Secrets of the Milky Way
Our Galaxy jealously guards its secrets. And if it lets you discover any, it’s very slowly.
We live on a huge island of glowing matter surrounded by darkness, which only a hundred years ago was the entire universe for us. That it is only one of many similar objects separated by millions of light years of vacuum, we learned only in 1923. This groundbreaking discovery, second only to the Copernican Revolution, was made by Edwin Hubble, who worked at the Mount Wilson Observatory near Los Angeles.
In the part of the universe under modern research there are several hundred billion similar islands, called galaxies in astronomy. The one we live in is referred to as “our Galaxy” or simply “the Galaxy”, traditionally written with a capital letter. There is also the confusing term “Milky Way,” the proper meaning of which must be read in the context of a sentence.
This term comes from the myth about Herakles, the offspring of Zeus and the mortal earthling Alcumene. According to one of its versions, the ruler of Olympus decided to gain immortality for his son, and in order to achieve this, he put him to the breast of his sleep-stricken wife Hera. Herakles suckled so violently that the goddess awoke and pulled his breast out of his mouth. The milk (γάλα) gushed out into the sky and spilled into a stream called in ancient Greece the Milky Circle (Γαλαξίας Κύκλος). The Romans assimilated this name under the form Via Lactea (Milky Way), which became established in most European languages.
A Mistake Under the Cloud
From the outside, our Galaxy looks like a thin disk with a diameter of 100,000 and a thickness of just 1,000 light-years. We are looking at it from the inside, so when we look along the disk’s radius, we see many more stars than if we were looking along its axis. There are so many that the eye literally gets bogged down in the stellar myriad that merges into the pale silver belt of the Milky Way proper.
In ancient times it was visible from every place on Earth. Today — it is drowning in the increasingly bright lights of our civilization and hiding behind the increasingly dense smog. If you want to see it, go on a moonless night to a place where the sky is still really dark and the air is clear. It is also worth taking even a small pair of binoculars with you — in its eyepiece the Milky Way will turn into a swarm of faint stars, which were first seen 400 years ago by Galileo.
In December, between 6 and 8 p.m., this stellar stream rises from the horizon in the east, reaches the zenith (the point directly above the observer’s head) and disappears below the horizon in the west. Two bright stars shine against its background at nearly equal distances from the zenith: the yellowish Capella on the eastern side and the white-blue Deneb on the western side. Deneb is a particularly important landmark because it points in the direction in which we are speeding along with the whole Solar System, covering 230 km every second. At a speed that would make a trip from the Earth to the Moon take less than half an hour, we orbit the center of the Galaxy, which is 25,000 light-years away. It will take us 250 million years to complete the orbit begun at the turn of the Permian and Triassic Periods, when the Earth was shaken by a cataclysm of unknown nature, the cause of the largest extinction of species in the history of our planet.
All the stars of the Milky Way, estimated by astronomers to number two hundred billion, follow sun-like orbits. The latest map of this swarm was created a few months ago based on data from the European satellite Gaia. Each pixel of the map shows the brightness and color of the collective starlight detected in the corresponding small patch of sky. The dark spots and streaks are relatively close and very dense clouds of interstellar matter that completely obscure the stars behind them. The rarer clouds filter the light, letting mostly red through, making those distant stars appear redder than those close by.
Stars located in a large range of distances from the center of the galactic disk have almost equal orbital speeds. They move as fast as the Sun. In our planetary system we observe a completely different regularity, according to which the further a planet orbits from the Sun the slower it moves — Mercury reaches a speed of almost 50 km/s, while Neptune, the farthest from the Sun, only 5.5 km/s. The reason for this difference is the different shapes of the gravitational fields that govern the motions of stars and planets. The Solar System field has such a simple structure that it is very easy to “weigh”, and more precisely to determine the mass of its dominant source, the Sun: it is enough to measure the speed and the radius of the orbit of one of the planets. Similarly — by studying the motions and distribution of stars, it is possible to determine the mass of the sources of the Galactic field, although such a task is incomparably more complicated.
The constancy of the orbital velocity in the Galactic disk suggests that there is no gravitationally dominant counterpart to the Sun at the center of the Milky Way. There is indeed a giant black hole by our standards, but its mass is only one hundred-thousandth that of the galactic disk. The stars therefore appear to be orbiting in a field that they produce themselves, which means that their combined mass should be equal to the mass of the gravitational sources.
Nothing to see
To an astrophysicist, a star is a simple fusion reactor whose mass can be immediately given by knowing how much power it develops, that is, how much energy it emits per second. When the appropriate observations and calculations were made, a disturbing deficit was revealed. It was found that the “luminous” mass contained in stars is several times smaller than the “gravitational” mass producing the field, and what’s more — the latter is distributed differently. Similar disproportions have also been observed in other galaxies. In their study distinguished American astronomer Vera Rubin, who almost half a century ago wrote in the prestigious weekly magazine “Science”:
“A growing body of data supports the idea that we cannot even see 90 percent of the mass of the universe. There is dark matter concentrated around galaxies, the composition of which we have no idea about. This can range from light elementary particles to massive black holes.”
Its discovery was greeted with incredulity (“The great astronomers thought it was irrelevant,” Rubin recalled), but as time passed, other evidence of dark matter emerged, eventually gaining status as a legitimate component of the universe. Unfortunately, its nature remains a great mystery to modern science, although of the numerous hypotheses about it only one survives: that it is unknown elementary particles that interact with ordinary matter only through gravity. Astronomers estimate that the Galaxy’s invisible envelope of particles is an almost spherical cloud with a diameter of at least six hundred thousand light-years.
Viewed from above, the Galaxy’s disk looks like a giant cyclone with clearly visible spiral arms. The spiral pattern rotates in such a way that over a large range of distances from the center, its speed is less than the orbital speed of the stars. The stellar swarm thus “flows” through the arms, at the same time as it thickens and, leaving them, disperses again. An analogous situation can be observed on a multilane road, when one of the lanes is blocked by a slowly moving tractor. The river of cars then becomes a congestion that moves slower than the cars themselves, with its contents constantly being replaced. In the Galaxy, the role of the moving obstacle is played by a gravitational field disturbance, probably generated by three small galaxies in close proximity to the Milky Way (these are the Large and Small Magellanic Clouds and the galaxy in Sagittarius). According to another hypothesis, the generator of the disturbance is what astronomers call the “crossbar”, a dense cluster of spindle-shaped stars whose center coincides with the center of the Galaxy. Its existence was only realised in the early 1990s. The 20,000 light-year-long bar spins in the disk like a blender blade, at a rate of one rotation per 100 million years.
Disturbances in a blender
In addition to stars, interstellar matter flows into spiral arms and is compressed so strongly that its clouds collapse under their own gravity and turn into stars with planetary systems. Among nascent stars, most are objects with masses no greater than that of the Sun. High-mass stars are few in number; however, they shine so brightly that they are the ones that give the galactic spiral its unusually grand appearance. By observing them, it has been found that the Sun is about halfway between the arms bearing the names of Perseus and Sagittarius. Their outlines can be clearly seen in the three-dimensional map of our galactic environment that the aforementioned OGLE team published in Science.
It takes a few hundred thousand to a few million years before the cloud condensed in the arm turns into stars. During this time, chemical synthesis proceeds in its calm and shielded from harmful factors. Observing such clouds, a hundred and several dozen different compounds have been identified, including glycolaldehyde (the simplest sugar), formamide (a compound involved in possible reactions in interstellar space whose products are nucleic bases and amino acids), and glycine (the simplest amino acid; its identification, however, needs to be confirmed). In the swirling around young stars and much more densely compacted protoplanetary disks, even more complex molecules are formed. This is evidenced by the meteorites formed early in the history of the Solar System, in which tens of thousands of organic compounds have been found, including dozens of amino acids (including those not found in the Earth’s biosphere) and adenine, guanine and uracil — three of the five “letters” of our genetic code.
However, the idyllic image of the Galaxy swirling peacefully in its invisible envelope is far from the truth. From time to time, the central black hole engulfs the clouds and stars that approach it, and a massive explosion occurs. The last such cataclysmic event took place a few million years ago, and its traces are two nuclear mushroom-like growths above and below the galactic disk that have already reached a size of 25,000 light years. The disk itself is marked by numerous scars from collisions between the Galaxy and its small neighbors. A collision with a galaxy in Sagittarius (completely harmless for us) is going on before our eyes.
In two billion years the Galaxy will collide with the Large Magellanic Cloud and in four to five billion years with the Andromeda Galaxy. However, these prospects are so distant that they are not worth bothering with when admiring the Milky Way.
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