School of Astronomy

Vasil Levski-School, Trojan��

�� Pavlina Bojidarova Vasileva

�� Bogomila Toshkova Todorova

�� Mirena Milkova Shotekova

Scientific instructor:� Nadka Dankova

THE SUN

Physical nature and activity of the sun. The sun and the remote stars. Some modern aspects of research.

Practical lesson: an observation of the sun and determination of the sun activity.

�� The rays of the rising sun light up the mountain hills. The stars run away from the sky at the sight of the Sun god. Up goes Hellios’ chariot. He travels along the sky and pours his light-giving rays upon Earth, giving it light, warmth and life.

�� From an ancient Greek legend

�� The Sun is the nearest star to the Earth. Life on Earth would be extinct without the invigorating sun energy and it would turn into one of the desert islands in space.

�� Historical information

�� The closer we get to the Sun’s secrets, the more we admire the luminous body. The ancient Greek philosophers Anaxagorus (500 – 428 B.C.) and Democritus (460 – 370 B.C.) thought that the Sun was a crag, getting hot by its fast rotation.� Heraclitus (544 – 470 B.C. ) estimated that the Sun was not bigger than a footstep.

�� According to Aristotle the Sun and the other luminous bodies consist of constant and everlasting matter – ether.

�� There are spots on the Sun according some Chinese chronicles from 301 B.C. In 1611, Christopher Shainer, a famous mathematical professor, talked about spots on the surface of the heavenly sun. The Italian Galileo, the Dutch Goldsmidt, the English Harriot reported some sunspots nearly at the same time.

�� William Harshell supposed that these sunspots were gaps in the clouds and his son John Harshell accepted them as sun whirls.

�� All questions connected with the condition and structure of the sun were clarified at the end of the 19th century, when the spectral analysis method entered into astronomy.

�

Physical nature of the Sun

Sun structure

�� The Earth is 150 million km away from the Sun. Sunshine covers that distance in approximately eight minutes, at the rate of 300 000 km per second.�

�� Physical charestic of the Sun

�� Weight:� M = 2.10 30 kg

�� Radius: R ~ 7.10

�� Temperature on the top: T ~ 6000 Kelvin (K)

�� Luminosity: L ~ 4.10 W

�� The Sun moves around its orbit round the centre of the Milky Way galaxy at the rate of 250 km per second, making a complete circuit in 250 million years. Its chemical structure is very complex: 70% hydrogen, 29% helium and about 1% other elements.

�� The sun core spreads out to a distance of 0.3 Rsun. Some assert that the temperature at the core of the Sun is 10 7 K. And it is there that in the process of thermo-nuclear synthesis the sun’s energy is generated. Two types of reactions play a major role.� In the first, the proton reaction, four hydrogen atoms formed one helium atom and energy is given.� In the second, the carbon cycle, four protons form one helium atom. This process needs carbon as a catalyst. From the core, the sun’s energy transfers into the zone of a radiant heat transfer. It stretches from 0.3 to 0.7 Rsun. Here, the energy moves to the top by moving the matter itself. This is called the convection zone

�� The structure of the Sun is similar to the structure of cells. There are three different regions in cells as well: the first one makes energy; the second one accumulates it and the last one, the internal, reprocesses it and turns it into vital energy.

�� The Sun is a star of the major succession of the chart of Herzsprung and Russel, a star on a small scale and mass from the spectral class G-figure 1. Usually, the Sun is said to be a gas sphere. To be more precise, it is a sphere of plasma.

��

The solar atmosphere

�� Immediately outside convection zone is the photosphere,� 300 km deep. It consists of granules about 1000 km thick. The most remarkable objects on the photosphere are the sunspots.� These are regions where the magnetic field is very strong, whose temperature is about 4000-4500 k.

�� According to B. Duno, sunspots are openings through which accumulated energy moves to the earth.� Magnetic storms result from the sunspots. When there are more sunspots, the year is more fruitful in agriculture, but the human nervous system can be disturbed.

�� The sunspots, together with the protuberances, the sunbursts and some others, are the most popular phenomena of the Sun’s activity. The cycle of the Sun’s activity is usually called the 11 years cycle.

�� According to the predictions of the Czech heliophysicist Kopetski, the new century’s weather is likely to be hot and dry, with higher sun activity expected.

�� The gas over the photosphere is gauzy and more rarefied. The chromo sphere and the corona (the outside layers of the solar atmosphere) are made of that gas.

�� Above the chromo sphere is the sun corona (figure 2). During complete solar eclipses, it looks like a fine white radiance around the Sun.� Scientists have developed special telescopes with which complete solar eclipses can be artificially simulated.� These telescopes must be located at high altitude, in mountain regions

�� Far away from the centre of the Sun, the corona gradually spreads out into interplanetary gas.

�� The most impressive forms in the internal layers of the corona are the protuberances, gigantic fountains of glowing gas. Research on the corona is being carried out using the NASA-TRACE� probe that was put into orbit in April 1998. The probe has sent information about enormous tongues of plasma, thousands of kilometers high.

�� There are some other forms in the corona as well as the protuberances: these are rays, holes, polar brushes, tubes, arches and arcades.

�� The plasma of the corona rushes in all directions of cosmic space at of hundreds of km per second. This plasma is called solar wind. Its consists of 96% Hydrogen and 4% Helium with other gases in insignificant quantities. The solar wind spreads out to a distance of about 100 astronomical units (a.u.) away from the Sun, where its dynamic pressure adjusts the balance with the pressure of the surroundings

�� In August 1999, the Bulgarian Astronomical Association and the astronomical school in Troyan observed a complete solar eclipse, intending to research the physical properties of the solar atmosphere. Two of our photos from the observation are shown (figures 2 and 3).� Below are our conclusions about the physical properties of the solar atmosphere.

��

�� The solar Sun’s magnetic field gives clearly defined structure to the solar atmosphere. The plasma of the atmosphere is both inside and outside the regions resembling tubes around the magnetic field lines. The corona has luminous structure. The curvature of the beams of the corona resembles the curvature of field lines around a magnetized sphere.� The medium brightness of the corona depends its distance from the limb: the internal part of the corona is much brighter. The movement of gas over the field lines affects many of the beams of the corona, as well as the protuberances.� That is why they go out from the surface of the Sun. The chromo sphere, which can be seen only during complete solar eclipse, is pink in colour. This colour is due to the radiating of hydrogen (the biggest quantity of hydrogen radiates in the chromo sphere).

�

The Sun and the remote stars

�� Research work on the Sun is of considerable importance for our knowledge of the nature of the remote stars which cannot be observed in such detail.

�

�The table below defines the position of the sun

�

Color

Star

Temperature

Spectral class

� Blue

Bluish-white

White

Yellow-white

Yellow

Orange

Red

Zeta-Orion

Spica

Sirius

Procion

Sun

Albedaran

Arctur

350 o C

21 000 o C

10 000 o C

7 500 o C

6 000 o C

4 700 o C

3 300 o C

�� The Sun is considered to contain enough hydrogen to keep on giving energy for millions of years. When it uses up its fuel, our star will change.

�� The destiny of the Sun

�� In its evolution, every star passes through the following main stages: an archstar, a star of the major succession of the chart of Herzsprung and Russel (figure 1), and a final stage.

�� According to current theory, the age of the Sun is about 4,5 millions of years. The Sun is in the middle of its evolution as a star of the major succession.� It is almost certain that the Sun will keep shining for a few billion years more.

�� After 5 billions of years, the huge energy of the Sun’s nucleus will force the external layers to expand, turning it into a gigantic red star. After this ‘red giant’, which will continue about 100 million years, the Sun will contain no nuclear fuel. It will shrink and will become a ‘white-pigmy’ star.

�� When a star as massive as the Sun exhausts its supply of helium and reaches the last stage of its life as a red giant, the star emits a part of its substance. The most external layer of the star, consisting of hydrogen, whirls away in the space in the shape of a gauzy, expanding cover of gas-called a ‘nucleus star’.

�� The white-pigmy stage comes in cases when the stars are not so heavy ( M < 1,4 Msun), where Msun is the weight of the Sun.

�� The star shrinks to a size of a planet like the Earth

�� The white pigmy gradually cools down, sometimes turning into a ‘cold black pigmy’.

Some contemporary aspects of research work

�� The cosmic probe Julius, launched in 1990, used Jupiter’s gravitation. The probe flew near Jupiter in 1992. It reached the Sun in 1995, recorded the solar wind and solar magnetism, and passed over the both poles of the Sun:

��

�� *�� *�� *�� *

�� The research satellites TRACE and SOHO are examining the Sun. One of the latest disclosures from these satellites is about the knots of the corona: these are huge forms in the shape of arches, compound of electric gas. It was concluded that the knots of the corona are dynamic forms of plasma, turning at the rate of sound.

�� *�� *�� *�� *�

�� In Keet Pic, Arizona, there are twenty-four big telescopes and one solar telescope built on a tower that is 30 meters high. The diameter of the flat mirror is D=2m; it always follows the Sun. Sun phenomena are under constant observation here, resulting in new conclusions about the nature of the Sun and solar activity.

�� *�� *�� *�� *

�� At Stanford University research in the field of solarseizmology is under way. In this way according to movements inside the Sun, some conclusions about its structure can be made.

�� *�� *�� *�� *�

�� The Japanese telescope “Jocko” examines the sunspots and solar wind in the x-ray-sphere.

�� *�� *�� *�� *

�� At Mount Wilson Observatory, Dr. Sally Balguinous is examining thirty stars similar to the Sun. She is looking for answers to questions of surrounding the decline in solar activity since 1985.

��

A practical lesson

�Sun observation. Determination of the sun activity.

Equipment Required: a school telescope with� solar shield or binoculars with a device in the shape of “Г”, a white sheet of paper, a pencil, a pillar, a line, a clock and a pair of compasses.

Theory: It is well known that the distance over the horizon changes yearly. In summer afternoons it raises very highly. It reaches its highest position on the 22 of June. In the northern hemisphere the longest day and the shortest night fall. On that day the longest night and the shortest day fall be seen in the southern hemisphere. After this, every day the Sun raises higher and higher over the horizon: it reaches its highest position on the 22 of December. This day is the shortest day and the longest in the Northern hemisphere, and in the Southern hemisphere, the longest day and the shortest night.

�� There is a gradual passage between these two extreme positions. Important dates, during this passage are 21 st March and 23 rd September. These are the days of the vernal equinox and the autumnal equinox. On these days the day and the night are equal. From the 23 of September in the Northern hemisphere autumn starts, in the Southern hemisphere, spring.

�� In summer, in the middle latitudes of the Northern hemisphere, the Sun rises from north-east and goes down to north-west; in winter, respectively from south-east and to south-west.

�� These special features result from the motion of the Earth round the Sun. It moves from the west to the east on a trajectory called Earth orbit. During its motion round the Sun, the axis of the Earth is always inclined, compared to the flatness of the orbit.

�� The Earth completes its circuit around the sun 365,25 days and nights. This period of time is called a solar or a tropical year. One civil year has 365 days (a leap year has 366 days).

�� In our practical work there are occasionally problems determining the north-south direction. If you do not have a compass you could use one comparatively easy method, connected with the visible motion of the Sun.

�� To do this, we use one of the most ancient instruments, a pillar.� It is a small pillar fixed vertically on a horizontal top, figure 5.

�� Practically, the north-south direction is a projection of the celestial meridian.

�� The relative Bolf numbers are one of the static characteristics of the sun activity. They are defined by the number of the sunspots and the total visible area of the spots in parts of the visible area of the solar disc. To determine this, we project the image of the sun onto a screen using a school telescope or binoculars, supplied with a frame in the shape of Г. The image of the Sun should ideally be 10 centimeters in diameter. We can get that size of the picture by moving the screen and fixing, using the eyepiece of the telescope till the shape of the Sun coincides with the circle, drawn in advance, on the screen. We outline the spots that are projected on the sheet of paper, and number them: the single spots and the groups of spots with numbers, the spots in the groups with small letters. We get the relative Bolf number from the formula: W=K (10g+F), where g is the number of the groups and the single spots, f –the number of all spots, single spots and spots in the groups.� K is taken as 1.

Tasks

Using a pillar, define the meridian in the place of the observation.
Make several observations of the sunrise and the sunset. Record the time and place of sunrise and sunset.
Draw a picture of the sunspots during observation.
Define Bolf”s relative numbers as a quantitative characteristic of the sun activity.

��

NB: Do not observe the Sun without protective glasses or a telescope, which is not supplied with a special filter.�� Ignoring these rules can result in blindness.

��

Report from the practical lesson

�� The lesson was conducted in two stages:

- The meridian line in the school yard was detected and an observation of sunspots carried out

- The sunset was observed from the ‘Astronomical Stage’, at Baba Stana, near Troyan

��

�� The lesson was taken on 22 nd June 2002

�� We successfully complete the following tasks:

The Sun was observed, using a school telescope and a filter, and the sun activity was defined.
We went on an excursion to the astronomical stage and made an observation of the sunset-figure 5, application 1 According to the position of the sunset, we defined points of the compass
We listened to the story of the 100 year-old Ivan Marinsky, about using that stage as a post of observation�� (figure 7)

�