Our Milky Way Galaxy is very old. Indeed, at 13.21 billion years of age, it is almost as old as the 13.8 billion year old Universe itself, which was born in the wild exponential inflation of the Big Bang. In fact, the oldest stars inhabiting our Galaxy were likely part of the brilliant stellar fireworks display that brought to an end the strange Cosmic Dark Ages–the era that occurred soon after the Big Bang when our Universe was a featureless swath of barren darkness. Today, we can observe our Galaxy as a fuzzy band of white light that stretches across the night sky, from horizon to horizon, like an upside down smile, reminding us that we are only a small part of something else–something vast, mighty, and mysterious. In April 2016, an international team of astronomers announced they have discovered that the central 2,000 light-years within our Galaxy hosts a population of primordial stars that are more than 10 billion years old–and their orbits in space preserve the ancient long-lost story of our Milky Way’s birth.
When we look up at the sky on a clear midnight, we see that it has been set on fire by the furious, faraway flames of billions and billions of sparkling stars. Our Milky Way Galaxy, that contains our Solar System, was given its name because of its appearance as a faint glowing band stretching across this midnight sky, whose stars cannot be distinguished as individual objects by the naked eye. The term “Milky Way” is a translation from the Latin via lactea, and from the Greek “milky circle”. From our planet, our Galaxy appears as a band because its disk-shaped structure is being observed from within. The great Italian astronomer Galileo Galilei first resolved this glowing, fuzzy band of light into individual stars with his telescope in 1610.
Until the early 1920s, most astronomers believed that our Galaxy contained all the stars dwelling in the Universe–in fact, they thought that our Galaxy was the entire Universe! However, following closely on the heels of the 1920 Great Debate between the two American astronomers Harlow Shapley (1885-1972) and Heber Curtis (1872-1942), came the historic discoveries of the American astronomer Edwin Hubble. Hubble, the “father of modern observational astronomy”, was able to demonstrate that our Milky Way is really only one of billions of galaxies. Indeed, it is now estimated that the number of galaxies dancing around in our observable Universe could be as great as 200 billion.
The Great Debate centered on the identity of spiral nebulae. The main issue of the debate was whether these distant nebulae were really relatively small objects that inhabited the outer limits of our Milky Way, or whether they were independent galaxies in their own right. Edwin Hubble–after whom the Hubble Space Telescope is named–settled the issue once and for all. Our Galaxy is not the entire Universe.
Our Milky Way is a large barred-spiral galaxy that sports an impressive diameter that is usually estimated to be approximately 100,000 to 120,000 light-years–but it may be as much as 150,000 to 180,000 light-years. Our Galaxy is also thought to host an estimated 100 to 400 billion stars, although this stellar number may really be as high as one trillion. In addition, there very well may be at least 100 billion planets inhabiting our Galaxy.
Our Sun, and its familiar family of planets, moons, asteroids, and comets, are all located within the Galactic disk, approximately 27,000 light years from our Milky Way’s secretive heart, or core. Our entire Solar System is situated on the inner edge of one of our Galaxy’s spiral-shaped collections of gas and dust named the Orion Arm. The stars that inhabit the inner 10,000 light-years, or so, form a bulge. Also, one or more bars radiate outward from this bulge. At the very center of our Milky Way, there lurks a powerful radio source, named Sagittarius A* (Sagittarius-a-star), which is likely a supermassive black hole that weighs-in at millions of suns.
The stars and gases at a variety of distances from the Galactic Center all orbit at about 220 kilometers per second. The constant rotation speed is at odds with the Keplerian laws of dynamics, and so it suggests that much of the mass of our Galaxy does not emit or absorb electromagnetic radiation. This mass is thought to be composed of the dark matter, which is a bizarre substance theoretically composed of exotic non-atomic particles that do not interact with visible light or any other form of electromagnetic radiation. It is generally thought that the dark matter accounts for most of the material content of the Universe.
The period of rotation is approximately 240 million years at the position of our Solar System. As a whole, our entire Galaxy is traveling at a speed of 600 kilometers per second with respect to extragalactic frames of reference. Our Milky Way’s center is located in the direction of the constellations Sagittarius, Scorpius, and Ophiuchus–where it appears brightest. The most ancient stars in our Milky Way likely ignited soon after the cosmological Dark Ages came to an end.
Our Galaxy has several galactic satellites and is a member of the Local Group of galaxies, which is itself a constituent of the Virgo Cluster–which is itself a component of the immense Laniakea Supercluster.
The Milky Way’s Heart
Because of interstellar dust, swirling smoke-like along the line of sight, the Galactic Center cannot be observed at visible, ultraviolet or soft X-ray electromagnetic wavelengths. Therefore, the information available to explain the Galactic Center is derived from observations at gamma ray, hard X-ray, infrared sub-millimeter and radio wavelengths.
Shapley noted in 1918 that he believed the halo of globular clusters circling around our Milky Way appeared to be centered on the stellar swarms in the constellation Sagittarius. However, giant, frigid, dark–and beautifully eerie– molecular clouds, haunting the area, shrouded the view for frustrated optical astronomers. In the first decades of the 1940s, the German astronomer Walter Baade (1893-1960) of the California Institute of Technology, in Pasadena–where he did his most important work–took advantage of World War II blackout conditions in nearby Los Angeles to go on the hunt for the center of our Galaxy. To accomplish this, Baade used the 100 inch Hooker Telescope at Mount Wilson Observatory, and spotted a one-degree-wide void in the interstellar dust lanes near the star dubbed Alnasl (Gamma Sagittarii)–providing a clear window through the spiral arms of our Milky Way to the swarms of sparkling stars surrounding its nucleus. The gap has been known as Baade’s Window ever since. Nevertheless, Baade was not convinced that he really had located the Galactic Center, and when the Mount Palomar Telescopes in California were commissioned around 1950, he went on to use them for additional searches for this Galactic holy grail–but with no success.
In the 1970s, a team of Australian astronomers from the Division of Radiophysics at the Commonwealth Scientific and Industrial Research Center (CSIRO), led by Dr. Joseph Lade Pawsey, used what they called a “sea interferometer” to detect some of the first interstellar and intergalactic radio sources. By 1984, they had successfully constructed an 80-foot fixed radio dish antenna, and had used it to conduct a detailed study of an extremely powerful extended belt of radio emission spotted within Sagittarius. The astronomers named a particularly intense point-source, near the center of this belt, Sagittarius A and realized that it was situated at the very center of our Galaxy–despite being about 32 degrees south-west of the conjectured Galactic Center of the time.
A study conducted back in 2008, using the Very Long Baseline Interferometer (VLBI), that links radio telescopes in Arizona, California, and Hawaii, measured the diameter of an especially intense compact radio source, Sagittarius A*, located within Sagittarius A–which coincides with a supermassive black hole at the very heart of our Galaxy. Accretion of gas onto our Milky Way’s resident supermassive black hole, probably involving a glaring disk circling it, would emit energy to power the radio source–which itself is considerably larger than the black hole. The black hole itself is too small to be observed with today’s instruments.
The 2008 study measured the diameter of Sagittarius A* to be 44 million kilometers. As a comparison, the radius of our planet’s orbit around the Sun is about 150 million kilometers, whereas the distance of Mercury from our Sun at closest approach is 46 million kilometers. Therefore, the diameter of the radio source is only slightly less than the distance of Mercury from our Star.
Astronomers at the Max Planck Institute for Extraterrestrial Physics in Germany have confirmed the existence of a supermassive black hole at the heart of our Galaxy, that is about 4.3 million times the mass of our Sun.
Our Galaxy’s Hidden Heart Reveals Its Ancient Secrets
In April 2016, the team of international astronomers announced that they had kinematically disintangled the primordial component of our Galaxy’s hidden heart for the very first time–separating its stellar population from the stars that currently dominate the mass of our Milky Way. Using the AAOmega spectrograph on the Anglo Australian Telescope near Siding Spring, Australia, the astronomers focused on a very ancient and well-known class of stars, termed RR Lyrae variables. RR Lyrae variables pulsate in brightness approximately once daily, which makes them more difficult to observe than their non-pulsating stellar siblings. However, RR Lyraes have the definite advantage of being standard candles. In astronomy, standard candles are astrophysical objects, such as variable stars like RR Lyrae, which possess a known luminosity as a result of some characteristic quality possessed by the entire class of objects. Therefore, if a distant celestial object can be identified as a standard candle, then the absolute magnitude (luminosity) of that object is known. By knowing the absolute magnitude, astronomers can calculate the distance from the apparent magnitude.
RR Lyrae stars enable astronomers to make precise distance estimates. These variable stars are found only in ancient stellar populations that are about 10 billion years old, and inhabit primordial halo globular clusters. Globular clusters are collections of stars that orbit around the core of a galaxy as satellites. Tightly bound together by gravity, they are pulled together by this force into spherical shapes, and have relatively high stellar densities toward their centers.
The velocities of hundreds of stars were simultaneously obtained by the astronomers, who observed them in the direction of the constellation Sagittarius over an area of the sky larger than the full Moon. As a result, the scientists were able to use the age of these stars to explore the environment in the central part of our Milky Way when it was first forming.
Our Galaxy hosts multiple generations of stars that span the time from its ancient formation to the present. Because heavy atomic elements are cooked up in the searing-hot hearts of stars, younger stellar generations become the recipients of the metals formed in the furnaces of previous generations of stars. In this way, successive stellar generations become increasingly rich in heavy metals that were formed in the hot hearts of older generations. All of the atomic elements heavier than helium, that are listed in the familiar Periodic Table, are called metals by astronomers. The Big Bang produced only the lightest of atomic elements–hydrogen, helium, and traces of lithium and beryllium (Big Bang nucleosynthesis). All of the atomic elements heavier than helium were manufactured in the seething-hot flames of the stars by way of the process of nuclear fusion–or, alternatively, in the fatal supernovae blasts of the more massive stars when they perished in these explosive stellar tantrums. In the intense and powerful fires raging in the hot hearts of the stars, increasingly heavier and heavier atomic elements are fused from lighter ones (stellar nucleosynthesis).
Therefore, the term metal, in the jargon of astronomers, carries a different meaning than the same term when it is used by chemists. Metallic bonds are absolutely impossible in the extremely hot cores of stars, and the very strongest of chemical bonds are only possible in the outermost layers of the coolest of sub-stellar objects, such as brown dwarfs. Brown dwarfs are stellar failures, and are not even stars in the strictest sense because, even though it is commonly thought that they are born in the same way as normal stars, they are much too small for their nuclear-fusing furnaces to ignite.
The metallicity of a star provides a precious tool for astronomers to use because its determination can reveal the star’s age. The oldest stars, designated Population III, were depleted of metals because there had been no previous generation of stars to produce them. Population I stars, like our Sun, represent the youngest stellar generation, and have been gifted with the metals that had been produced by earlier generations of stars. Population II stars are very ancient–but not as ancient as Population III stars. However, Population II stars are much older than Population I stars like our Sun, and they carry the metals manufactured in the hot hearts of Population III stars–but they do not contain the higher metal content of stars like our Sun. Nevertheless, Population II stars possess very low metallicities.