Since the XVII century, the most important goal of astronomers has been the study of the Milky Way – this giant collection of stars that Galileo saw through his telescope. The efforts of many generations of observer astronomers were aimed at finding out what is the total number of stars of the Milky Way, determining its actual shape and boundaries, and estimating its size. It was only in the 19th century that it was possible to understand that this is a single system that contains all the visible stars. On an equal footing with all, our Sun enters into this system, and with it the Earth and planets. Moreover, they are far from being located in its center, but on its outskirts.
It took many decades of careful observation and deep thought before the structure of the Galaxy was revealed to astronomers in its entirety. So they began to call the star system that we see – of course, from the inside – like a strip of the Milky Way. (The word “galaxy” is derived from the Greek “galacticos”, which means “milky.”)
It turned out that the Galaxy has a fairly regular structure and shape, despite the apparent raggedness of the Milky Way, the disorder with which, it seems to us, the stars are scattered across the sky. It consists of a disk, a halo and a crown. As can be seen from the schematic drawing, the disk is like two plates folded by the edges. It is formed by stars that, inside this volume, move in almost circular orbits around the center of the galaxy.
The diameter of the disk is measured – it is approximately 100 thousand light years. This means that it will take light one hundred thousand years to cross the disk from end to end in diameter. That’s how huge the Galaxy is! And the number of stars in the disk is about one hundred billion.
The halo contains a comparable number of stars. (The word “halo” means “round.”) They fill a slightly flattened spherical volume and move not in circular but in strongly elongated orbits. The planes of these orbits pass through the center of the galaxy. In different directions they are distributed more or less evenly.
The disk and the halo surrounding it are immersed in the crown. If the radii of the disk and the halo are comparable in size, then the radius of the corona is five, and maybe ten times more. Why maybe”? Yes, because it is invisible – no light comes from it. How did astronomers then learn about it?
All bodies in nature create gravitation and experience its action. This is evidenced by the Law of universal gravitation, discovered by Newton. So they found out about the crown not from the light of the world, but from the gravity it creates. It acts on visible stars, on gas clouds emitting light. Observing the movement of these bodies, astronomers noticed: in addition to the disk and the halo, something else acts on them.
A detailed study of this “something” and finally allowed to discover the crown, which creates additional gravity. It turned out to be very massive – several times more than the mass of all the stars included in the disk and halo.
Such is the information received by the Soviet astronomer J. Einasto and his collaborators at the Tartu Observatory.
Of course, studying the invisible crown is very difficult. Because of this, estimates of its size and mass are not very accurate yet. But her main mystery lies elsewhere: we do not know what it consists of. We do not know if there are stars in it, even if they are some unusual ones, which do not emit light at all.
Now many people assume that its mass does not consist of stars at all, but of the smallest elementary particles – neutrinos. Physicists have known these particles for a long time, but by themselves they also remain largely mysterious. It is not known about them that one can say the most important thing: do they have a rest mass, that is, such a mass that a particle possesses in a state when it does not move, but stands still. Most elementary particles have such a mass.
This, for example, is an electron, proton, neutron, of which all atoms are composed. But the photon, the quantum of light, does not have it. Photons exist only in motion. Neutrinos could serve as material for the crown, but only if they have a mass of peace.
It is easy to imagine how eagerly astronomers expect news from physical laboratories, where special experiments are now being conducted to find out if the neutrino has a rest mass or not. Perhaps it is physicists who will solve the riddle of the invisible crown.