Ghost Stars

Somewhere in space there may be a distant world whose clear, dark night sky is bereft of the fabulous fires of a crowd of thousands of stars--like the starry sky above our own Earth. Such a dismal, black night sky may belong to a planet that orbits a ghost star; a star that was ejected from an ancient galaxy that was gravitationally torn to shreds billions of years ago. In October 2014, astronomers announced that NASA's Hubble Space Telescope (HST) has spotted the faint, ghostly light emanating from stars that are the ejected and wandering orphans of galaxies that were ripped apart long ago and far away. The mayhem occurred 4 billion light-years away, within an enormous collection of almost 500 galaxies playfully nicknamed Pandora's Cluster--also known by the more dignified name of Abell 2744--and these tragic, orphan stars are no longer bound to any one galaxy, but instead drift freely, lost and alone, between the constituent galaxies of that distant cluster!

By watching the ghostly, glowing light from the orphaned stars, HST astronomers have put together forensic evidence that indicates that as many as six galaxies were ripped apart gravitationally inside Pandora's Cluster over a span of 6 billion years. Supercomputer modeling of the gravitational dance between the galaxies, inhabiting the cluster, indicates that galactic constituents as large as our own starlit, barred-spiral Milky Way Galaxy are probably the source of the ghostly stellar orphans. The tragic galaxies would have been ripped to shreds if they hurled themselves through the heart of the galaxy cluster where gravitational tidal forces are the most powerful. Astronomers have long proposed that the light flowing from scattered stars should be detectable after their doomed host galaxies are fragmented. Unfortunately, the predicted "intracluster" ghostly light of orphaned stars is very dim and difficult to detect.

Pandora's Cluster has a violent history. It is a giant cluster that formed as a result of the simultaneous pile-up of at least a quartet of smaller, separate galaxy clusters that occurred over a span of 350 million years. The galactic constituents of the cluster account for less than five percent of its mass. The gas floating around in the cluster is so searing-hot that it glows only in X-rays, and it accounts for about 20 percent of the cluster's mass. The lion's share of the mass of Pandora's Cluster--75 percent of it--is made up of the mysterious dark matter. Dark matter is weird stuff that accounts for most of the matter content of the Universe. Although its identity has not yet been determined, dark matter is generally thought to be composed of exotic non-atomic particles that do not interact with light or any other form of electromagnetic radiation--and is, as a result, transparent and invisible. However, astronomers generally agree that it is really there because it does exert an observable influence on both "ordinary" atomic matter and light through the force of gravity. So-called "ordinary" atomic matter is much less abundant than the exotic dark matter. Though relatively sparse, atomic matter accounts for literally all of the elements listed in the Periodic Table, and it also represents what we on Earth perceive as familiar--for example, it is the stuff of stars, planets, moons, and people. So-called "ordinary" atomic matter--which is actually very extraordinary stuff--constitutes a puny 4 percent or so of the mass-energy of the Cosmos.

Pandora's Cluster also displays a radio halo along with a handful of other Abell clusters. It exhibits a strong central halo, along with an extended tail. The tail could either be an extension of the central halo, or relic radiation. It received its nickname because so many bizarre and different phenomena were let loose as a result of the pile-up collision.

Clusters Of Galaxies

A host of fiery, brilliant stars dazzle the more than 100 billion galaxies that dance around in our visible, or observable, Universe. The visible Universe is that relatively small region of the Universe that is observable. Most of the unimaginably gigantic Cosmos is situated far, far beyond what is observable from where we are situated. This is because the light that travels to us from those very distant regions has not had sufficient time to reach us since the inflationary Big Bang birth of the Universe about 13.8 billion years ago.

Most galaxies reside in groups or clusters--with groups being quite a bit smaller than clusters. Clusters and superclusters of galaxies are the largest structures that are known to exist in the Universe, and they are commonly made up of hundreds to thousands of separate galaxies that are all tied together by the force of gravity--thus forming the densest component of the large-scale structure of the Universe.

Our own Milky Way is an inhabitant of the Local Group that hosts over 40 galaxies, of which our Galaxy and another spiral, the Andromeda Galaxy, are the largest members. Our Local Group, in turn, is situated near the outer limits of the Virgo Cluster of galaxies, whose core is 50 million light-years from us. The starlit galaxies of our Cosmos trace out for us mysterious, enormous, and very massive web-like filaments composed of the exotic dark matter. The fiery galaxies that dance around together in groups and clusters light up this transparent, weird Cosmic Web, shedding wonderful light on that strange structure which would otherwise be invisible.

In the very ancient Universe, opaque clouds of gas met together along the massive and enormous dark matter filaments of the great Cosmic Web. The densest regions of the heavy dark matter web snatched at the pristine clouds of wandering primordial gases with the irresistible force of gravitational attraction. Because dark matter does interact with atomic matter gravitationally, and it warps, distorts, and bends light (gravitational lensing), it reveals its strange phantom-like presence to the curious eyes of observers. Gravitational lensing is a phenomenon proposed by Albert Einstein when he realized that his calculations showed that gravity could warp light and therefore produce lens-like effects.

With your mind's eye envision how this invisible, mysterious, phantom-like form of exotic matter snatched at the clouds of pristine gas--that was mostly hydrogen--with its relentless gravitational embrace. The pools of gas became nurseries for the first generation of fiery baby stars to ignite the ancient Cosmos with their fabulous shrieks of newborn light. The fierce gravity of the Cosmic Web pulled in its prey until the snared gas clouds created blobs within the transparent halos of the exotic dark matter. The floating, tumbling clouds of ancient, primordial gases sunk down, down, down into the mysterious hearts of these transparent halos, that were strung out on the mysterious Cosmic Web like black pearls on a strange necklace.

Astronomers believe that the first galaxies to dance in our Universe were opaque and dark clouds of gas, merging together at the hearts of dark matter halos, and that they hoisted in the very first batches of incandescent neonatal stars with the powerful lure of their gravity. The sparkling baby stars and searing-hot glaring gas lit up what was previously a swath of incredible darkness--a dismal expanse, now set to shine with new and dazzling fires.

Ghost Stars

"The Hubble data revealing the ghost light are important steps forward in understanding the evolution of galaxy clusters. It is also amazingly beautiful in that we found the telltale glow by utilizing Hubble's unique capabilities," noted Dr, Ignacio Trujillo in an October 30, 2014 HUBBLESITE Press Release. Dr. Trujillo is of the Instituto de Astrofisica de Canarias (IAC) in La Laguna, Tenerife, Spain, and one of the astronomers involved in this study of Abell 2744.

"The results are in good agreement with what has been predicted to happen inside massive galaxy clusters," added Dr. Mirela Montes in the same HUBBLESITE Press Release. Dr. Montes is also of the IAC, and is lead author of the paper describing the research published in the October 1, 2014 issue of The Astrophysical Journal.

The team of astronomers used HST's steady stare to put together visible-light and near-infrared color images of Pandora's Cluster. They split these color images according to brightness, in order to study the color not just of the brilliant and luminous galaxies but also of the considerably dimmer intracluster light.

The authors of the study believe that the intracluster light is not a particularly well-defined quantity, from an observational perspective. While theorists are able to track every stellar sparkler as they model a galaxy cluster's evolution through the passage of time, observational astronomers must define intracluster light as any light below some threshold in surface brightness.

The astronomers estimate that the combined light emanating from approximately 200 billion orphaned ghost stars contributes about 10 percent of the Pandora Cluster's brightness. Because these extremely faint stars are brightest at near-infrared wavelengths of light, the team noted that this particular type of observation could only be accomplished by using HST's infrared sensitivity to exceptionally faint light.

HST measurements determined that the phantom ghost stars harbor an abundance of heavier atomic elements such as oxygen, nitrogen, and carbon. Essentially, this means that the wandering and widely dispersed stellar sparklers much be either second- or third-generation stars that were enriched with the heavier elements forged in the seething-hot hearts of the Universe's first-generation of stars. Spiral galaxies--similar to the ones thought to have been ripped apart--can still produce chemically enriched stellar babies.

Harboring the truly impressive mass of 4 trillion suns, Pandora's Cluster is a target in the Frontier Fields program. This very ambitious three-year project teams HST and NASA's other Great Observatories to stare at select massive galaxy clusters in order to help astronomers study the very distant Universe. Galaxy clusters are so extremely massive that their powerful gravity deflects the light that is passing through them, magnifying, brightening, and distorting this light as a result of gravitational lensing. Astronomers use this handy gift provided by Mother Nature--this highly useful property of space--as a kind of Cosmic zoom lens to magnify the images of long ago and faraway galaxies that otherwise would be much too faint to be observed.

Dr. Montes' team used the HST data in order to study the environment of the foreground cluster itself. There are five other Frontier Fields clusters in the program, and the team is planning to search for the eerie, phantom ghost light haunting these clusters, too.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.

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