Spiral Galaxy Arms

1991/05/01 Arregi Bengoa, Jesus Iturria: Elhuyar aldizkaria

• Astronomia

In the last two articles we have used two themes that can be considered small details of the structure of spiral galaxies. For the same reason they can be unknown details. This time we talk about the same spiral structure. Although the problems of the creation of spiral arms and the subsequent dynamics have not yet been solved, in the last two years there have been news in both the theoretical and experimental fields, to which we will refer.

B. Previous year Elmergreen and his partner, through sophisticated computer programs, have studied the intensified photo of the galaxy M81 to get an image of the structure we would see if the galaxy were in front. In addition to the M81, they have also processed photographs of the M51 and M100 galaxies. As will be seen later, the results obtained with this method confirm the density wave theory to explain the stability of the spiral arms. The latest version of this theory was published in 1989, C. C. Lin, G. Bertin, S. A. Lowe and R. P. In an article signed by Thurstans. But before we talk about these latest advances, we will make a brief analysis of the genesis and essence of the theory.

The aim of the theory is to explain the shape and stability of the spiral arms, since based on the movement of the galaxy they cannot exist as we know them. The spinning motion on its axis of the galaxy is different from that of the rigid solid. Angular velocity is not the same for all points. Except in the vicinity of the core, the angular turning speed decreases as the radius increases. This movement is called differential rotation. Consequently, if at one point a radius of luminous matter was generated, in 200 or 300 million years the outer matter would turn around, but around the nucleus two or three. Therefore, at that time the structure of a typical spiral galaxy would be constituted. If we take into account the age of galaxies, at that time a spiral of more than ten turns would form. This problem is called a spiral problem.

C. C. Lin and F. H. In were those who developed the first formulation of density wave theory (more than 25 years ago, B. Based on an idea by Linblad). According to this theory, spiral arms are sets of high-density mates that slowly move around the core. This does not mean that the matter generated by this high density is always the same. Stars and clouds of dust and gas fill their orbits around the core with the corresponding speed. When they are on this path with an arm, they suffer a braking and cause an increase in the concentration of matter.

After crossing the arm leave the front to accelerate again and follow the journey. Those who come from behind take the place of the stars and clouds that have passed by. To understand this process is usually compared with the jam generated by a truck on the road. The truck and the row of cars that chase him go slowly, but the cars that form that row are not the same. The strikers pass the truck and go ahead and behind arrive more cars that form the row.

In addition to braking, clouds of gas and dust suffer a strong compression when encountering the arm. This compression favours the formation process of the stars. If the core is abandoned, all stars form in arms. Among them, the spectral types 0 and B are the ones that show the shape of the arms, since thanks to their large mass they are the most luminous. The compression generated by the mass and the high interior temperatures make thermonuclear reactions more effective, generating high temperatures and luminosity (but also with a rapid fuel exhaustion).

Consequently, we have short-lived stars that do not have time to cross the entire arm before turning them off. Smaller mass stars have a longer life and have time to rotate around the galaxy. The Sun, for example, was born in a stellar cluster some 4.5 billion years ago, when a cloud of gas entered an arm. Since then the Sun has escaped from that cluster and has revolved around the nucleus of the galaxy about 200 laps across each arm of the galaxy so many times. Undoubtedly, the effect of the stars when crossing the arms is much less than that of the clouds.

The relationship between arms and stars described above was fully confirmed in the 1970s. It was then that they discovered, through radio telescopes, the first giant molecular clouds that were creating stars in the arms of our galaxy. These clouds are mostly formed by molecular hydrogen, but there are also small amounts of other molecules such as CO and H ₂ O. Molecular hydrogen does not emit radiation, but CO does excite. Excitation occurs by collisions with H2 molecules. Therefore, from the point of view of CO distribution, these enormous molecular clouds can be studied. A cloud that we could take as a model could have a ^{mass 10 6} times that of the Sun and would probably form when the gas meets the arm. 30% of the mass is in the process of star formation.

The theoretical improvement proposed by the aforementioned authors predicts two density waves that could interfere. The theoretical model, based on these two waves and elaborated by computers, provides a spiral structure that would last many laps for the galaxy. B. In the photos processed by Elmergreen you can see some holes or cuts in the arms of galaxies. These discontinuities would result from the interference of the top of one wave and the valley of the other.

In these places there would be no increase in density, and its star and matter would remain unhindered leaving a gap in the travel arm. It seems that the sessions of Elmergreen C. C. They confirm Lin's theory. Another interesting detail is that the photos show small bags of stars that come out or create by the arms. These would be sets of stars deviated by wave resonance and their study would allow astronomers to calculate the velocities of both density waves.

However, there is a fundamental problem that remains unanswered: how did those waves of density arise?

EPHEMERIS SUN : June 21 21 hours in 18 minutes enters Cancer: SUMMER begins. LUNA Below PLANETS MERCURY: is in superior conjunction on June 17, making it invisible. At the beginning of the month in the morning and at the end in the evening it is not easy to see because its elongation is low. VENUS: reaches its maximum elongation (45\n) on day 13. So we'll see it very well when we get dark. MARTITZ: early in the month elongation slightly higher than that of Venus, but lower for the end. On the 23rd, specifically, it will be 0.3 <unk> south of Venus. However, its magnitude is only 1.7. JUPITER: will also revolve around the other two. Day 17 is 1.2> south of Venus. Its magnitude is -1.9. SATURN: At the beginning of the month we have to wait an hour after it darkens to leave, but at the end of the month it appears almost at dusk to the East.