One of the European Space Agency’s most ambitious space missions is Gaia , which aims to provide us with a three-dimensional map of our Galaxy, the Milky Way. Since 2013 it has been dedicated to determining the positions, distance indicators and movements of more than a billion stars. The mission is of such depth that since it began there are about 400 million stars that we know are out there for the first time. 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 width of a human hair seen from 30 km away. ; on the other, Gaia is able to measure the annual displacement of a star in the sky that corresponds to less than the size of a pinhead on the Moon as seen from Earth.
During these years this space observatory has made hundreds of billions of stellar measurements, sending 40 gigabytes of data to Earth every day. The numbers can make us dizzy, but they tell us about the immensity of the Milky Way: those billion stars represent only 1% of the total population of the Galaxy .
History of the Milky Way
Much has changed over time in 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 which, due to their concentration and smallness, appear like diffuse pieces of cloth. . That is why its color has been compared to milk .” For centuries the Milky Way has been a beautiful fixture in the sky, a creamy ribbon of light flecked with deep black that crisscrossed the vault of heaven. For our great-great-grandparents, that “starlight plume” – as astronomer Charles Whitney defined it – was the entire universe: a finite system of stars surrounded, perhaps, by a limitless void. In the 19th century it was hard to imagine that those other fuzzy points , some spiral-shaped, that could be observed above and below the plane of the Milky Way were other much more distant “island-universes” . The change in perspective has been rapid and today we easily speak of a huge cosmos filled with billions of galaxies.
Ever since Galileo pointed his telescope at the Milky Way discovering “a great multitude of stars that present themselves immediately to view”, the universe has gone from being perfect to being in continuous change and evolution. In 1750 a very religious Englishman named Thomas Wright proposed that the universe was organized in a series of concentric circles with the Sun on one side , not in the center, of one of them. Fast-forwarding 200 years, he explained that the Milky Way’s milky band could be an optical effect because we are looking at the disk of the universe edge-on.
But it was the 18th-century prince of astronomy, the self-taught William Herschel , who was the first to attempt to determine its structure observationally. 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 almost half a meter in diameter – doing what the Gaia satellite has done: meticulously counting the visible stars. in more than 600 regions of the sky. Knowing that the fainter the light from a star, the farther away it is, he deduced that the Milky Way was shaped like a convex lens, like a lentil . However, Herschel did not know that there are clouds of gas and dust that hide the center from us, and that is why he thought that the Sun was near the center of that disk . He even deduced the size of this lentil: 850 times greater than the distance that separates us from the brightest star in the northern hemisphere, Sirius. This means a size of 7,400 light-years, just under a tenth of the size of the Milky Way we now estimate.
We had to wait for the appearance of Howard Shapley , the son of a peasant couple from the American 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 the one Shapley came from. He did this by determining the position of a special type of star groupings, globular clusters , tight, round aggregates of stars that look through the telescope like fluffy snowballs. More than a hundred of them are scattered above and below the plane of the galaxy, and are positioned around a distant point in the constellation of Sagittarius. The young astronomer guessed that this point must be the center of the Milky Way. Astronomers were obviously not very comfortable with Shapley’s conclusions , but eventually they were forced to accept the outlying position of our Sun, about 30,000 light-years from the center, which is moving about 40 km /s and it takes 240 million years to make a complete revolution. This means that since the time the dinosaurs ruled the Earth, the Sun has only completed between 18 and 20 revolutions. On this time scale our star has just come of age, 18.4 galactic years, and the universe, 61 .
size issue
The size of our Galaxy has been adjusting as we have obtained more precise measurements. Today we accept that the more than 100,000 million stars it contains occupy a region of space about 100,000 light-years in diameter (950,000 billion km) and 1,000 light-years thick. If we sent the ancient space shuttle 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 km, more or less the distance from Madrid to Lima . And taking into account that the oldest discovered stars in the Galaxy are about 13.4 billion years old, we can accept that our cosmic city has lived for 13.6 billion years. But not all stars have been there forever, new 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 7 stars per year.
Galactic arms in discussion
If we could see the Galaxy from above, we would discover that it is a spiral, as is 77% of the galaxies in the universe. Now, the number of arms it has is something that is still under discussion . In the 1950s it was believed that it had 4 arms, but observations made from the 1980s reduced them to 2, that of Shield-Centaur and that of Perseus (their names are referred to the constellations through which they cross), a model that was confirmed in 2013 thanks to observations made by the Spitzer Infrared Space Telescope. But a year later, at the end of 2014, data obtained by the Wide-Field Infrared Survey Explorer (WISE) also put the four-arm model into play . Apparently, our galaxy has the two aforementioned arms made of old stars and two others, the Sagittarius and the Norma-Swan, with young stars. Why this is so is something that still has no explanation. Our Sun is located in a small appendage known as the Orion arm , situated between the Sagittarius arm and the Perseus arm. But make no mistake, the geography of the Milky Way is not something that is well established. The only thing that seems to be clear is that, regardless of the number of arms it has, we live in a barred spiral galaxy, since a “bar” of matter crosses its center with a length of 27,000 light-years. Only 30% of spiral galaxies have it.
Why does it have this peculiar spiral structure? The best explanation we have is the density wave model, proposed in the 1960s by Swedish Bertil Lindblad and developed by Asian-Americans 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. However, this hypothesis had a serious problem: due to the differential rotation that the galaxy presents (that is, the stars closer to the center move faster) the arms would end up disappearing after a few orbits .
To solve this Lindblad problem, Lin and Shu proposed that the arms were not fixed concentrations of matter, but the product of density waves that revolve 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 (stars) move at 120 km/h but when they reach it, they reduce their speed adapting to that of the traffic jam (density wave), which increases the number of cars (the density of stars). Once passed, the cars (stars) return to their previous speed.
References
Croswell, K. (1996) The Alchemy of the Heavens, Anchor
Overbye, D. (1992) Lonely Hearts in the Cosmos, Salvat
