Our spiral Milky Way Galaxy is a hazy band of light when seen in Earth’s clear, dark night-time sky. This glowing band of nebulous light, stretching from horizon to horizon, is formed from a host of fiery stars that cannot be seen individually with the unaided human eye. Our 4.56 billion-year-old Sun is one of billions of other brilliant stars that perform their fantastic, joyful dance within this large Galaxy that is our home. Our Star is situated in our Milky Way’s far suburbs, in one of its whirling spiral arms. But, the ancient history of the star-birth, that occurred deep within our Galaxy’s heart, has remained a long-standing mystery. In December 2019, astronomers at the Max Planck Institute-Gesellschaft in Germany, released their findings proposing that there were two intense bursts of activity that led to the birth of stars in the center of our Milky Way.
The new observations indicate that star-birth peaked in our Galaxy’s heart around eight billion years ago. However, the observations also suggest that there was a second round of stellar birth that occurred about one billion years ago. Many astronomers had previously proposed that the stars inhabiting our Milky Way’s relatively small central disk had been born continuously. This scenario will inspire new theoretical work explaining the origin and properties of the bar-shaped feature within our Galaxy’s disk.
According to the new observations, more than 90% of the disk stars formed during the first round of star-birth at least eight billion years ago. However, the second round of stellar-birth, that was responsible for the formation of approximately 5% of the disk stars, occurred much later–within a relatively brief span of time only about one billion years ago. Between the two episodes of intense stellar birth, there was a long period of celestial peace and quiet, when hardly any bright new baby stars were born.
The stars observed in this study populate a dense, disk-shaped region of our Galaxy, termed the nuclear disk. This disk encircles the Milky Way’s innermost cluster of stars and its central, resident supermassive black hole, dubbed Sagittarius A* (pronounced Sagittarius-a-star). Our Galaxy’s central black hole is a relative light-weight– at least as far as supermassive black holes go–and weighs in at mere millions of times solar-mass, as opposed to the billions of times solar-mass sported by many others of its bizarre kind.
With their observations of two intense episodes of star-birth, the astronomers have suggested a revision of part of our Galaxy’s mysterious ancient history. Many astronomers have assumed that the stars populating the heart of our Milky Way were born gradually over the past millions of years. However, the new findings suggest that there may be a different timeline. If so, this could have consequences for a number of other astronomical phenomena.
The new scenario is also particularly interesting because it sheds new light on the growth of Sagitarrius A*. Gas floating into the mysterious heart of our Galaxy results both in star-birth and an increase of our resident supermassive black hole’s hefty mass. This newly proposed revision of our Milky Way’s star-formation history suggests that Sagittarius A* probably attained most of its present mass before eight billion years ago.
A Brief History Of Our Galaxy
Our starlit, barred-spiral Milky Way is just one of billions of other galaxies inhabiting the observable Universe. Before 1920, astronomers thought that our Galaxy was unique–and that it was the entire Universe.
Our Milky Way has an impressive diameter that is between 150,000 and 200,000 light-years, and it is estimated to be the home of 100-200 billion stars–as well as more than 100 billion planets. Our Solar System is situated at a radius of approximately 27,000 light-years from the Galactic center, on the inner edge of the Orion Arm, which is one of the spiral-shaped concentrations of gas and dust that make our Milky Way look like a twirling gigantic starlit pin-wheel in the vastness of Spacetime. The fiery, brilliant stars situated within the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge.
Brilliant stars and clouds of gas, located at a wide range of distances from our Galaxy’s heart, all circle at approximately 220 kilometers per second. This constant speed of rotation contradicts the laws of Keplerian dynamics and indicates that about 90% of the mass of our Galaxy is invisible to our telescopes–and that it neither emits nor absorbs electromagnetic radiation. This invisible, ghostly material has been called the dark matter, and it is thought to be composed of exotic non-atomic particles. The mysterious dark matter plays the important role of the gravitational “glue” that holds galaxies together, and its existence explains why objects at varying distances all rotate at a constant speed around the Galactic center, thus defying Keplerian dynamics.
Our Milky Way, as a whole, is soaring through Spacetime at a velocity of approximately 600 kilometers per second with respect to extragalactic frames of reference. The most ancient stars inhabiting our Galaxy are almost as old as the 13.8 billion-year-old Universe itself, and therefore likely formed shortly after the cosmological dark ages following the Big Bang. The cosmological dark ages refer to a very ancient era before the birth of the first generation of stars.
When we use the term “Milky Way”, we are referring only to the band of glowing light that we see stretching from horizon to horizon in our sky at night. The dark areas within this nebulous and gently luminous band, such as the Great Rift and the Coalsack, are actually regions where interstellar dust is blocking the light emanating from distant stars. The part of the night sky that our Galaxy obscures is referred to as the Zone of Avoidance.
Our Milky Way has a low surface brightness, and its visibility can be signficantly reduced by background light flowing out from light pollution or moonlight. Our Galaxy is difficult to see from brightly lit cities, but it shows itself off very well when observed from rural areas when Earth’s Moon is below the horizon. Indeed, one third of the human population cannot see the Milky Way from their homes because of this background light.
Our Galaxy is the second largest galaxy inhabiting the Local Group. The slightly larger spiral galaxy, named Andromeda, is the largest. Our Milky Way is also circled by several small satellite galaxies, such as the amorphous Large and Small Magellanic Clouds. As a member of the Local Group, our Galaxy and its satellites form part of the Virgo Supercluster, which is itself a component of the Laniakea Sypercluster.
Two Brilliant Blasts Of Baby Star-Birth
The intense, but short-lived, episode of baby star birth one billion years ago, is believed to be one of the most energetic events in our Galaxy’s history. Hundreds of thousands of newly formed massive stars probably exploded as supernovae within a span of only a few million years.
Because of these new observations, astronomers will go on to study an important feature of our Milky Way. Our Galaxy is a barred spiral. This means that it sports an elongated region calculated to be somewhere between 2,000 and 15,000 light-years in length, binding together the inner ends of its two main spiral arms. These galactic bar structures are believed to be quite efficient when funneling gas into a galaxy’s central region. This would result in the birth of fiery new baby stars.
Astronomers will likely come up with new scenarios to explain the quiet billions of years that were barren of baby star birth in the nuclear Galactic disk. During those many peaceful years, gas was evidently not funneled into the Galactic center in sufficient quantities to form new stars. Dr. Francisco Nogueras Lara, lead author of the paper describing this research, noted in a December 16, 2019 Max Planck (MPIA) Press Release that “Either the Galactic bar has come into existence only recently, or such bars are not as efficient in funneling gas as is commonly assumed. In the latter case, some event–like a close encounter with a dwarf galaxy–must have triggered the gas flow towards the Galactic center about one billion years ago.” Dr. Lara was formerly at the Astrofisica de Andalucia, and is currently a post-doctoral researcher at MPIA.
This proposed reconstruction of the history of the nuclear Galactic disk is based on certain known properties of star formation. Stars can only “live” on the hydrogen-burning main sequence for a set span of time. For example, our almost 5 billion year old Sun has a “life” span of 10 billion years, and it is still in mid-life. The “life” span of a particular star is based on its mass and chemical composition.
Whenever a large number of stars have been born at the same time–which is common in the Cosmos–astronomers can observe the ensemble, plot stellar brightness against the reddishness of color, and go on to determine how long ago the stellar siblings were born. One indicator of stellar age is referred to as the red clump. The red clump stars have started to fuse helium in their cores–which means that they have already fused their necessary supply of hydrogen into helium. By determining the average brightness of stars in that clump, astronomers can deduce the age of that group of stars.
However, there is a “catch”. All of those techniques demand that astroomers study separate stars. For our Galaxy’s central regions, that presents quite a challenge. This is because, when observed from Earth, the Galactic center is hidden behind enormous clouds of obscuring dust, thus requiring infrared observations in order to peer through these blanketing dust clouds.
Also, such studies are bound to observe too many stars in our Milky Way’s center. The Galactic disk is very dense, packed with between a thousand and a hundred thousand stars in a cube with a side-length of one light-year. When astronomers observe very dense star fields of this type, those stellar disks will overlap in the telescope image. Separating such fields into individual stars is extremely difficult–but necessary if an observer wants to reconstruct the formation history of the Galactic center.
Taking all of those challenges into consideration, Dr. Rainer Schodel (Instituto de Astrofisica de Andalucia, PI of the Galactic Nucleus Survey), MPIA’s Dr. Nadine Neumayer, and their colleagues started to plan how to tackle the problem. The astronomers realized that they would have to find the right instrument for this difficult task. As Dr. Neumayer explained in the December 16, 2019 MPIA Press Release “We needed a near-infrared instrument with a large field of view, able to observe the Milky Way’s central region which is in the Southern Sky.” The European Southern Observatory’s (ESO’s) HAWK proved to be an ideal instrument for them to use for their survey. HAWK is an infrared camera at the Very Large Telescope (VLT) at the Paranal Observatory of the ESO in Chile.
For their Galactic Nucleus Survey, the astronomers observed our Milky Way’s central region using HAWK-1 for 16 nights. By doing this, they managed to obtain accurate photometry of more than three million stars. Using a special technique termed holographic imaging, the astronomers were able to distinguish between stars that were a mere 0.2 arc seconds apart. With this high degree of accuracy, it is possible to distinguish two separate pennies when viewed from a distance of more than 8 kilometers. A duo of clearly visible red clumps in the resulting color-magnitude diagram enabled the astronomers to reconstruct the formation history of the Galactic nuclear disk.
The astronomers are currently studying the influence of dust on their observations (extinction and reddening). Taking into account the effects of dust will help them make even more precise reconstructions of the history of our Milky Way’s central regions in the future.
