Core of French

1991/02/01 Arregi Bengoa, Jesus Iturria: Elhuyar aldizkaria

• Astronomia

In the night number we presented the French Way as a spiral galaxy. On this occasion we will try to make a more detailed description of it and present the problems it poses.

In the December issue we present the French Way as a spiral galaxy. Among the things discussed at the time, there was very little information about the galaxy's core. But perhaps this part is the most important and attractive of spiral galaxies. Therefore, on this occasion we will try to make a more detailed description of it and present the problems it poses.

The exact location of the core of the French Way was not achieved until it had the help of radio astronomy. As is known, interstellar dust absorbs optical radiation. Therefore, the depth of the field of observation of optical telescopes is relatively small in the plane of the disk of the galaxy, due to the abundance of dust in it. However, interstellar dust is transparent at both ends of the electromagnetic spectrum: on the one hand in the X-ray and gamma band and on the other in the infrared bands and radio waves. Therefore, what we know about the nucleus of our galaxy has been analyzing the radiation of these three areas of the spectrum. Among them, the emission of 21 cm of neutral hydrogen (emission of H I clouds) has been especially important.

Until the radio telescopes began to be used it was thought that the center of the French Way was around the constellation of Norma, but when maps of H I clouds were obtained, it was confirmed that the spiral arms were produced from a point that, at 28,000 light-years from our position, would be in the direction of the constellation of Sagittarius. Therefore, the area of the galaxy should be there. In this direction was found a powerful radio station that would be called the first Sagittarius A. Once the resolution of the radio telescopes was improved, it was confirmed that Sagittarius A was composed of different contiguous sources.

The nucleus of the galaxy, specifically, was identified with the Sagittarius source A-West, in the position of straight ascension 17 h 42 m 29 s, declination 28é 59’ 48” (1950, 0). These studies were confirmed by measurements made in the infrared band when special photographic plates were designed to collect these radiations. The IR16 infrared source thus discovered was completely adjusted to Sagittarius A-West.

In short, we can say that the nucleus of the galaxy is a 3-year light diameter sphere, composed of millions of stars (considering that the closest star to the Sun is 4.3 light-years), with interstellar dust and neutral hydrogen. In the area of this sphere we have a very compact radio source. Its diameter is 10 U.A. (the astronomical unit is about 150 million km away from the Sun to the Earth) and its mass is 5 million times greater than that of the Sun.

This striking image can extend to a region of 20,000 light years in diameter around the core. Here is the violent radio station Sagittarius B2. Its diameter is 100 light years and its mass is 3 million times greater than that of the Sun. It is also a thin rotating hydrogen disc. The speed of the edge gas is 250 km/s. It is also a hydrogen that expands out at 150 km/s.

What is the origin of the radiations collected in the different bands? As for radio waves, most of the radiation is synchrotron radiation. This is a special radiation emitted by the charged fractions (especially electrons) that rotate very quickly around the lines of a magenic field. In this emission zone, only the nucleus emits 1030 watts, that is, thousands of times more than the total emission of the Sun. Along with the synchrotron emission, emission lines such as H I clouds arrive.

Emission in the infrared zone is considered to be the thermal emission of interstellar dust. The nucleus stars would heat the surrounding dust and molecular clouds and these would emit wavelength radiation corresponding to their temperatures. The energy emitted by the nucleus of the galaxy in this field is about 3x1036 watts, that is, 100 million times the emission of the Sun.

Part of the emission in the X-ray and gamma band is synchrotron emission. Another part is due to the reverse dispersion of Compton. This occurs when one of the electrons that produces synchrotron radiation collides with a low energy photon, making it more energy and frequency. Part of the X-rays can also be a very hot cloud emission.

We can say that the processes that generate emissions are known, but we still have a basic problem that is not entirely known. What are the phenomena that can give rise to the enormous energies we have mentioned in such a small space within the dimensions of the nucleus? One or another hypothesis has been made, but they are all just speculation and there is still no successful theory that scientists have accepted.

EPHEMERIS SUN: March 21 enters Aries in 3 hours and 1 minutes (UT). Spring begins. LUNA Recent searches New moon Growing room Full moon • 8 hours 32 m 168 h 10m 23 hours 3 m 307h 17m PLANETS Mercury: Day two is in higher conjunction. So it is invisible, but at dusk in the last third of the month. The 27 passes through its maximum elongation. Venus: Its elongation is growing. So every time we can see it higher at dusk. Tuesday: It still looks good, but every time it hides before. At the end of the month it will be saved shortly after 1 in the morning (UT). Jupiter: It is visible almost all night before it darkens. On the margin of Venus, it is the brightest star in the sky between stars and planets. Saturn: We can see it in the morning, every time it will come out before. It leaves about five and a half in the morning at the beginning of the month and two hours before the end. Day 12 can be seen when it hides behind the Moon.