RIGEL AND BETELGEUSE Roberto Bartali

The constellation of Orion (the Hunter) contains two giant stars: Betelgeuse (alpha Orionis) and Rigel (beta Orionis). Both are very bright objects, we perceive them as a first

magnitude star. The first star I will describe is Betelgeuse. The real name is "Ibt al Jouzah", but there are many different names and description for this star, because the constellation was known by many ancient cultures both in Europe, America and Africa. It is a cool red supergiant star of class M2 Ia, and the superficial temperature is 3,500 K; there are many stars like it, a good example is Antares (alpha Scorpius). Betelgeuse is not on the Main Sequence curve of the HR diagram, but is on the right, this means that it is at the end of the evolution of a

low mass star. When it was on the main sequence, it was not much different to our Sun, and in a few

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Orion Constellation with star names. From: http://home-3.worldonline.nl/~ppsmeets/Sterrenbeelden/Orion/Orion.gif

billion years our star will be a red upergiant too ending it life expelling its outer envelope forming a planetary nebula. Not far rom now, we can see the planetary nebula around Betelgeuse. ven when it is a very luminous star, its mass is not very large. We know that the

luminosity of a star depends on the gravity at the surface, so g=GM/RR (g=gravity, G universal gravitation constant, M=mass, RR= square of radius) really the mass of supergiant are low, but the radius is very large, so g is very low, then the gas experiment a very low pressure and can escape off from the star. The visual magnitude of Betelgeuse is variable in irregular manner from 0.4 to 1.3.

The constellation of Orion and the image of the star Alpha Orionis (Betelgeuse) From: http://zimmer.csufresno.edu/~fringwal/betelgeuse.gif

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Only a few constellations has a pair of bright stars of same magnitude. Gemini and Orion are 2 of that kind, and they have in common that star alpha is less bright then beta. Maybe when it was classified the magnitude was near the minimum. Searching in my library I find a value for the magnitude from 0.4 to 0.7 depending on the book and the year of publication. The spectral type is M2. the

spectrum present lines from titanium oxide. The photosphere of Betelgeuse.

From: http://imagine.gsfc.nasa.gov/YBA/HTCas-size/betelgeuse.html The titanium combined with oxygen

form a molecule of titanium oxide which absorbs efficiently photons at visible

ing of de He and the burning of de H, sometimes one nergy is greater than the other.

wavelength. This rises some questions: If the star is M2, so most of the energy came out in the red and IR region of the spectrum, why it is more visible in UV light? Maybe, is the presence of a wide calcium line ( 422.7 nm, corresponding to blue violet color, near UV ) in spectra the reason why we can see a greater Betelgeuse in UV light then in visible light. Right? If the titanium oxide absorbs most of the light in visible spectrum, is this the reason for the relative low magnitude for such a giant star? Betelgeuse is one of a few variable stars we can see with naked eye, and the only one of the first mag. The first star resolved optically was Betelgeuse, the HST take a picture of the star photosphere in UV light in 1995. the luminosity is much more than of the sun and the size is more than the sun too.( some 400 times the sun radius and 20,000 times the sun luminosity) The distance of the star is about 520 ly, the angular size is 0.044 arcseconds. This angular size is more than the resolution of most big telescope on earth, 5 meter aperture or bigger can theoretically resolve the disc, but refraction, pollution, turbulence, and other factor reduce the resolving power to a value less then the angular size of the star. Only HST in space can resolve such a disc (with much less diameter telescope). Perhaps the VLT on Cerro Paranal (Chile) can resolve the disc, because is one of the better place on earth for atmosphere stability. The great luminosity is due to the enormous size of the star, not to the energy emitted because it is a cool star. There are a few stars close to Betelgeuse, but it is not clear if they are companions. The star is at a critical point in his history, close to de beginning of the end, it is on the final evolution of a giant. The pulsation observed in Betelgeuse, probably is due to the conflict between the burne

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sky (7 ranked). the former name, as B

Betelgeuse. Supergiants like Rigel, leave the main sequence of the HR diagram soon, and go to the right on the supergiant region. The nucleus increase in density quickly. After burning all the H, is the time for a nucleus of He, then one of C, next O, next Si and finally Fe. After the star die as a supernova, the process to convert H to He for a massive star like Rigel

lprocess start with C that generate N that generate

e

The spectral type is B8 Ia, white blue color, is a very

II and MgII. Rigel is a young star, in this phase of

B is a b type star too, of mag. 6.7, at a distance of

the life of a star like

take place in a different way than for

proton generate a He nucleus and C, an

Now is the turn of Rigel. Rigel is the beta of Orion, a bright star of visual mag=0.18, one of the most bright in the

telgeuse came from Arabic, is "Rijl Jauzah al yusra". So it is the brightest star of Orionis. The value of the visual magnitude varies as the author and the catalog (as for Betelgeuse) from 0.34 to 0.18, but the latest could be better because was measured by Hipparcos.

hot supergiant some 12,000 to 15,000K. the spectra of this type of str present lines of Ca II, H I, O II, Si

star life, predominate ionized atoms. It is one of the most luminous star in the galaxy, the absolute magnitude is varies from -6.69 to -7.1 depending on books. the distance of Rigel is 773 ly, so if we put Rigel at the same distance of Alpha Cen, the brightness would be so high that could be capable to generate a shadow of an object illuminated by the star. Rigel is a multiple star system. Rigel

9", but this star is already a double, but very close, so difficult to see in telescopes less than 100 cm in aperture. The star has a mass about 50 times then the mass of sun, so it ´s life is very different to

fusion with a

A beautiful image of the Orion Constellation, we can see easily the red-orange star Betelgeuse on the upper left and the bright blue-white star Rigel on the

/www.imakinaria.com/archives/orion

bottom right. From: http:/.jpg

A picture of Rigel, beta Orionis showing its blue-white color typical of a very hot massive and young star. From: http://rst.gsfc.nasa.gov/Sect20/A5a.html

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ess massive stars like Betelgeuse. The O and finally decays to N again and d start again.

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eferences:

blications • Freedman R, Kaufmann III W, Universe, 2002, Freeman ed..

R

• Karttunen et al., Fundamental Astronomy, 2000, Springer • Burnham R., Burnham´s Celestial Handbook, 1978, Dover Pu