ORION  
(April schedule)

     Orion constellation is winter constellation and it is in best position for evening observing in February when it is on the southern sky.
     In January it is rising and is low on the southeastern sky, in February it is in best position for evening observing on the southern sky and in March is low on the southwestern sky ready to set behind the horizon.
     We will observe it in April early in the evening when it is very low on the southwestern sky and try to catch it before it sets behind the horizon.

Double and multiple stars

     Beta Orionis - Its name is Rigel. It is supergiant star with blue-white color with visual magnitude of 0.1. It is young star by age by already late stages of its stellar evolution. It has already stopped fusing hydrogen into helium in its core and thus left the main sequence (evolved away from the main sequence). Now is fusing helium into heavier elements in its core and hydrogen into helium in a shell outside its core. This causes the star to expand, and Rigel diameter is already about 80 times bigger than our Sun diameter. Because of the large size its luminosity is enormous. In the near future will become red supergiant - because of its expansion, its surface will become cooler and its size even larger and most likely will finish its evolution like supernova. Now Rigel mass is about 20 solar masses, its surface temperature is about 12,000 K (stars do not have surface - it is in fact their outer atmosphere which we can see visually) and is only about 8 million years old. Its distance is about 900 light years away. 
     The companion is true companion, gravitationaly connected with Rigel, and their separation on the sky is about 10" ( ten arc seconds). Its is much dimmer with visual magnitude 6.8 and in fact is composed of two main sequence stars, with blue-white color. They cannot be resolved optically, but only with spectrometer, thus such binary stars are so-called spectroscopic binaries or spectroscopic binary system. Their orbital period is about 10 days. Their mass is 2.5 and 2 times bigger than that of our Sun. The companion is about 330 billion km away from Rigel. 
     Delta Orionis - Its name is Mintaka. It is spectroscopic binary system (and also eclipsing binary system) with orbital period of about 6 days. Consists of hot bright giant star and main sequence star. Their mass is about 25 and 8 solar masses, their diameter is about 17 and 7 solar diameters and their surface temperature is about 30,000 K and 26,000 K. The luminosity of the first star is enormous. Very close there is third star, sub-giant, only about 0.25" away. Its mass is 23 solar masses, its diameter is 10 solar diameters, and its surface temperature is about 28,000 K and its luminosity is enormous. The total visual magnitude of all these three stars is 2.2. Its distance is about 1,200 light years.
     The visual companion which we can observe is magnitude 6.9 star (HD36485) and is about 50" away. It is also spectroscopic binary star - one main sequence star and companion with 30 days orbital period. Its mass is 9 solar masses, its diameter is 6 solar diameters and its surface temperature is about 18,000 K.
     Also there is another visual companion in between with 14 magnitude, but it is not known if it is true physical companion to Mintaka or only star in our line of sight. 
     Lambda Orionis - Its name is Meissa. Its visual magnitude is 3.5 and it is hot blue giant star with surface temperature of 35,000 K, 28 solar masses, 10 sun diameters and enormous luminosity. Lies in a large star forming region (covers more than 7 degrees on the sky around Meissa) with the oldest stars only about 5 million years in age. The open cluster of formed stars is called Collinder 69 and Meissa is its most prominenet member. The cluster and also Meissa are about 1,100 light years away from the Earth. Astronomers for such star forming regions use the name HII regions - those are regions of interstellar hydrogen that is now ionized - it is cloud of partially ionized gas where stars are formed and born. The gas was neutral hydrogen which is now ionized from the newly formed hot luminous stars like Meissa with their strong ultraviolet and even weak X-ray radiation. In such regions in our and other galaxies usually thousands of stars are born in a few million years. The estimated age of Collinder 69 is about 5 million years and contains a lot of stars.
     Meissa companion is about 4 arc seconds away and 5.6 visual magnitude. It is a main sequence star with 10 solar masses, 4 solar diameters and surface temperature of 25,000 K. Also two more stars of 11 magnitude close to Meissa can be noticed - one 30 arc seconds away and another 80 arc seconds away.
     Theta1 Orionis - in fact this star on the sky is small open cluster of eight stars. How astronomers observed theta1 with more powerful telescopes, they discovered more stars. The cluster is called Trapezium because its four bright stars form a trapezoid when we look at it through a telescope. The cluster is in the center of the well-known Orion Nebula - one of the most beautiful objects on the sky. It is cluster of very young main sequence stars - only about 380,000 years ago these stars were formed! They are still inside the Orion Nebula in which they were born. Most of these stars are enormous with mass up to 30 solar masses and outer atmosphere temperature of up to 60,000 K. The cluster diameter is only 1,5 light year. These massive stars have enormous radiation and illuminate and ionize the Orion Nebula. When we observe the Orion Nebula with a telescope we can see only small number of stars visually, but on the infrared images of the nebula (because infrared light can penetrate easier through the gas clouds of the nebula) we can count more than thousand stars - all young newly formed stars inside this starbirth region of hydrogen gas. Probably their total number is about 2,000 stars. This and hundreds or more other such spots is where the stars are being born in our galaxy. The diameter of the Orion Nebula is about 25 light years. It is about 1,300 light years away. The above mentioned Trapezium cluster also contains so-called brown dwarfs - these are objects which does not have enough mass to ignite fusion of the hydrogen atoms in its core - these are objects which unfortunately did not have enough lack to become stars! 
     The visual magnitudes of the four brightest stars in the Trapezium which also form the trapezoid are 6.6 / 7.5 / 5.1 / 6.4 and their separation is 9 / 13 / 13 arc seconds.
     When we look at Theta1 Orionis in a telescope, in the same field of view we should also see Theta2 Orionis. It is double star with visual magnitudes 5.0 and 6.2 and separated about 50 arc seconds. 
     Iota Orionis - This star is about 1,300 light years away and their components have visual magnitudes of 2.9 and 7.0 and are separated by about 11 arc seconds. The first 2.9 magnitude component is also binary, but spectroscopic binary - components are too close to each other to be observed visually - in any telescope they will appear like one star. Their period is about 29 days. They are giant star and giant or sub-giant star. Their mass is 23 and 13 times of our Sun, their diameter is 8 and 5 times of our Sun and their temperature is 33,000 K and 27,000 K. They are young but already evolved stars. 
     The 7.0 magnitude companion is giant star about 750 billion km away from the main component. This is also young star.
     Also there is third companion 50 arc seconds away.
     When we observe Iota Orionis, in the same field of view in the telescope we should see another double star Struve 747, with large main sequence components 4.7 and 5.5 visual magnitude and separated 35 arc seconds.
     Sigma Orionis - It is multiple star system. Its visual magnitude on the sky is 3.7 and is about 1,200 light years away. The first (and main) component is in fact binary system separated only 0.25 arc seconds, and very hard to resolve visually with a telescope. They are hot, young main sequence stars, with surface temperatures of 32,000 K and 30,000 K, with 18 and 14 solar masses, They orbital period is 170 years and are 15 billion km apart.
     Next component is a main sequence star with 7 solar masses and about 700 billion km apart from the main component. Its visual magnitude is 6.6 and is separated on the sky about 13 arc seconds from the main component.
     Next component is also a main sequence star with 7 solar masses and about 2,200 billion km apart from the main component. Its visual magnitude is 6.7 and is separated on the sky by 42 arc seconds from the main component.
     The last component is also a main sequence star about 600 billion km apart from the main component. Its visual magnitude is 8.8 and is separated on the sky by 12 arc seconds from the main component.
     Zeta Orionis - Its name is Alnitak. It is part of the open cluster of stars Collinder 70. Its total visual magnitude is 1.8. It is 1,000 light years away. Its main component is in fact triple star system consisting of blue supergiant star, one blue sub-giant which orbits very close to the supergiant with 3 days period and cannot be resolved visually and another companion - giant star with orbital period of 1,500 years which can be resolved and seen on the sky about 3 arc minutes away from the pair and with a visual magnitude of 3.7. All these three components are young stars with 33, 14 and 16 solar masses, 20, 7 and 7 sun diameters and surface temperatures of 28,000, 28,000 and 29,000 K.
     There is another star which can be seen visually close to the system.
     Eta Orionis - Its name is Saiph. It is about 1,000 light years away from the Earth with a visual magnitude 3.3. The main component is in fact eclipsing binary star system with period of 8 days which can not be resolved visually. The second component have period of 9 years and is separated on the sky about 1.7 arc seconds from the main component. It is 60 billion km away from the main component and has visual magnitude 4.9. All three stars are massive blue-white main sequence stars. 

Deep sky objects
(will be observed only from dark location outside of the city)

     M 42 - Messier 42 from the Messier Catalog. It is also called Orion Nebula. It is one of the most beautiful celestial objects on the sky within the reach of amateur astronomers. It is diffuse nebula - a cloud of interstellar gas (mostly hydrogen and helium) and dust located in our galaxy and other galaxies with no defined shape. Can be from several light years to up to several hundred light years across in size. These nebulae are place where the star formation takes place in our and other galaxies. The gas and dust in such nebulae are with very low density, but with time measured in million years will start grouping randomly in many spots. In time scale of several hundred thousand years, because of the gravitation the clumps will attract more and more material from the cloud. Some clumps will reach enough high pressure and temperature inside their cores for the fusion of hydrogen atoms to start. The star is born. It will spend some time as a pre-main sequence star until all processes inside stabilize and start radiating energy and light like normal star. Astronomers discovered a lot of such clumps - stars in forming in the Orion Nebula and other such types of nebulae. These nebulae are also called star-forming regions. There are many hundreds if not thousands such star-forming regions in our galaxy. Thousands of stars can be formed in one such nebula over a period of several million years. As soon as the first massive stars are formed in the nebula, because of their enormous ultraviolet radiation they ionize the hydrogen from which the diffuse nebula is formed. The nebula will also become emission nebula at the same time because of the radiation of the ionized hydrogen (and other gasses which are in very small amount). Such regions in our galaxy and other galaxies astronomers call HII regions and these regions (regions of ionized hydrogen) are sure sign that star forming is going on in that parts of the galaxies.
     Orion nebula is located about 1,300 light years away from the Earth, and is one of the closest regions to the Earth where star formation is taking place. On the infrared images of the nebula up to thousand stars can be counted, and in reality probably the number of stars created inside the nebula is 2,000 or more. In the heart of the nebula the most massive stars are formed recently, only 300,000 years ago (see Trapezium star cluster above). The diameter of the Orion Nebula is about 24 light years and have mass several thousand larger than the mass of our Sun. On the sky the nebula has diameter of more than one degree and is visible with naked eyes from dark locations away from the light polluted urban areas. Studying the Orion Nebula astronomers can learn the processes that lead to answers about stars and planets formation. Here astronomers have already observed hundreds of very young stars, pre-main sequence stars, unborn stars in various stages of forming, brown dwarfs, protoplanetary disc and a lot of other objects and phenomena connected with stars and planets formation. In the future after many more stars will be formed in the nebula, some of the nebula material will be partially spent and the rest of the gas will be dispersed because of the strong ultraviolet radiation from the many stars that were born inside the nebula. The stars will destroy the place they were born. What will remain will be a beautiful open young star cluster of several thousand stars to orbit around the center of our galaxy like thousands other.
     Very close to M42 is M43 nebula. M43 is divided by M42 by a dark line of dust on the sky. In fact, M42 and M43 are one nebula in reality, but the dark line divides the nebula and gives us impression that there are two nebulae close to each other on the sky.