Our Galaxy’s Stolen Stars
By Judith E Braffman-Miller
Our star-blazing, spiral Milky Way Galaxy spins majestically like a giant pin-wheel swirling in space. A very ancient structure, our Galaxy is thought to have started out as either one or several small over-dense regions in the mass distribution of the baby Universe, soon after its inflationary Big Bang birth almost 14 billion years ago. Some of these primeval, over-dense regions served as the seeds that eventually grew to become glittering globular clusters–spherical clusters of ancient stars–that now inhabit the stellar halo of our Galaxy, and contain the oldest surviving stars to be born in what eventually became our Milky Way. The eleven most distant stars known to dwell in our Galaxy are located approximately 300,000 light-years from our planet–far beyond our Milky Way’s spiral disk. In January 2017, new research released by Harvard University astronomers reveals that about 50% of those ancient stars might be the stolen stellar children of another galaxy–the ill-fated, nearby Sagittarius Dwarf galaxy. In addition, the stolen stars are members of a very long stream composed of fiery stars that extends one million light-years across space–or an almost unimaginable 10 times the width of our Milky Way!
“The star streams that have been mapped so far are like creeks compared to the giant river of stars we predict will be observed eventually,” study lead author Ms. Marion Dierickx explained in a January 2, 2017 Harvard Center for Astrophysics (CfA) Press Release. Marion Dierickx is a doctoral student at the CfA in Cambridge, Massachusetts.
The Sagittarius Dwarf is one of literally dozens of tiny galaxies that dance around our Milky Way. Over the age of our Universe, it has managed to make several loops around our Galaxy. Unfortunately, for the poor little Sagittarius Dwarf, at each passage the mighty gravitational tides of our Milky Way mercilessly pull on the smaller galaxy–tearing it apart like a wad of dough being stretched and pulled in opposite directions by a pastry chef.
A Little Galactic Runt
The Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG) is a loop-shaped elliptical satellite galaxy of our Milky Way. The entire galaxy is made up of only a quartet of globular clusters, with the main one having been discovered in 1994. Little Sgr dE is approximately 10,000 light-years in diameter, and is now about 70,000 light-years from Earth, wandering around in a polar orbit at a distance of approximately 50,000 light-years from the Milky Way’s core. A polar orbit is one that passes over our Galaxy’s galactic poles. This means that the Sagittarius Dwarf is about 1/3 the distance of the Large Magellanic Cloud (LMC), which is also a satellite galaxy of our Milky Way.
Sgr dE was discovered back in 1994 by Dr. Rodrigo Ibata, Dr. Mike Irwin, and Dr. Gerry Gilmore, who immediately recognized this badly bullied small, starry galaxy as the nearest known neighbor of our own giant Galaxy at the time of its discovery. However, since 2003, the tiny Canis Major Dwarf Galaxy has been recognized as our Milky Way’s nearest known galactic neighbor in space. Even though Sgr dE is one of the nearest companion galaxies to our own, the primary parent-cluster is situated on the opposite side of the Galactic core from our planet. As a result, Sgr dE is extremely faint, even though it covers a large area of Earth’s sky.
Additional observations conducted by teams of astrophysicists from both the University of Virginia and the University of Massachusetts (Amherst), that used data derived from the 2MASS Two-Micron All Sky Infrared survey, unveiled the entire loop-shaped structure of Sgr dE. In 2003, using infrared telescopes and supercomputer simulations, Dr. Steven Majewski, Dr. Michael Skrutskie, and Dr. Martin Weinberg created a new star map. This new stellar map picked out the entire Sagittarius Dwarf–its position, looping shape, and presence–using the tattle-tale mass of background stars. It also showed that this much smaller galactic neighbor is at a near right angle to the plane of our own Milky Way.
Globular clusters are beautiful and glittering spherical collections of stars that orbit the core of their galaxy as a satellite, and are very tightly held together by gravity–which is why they have their spherical shapes and relatively high stellar densities toward their centers. Usually found in the halo of their galactic host, globulars hold considerably more stars, and are much older, than less dense open clusters–that are normally denizens of their galaxy’s disk. Globulars are common inhabitants of the Cosmos, and there are approximately 150 to 158 currently known globular clusters in our Milky Way–with, possibly, 10 to 20 more awaiting discovery.
Every galaxy of sufficient mass in the Local Group of galaxies–of which our Milky Way is a member–has its own accompanying retinue of globulars. The Sagittarius Dwarf and the Canis Major Dwarf are apparently in the process of contributing their own associated retinue of globulars to our large Milky Way. This observation reveals that a large number of our own Galaxy’s globulars may have been stolen from other galaxies in the past.
The Sagittarius Dwarf displays a sparkling quartet of known globulars–with one, named M54, apparently situated at its core. It is also connected dynamically to the “youthful” globular dubbed Terzan 7–as well as to Terzan 8 and Arp 2.
Many astronomers originally thought that the Sagittarius Dwarf had reached an advanced stage of its own destruction, and that a large amount of its original matter had already been snatched up by our Milky Way. However, this badly bullied little galaxy still shows coherence as a dispersed elongated ellipse, and also apparently travels in a roughly polar orbit around our own Galaxy as close as about 50,000 light-years from the Galactic core. Even though it may have been born as a spherical object before tragically tumbling towards our Milky Way, the Sagittarius Dwarf is currently being ripped to shreds by the immense tidal forces of our Galaxy–a slow process that has occurred over the passage of hundreds of millions of years. Indeed, numerical simulations have indicated that stars stolen from the little galaxy would spread out in a long stellar stream along its path–and this glittering stellar stream has been detected.
Some astronomers think that the Sagittarius Dwarf has circled our Galaxy for billions of years, and has already orbited it about ten times. Furthermore, a supercomputer simulation, published in 2011, indicates that our Milky Way may have received its lovely spiral structure as a result of repeated collisions with its starlit tiny neighbor, the Sagittarius Dwarf.
In February 2013, a team of astronomers using the Hubble Space Telescope (HST), announced that their observations of our Milky Way’s outermost regions uncovered a shell of mysterious stars. The astronomers proposed that these stars are the tragic remnants of an ancient act of celestial cannibalism by our Galaxy, when it greedily devoured smaller satellite galaxies.
Our Milky Way and the Andromeda Galaxy (M13) are the two largest members of the Local Group of galaxies, which also hosts more than 30 smaller galaxies, and is spread out over an area extending more than a few million light-years. The Andromeda Galaxy, like our Milky Way, is a magnificent starlit spiral. Currently, the Andromeda Galaxy is a comfortable 2 million light-years away from us, but that is going to change. The relentless pull of gravity is carrying Andromeda towards our Milky Way at a speed of approximately 100 kilometers per second. In about 4 billion years our Milky Way will collide with and merge with Andromeda, and the two galaxies will become one. The new Galaxy will be an elliptical, possessing a mass that is equivalent to that of the Milky Way and Andromeda combined. One light-year is the distance that light can travel through a vacuum in one year–which is about 5,878,635 miles.
Our entire Local Group is located close to the outermost region of the Virgo Supercluster, whose glowing heart is situated about 50 million light-years from our Galaxy. The numerous groups of galaxies and galaxy clusters are smaller constituents of enormous web-like filaments and thin, broad expanses. The Laniakea Supercluster is the supercluster that hosts our Milky Way, as well as about 100,000 other nearby galaxies. The Laniakea Supercluster was revealed in September 2014, when a team of astronomers published a new way to define galactic superclusters according to the relative velocities of galaxies. This new definition of the local supercluster includes the earlier defined local supercluster, the Virgo Supercluster, which is now classified as an appendage.
Our Galaxy’s Stolen Stars
Ms. Dierickx, and her doctoral adviser, Harvard theorist Dr. Avi Loeb, used supercomputer models to simulate the motions of the Sagittarius Dwarf over the past 8 billion years. For their model, they varied its initial velocity and angle of approach to our Galaxy in order to determine what best correlated with current observations.
“The starting speed and approach angle have a big effect on the orbit, just like the speed and angle of a missile launch affects its trajectory,” Dr. Loeb explained in the January 11, 2017 CfA Press Release.
At the start of the simulation, the Sagittarius Dwarf weighed-in at approximately 10 billion solar-masses–or about one percent of our Milky Way’s current mass. Dierickx’s calculations revealed that, as time passed, the very unfortunate little galaxy was stripped of about a third of its stars and a full nine-tenths of its dark matter. Dark matter is an invisible and mysterious form of matter that is thought to be composed of exotic non-atomic particles that do not interact with light or any other form of electromagnetic radiation–which is why it is invisible. The dark matter is much more abundant than so-called “ordinary” atomic matter that composes a mere 4% of the mass-energy of the Universe.
According to the simulation, the loss of both a third of the Sagittarius Dwarf’s stellar inhabitants, as well as most of its dark matter, resulted in the formation of three distinct streams of stars that extend as far as one million light-years from our Galaxy’s core. They reach all the way out to the very edge of our Galaxy’s halo and show one of the largest structures observable on the sky.
In addition, five of the 11 most remote stars in our Milky Way show velocities and positions that match what is predicted of stars that have been stripped from the unlucky Sagittarius Dwarf. The other six stars do not appear to have been stolen from Sagittarius–but may have been stolen from yet another unfortunate dwarf galaxy.
Mapping projects such as the Sloan Digital Sky Survey (SDSS) have tracked one of the trio of star-streams predicted by these simulations, but not to the full extent that the models indicate. Upcoming future instruments like the Large Synoptic Survey Telescope, which is designed to detect much fainter stars across the sky, should be able to identify the two other stellar streams.
“More interlopers from Sagittarius are out there just waiting to be found,” Ms. Dierickx commented in the January 11, 2017 CfA Press Release.
This study has been accepted for publication in The Astrophysical Journal.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. 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.