A Tiny Ancient Star Reveals Our Galaxy’s True Age

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As we gaze up at the night sky, while standing far from the interfering glare of bright city lights, we can see our Milky Way Galaxy stretching from horizon to horizon like a sparkling starlit smile–telling us that we are only a small part of something vast, ancient, and mysterious. Astronomers have long thought that our Galaxy is very old. Indeed, scientists have proposed that it may be almost as old as the Universe itself. In November 2018, astronomers using the Gemini Observatory announced that they have discovered a tiny tattle-tale star that is likely the oldest known star dwelling in the disk of our Milky Way. Despite its unimpressive size, this diminutive star could play a disproportionate role in our scientific understanding of the true age and history of our Galaxy. The ancient star also sheds new light on the mysterious conditions that existed in the newborn Universe soon after its birth in the Big Bang almost 14 billion years ago.

The Gemini Observatory is composed of twin 8.1-meter diameter optical/infrared telescopes that can together scan the entire sky. Gemini North and Gemini South are situated at two separate locations in Hawaii and Chile, respectively.

The tiny tattle-tale star has a very interesting story to tell. It is old, small, and most importantly composed of elements very similar to those that formed in the Big Bang. In order to host a star like this, the disk of our Milky Way could very well be up to three billion years older than previously thought. Our Galaxy’s age has been calculated to be approximately 13.51 billion years, while our Universe is thought to be about 13.8 billion years old.

„Our Sun likely descended from thousands of generations of short-lived massive stars that have lived and died since the Big Bang. However, what’s most interesting about this star is that it had perhaps only one ancestor separating it and the beginnings of everything,“ commented Dr. Kevin Schlaufman in a November 5, 2018 Gemini Observatory Press Release. Dr. Schlaufman is of Johns Hopkins University in Maryland, and lead author of this study published in the November 5, 2018 issue of The Astrophysical Journal.

The Big Bang theory suggests that the first generation of stars were composed almost entirely of hydrogen and helium. The Big Bang birth of the Universe formed only the lightest of atomic elements–hydrogen, helium, and small quantities of lithium (Big Bang Nucleosynthesis). All atomic elements heavier than helium–termed metals by astronomers–were created by the stars in their nuclear-fusing furnaces (Stellar Nucleosynthesis). Alternatively, in the case of the heaviest atomic elements of all–such as gold and uranium–in the powerful and fiery supernovae blasts that heralded the explosive demise of massive stars (Supernova Nucleosynthess).

When stars perish, their stellar material is recycled to be used in the formation of new baby stars. Newborn stars receive–as their legacy from earlier generations of stars–all of the elder stars newly forged heavier atomic elements. The oxygen you breathe, the iron in your blood, the calcium in your bones, the sand beneath your feet, the water that you drink, were all formed in the nuclear-fusing hearts of the Universe’s myriad stars.

Astronomers refer to stars which are depleted of atomic elements heavier than helium as low metallicity stars. „But this one has such low metallicity it’s known as an ultra metal poor star–this star may be one in ten million,“ Dr. Schlaufman continued to explain in the Gemini Observatory Press Release.

The birth of the first generation of stars is one of the most fascinating mysteries haunting cosmologists. The most ancient stars are believed to have ignited as early as 100 million years after the Big Bang. However, the first stars to form in the Universe were unlike the stars we know today. This is because they formed directly from the pristine primordial gases churned out in the Big Bang itself. These primordial gases were primarily hydrogen and helium, and these two lightest of atomic elements are believed to have gravitationally pulled themselves together to form ever tighter and tighter knots. The cores of the first generation of protostars to emerge in our ancient Universe first caught fire within the mysterious dark and frigid hearts of these extremely cold dense knots of pristine ancient gases–which ultimately collapsed under their own relentless, heavy gravitational pull. The first stars did not form the same way or even from the same elements as stars do now. The first stars are referred to as Population III stars. Our own Sun is a member of the youngest stellar generation, and is classified as a Population I star. Sandwiched between the youngest and oldest stellar generations are the Population II stars.

It has been proposed that the massive primordial Population III stars were brilliant, and their existence is considered to be responsible for causing the Universe to change from what it once was to what it now is. These mysterious, dazzling first stars altered the dynamics of the Universe by heating it up and ionizing the existing gases.

Star Light, Star Bright

The metallicity of a star refers to the fraction of its material that is composed of atomic elements–metals–that are heavier than hydrogen and helium. Stars account for most of the atomic (visible) matter in the Cosmos, being composed primarily of hydrogen and helium. A star, no matter which of the three stellar generations it belongs to, will be a gigantic roiling, searing-hot sphere composed mostly of hydrogen gas. The term metal in astronomical jargon does not mean the same thing that it does in chemistry. Metallic bonds cannot exist in the extremely hot cores of stars, and the very strongest of chemical bonds are only possible in the outer layers of cool „failed stars“ known as brown dwarfs. Brown dwarfs may be born the same way as true stars, but they never quite manage to attain the necessary mass to light their nuclear-fusing stellar fires.

The metallicity of a star provides an important tool that astronomers use to determine a particular star’s true age. When the Universe was born, its „ordinary“ atomic matter was mostly hydrogen which, by way of the process of primordial nucleosynthesis, went on to create a large amount of helium along with much smaller quantities of beryllium and lithium–but nothing heavier. For this reason, the ancient Population II and Population III stars have much lower metallicities than younger Population I stars like our Sun. The term nucleosynthesis itself is defined as the process by which heavier atomic elements are created out of lighter ones, as the result of nuclear fusion (the fusion of atomic nuclei.

Therefore, the stellar Populations I, II, and III, display an increasing metal content with decreasing age. Population I stars, like our Sun, have the highest metal content, while Population III stars are depleted of metals. Population II stars have only trace quantities of metals.

A Big Starlit Smile

Galaxies like our Milky Way, are gravitationally bound systems composed of stars, interstellar gas, dust, stellar relics, and dark matter. Dark matter is thought to be composed of exotic non-atomic particles that do not interact with light or any other form of electromagnetic radiation, making it invisible. However, most astronomers think that it really exists in the Universe because it does interact gravitationally with objects that can be observed. Dark matter is a much more abundant form of matter than the „ordinary“ atomic matter that composes the Universe that we are most familiar with.

The word galaxy itself is taken from the Greek galaxias, translated literally as „milky“. Galaxies can range in size from dwarfs that host only a few hundred million stars to galactic behemoths that contain an astounding one hundred trillion stellar inhabitants, each orbiting around its galaxy’s center of mass.

Relatively small, spherical, and tightly bound collections of stars termed globular clusters are among the most ancient objects in our Milky Way. The ages of individual stars in our Galaxy can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238. Astronomers can then compare the results to estimates of their original abundance, by way of a technique termed nucleocosmochronology.

Several individual stars have been discovered in our Galaxy’s halo with ages measured very close to the 13.80-billion-year-old Universe. In 2007, a star inhabiting the Galactic halo, dubbed HE 1523-0901, was estimated to be approximately 13.2 billion years old. As the most ancient known object inhabiting our Milky Way at that time, this measurement placed a lower limit on our Galaxy’s age.

The age of stars dwelling in the Galactic thin disk was also estimated by astronomers using nucleocosmochronology. Measurements of stars inhabiting the thin disk indicate they were born approximately 8.8 billion years ago–give or take about 1.7 billion years. These measurements indicate that there was an interval of almost 5 billion years between the formation of the Galactic halo and the thin disk. More recent studies of the chemical signatures of thousands of stars indicate that starbirth might have plummeted by an order of magnitude at the time of disk formation, 8 to 10 billion years ago, when interstellar gas was much too hot to give birth to new baby stars at the same rate as before. Although it seems counterintuitive, things have to get very cold in order for a fiery new stellar baby to be born.

Satellite galaxies surrounding our Milky Way are not dispersed randomly. Indeed, they seem to be the result of an ancient break-up of some larger system that produced a ring structure about 500,000 light-years in diameter and 50,000 light-years wide. Close and catastrophic encounters between galaxies tear off enormous tails of gas which, over time, can coalesce to create dwarf galaxies.

In November 2018, astronomers reported the discovery of that tiny tattle-tale star that is one of the oldest inhabiting the Universe. This little star may also be one of the very first stars to be born in the Cosmos, and it is classified as an ultra-metal-poor (UMP) star composed almost entirely of matter formed in the Big Bang. Astronomers refer to such stars which are depleted of heavy metals as low metallicity stars. „But this one has such low metallicity, its known as an ultra metal poor star–this star may be one in ten million,“ Dr. Schlaufman commented in the November 5, 2018 Gemini Observatory Press Release.

2MASS J18082002-5104378 B

The tiny tattle-tale star, dubbed 2 MASS J18082002-5104378 B, also weakens the prevailing assumption that the first stars to be born in the Universe were behemoths–exclusively high-mass and short-lived stars. Indeed, this star’s location within our Milky Way’s disk–which is usually both crowded and extremely active–is a surprise.

2MASS J18082002-5104378 B is a member of a binary stellar system. It is the smaller companion of a low-metallicity star observed in 2014 and 2015 by the European Southern Observatory’s (ESO’s) Very Large Telescope UT2. Prior to the discovery of the tiny tattle-tale star, astronomers had mistakenly assumed that the binary system might host a stellar mass black hole or a neutron star. Stellar mass black holes and neutron stars are the relics that massive stars leave behind after they have gone supernova. From April 2016 to July 2017, Dr. Schlaufman and his colleagues used both the Gemini Multi Object Spectrograph (GMOS) on the Gemini South telescope in Chile and the Magellan Clay Telescope located at Las Campanas Observatory, in order to study the stellar system’s light and measure its relative motions, in this way discovering the tiny UMP by spotting its gravitational pull on its stellar partner.

Gemini was critical to this discovery, as the flexible observing modes enabled weekly check-ins on the system over six months,“ Dr. Schlaufman commented in the November 5, 2018 Gemini Observatory Press Release.

„Understanding the history of our own Galaxy is critical for humanity to understand the broader history of the entire Universe,“ noted Dr. Chris Davis in the same Press Release. Dr. Davis is of the U.S. National Science Foundation (NSF). NSF provides funding for the Gemini Observatory on behalf of the United States.

2MASS J18082002-5104378 B contains approximately a mere 14% of the mass of our Sun making it a red dwarf star. Tiny red dwarf stars are both the smallest and longest-lived of true stars that have gained sufficient mass to light their nuclear-fusing fires. Red dwarfs are also the most numerous stars inhabiting our Galaxy. This is because, while average-sized stars like our Sun „live“ for approximately 10 billion years on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution–before finally consuming their entire necessary supply of nuclear-fusing fuel–smaller red dwarf stars take „life“ easy, and burn brightly for trillions of years.

„Diminutive stars like these tend to shine for a very long time. This star has aged well. It looks exactly the same today as it did when it formed 13.5 billion years ago,“ Dr. Schlaufman said in the November 5, 2018 Gemini Observatory Press Release.

The discovery of 2MASS J18082002-5104378 B is important because it provides astronomers with new hope for detecting more of these ancient stars which shed new light on what occurred in the primordial Universe. Only about 30 UMPs have been identified so far. But, as Dr. Schlaufman concluded, „Observations such as these are paving the way to perhaps one day finding that ever elusive first generation star.“

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Source by Judith E Braffman-Miller

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