed and thus how it will continue to change.  Tedious as the work could be, Brown enjoyed it.  Examining and analyzing the raw data fills in just a bit more of the universal jigsaw, and like any good scientist, that’s how Brown sees his role.

Stumbling upon a corner piece of the puzzle is rare, but early one morning Brown did just that, and at first didn’t even notice.  While typing up yet another star, he noticed that for some reason this star’s numbers didn’t align with all the others he had catalogued.  Though similar to the stars he was looking for, this particular star seemed to be moving oddly.  “When I went to write it up I realized it was impossible to explain,” he said, even now wrinkling his forehead in remembered perplexity. Sifting through the mass of papers and astronomy reference books that covered his computer desk and overflowed onto the small table in a corner of the room, he searched for an explanation but none came readily.  Despite thorough searches through reference books and star catalogs, Brown could not find any mention of this star or an explanation for its eagerness to leave the Milky Way behind.

Once again chance played its part in science as Brown, trawling for answers in the dusty electronic archives of scientific papers, came upon Hills’ speculative work of nearly two decades before.  Surprise and excitement are what Brown remembers now upon reading the paper for the first time.  Checking his star against Hills’ scenario, Brown found it matched in almost every particular.  “It was an amazingly prescient paper,” he said, his voice rising in pitch as he conveys how incredible the prediction was.  Brown called his discovery HVS-1, hypervelocity star one, because of Hills’ paper.  At a stroke, Brown had just helped confirm that there is indeed just one giant black hole at the center of the galaxy.

As fast as he could, Brown found his project supervisor, noted astronomer Margaret Geller, and told her about what he had seen, showing her Hills’ old paper and the star that fit his speculation.  “She thought it was funny I wanted to do a paper about just one star after working on a survey with thousands of stars,” Brown laughed.  Despite Geller’s teasing, Brown managed to get her approval to shift his research from the survey to trying to figure out as much as possible about the runaway star.  Several months analyzing the star’s properties, including its age, composition and likely path.  This culminated in a paper with the subdued title: Discovery of an Unbound Hypervelocity Star in the Milky Way Halo in the Astrophysical Journal in February 2005.

Within days of publication, Brown and his teammates were bombarded by the media.  Brown proudly shows off a wood-framed copy of the story published in the New York Times.  Tracing the “Going, Going, Gone!” headline with a finger, he hangs the frame back on the wall.  Reporters soon moved on to other topics, but Brown, and others including teams at the University of Nuremberg in Germany and the Astronomical Institute in Amsterdam, started to devise plans to find more hypervelocity stars and build a picture of their origin and meaning.  Based on Hills’ original calculations and later data about the stellar population of the Milky Way, Brown estimated only a one in a hundred million chance that any random star in the sky is a hypervelocity star.  The key for astronomers was to develop ways of quickly discarding stars without the speed and other characteristics that first attracted Brown’s attention.

In the very first attempt to look specifically for hypervelocity stars, Brown found two more.   Over the next few years, astronomers at both Harvard and other research institutes have found a total of 16 hypervelocity stars.  “There’s only supposed to be 1,000 of them in the galaxy,” Brown said.  The chances of a star not only facing the right circumstances to become hypervelocity but be in the right spot to be found are vanishingly small.  Finding 16 so quickly and relatively easily raises questions about the data used to calculate the odds, and whether broad predictions about the galaxy based on that data are inaccurate.

Eventually the star will make its way entirely from the shore of light that is the Milky Way, and sail on into the oceanic abyss of intergalactic space.  Eventually, after thousands, or maybe even millions of years, it may get captured by the gravity of another galaxy and join a new dance.  One of the more recently discovered hypervelocity stars is suspected to have made that journey already to the Milky Way from another galaxy.

Brown’s discovery, like the star itself, has gone far beyond where it began.  Since its publication his original paper on hypervelocity stars has reached the top 1 percent of most cited articles in other astronomy publications.  “I thought it would die down, but it just keeps getting bigger,” Brown said.  And while there are plans to continue surveying for more hypervelocity stars, there is a shift in the research to applying the stars to other parts of the universal jigsaw puzzle.

One of the biggest issues for astronomers looking for pieces of the puzzle in the Milky Way is that most of the pieces are apparently invisible.  Based on the mass of all the visible matter in the galaxy, the spiral we call home should not exist.  Stars should be flying apart with as much enthusiasm, if not as much speed, as the hypervelocity stars.  The most commonly agreed upon theory to explain this contradiction is an invisible undetectable and unverifiable substance called “dark matter.”  “Dark matter is little more than fancy name for our ignorance,” said Samantha Penny, an astronomer at the University of Nottingham in England.  Dark matter makes up an estimated 95 percent of the mass of the galaxy, and a large majority of the rest of the universe too, Penny said, yet it is entirely theoretical and can only be inferred based on the way the Milky Way and other galaxies hold together.  “You get different answers from different people about dark matter,” Penny said, chuckling almost ruefully over the endless arguments in the scientific world.  “Dark matter is basically just a label.”

Hypervelocity stars could change all that, at least somewhat.  If the current scientific consensus is accurate, there is a huge dark matter halo around the Milky Way holding the spiral together.  “There should be a big clump of dark matter above the center of the galaxy,” Penny said.  Hypervelocity stars cruising through the vacuum will start to be affected by the immense gravity of all the dark matter around them.  No human made experiment could even begin to do what the stars are doing.  “The stars are amazing test particles for dark matter,” Brown said.

The actual amount of dark matter is uncertain, but current estimates indicate there’s at least six times as much dark matter in the halo as there is visible matter in the whole galaxy, and possibly as much as thirty times.  In cosmology though, numbers are so large that they don’t even seem real.  For instance the sun is a little less than 333,000 times the mass of Earth, and there’s an estimated 90 billion solar masses making up just the visible portion of the galaxy, which is itself a small fraction of the necessary dark matter.  “It’s exciting to work on partly because it’s all so big,” Brown said.

Research teams around America, Europe, and Asia are all racing to be the first with workable arrangements that can use the hypervelocity stars to test for dark matter.  It’s not just scientific curiosity either.  Proving (or even disproving) dark matter’s reality would be a feat comparable to Copernicus explaining the motion of the Earth around the sun, or Hubble discovering the expansion of the universe.

At its heart, that’s where the attention to hypervelocity stars originates.  Astronomical discoveries and breakthroughs don’t necessarily change the way we live, but they absolutely change the way we think, sometimes so quickly it can only be called hypervelocity.

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