The starlit galaxies of the observable Universe were born very long ago, and began to cast their brilliant, beautiful, starlit fires into Space less than a billion years after the Big Bang birth of the Universe almost 14 billion years ago. Our own large barred-spiral Galaxy, the Milky Way, is a very ancient structure that houses our Solar System, which is located about 27,000 light-years from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. The stars that dwell in the innermost 10,000 light-years of our Galaxy create a bulge and one or more bars that radiate out from the bulge –where there lies in wait, at the very heart of our Galaxy, an intense radio source, named Sagittarius A * , or Sgr A * (pronounced saj-a-star) , for short. Sgr A * is thought to be a supermassive black hole that weighs-in at millions of times the mass of our Sun. In November 2016, a team of astronomers announced the happy news that they have discovered a new family of stars living in our Milky Way's heart. This new family of stellar sparklers are a welcome addition to our neighborhood because they can shed new light on our Galaxy's birth in the primordial Universe.
This new discovery can solve some of the haunting mysteries surrounding globular clusters –which are spherical concentrations of about a million stars that formed at the very beginning of our Galaxy's existence. Liverpool John Moores University (LJMU) researcher, Dr. Ricardo Schiavon, led the project responsible for discovering the tattle-tale family of stars. LJMU , in Liverpool, England, is a member of the Sloan Digital Sky Survey (SDSS) –an international collaboration of scientists at numerous institutions. One of the projects of this collaboration is the Apache Point Observatory Galactic Evolution Experiment (APOGEE) which gathers infrared data on literally hundreds of thousands of stars dwelling in our Milky Way Galaxy.
By observing stars in the infrared towards the Galactic Center , the team of astronomers were able to discover the new family of stars – the likes of which had previously been observed only within globular clusters.
Globular clusters are beautiful spherical collections of stars that orbit around the core of a galaxy, and are very tightly glued together by their own gravity – which is why they have spherical shapes and relatively high stellar densities towards their centers. These lovely collections of stars are usually found in the halo of a galaxy, and they harbor considerably more stars – and are also much older – than open clusters , which are considerably less dense than their globular cousins. Open clusters are also usually seen in a galaxy's disk , rather than in the halo.
There are approximately 150 to 158 globulars known to inhabit our Galaxy, and they are considered to be fairly common objects. In addition, there are perhaps about 10 to 20 more still undiscovered globular clusters within our Milky Way. These globulars orbit our Galaxy at radii of about 130,000 light-years– or more! Galaxies that are larger than our Milky Way can also play host to more globulars. For example, the slightly larger largest spiral, the Andromeda Galaxy , may host as many as 500 of these clusters. Some of the giant elliptical (football-shaped) galaxies – especially those that are located at the centers of galaxy clusters, such as M87 –can host as many as 13,000 globular clusters.
Our Galaxy is a denizen of the Local Group . Every galaxy of sufficient mass dwelling in the Local Group has an associated collection of globular clusters. The Sagittarius Dwarf galaxy, and the controversial Canis Dwarf galaxy, have both been observed to be in the midst of contributing their associated globulars –such as Palomar 12 –to the Milky Way. This sheds light on how many of our Galaxy's globulars might have been snatched up in the past.
Globulars contain some of the first stars to be born in a galaxy like our own. Nevertheless, how these clusters were born, and the role they once played in galactic evolution, are not well understood. Despite all this, it is generally thought that globular clusters formed in concert with the star-birthing process that occurred within their primordial parent galaxies – rather than as separate and distinct galaxies in their own right. Also, in many globular clusters , most of the constituent stars appear to be at the same stage of stellar evolution. This observation suggests that they were all born about the same time. However, the star-birth history varies from cluster to cluster, with some clusters showing distinct populations of stars.
Milky Way Matters
A sparkling host of brilliant stars hurl their fabulous light out into Space from where they dwell within the more than 100 billion galaxies of the observable Universe. The observable Universe is that relatively small portion of the unimaginably vast Cosmos that we are able to observe– most of the Universe exists far beyond what we can see. This is because the light streaming towards us from those mysterious and remote regions has not had enough time to reach us since the Big Bang. The starry galaxies of our observable Universe trace out for us enormous, and otherwise invisible, heavy filaments composed of transparent dark matter . The identity of the dark matter is not known, but many astronomers strongly suspect that it is composed of exotic, non-atomic particles that cannot interact with light, or any other form of electromagnetic radiation, which is why it is invisible. The starlit galaxies that mingle together to form groups and clusters of galaxies light up the transparent filaments that compose what is called the Cosmic Web. In this way, the stellar constituents of galaxies outline, with the bright light of publicity, that which we would otherwise not suspect is there.
The most widely accepted theory of galactic formation and evolution is frequently referred to as the bottom up model. The bottom up model proposes that large and majestic galaxies – like our own Milky Way – were rare in the early Universe, and that galaxies only gradually attained these impressive sizes as a result of collisions and mergers between much smaller, amorphous protogalactic blobs. It is generally thought that the most ancient galaxies were only about one-tenth the size of our own Galaxy, but as a result of their rapid production of fiery new and dazzling baby stars, they were just as brilliant. These relatively small, but extremely bright, very ancient galaxies served as the "seeds" from which the large galaxies inhabiting the Universe today grew and flourished.
In the primordial Universe, opaque clouds of mostly hydrogen gas bumped into one another and then coalesced along the massive, enormous filaments of the invisible Cosmic Web –composed of the ghostly dark matter. Even though scientists have not as yet identified what the dark matter really is, they have a good idea of what it probably is not. Dark matter is most likely not made up of the "ordinary" atomic matter that composes all of the familiar elements listed in the Periodic Table– the so-called "ordinary" stuff of stars, planets, moons, oceans, sand, trees, and people. "Ordinary" atomic matter is really very extraordinary – even though it only accounts for about 5% of the mass-energy of the Universe. Atomic matter is what brought life into the Universe. We are such stuff as stars are made of. The stars created literally all of the atomic elements heavier than helium, that made life possible, in their nuclear-fusing hearts. In this way, the stars progressively created heavier and heaver atomic elements out of lighter ones. The iron in our blood, the calcium in our bones, the oxygen we breathe, and the carbon that is the basis for life on Earth – the dirt, stone, and sand beneath our feet – were all manufactured in the searing-hot nuclear-fusing furnaces of the stars or, alternatively, in the explosive supernova demise of the more massive stellar denizens of the Cosmos.
Our Galaxy's oldest stars have been estimated to be 13.6 billion years old. This would suggest that the Milky Way is almost as old as the Universe itself, which is about 13.8 billion years old. Stars and gases at a wide range of distances from the Galactic Center of our Milky Way all orbit at about 220 kilometers per second. This constant speed of rotation conflicts with the laws of Keplerian dynamics and hints that much of our Milky Way's mass does not emit or absorb electromagnetic radiation – an indication of the existence of dark matter.
In the primeval Universe, little by little, the wandering clouds of primordial gases and the invisible, ghostly dark matter did their fantastic dance together, combining to create the familiar structures in Space that astronomers observe today. Dense collections of the dark matter came to fill the entire ancient Cosmos, thus becoming the "seeds" from which the galaxies formed and evolved through Time. The powerful gravitational tugs of those ancient protogalactic "seeds" squeezed the primeval gases into ever tighter and tighter clouds. The clouds intermingled in a mysterious ancient dance, colliding and merging with one another to create these very ancient galactic building blocks. The primordial building blocks formed when halos of dark matter collapsed under the powerful and heavy weight of their own gravity. The protogalaxies did their ancient waltz together, eventually forming ever larger structures that became immense, majestic, starlit galaxies like our own Milky Way. The very ancient Universe was much smaller than it is today because of the accelerating expansion of Spacetime. The protogalaxies were relatively close to one another and, as a result, frequently bumped into one another and merged. This is how galaxies like our Milky Way were born.
From Earth the Milky Way can be seen as a fuzzy band of soft white light about 30 degrees wide, creating an amazing arc across the sky. The light emanating from this band originates from the accumulated light of unresolved stars and other material situated in the direction of the Galactic Plane . Darker segments of the band, such as two areas named the Great Rift and the Coalsack , are really regions where light from distant stars is blocked out by shrouds of obscuring dust swirling around in the space between stars. The area of the sky blocked by our Milky Way is called the Zone of Avoidance. Our Galaxy is the second-largest inhabitant of the Local Group , after the spiral Andromeda Galaxy .
Our Milky Way plays host to between 200 and 400 billion stars, and at least 100 billion planets. By comparison, the Andromeda Galaxy is thought to harbor approximately one trillion stellar inhabitants. However, most of the mass of our Milky Way appears to be composed of the dark matter , which interacts with "ordinary" atomic matter only through the force of gravity. A dark matter halo is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center.
The disk of stars in our Galaxy does not display a sharp edge beyond which there are no stars. Instead, the concentration of stars decreases with distance from the center of our Milky Way. Surrounding the Galactic Disk is the spherical dark matter halo, containing stars and globular clusters, that extends outward. However, the halo is limited in size by the orbits of a duo of amorphous Milky Way satellite galaxies, the Large and Small Magellanic Clouds. The Magellanic Clouds make their closest approach to the Galactic Center at approximately 180,000 light-years.
Welcoming A New Family Of Stars Into Our Galaxy's Heart
The lovely new family of bewitching stars possibly once belonged to globular clusters that were destroyed during the violent beginning of the formation of the Galactic Center. In this case, there would have once been approximately 10 times more globular clusters in our Milky Way, during its formative early years, than there are today. This means that a large percentage of the elderly stars, now dwelling within the inner portions of our Galaxy, may have been born in globular clusters that were eventually destroyed.
"This is a very exciting finding that helps us address fascinating questions such as what is the nature of the stars in the inner regions of the Milky Way, how globular clusters formed and what role they played in the formation of the early Milky Way– and by extension the formation of other galaxies. The center of the Milky Way is poorly understood, because it is blocked from view by intervening dust. Observing in the infrared, which is less absorbed by dust than visible light, APOGEE can see the center of the Galaxy better than other teams, "explained Dr. Ricardo Schiavon in the November 21, 2016 LJMU Press Release.
Dr. Schiavon continued to note: "From our observations we could determine the chemical compositions of thousands of stars, among which we spotted a significant number of stars that differed from the bulk of the stars in the inner regions of the Galaxy, due to their very high abundance of nitrogen.While not certain, we suspect that these stars resulted from globular cluster destruction. They could also be the byproducts of the first episodes of star formation taking place at the beginning of the Galaxy's history. We are conducting further observations to test these hypotheses. "
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