One of the most ambitious missions of the European Space Agency (ESA) is the one carried out by the Gaia spacecraft : its objective is to provide us with a three-dimensional map of our galaxy, the Milky Way . Successor to the Hipparcos mission (active from 1989 to 1993), since 2013 it has determined the positions, distance indicators and movements of a myriad of stars. The project is of such magnitude that it has already mapped almost 1.7 billion stars. The accuracy of the measurements is incredible: on the one hand, the precision in the position is 300 microseconds of arc, which means placing a star on a map with an error less than the thickness of a human hair seen from 30 kilometers away; on the other, Gaia measures the annual displacement of a star in the sky corresponding to less than the size of the head of a pin on the Moon as seen from Earth.
This space observatory, located at Lagrange point 2 (L 2), 1.5 million kilometers from our planet, sends us 40 gigabytes of data a day . The dizzying numbers illustrate the vastness of the Milky Way: those 1.7 billion stars represent just over 1% of the total population of stars in our galaxy.
Much has changed over time our perception of that milky band that crosses the summer sky. As the Persian astronomer Nasir al-Din al-Tusi (1201–1274) wrote, the Milky Way “is made up of a great number of small and variegated groups of stars that, due to their concentration and smallness, appear like fuzzy pieces of cloth. . That is why its color has been compared to that of milk ” . Since we looked at the sky, we have seen that beautiful creamy ribbon of light with deep black spots that crosses the celestial vault. For our great-great-grandparents, that plume of starlight – as the American astronomer Charles Whitney defined it in the last century – was the entire universe: a finite system of stars surrounded, perhaps, by a boundless void. In the past, it was hard to imagine that those other fuzzy points – some spiral-shaped – above and below the plane of the Milky Way were other, much more distant island universes. The change of perspective has been rapid and today we are talking about a huge cosmos full of billions of galaxies .
Since Galileo pointed his telescope at the Milky Way and discovered “a great multitude of stars that appear immediately before the view”, the universe has gone from perfect to being in constant flux. In 1750, a very religious English astronomer named Thomas Wright proposed that the cosmos was organized in a series of concentric circles with the Sun on one side – and not in the center – of one of them. Fast forward two hundred years to his time, he explained that the milky band of the Milky Way could be an optical effect, because we see the disc of the universe on edge.
But it was the German William Herschel , a musician and later a self-taught astronomer, who first tried to determine its structure observationally. In the last quarter of the 18th century, already settled in the English city of Bath and with the help of his devoted sister Caroline, he spent many hours behind the eyepiece of his homemade telescope – more than 6 meters long and with a mirror of almost half a meter in diameter – doing what the Gaia satellite has done: meticulously counting the stars visible in more than six hundred regions of the sky. Knowing that the weaker the light from a star the farther it is, he deduced that the Milky Way was shaped like a convex lens, like a lentil. Herschel was unaware that there are clouds of gas and dust that hide the galactic center from us, and so he thought that the Sun was near the center of that disk. He even deduced the size of this bob: 850 times greater than the distance that separates us from the brightest star in the northern hemisphere, Sirius. This means an extension of about 7,400 light-years from one end to the other, less than a tenth of the size we now estimate that the Milky Way is.
We had to wait for the American Harlow Shapley (1885-1972), the son of a peasant couple from the Midwest, to dethrone the Sun from its regal position at the center of the known universe and banish it to a more rural location, like that one. from which he came. He did this by determining the position of a special type of star cluster, globular clusters, tightly packed aggregates of stars that look like fuzzy snowballs through the telescope. More than a hundred of them are distributed above and below the plane of the galaxy, and they are around a distant point located in the constellation of Sagittarius. The young astronomer ventured that this point must be the center of the Milky Way.
His veteran colleagues were not comfortable with Shapley’s conclusions, but irrefutable evidence soon appeared: in 1927, the Dutchman Jan Oort showed that the stars of the galaxy revolved around a point in the direction of the constellation Sagittarius, the same one around which the globular clusters of Shapley were distributed. We had to accept the subterranean position of our star, about 26,000 light years from the galactic center, which takes between 225 and 250 million years to orbit completely, moving with respect to it at a speed of about 251 km / s. Since the dinosaurs appeared, the Sun has made one turn, and it is estimated that it has made between twenty and twenty-five since it formed.
The size of our galaxy has been adjusting as we have obtained more precise measurements. Today we accept that the more than 100 billion stars it contains occupy a region of space some 100,000 light-years across (950,000 trillion kilometers) and a thousand light-years thick. If we were to send one of NASA’s ancient space shuttles across the galaxy, it would take 4 billion years to do so. Or put another way, if the Earth were the size of a microbe, the Milky Way would extend up to 9,500 kilometers, more or less the distance from Madrid to Lima. And taking into account that the oldest stars in the galaxy are about 13.4 billion years old, we can accept that our cosmic megalopolis is about 13.6 billion years old. But not all the stars have been there forever; births occur all the time. Of course, age takes its toll: if in youth there were true bursts of star formation, today the galaxy only gives birth to just over seven a year.
If we could look at the Milky Way from above, we would find that it looks like a spiral, like 77% of known galaxies. Now, the number of arms it possesses is still debated. In the 50s of the last century it was believed that it had four main ones, but observations made from the 80s reduced them to two, that of Shield-Centaur and that of Perseus (their names allude to the constellations through which they cross), a model that was confirmed in 2013 thanks to NASA’s Spitzer infrared space telescope. But at the end of 2014, data obtained by another infrared space telescope from the US agency, the Wide-Field Infrared Survey Explorer (WISE), put the four-armed model back into play.
Apparently, the galaxy has the two main arms mentioned above, with a higher density of old stars; and two other minors, Sagittarius and Norma, with a predominance of young stars. Why? We do not know. Our sun is in a small appendage known as the arm or spur of Orion, located between that of Sagittarius and that of Perseus. But make no mistake: the geography of the Milky Way is not yet well established, and that is why missions like Gaia’s are necessary . The only thing that seems clear is that, no matter how many arms it has, we live in a barred spiral galaxy, since a bar of matter 27,000 light-years long crosses its center . Only 30% of spiral galaxies have it.
Why does it have this peculiar structure? The best explanation is the density wave model, proposed in the 1960s by Swedish astronomer Bertil Lindblad and developed by his Chinese-born American colleagues Chia Chiao Lin and Frank Shu. Until then, astronomers believed that the spiral arms of galaxies were material, that is, made up of stars and gas that were always there. This hypothesis had a problem: due to the differential rotation that the galaxy presents (that is, the stars closest to the center move faster), the arms would end up disappearing after a few orbits.
To solve this problem, Lindblad, Lin and Shu suggested that the arms were not fixed concentrations of matter, but the product of density waves that rotate around the galactic center at a slower speed than stars and gas clouds. In essence they work like a traffic jam on a highway: the cars (the stars) travel at 120 km / h, but when they reach the traffic jam they slow down and adapt to that of the traffic jam (density wave), which causes it to increase the number of vehicles in a given space (the stellar density). Once the agglomeration is over, the cars (stars) regain their previous rhythm.
But not only the galactic disk challenges the explanatory capacity of astrophysicists: its center is even more mysterious . If, on a clear and moonless summer night, we look to the east, where the Milky Way cuts the horizon, we will be contemplating the heart of it. Located in the constellation Sagittarius, it is hidden behind an impenetrable fog of interstellar gas and dust. Such clouds, several light years in size, are a kind of cosmic alembic that distills complex molecules that we never dreamed could be found in cold space. Thanks to chemical reactions that have taken place for millions of years, radio astronomers have found not only the ammonia we have under the sink, but also the acetylene from solders, the antiseptic formaldehyde, enough alcohol to fill more than a thousand quadrillion bottles of whiskey , sugar (more specifically glycolaldehyde) … and so on up to 120 types of molecules.
If we get closer to the galactic core, we will see frenzied activity. More stars are born there than in the suburb where we live, and the most massive ones expel their outer layers in the form of fearsome winds. Meanwhile, supersonic shock waves, product of the gigantic explosion with which stars of mass several times higher than that of the Sun die, are heard throughout the region. It’s a sight no one expected in the 1970s, when Bart J. Bok, the Dutch-American astronomer specializing in the Milky Way, who liked to describe himself as “the galaxy’s night watchman,” was pretty sure of everything. it happened in her. It was the arrival of radio astronomy and infrared astronomy that showed us this new vision of our bustling galactic center.
One of the research centers that has contributed to lifting the veil that hides its mysteries is 80 kilometers west of the city of Socorro, in New Mexico. After crossing a desolate landscape and after climbing a slope of Highway 60, we will find there a plantation of antennas of 25 meters in diameter each, arranged on the old bed of a lake. It is the Karl G. Jansky Very Large Array (VLA) radio astronomical observatory, and its 27 dishes move in unison following the instructions of the Chief, the powerful computer in charge of their orientation.
These radiogrids only pick up two emitters: that of hot gas from which the radiation from one of the numerous star swarms in the central region of the Milky Way has removed part of its electrons; and that of the electrons themselves, which move at high speeds due to the magnetic field existing there, much more intense than that of the rest of the galaxy, but 500 times weaker than that of the Earth. They are not overly melodious stations: they sound like steam coming from a radiator.
Near the galactic center, gamma radiation – which we can create in nuclear reactors – floods space with an energy 250,000 times greater than that of visible light. It comes from the annihilation of an electron with its antimatter twin, the positron, at an inconceivable rate: ten billion tons of antimatter are consumed every second. Such a monster has been dubbed the Great Annihilator , although its catalog name is far less poetic, 1E 1740.7-2942. It is likely that this antimatter factory is a black hole that, hidden behind a huge cloud of gas, emits two geysers of matter five light years long in opposite directions. But the Great Annihilator is not at the core of the galaxy, but very close: 350 light years away.
Moving through the center of our cosmic city, a 400 by 900 light-year space called Sagittarius A , is like moving through a megalopolis. It is densely populated with stars and gas clouds moving at three and a half million kilometers per hour. In addition to carrying that devilish speed, they have a temperature of 10 million degrees Celsius, which causes the gas to emit a large amount of X-rays. Thanks to NASA’s Chandra X-ray observatory, launched into space in 1999 and still active, Almost a thousand X-ray sources have been identified in Sagittarius A, mostly objects such as white dwarfs, neutron stars, black holes, and very hot gas clouds .
Surrounding them are also distinguished, like galactic cava bubbles, the remnants of supernovae that exploded long ago, and two zones of star formation, Sagittarius B1 and B2. And if we look away a little we will discover IRS 16, a group of about twelve stars that illuminate the surroundings with their bluish light and that constitute a true mystery: they are young, between 3 and 7 million years old, and they are crammed into a space of one or two light years. At first glance they look like stars, but they are too big and too bright. What are they? It is not known. Another enigma is strange gas clouds that somehow resist the intense gravity of the galactic core. The story begins in 2012, with what appeared to be a cloud of gas baptized with the bland name of G2 and approximately three times the mass of Earth. When it passed just over five times the distance from the Sun to Pluto from the black hole at the center of the galaxy, astronomers expected it to shatter. To his surprise, the mysterious object survived. And to add fuel to the fire, they later realized that another similar cloud, G1, discovered in 2004, had passed even closer to the black hole and went on like nothing.
Astronomers from the University of California and the Andalusian Institute of Astrophysics have identified three other objects of this type thanks to the WM Keck observatory in Hawaii: they have been called G3, G4 and G5. The hypothesis they are now considering is that they are not clouds, but stars surrounded by a layer of gas : they may have swelled when they were near the black hole, but since their cores are massive enough, gravity did not break them. Others suggest that they appeared after a fusion of stars … How they got there and what they will become is a mystery to answer.
Buried in Sagittarius A we find the true nucleus of the galaxy, Sagittarius A *, a very powerful source of radio waves that corresponds to a super black hole with a mass four million times greater than that of the Sun, compressed into a smaller volume than that of the Earth’s orbit to the star king. If the X marks the place where the treasure is buried, the super black holes indicate where the center of the galaxies is located. Sometimes Sagittarius A * goes off the hook with X-ray flashes like those detected on January 5, 2015, four hundred times brighter than usual; or with those registered in 2019 in the infrared range. It is not known what caused them: some astronomers think that those of 2015 could be due to the fall towards the black hole of an asteroid, others that they were due to a coupling of the magnetic field lines with the jets of gas that are directed towards it. In the case of 2019, things are even weirder: Some point out that it could have been a late black hole reaction to G2’s approach a few years earlier.
From this center also sprout, as in a fountain, two jets of matter up and down the galactic plane, which cool down as they fall towards the disk, in the outer neighborhoods of the Milky Way. This shower of matter influences what will happen to the galaxy: as in the civilizations of yesteryear, the capital of the empire decides the future of the rest.
If these are the mysteries that lurk in the core, the outskirts are not short: there we find two filaments of stars and gas. The best known is the Sagittarius current, which circles the Milky Way in polar orbit. But the most fascinating thing is its origin: it is supposed to be a product of cannibalism. Beautifully shaped, large and symmetrical, our galaxy looks like a finished product, but in recent years we have discovered that it is not. Keep collecting cosmic material: gobble up tiny, faint galaxies that venture nearby and risk falling into your gravitational well. In fact, one of the most fashionable theories is that galaxies like the Milky Way grew thanks to a slow but continuous work of collecting smaller ones. The process remains active, but at a slower pace. A dramatic example is the small spheroidal dwarf galaxy Sagittarius, which is falling on the disk of ours, fortunately on the other side of where we are. It was discovered by accident in 1994, and its mass is 1% that of the Milky Way. Today we know that its gravity stretches it like a pastry chef does with his dough. In fact, astronomers from the Instituto de Astrofísica de Canarias discovered some of the debris of the phenomenon a few years ago, just over 18,000 light years from the center of the Milky Way.
As if this feast were not enough, Omega Centauri, the largest and brightest globular cluster in the galaxy that welcomes us, is not what it appears to be , a clustered set of some 10 million old stars that formed at the same time as the Milky Way, but what remains of the heart of a dwarf galaxy that we devoured long ago. Similarly, the Canis Major dwarf galaxy, which contains one billion stars and is located some 25,000 light years distant from the Solar System and 42,000 from the galactic center, is believed to be part of this lengthy free buffet with the that the Milky Way is growing and getting fatter. Will this intergalactic feast ever end? No, at least for the next 4.5 billion years. At that time, the Great Andromeda Galaxy, M31, and ours will merge in a long and deep cosmic embrace.
