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The sun, our star – Part 1

27 Jan

“When pain brings you down, don’t be silly, don’t close your eyes and cry, you just might be in the best position to see the sun shine.”  – Alanis Morisetti

“If I had to choose a religion, the sun as the universal giver of life would be my god.”  
―   Napoleon Bonaparte

Without our star, the Sun, we were not here on this planet, or any form of life we know, it is common knowledge. Its importance is clear in the myths of the sun and of the cycles of nature. Osiris, Adonis, Dionysus, Mithras, Buda, Bachus, Jesus Christ, they all were born on the 25th December, since it is …and they all die around Easter, when nature reborn from the cold winter (in north hemisphere).

Below it is shown the decorative theme in the tomb of Ramesses IX showing the king’s adoration of the sun disk, accompanied by Isis and Nephthys on the lintel over the entrance.

PLASMA: THE FOURTH STATE OF MATTER

To understand our sun we need to recall what is a plasma. The plasma is considered to be the fourth state of matter, after the gas, liquid and solid states. At ambient pressure and temperature, a gas is a good electrical insulator. For example, if we connect two electrodes separated by a few millimeters to an electric generator (let us say, feeding the circuit with 220 Volts), apparently there is no net current crossing the space between them. However, if you do the same experiment, with the electrodes immersed in a gas at a lower pressure, you will notice na electric current across the gas, and at the same time, the gas start to glow with a color that depends on the spectral composition of the gas and the current value and gas pressure (VIDEO 1). The current is mainly transported by free electrons that succeed to cross the gas from one electrode (cathode) to the other (anode) by taking energy from the electric field acting between the two electrodes. We say that the gas become a plasma, a phenomena that currently can be seen almost everywhere (VIDEO 2).

Another type of plasma discharge occurs around high-tension distribution lines that fed sub-stations and cities with electric energy. The electric field near electrodes (the role of electrodes being in this case played by two cables, or by a cable and electrical connection to earth). Electrons are created by several processes and are strongly transported by the field, producing streamers of electrons that originate from the ionization process of molecules and atoms. This type of discharge is called corona discharges, and are accompanied by a characteristic noise, and it is shown in Fig.2. Artificial sunlight can be obtained using small bulbs (for example, with a small drop of Mercury inside a quartz tube filled with a rare gas at a low pressure, typically a few millimeters of Mercury or one hundred pascal, see pressure units here). Seasonal affective disorder can be treated with sun lamp light therapy {1}.

enseignes lumineuses

Fig.1 – Tokyo is full of illuminated advertisements. Image credit: http://whisky.centerblog

When the voltage applied to the electrodes inside the bulb increases, a small current cross the gas, ionizing molecules and atoms (neutral particles) and a feeble luminosity appears – that’s what is called a glow discharge (see VIDEO). The word plasma was coined by Irving Langmuir (1881-1957), a term that ensued from his observations of the separation of the plasma into cell-like regions with boundaries formed by charged particles sheathes, whenever regions with different densities, temperatures, or electromagnetic fields inhomogeneity are present. [1]. In general terms, plasma is a state of matter composed by ions (positive and negative) and free electrons subject to collective Coulonbian forces in a médium composed by neutral particles (atomes and/or molecules).

If the electrons and ions densities are much lower than the desnity of neutral particles, the plasma is said to be weakly ionized (glow discharges, lightning); if, by the contrary, the density of neutral particles is much lower than thecharged particles density, the plasma is said to be strongly ionized (stars, thermonuclear reactors). If ñe    represents the average number density of electrons (average number of electrons per unit of volume) and ñZ is the average number density of ions with ionic positive charge , then the global condition is satisfied:

Globally, the plasma state is characterized by equal number of positive and negative charges.

Eq1

The electromagnetic field is well described by the set of Maxwell’s equations:

Eq2Eq3Eq4Eq5

This set of equations are defined when the Lorentz force is given by:

Eq6

and the constitutive equations are defined by D=εE  and B=μH, relating the electric field vector E and the displacement vector D, and as well the magnetic flux density vector B and magnetic field vector H, while ε and μ are the permittivity and permeability of the medium, and ρ and J are the electric charge and current densities, respectively. m and q are the mass and charge of the particle.

The sun magnetic field gives rise to sunspots, coronal loops, faculae, solar flares, solar wind and prominences, solar cycle, irradiance variability. Usually the magnetic field near the solar surface is measured using the Zeeman effect. Until now, the vast majority of all recordings of the magnetic field still refer to measurements of the Zeeman effect of the photosphere.

Image credit: http://www.daviddarling.info/

Coronal loops. Image credit: http://www.daviddarling.info/

The sun’s magnetic field is responsible for generating self-excited dynamos [2], magnetoconvection phenomena, interaction of radiation with magnetized gas, magnetic reconnection.

The sun rotates around its axis in 26 days (28 days when viewed from the Earth) in the equatorial region, while in the polar regions it takes 37 days (40,5 days when seen from Earth). The spacecraft Mariner II in 1962 detected the solar wind. the speed distribution, direction, temperature, composition, and spatial structure of the solar wind were mapped from a number of spacecraft, mostly sampling at low solar latitudes (few degrees from the plane of the ecliptic). It is the solar wind that stretches the interplanetary magnetic field. The source of the interplanetary sector structure is invariant with time. This means that the same boundary might be observed anew after 27 days. This is just na average value, since the solar wind velocity can modify the time of arrival of a sector boundary in 1 or 2 days.

Faculae, the birghtest region around a sunspot. Image credit: earthobservatory.nasa.gov

Faculae, the brightest region around a sunspot. Image credit: earthobservatory.nasa.gov

What is amazing is the Archimedes spiral that the magnetic field lines of force of the solar wind depict in space, and shown in the document shown below [4].

What scientists found is na Archimedean spiral like figure of the Interplanetary Magnetic Field that reminds the swastika (卐) (Sanskrit: स्वस्तिक), i.e., an equilateral cross with four arms bent at 90 degrees, sacred symbol in Hinduism, Buddhism, and Jainism, which literally means “to be good”, or “being with higher self”.Sun001

fig3-pdf

The Interplanetary magnetic field lines of force separated in sectors, as seen by Mariner II.

lord-ganesha-on-swastika-HG75_l

Swastika, means “to be good”

The resemblance between the IMF and the swastika suggest the following question: Did the ancients had knowledge that have been lost to time?…

Below it is shown the solar structure.

structure

The structure of the sun, our star. Image credit: astro-observer.com

SUN OBSERVATORY

You can check out in real-time the current status of our Sun here:
http://sohowww.nascom.nasa.gov/

To be continued…
REFERENCES:

[1] Mario J. Pinheiro, Plasma: the origin of the word, in Philica.com article 363

[2] Dynamo effect, page of the university of Oregon

[3] Geodynamo theory and simulations, Paul H. roberts and Gary A. Glatzmaier, Rev. Mod. Phys. vol. 72, Nº4, October 2000

[4] Large scale properties of the Interplanetary magnetic field, Kennett H. Schatten, NASA Report

SITES:

{1} Sun lamps.

Detectors in Optical Astronomy

15 Oct

“Astronomy compels the soul to look upwards, and leads us from this world to another.” – Plato (4277-3477 BC), The Republic.

“Correctly understood, the stars were proof of a higher design in the Cosmos.”Plato, in Exploring Ancient: A Survey of Ancient and Cultural Astronomy, by David H Kelley, Eugene F. Milone, Anthony F. Aveny (Springer, New Yoork, 2011) 

Humankind always has sustained, since the dawn of times, a major interest in observing the sky and gaining new knowledge and insights on the working mechanism of Nature (and for some, a glimpse of God). For this reason astronomy is the oldest science developed by humankind and we have been using the discoveries made by the use of this science to improve our civilization, embedding this knowlegde in our culture, art, religion, and our present notions of space and time.

But, how can we access to this knowledge, understand how the universe looks like? The first mean we have is just the bare eye able to separate image details with about 3 minutes of arc. This is equivalent to to 1/10 of the moon diameter, or the airplane wing span (10 m) flying at 10 km high.

...

What is called angular resolution is the smallest detail detected by a telescope (or an eye), which is limited by aberrations and diffraction pattern (a series of concentric rings of light and darkness due to interference).

Fig. 1 -- Eye structure.

Why it is not possible to obtain an angular resolution with infinite value? Because unfortunately any optical detector (eye, telescope, camera,…) gives of a point light source a diffuse image of what it is called a diffraction pattern (Fig.2) caused by the light waves diffracted at the fringes of what is called the “apertures” (diaphragms that confine the circular beam area). Therefore, the view of the smallest point of the object emmiting light is limited by this pattern, which may be described by parameters such as the diameter of the inner structure (defined as the first zero of light intensity), and other parameters. But to simplify, usually it is prefered to use a new definition, actually a convention proposed firstly by Ernst Abbe in 1874:

d=lambda/2NA

Here, d is the size of the finest detail that can be resolved with a camera, lambda is the light wavelenght, and NA denotes the numerical aperture of the objective lens, which can be defined by the ratio:

NA=nD/2f

This means that a telescope with 12 cm of diameter can separate at maximum stars distant one second of arc. The formula is this one:

AR(angular resolution)=250,000 x lambda/diameter

AR=250,000 x 6500Angstrom/0.12= 1.35 seconds of arc

We used for lambda the value 6500 angstrom=10^(-10) meters, a common value in the optical range. And what size eye (AR=1′, one minute) would you need to detect radio waves (lambda=0.1 m) ?

d=250,000 x 0.1/ 1’=450 m !

Sunset at the fingertips of my friend Americo Jones. The bridge links Lisbon to the South of Portugal and it is similar to the Golden Gate Bridge that cross San Francisco Bay.

………………….

That is, your eyes should be apart 450 meters to detect radio waves…, for which the atmosphere is transparent. Unfortunately, our eyes cannot strecht so much apart. Otherwise, human eyes, working together with the human brain, are extraordinary matural detectors in its range of color sensitivity, sensitivity to dim light and adaptability. Painters are highly sensitive to colors, see this article about impressionists French painters, and the eye was the principal help classical astronomers had before and served to observe planets and the most brilliant stars in the sky.

...

Hence, this represents a theoretical resolution, since atmospheric turbulence interposed between the star and the observer forbid this possibility. This is why telescopes are built at the top of mountains in order to gain a factor of 3 or 4 in this resolution. For example, at Mauna Kéa, Hawaii, at 4200 meters of altitude, it was built the world’s largest observatory for optical, infrared, and sub-millimeter astronomy. Among the equipment, France built a telescope 360 meters of diameter, so huge in order to increase the luminosity coming from the stars.

Mauna Kéa, Hawaii, tha world's largest observatory for optical astronomy. Image credit: http://www.ifa.hawaii.edu/mko/

In the actuality, astrophysicists need more sensitive detectors to observe quantitatively very faint stars. But notice that at the end it is again the eyes that remain the ultimate detectors, since astronomers need to look at the pictures…

...

The next detector used in astronomoy is the analog camera. Although less sensitive than the eye, its advantage stays in the possibility to let the shutter open for a long time of exposure, it is simple to handle and possess a great capacity to stock information. But its main disadvantage is the difficulty to digitalize the image for computer analysis. For example a black-and-white roll film has one side more brilliant than the other.  The least brilliant side has an emulsion of gelatin with suspended array of silver halide crystals which determine the sensitive of the film to light exposure. The reaction taking place is the following:

Ag Br + h f -> Ag^+ + Br + e^- [when exposed to light with energy hf, Bromure is liberated and retained in the gelatine]

Ag^+ + e^- -> Ag  [the liberated silver atom is sequentially converted in metallic silver by electronc capture.]

In particular, the size of the grains determine the time of exposure necessary: larger grains needs faster exposure but gives a grainier appearance; smaller needs more extended time of exposure but the images are finer looking. The ISO factor traduces the graininess of the roll film, appearing as a multiple of 10 or 100. For example, lower ISO numbers produce finer grain but slower film, and vice versa.

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However, nowadays the amateur and professional have acess to electronics registreurs, which has as precurseurs the electronographic camera invented by Prof. Lallemand and his team at the Observatoire de Paris in 1967. Although this detector was capable to detect just one photon and has a magnetic focalisation of the photo-electrons, it has a low performance. with the advent of integrated circuits (IC) and semiconductors, at 1970’s Williard S. Boyle (received in 2009 the Nobel Prize for this invention) and Smith invented a camera called CCD (charged coupled device) where successive metallic electrodes were capable to locally confine electric charge into silicium. See how it works here.

This operational mode allows that pratically each photon incident on the telescope can be enregistered and its properties made CCD cameras of great use in telescopes at the surface of Earth and in satellites.

After this short digression on the means to watch the sky by yourself, I invite you to watch these wonderful programs broadcast by BBC and presented by Sir Patrick Moore (here) along the great British traditions of scientific divulgation.

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