Rigel - the beta of Orion
People have different stars. For those that travel they show the path.
For others, they are just small lights.
For scientists they are problems.
Antoine de Saint-Exupery


“The Little Prince” - Antoine de Saint-Exupery

Do you remember the last time you looked at the stars? Maybe it was long ago in the days of your childhood? Maybe you looked at them in the chili winter nights. Or maybe yesterday, tired from the stressful day, you asked yourself one of the eternal questions while looking at the silent bright starts… Look at them again… Every one has a small part of the eternal mystery and wisdom of the Universe. At the same time each is a remote and exciting world. Have you experienced at least for a moment the irresistible desire to understand what those distant suns are really like, to touch their unknown worlds? Maybe… The skies have always attracted people with peculiar figures made up by the brightest starts. The poetic fantasy of the ancient has immortalized many legendary heroes in the images of the constellations. Many millennia ago the stars have sealed the famous scene of the celestial hunt. During the entire night in the south region of the winter sky shines a splendid constellation, named after the handsome and quick mythical hero, Orion. He was hunting in the mountains guarded by his two faithful dogs, when suddenly the The Greater Dog (Canis Major) started chasing after a rabbit. The rabbit found a protection next to Orion, huddling by his feet in the sky. On the ancient sky maps to the right and above of Orion a swooping down enraged bull is depicted during the hunt. The brightest star of the Taurus (The Bull) constellation is Aldebaran, also called the eye of the bull. That is why the hunter is portrayed holding high a heavy mace an instant before killing the bull. Next to the The Greater Bear (Ursa Major) is the most beautiful constellation in the night sky – the powerful warrior Orion. He cannot remain unnoticed even when there are interfering lights. According to the Greek mythology Orion was a great hunter who claimed superiority over all creatures on Earth. Infuriated by his arrogance the Gods send a scorpion to punish him. The scorpion bit the mighty hunter on the heel and he died. The moon Goddess, Diana, felt sorry for Orion and asked the Gods to put him in the sky, so that everyone can admire him. The Gods agreed. However, to remind everyone that no one is perfect, they placed in the sky a constellation called The Scorpion (Scorpius). This constellation is best viewed during the summer. But the Gods made such that Orion could never meet his murderer. The two constellations can never be seen in the sky at the same time. Orion may have not been superior to all creatures on Earth but Orion definitely surpasses all other constellations, it has more bright stars than any other. There are many other legends about the constellation Orion. According to one of them one day Scorpion bit Orion and he died. But the son of the God Apollo, Eskulap, found out about Orion’s death and wanted to resurrect him. However, Hades, the ruler of the underground kingdom became angry at Eskulap for taking away his shadows. Hades complained to his brother Zeus. Zeus killed Eskulap with a lightning bolt and turned the heroes from the legend into stars. According to another legend, during a hot summer day, the legendary hunter Orion went to the thick wood of Kiteron. It was as hot as if Helios was pouring down glowing embers onto him and Orion searched for a stream to cool himself down. After wandering for a long time he reached the evergreen valley of Gargafia, land of the goddess of the hunt, Artemida, where he saw a clear creek. After refreshing himself, Orion went down to the stream and found a cave in which there was a crystal-clear lake. That was Artemida’s cave and no mortal new about its existence. The splendid goddess was just going to take a bath in the crystal-clear water, when Orion appeared at the entrance of the cave. Nymphs started screaming and surrounded the goddess so that the mortal could not see her. Enraged Artemida turned Orion into a beautiful deer with huge antlers but left him with a human mind. The scared deer started running in the woods crying bitterly. His both favorite dogs, The Big Dog and The Little Dog sniffed him and started chasing the deer. They did not recognize their master. They caught up with him and tore him to pieces. According to this legend, that was the way Orion tragically died because he was the only one among the mortals to see, even if involuntary, the fascinating beauty of the splendid Artemida. The Gods immortalized him by turning him into one of the most amazing constellations noticeable with its bright stars. On the ancient sky maps Orion is shown with a lion hide thrown over his left arm and with a huge mace in his right hand. Seven bright stars sketch the well-known outline of the constellation. Among them, representing the east shoulder of the Hunter - Betelgeuse is different from the others because of its red color. The other six stars: the west shoulder of Orion - Bellatrix, the stars from the belt- Alnilam, Alnitak and Mintaka, and both feet of the figure Saiph and the brightest of all Rigel are blue stars. Rigel is not only one of the brightest stars, but it is also one of the largest, the hottest, and one of the stars with the largest luminosity in our galaxy. Extraordinary and marvelous the stars like Rigel and all other bright blue stars in Orion are rulers of the sky. In comparison, the colder stars, which are many more than the bright stars, appear pale. There are no stars like Rigel among the three thousand stars that are closest to Earth. But you can find at least a dozen among the three thousand of the brightest stars. The origin of the name Rigel comes from the Arabian phrase “Rijl Jauzah Al Yusra”. Rigel’s Arabian names are Algebar or Elgebar. Rigel contains 25 solar masses and has a diameter that is 65 times larger than that of the sun. If Rigel had been in place of our Sun, it would have almost filled the space up to the orbit of Mercury. Rigel is much denser and hotter than Betelgeuse but it has a smaller diameter. Like all other blue super-giant stars, which live much more intensely than the cold red super-giants, Rigel is also a much bigger squanderer of nuclear fuel. The visible color of a given star is a result of its surface temperature. The hotter stars radiate out a larger portion of their energy in the shorter (Blue) wavelengths of light. Rigel’s surface temperature is about 10000 K. This is twice as much as the temperature of the yellow surface of the Sun and about three times as much as that of Betelgeuse. This high temperature determines the blue-white color of Rigel. It is also the reason for the high luminosity of the star. Every square centimeter of the surface of Rigel emits 15 times more energy than the equivalent surface area of the Sun and almost 100 times more than the same area of Betelgeuse. The brightness of a star is defined by two factors: its luminosity and its distance from us. The Sun is so bright only because it is so close to us and if it were at a distance similar to the distances of some of the closest stars to Earth it wouldn’t look so magnificent at all. The brightest star in the sky, Sirius has luminosity a little higher than that of the Sun, but it looks so bright only because it is on 9 ly (light years) from us. Rigel and all other blue stars are 1000 or more light years from us. Even Betelgeuse, which is 650 ly away, is almost our neighbor in comparison with Rigel. Astronomers define the luminosity of the stars by their spectra. After a long study of thousands stars’ spectra, scientists categorized the stars into different groups according to their luminosity. Rigel is from spectrum class B8. This means that there are spectral lines of neutral hydrogen in its spectrum. Also, as those lines are so strongly defined, Rigel is classified as a super giant with luminosity class Ia. These two characteristics give the astronomers the opportunity to figure out the luminosity of any star even if the distance to it is not known. The luminosity of Rigel is 50000 times greater than that of the Sun. Therefore, if Rigel were 10 parsecs (ps) (32.6 ly) away, the standard distance used by astronomers to compare the luminosity of different stars, it would have apparent magnitude –7.1. At the same distance the Sun would look as a not very bright star with a visible magnitude of 4.8. In addition, Rigel and the Sun are also very different from each other in their internal structure. The huge energy of Rigel is generated inside the star, where the temperature is 100 million degrees K. Inside the sun the temperature is “only” 15 million degrees K. This huge temperature difference is the reason for the different energy-generating processes in both stars. While the energy of the Sun is mainly the result of proton-proton reactions, the super hot Rigel produces energy using the triple alpha process (the fusion of three helium nuclei into one carbon nucleus). This powerful energy producing mechanism can happen on at very high temperatures and that is why it cannot exist inside the Sun. The two stars also differ from each other in one more significant factor. As we move from the nucleus of Rigel to its surface the temperature decreases much more quickly than the temperature gradient of the Sun. Near the nucleus of Rigel the cooling is so extreme that the energy is carried by moving currents of red gas. This process, called convection, is very effective in the core of Rigel. Far away from the center, however, the process slows down and the transmission of the energy is accomplished via radiation. In the Sun the situation is reverse. In the core of the Sun the energy is transmitted by radiation while convection cools down the surface layers. Observing Rigel with a telescope reveals something completely different than what one can see by simply watching the hot blue super giant with a naked eye. An intriguing story about triple Rigel started in 1830 when F. Struve first noticed that 9.5”(angular seconds) away from Rigel orbits a star satellite. The satellite, named Rigel B is about 2500 AU(astronomical units) away from the main star Rigel A. It is also classified as having spectral class B8 but it is from the Main Sequence and therefore it is smaller and much less bright than Rigel A. Up until now no one has noticed any changes in the relative position of the two stars. Even more, both stars have almost the same movements therefore they have the same visible movements. The same is true about their linear velocities (the velocities along the line of sight). It is probably very unlikely that their similar movements thru space to be just a coincidence. For this reason astronomers think that both stars are physically connected as they orbit one another, even if very slowly. In 1908 it looked like the system of Rigel obtained a new member. Canadian astronomer John Stanley Plaskett reported that the system is actually spectrally double. When a star is moving along the line of sight, the wavelength of the light that comes to us from the star changes proportionally to the speed of the star. When the star is moving towards us the wavelength of the light moves towards the blue end of the spectrum and when the star is moving away from us the wavelength of the light moves towards the red end of the spectrum. When a star orbits another star it moves towards us and then away from us regularly. As a result there is a change in the wavelength of the light from a star when there is a satellite moving around it even when the satellite is too dim to be visible. That is what Plaskett had observed in the spectrum of Rigel A. But, since he hadn’t detected any spectral characteristics of the satellite, he assumed that the satellite was too faint for the capabilities of his spectrograph. It looks like Plaskett had made an error. Decades later even the new technology does not indicate the existence of any spectral lines coming from the satellite of Rigel A. Today astronomers suspect that the observed changes in the line of sight velocity of Rigel A are the result of pulsating outer atmosphere of Rigel. In the meantime, in 1937, at the Mount Wilson observatory, astronomers clearly a very detailed spectrum of Rigel B, in which one could see two sets of spectral lines. As a big surprise Rigel B and not Rigel A turned out to be spectrally double. Rigel B and its satellite are both blue stars from the Main Sequence. They make one rotation around each other in about 10 days. This, however, was not the end of the puzzles surrounding Rigel. For many years, many astronomers have reported that Rigel B is visually double. The visible angular distance between the two stars was less then 0.2”. But these observations did not appear to belong to the spectral satellite. In 1947 the orbital elements of the spectrally double star was calculated. The calculations showed that the two stars are never more that 0.1” away from each other. The fact that Rigel B is not visually double star was later confirmed by observations from large telescopes. Like Plaskett, the observers of the visual satellite of Rigel B are also mistaken. In the vicinity of Rigel there is also a dust cloud, known as IC2118. From large dust clouds like this one, stars are formed when under the forces of gravity the clouds contract or break apart. Gas and dust nebulae in the Milky Way are in the spirals of the Galaxy. There are many young stars and looking at Orion we can see a large number of massive, fast burning blue stars. Rigel shines upon the dust in IC2118 even though the nebula is 2.5ly away in the Eridanus constellation. Edwin Hubble had calculated that the dust cloud had a diameter of 20 ly. According to him “…the dust cloud is so far away that it is difficult to image how the star can shine effectively upon the nebula”. His calculations indicate that the situation is similar to the one in the Pleiades star cluster, which is also surrounded by nebulae. We observe the scattered blue light coming from IC2118 thanks to the same process that is responsible for making the sky blue during the day. In the mid 60’s Canadian astronomers Cidney van Der Birg and Renee Rasin studied many such stars located in or near such reflective nebulae. They grouped together Rigel and seven other stars in what is today known as the T-association. This group is named after the T star of the Taurus constellation, which belongs to the first such identified group. The Rigel association is called Taurus-Orion R1 and it takes up volume far larger than the observed by Hubble. It is easy to imagine that there may be hundreds and maybe thousands other stars that belong to this association but they are too dim to be observed from Earth. Even all of the above is not everything that can be said about Rigel. For many decades a main object of research had been the significant gas cover around Rigel A. In 1986 Daniel Heis made a conclusion that Rigel is most likely under the effects of periodic disturbances due to the outflow or maybe inflow of matter. In other words, the matter in the outermost layers of the star is being blown away periodically and later parts of it fall down to the star again. Heis’s conclusion is based on the observation that the light coming from Rigel is being polarized. A photon particle (the carriers of light) may vibrate at any angle with respect to its direction of motion. However, if the photon vibrations lie mainly in a plane oriented at some angle with respect to their direction of propagation, the astronomers say that the light is polarized. Because of the large luminosity of Rigel, the angle of polarization of the light coming from Rigel can be determined very precisely. Heis’s observations made more certain that the light coming from Rigel is indeed polarized. This fact alone is very unusual. Many stars and especially those with strong magnetic fields emit polarized light. But in the case of Rigel it was more surprising that the angle of light polarization was changing every night. However, a more detailed analysis of the observations still cannot explain this change. In addition, using the data from his observations, Heis calculated that the strong magnetic fields are not likely to be the cause of the release of matter from the outermost layers of Rigel. Heis claims that the change in the polarized light is caused by pulsations of the star surface. In conclusion, we can say that Rigel A is very different from many starts that give off polarized light. Many stars lose matter by a process known as solar wind but the phenomena that take place on Rigel are different and strange. Most likely, Rigel A loses about 1% of its mass in several million years. The star blows away a part of its gas cover to about 100 million kilometers above its outermost atmosphere. If Rigel A were in the middle of our solar system, the gas cover, which expands at almost 200 km/s, would reach the orbit of Jupiter in just one month. The question of mass loss from blue super giants, like Rigel, came up again when 16 years ago in the Large Magellanic Cloud the 1987A supernova exploded. The common wisdom says that the most likely candidates for supernovae are the red super giants like Betelgeuse. However, astronomers determined that the 1987A supernova exploded from the blue super giant, Sendulek. –69^o 202. Scientists used computer modeling to explain why a blue super giant can become a supernova. They were very surprised to find out that a blue super giant in a late stage of development could indeed blow away a large part of its matter becoming a supernova. Even though the gas cover of Rigel A does not contain enough mass yet to become a supernova, the star could explode just like Sendulek. –69^o 202. Among the millions of stars similar to Rigel that were created near the spirals of the Milky Way galaxy, there are many bright stars but we can see only a few of them. Most of these stars are hidden from us by the dust clouds in the plane of our galaxy. So, in a particular part of the galactic diameter the blue super giants are kings in their own kingdom, but they are still princes in the cosmic continent.



Name: Rigel

Magnitude: 0.12
Spectral type: B8Ia
Celestial length: 5h14m32.3s
Aberration: -8^0 12'6"
Notes: variable quantity
Position: In the left foot of Orion
Distance: 10 ps(32.6 ly)

SUN

Mass /Sun = 1/..................1.0
Diameter /Sun = 1 /.............1.0
Diameter /mil.km/..................1.4
Visible stellar Magnitude..........-26.8
Absolutely stellar Magnitude.........4.8
Illuminating /Sun = 1/.............1.0
Spectral type.....................G2V
Superficially temperature........4 800K
Color..............................yellow
Rigel A

Mass /Sun = 1/...................25
Diameter /Sun = 1 /...............65
Diameter /mil.km/...................90
Visible stellar Magnitude............0.1
Absolutely stellar Magnitude........-7.1
Illuminating /Sun = 1/..........50 000
Spectral type....................B81a
Superficially temperature........9 500K
Color.........................blue-white
Rigel B

Mass /Sun = 1/..................2.5
Diameter /Sun = 1 /................2
Diameter /mil.km/..................2.7
Visible stellar Magnitude............6.8
Absolutely stellar Magnitude........-0.4
Illuminating /Sun = 1/.............100
Spectral type.....................B9V
Superficially temperature.......10 100K
Color.......................blue-white
Rigel C

Mass /Sun = 1/......................1.9
Diameter /Sun = 1 /.................1.5
Diameter /mil.km/..................2.1
Illuminating /Sun = 1/..............50
Ñïåêòðàëåí òèï.....................B9V
Superficially temperature.......10 100K
Color.......................blue-white