Billions of starlit galaxies wander through intergalactic space, and our own large, spiral Milky Way is just like others of its kind. When we look up at a clear night sky, we see a seemingly endless background of blackness, sparkling with the distant furious fires of myriad stars–and if we are lucky enough to be far from the glare of city lights, we can observe our Galaxy as a starry band, stretching across the sky, telling us that we are a small part of something vast and mysterious. Stars are sparkling tattle-tales, providing important clues about their ages, compositions, and even where they were born. The stellar inhabitants that dwell in the ancient heart of our Galaxy, its central bulge, offer a treasure trove of information about how our starlit-pinwheel-shaped Milky Way formed–as the island-in-space home of a multitude of stars–that whisper tantalizing clues about how they evolved over the course of billions of years. On January 11, 2018, at the winter meeting of the American Astronomical Society, held in National Harbor, Maryland, a team of astronomers announced their new findings that the Galactic bulge is a dynamic environment, hosting stars of various ages. These stars fly around at different speeds in our Galaxy’s secretive heart.

For a long time, astronomers thought that our Milky Way’s bulge was the quiet, peaceful abode of elderly stars. These old stars were the first of their stellar kind to populate our young Galaxy. However, the new study of about 10,000 normal Sun-like stars inhabiting the bulge, reveals that our Galaxy’s heart is really the abode of frantic stars in a state of chaos. This more recent proposal is based on nine years worth of archival data derived from the Hubble Space Telescope (HST). The swifter stars of the younger generation may have arrived at the Galactic heart as a result of our Milky Way’s terrible feast upon smaller, unfortunate galaxies. However, the younger stars dance around with the older, slower stars. At this time, only HST has sufficiently sharp resolution to simultaneously measure the movements of thousands of Sun-like stars mingling together at the bulge’s distance from our planet.

Because the secretive heart of our Galaxy is so crowded, it has always presented a challenge to curious astronomers. This is because it is extremely difficult to disentangle the chaos of stellar movement in order to investigate the bulge in detail. However, the HST study of our Galaxy’s wild, complicated, and chaotic heart, may provide important new clues about how our Milky Way evolved, according to the study’s authors.

Our Milky Way Galaxy hosts approximately two hundred billion stars, as well as an extremely large number of other objects. When observed with binoculars on a dark night, far away from a city, it is a majestic sight. Literally thousands of stars can be observed performing their blazing, mesmerizing dance in Space in every field of view.

Currently, our ideas about our Galaxy’s nature are built upon a more solid foundation than when our ancient ancestors created lovely fairytale myths and stories to explain this celestial starlit band’s existence. The Milky Way has been studied extensively by astronomers, turning their scientific instruments on the dazzling fireworks display in the sky.

Long before our Galaxy formed, the Universe was filled with mostly hydrogen gas–as well as with smaller amounts of helium. The gas eventually experienced a sea-change into fiery stars, their accompanying retinues of planets, and conscious beings, such as ourselves on Earth–and likely elsewhere. However, before this important metamorphosis could occur, our Galaxy had to come into existence.

Our Milky Way, in which we dwell, is now a very different domain than the frigid gas from which it emerged billions of years ago. In its beginning years, our Galaxy was a spherical cloud of hydrogen. However, currently astronomers using radio telescopes have been able to determine that our Milky Way is a large spiral galaxy–a starlit pinwheel whirling in Space–just one of a multitude of others. Nevertheless, astronomers are at a disadvantage. This is because we are inside our Galaxy, which makes it impossible for astronomers to observe it as a whole. There is a bright side to all this–astronomers can stare out into intergalactic space and study distant galaxies which are similar to ours.

Disk galaxies include galaxies with starlit spiral arms, like our own–as well as those with less well defined characteristics (lenticular galaxies). All disk galaxies are defined by their possession of pancake-shaped regions composed of gas and dust that separate them from their elliptical galaxy cousins. Elliptical galaxies are football-shaped structures, in which a population of mostly elderly red stars bob around chaotically–in sharp contrast to the well-organized behavior of the stars inhabiting disk galaxies, that host stars of all ages.

According to galaxy classification, spirals, like our Milky Way, are made up of flat, rotating disks, populated by stars, gas, and dust–as well as its central bulge. The bulge is surrounded by a much fainter halo of stars, some of which are denizens of globular clusters. Spirals are named for their spiral arms that extend out from the bulge to the disk. In contrast, ellipticals sport an ellipsoidal shape and an almost featureless, smooth brightness profile. Ellipticals display little in the way of structure. Indeed, the stars that dwell within ellipticals jitterbug around in, more or less, random orbits around their centers. Lenticulars are intermediate between spirals and ellipticals, and they share kinematic characteristics with both of these two galaxy types. Lenticulars are frequently referred to as “armless spiral galaxies” because they host a bulge–but no spiral arms.

According to the favored theory of galactic formation–the bottom-up model–large galaxies only eventually grew to their majestic sizes as a result of collisions and mergers between relatively small protogalaxies inhabiting the early Universe. The most ancient galaxies rapidly produced fiery, brilliant stars in blazing bursts of starbirth.

Spiral galaxies contain stars, gas and dust, and display five primary parts:

–Bulge: The large round structure that hosts primarily elderly stars, gas and dust.

–Disk: The flattened region that encircles the bulge in a spiral galaxy. The disk is shaped like a pancake.

–Spiral Arms: The spiral arms of a galaxy are curved extensions that begin at the bulge and of a spiral galaxy, bestowing upon it a “pinwheel” appearance.

–Halo: The halo primarily harbors individual elderly stars and clusters of elderly stars (globular clusters). The halo also contains dark matter, which is thought to be composed of exotic, non-atomic particles that do not interact with light or any other form of electromagnetic radiation–which makes it invisible. It is also the most abundant form of matter in the Universe–far more abundant than the atomic matter that accounts for every element listed in the familiar Periodic Table.

–Stars, Gas and Dust: Stars come a rich variety of types: old, newborn, middle-aged, youthful. Stars have differing temperatures, and they come in different sizes–massive, small, dwarf, and stellar “failure”. Stars differ from one another in respect to their stage of evolution, as well as their metallicity. Metallicity refers to the amount of elements heavier than hydrogen and helium a star contains. The most ancient stars in the Universe contained no elements heavier than helium, while the youngest generation of stars–of which our Sun is a member–contain the highest metal content. The stars of the generation between the oldest and the youngest–the stellar”sandwich generation”–have only small quantities of metals. However, this is somewhat misleading, since all stars, regardless of their generation, are composed mostly of hydrogen.

The new HST study will provide valuable clues about how our Galaxy evolved, according to the researchers. The research team, led by Dr. Will Clarkson of the University of Michigan-Dearborn, discovered that the movements of stars inhabiting the bulge are not all the same–and the difference depends primarily on a star’s chemical composition. Stars richer in elements heavier than hydrogen and helium–the metals, in the jargon of astronomers–have less disordered motions. However, these richly endowed stars are orbiting around the Galactic center faster than older, less well-endowed stars, that are deficient in these heavy metals.

“There are many theories describing the formation of our Galaxy and central bulge. Some say the bulge formed when the Galaxy first formed about 13 billion years ago. In this case, all bulge stars would be old and share a similar motion. But others think the bulge formed later in the Galaxy’s lifetime, slowly evolving after the first generations of stars were born. In this scenario, some of the stars in the bulge might be younger, with their chemical composition enriched in heavier elements expelled from the death of previous generations of stars, and they should show a different motion compared to the older stars. The stars in our study are showing characteristics of both models. Therefore, this analysis can help us in understanding the bulge’s origin,” explained Dr. Annalisa Calamida in a January 11, 2018 Hubblesite Press Release. Dr. Calamida is of the Space Telescope Science Institute (STSI) in Baltimore, Maryland, and a member of the HST research team.

The astronomers then went on to divide the stars according to their chemical compositions, and from there compared the motions of each observed group. They determined the stars’ chemical content by observing their colors, and then separated them into two main groups according to their heavy-element (iron) abundance. The astronomers found that the chemically enriched stars are traveling twice as fast as the other stellar population.

“By analyzing nine years’ worth of data in the archive and improving our analysis techniques, we have made a clear, robust detection of the differences in the motion for chemically deficient and chemically enriched Sun-like stars. We hope to continue our analysis, which will allow us to make a three-dimensional chart of the rich chemical and dynamical complexity of the populations in the bulge,” Dr. Clarkson explained in the January 11, 2018 Hubblesite Press Release.

The team of astronomers based their analysis on Advanced Camera for Surveys and Wide Field Camera 3 data derived from two HST surveys: the Wide Field Camera 3 Galactic Bulge Treasury Program and the Sagittarius Window Eclipsing Extrasolar Planet Search. Sets of spectra derived from the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT) in Chile were used in order to calculate an estimation of the chemical compositions of stars.

At this time, only HST has resolution that is sufficiently sharp to simultaneously measure the motions of thousands of sparkling Sun-like stars at the Galactic bulge’s distance from our planet. The heart of our Galaxy is about 26,000 light-years away. “Before this analysis, the motions of these stars was not known. You need a long time baseline to accurately measure the positions and the motions of these faint stars,” noted study team member Dr. Kailash Sahu in the Hubblesite Press Release. Dr. Sahu is of the STSI.

The team of astronomers studied Sun-like stars because they are both easily within HST’s reach, and are also very plentiful. Earlier observations peered at older, brighter red giant stars, that are not as abundant. This is because red giants represent only a brief shining moment in the “life” of a star. When stars that are similar to our Sun finally have depleted their necessary supply of hydrogen fuel, their looks change. The former Sun-like star swells to monstrous proportions and turns red. When our Star finally comes to the end of the stellar road, in approximately five billion years, it will also become bloated and red, and its fierce fires will engulf the inner planets Mercury and Venus–and possibly our Earth, as well.

“Hubble gave us a narrow, pencil-beam view of the Galaxy’s core, but we are seeing thousands more stars than those spotted in earlier studies,” Dr. Calamida noted in the January 11, 2018 Hubblesite Press Release. Our Galaxy’s bulge is roughly one-tenth the diameter of our pancake-shaped Galaxy. “We next plan to extend our analysis to do additional observations along different sight-lines, which will allow us to make a three-dimensional probe of the rich complexity of the populations in the bulge,” Dr. Clarkson added in the same Hubblesite Press Release.

The astronomers said that their research is also an important pathfinder for NASA’s upcoming James Webb Space Telescope to probe the archaeology of our Milky Way. Webb is scheduled for launch in 2019, and is expected to probe deeply into the stellar populations sparkling their way within the Galactic bulge

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