Fact-sheet on Orion and the Pleiades,

Transcription

1 Fact-sheet on Orion and the Pleiades, Constellation of Orion Orion is my all time favourite constellation and strides high into the southern evening sky this month, and is a fascinating area of the sky to observe.

2 Next to the Plough (the Big Dipper) Orion is the most recognisable constellation in the night sky, the three stars in a (almost) straight line for the Hunter's belt. These three stars have inspired civilisations for thousands of years. The three great pyramids of Gaza, Egypt, are laid out in the precise orientation as these three stars. Top left is Betelgeuse is the 12th brightest star in the sky, is called Alpha Orionis by astronomers. it is a semi-regular variable star whose apparent magnitude varies between 0.2 and 1.2, the widest range of any Illustration 1: Orion - Picture: Dr. Derek P. Blake. first magnitude star. The star marks the upper right vertex of the Winter Triangle and centre of the Winter Hexagon. The star is classified as a red super-giant because Betelgeuse is one of the largest and brightest stars known. If it were at the centre of our Solar System in place of our sun, its surface would extend past the asteroid belt and possibly to the orbit of Jupiter, wholly engulfing Mercury, Venus, Earth and Mars, a truly enormous star. Betelgeuse is currently thought to lie around 640 light years away, yielding a mean absolute magnitude of c Rigel [bottom right] is the 7th brightest star in the sky. It is called Beta Orionis even though it is brighter than Alpha Orionis (Betelgeuse), this is because Betelgeuse is a variable star. Again we do not have a firm calculation for its distance from Earth, estimates put it between 700 and 900 light years away. Rigel is a blue super-giant, of around 24 the mass of our sun. shining with around 85,000 times the luminosity of the Sun, some problems with sunburn here then. At its estimated distance, it seems to have a physical size of around 71 times the radius of the

3 Sun. The picture above right shows our Sun (Sol) and Rigel to the same scale. In stellar Illustration 2: Orion' Belt - picture Dr. Derek P. Blake navigation, Rigel is one of the most important navigation stars, since it is bright, easily located and equatorial, which means it is visible all around the world's oceans. Bellatrix [top right in the picture] or Gamma (γ ) Orionis, is the 22nd brightest star in the sky. The name Bellatrix is Latin for female warrior, it was also called the Amazon Star, a loose translation of the Arabic name Al Najīd, the Conqueror. It forms the left shoulder of Orion the Hunter. This is a massive star with mass of around 8.4 that the Sun. The temperature of the outer envelope of this star is 22,000 K, which is much hotter than the 5,778K on our Sun. This high temperature is what gives this star the blue-white colour that is typical of B-type stars. The diameter of this star, is 0.72 ± At an estimated distance of 250 light-years this gives a physical size of about six times the radius of the Sun. Saiph or Kappa Orionis makes up the left foot of the hunter, the name Saiph is from the Arabic saif al jabbar, literally sword of the giant however it does not form a part of Orion's sword, which includes the famously interesting Orion Nebula. Saiph is about 720 light years away and times the mass of our Sun, and is about same luminosity as Rigel. It has a much higher magnitude value (2.06) than Rigel due to the fact that, although it is a much hotter star (26,000 K), most of its energy is emitted in the ultraviolet, making it invisible to the human eye, paradoxically making it appear relatively dimmer. Large stars such as Saiph (and many other stars in Orion) are destined to collapse on themselves and explode as supernovae as they are dying stars. Meissa or Lambda Orionis, marks the hunter's head and has a traditional name of Heka, "Meissa" derives from the Arabic "Al-

4 Maisan" which means "The Shining One". Meissa is a blue spectral type O giant star and is part of the Collinder 69 star cluster. It has an magnitude One of the Collinder 69 (Lambda Orionis Association ociation) is a star cluster. Meissa is a double star. This is a wonderful and yet woefully neglected cluster for small telescopes. I've found many of the Cr clusters to be particularly suited to small telescopes. Now on to the very interesting area of Orion's sword, which drops at an angle from the left side of the belt, this is an area that I never tire of observing, it included the fascinating Orion Nebulae. As we have found previously nebula are the birth places of stars, nebulae are actually clouds of cosmic dust and gas, which often comes together over time to form stars and planetary systems. Back in the fifties a star was actually observed 'switching on' within this nebulae. Nebulae are beautiful because the stars within and behind the cloud, illuminates the dust and gas with their various colours. Illustration 3: Orian Nebula or M42 - picture Hubble Telescope The above picture from the Hubble Telescope shows the Orion Nebula to it's full extent. The nebula is a is a diffuse nebula, which means that they are extended and contain no well-defined boundaries, is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases, which glow making the nebula easy to see. The nebula is visible to the naked eye [as my picture above] in the night sky. M42 is about 1,344 and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light years across. It has a mass of about 2000 times the mass of the Sun. Older texts frequently referred to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula. Because of it's relative proximity to Earth, it has studied extensively and has given us a

5 great deal of knowledge on how stars and solar systems form. The Nebula is a part of a much greater nebula known as the Orion Molecular Cloud Complex. The Orion Molecular Cloud Complex extends throughout the constellation of Orion and includes Barnard's Loop, the famous Horsehead Nebula, M43, M78 and the Flame Nebula. Stars are continually forming throughout the Orion Nebula, and due to this heat-intensive process the region is particularly prominent in the infra-red. For anyone wishing to explore further there is a wealth of information on the internet. The Pleiades, M45 Another of my favourite sky objects [pictured right (right of centre)] can be seen with the naked eye and those with good eyesight will be able to see the principle seven stars. The cluster to the upper right of Taurus and, is often called the Seven Sisters, it is one of the brightest and closest open clusters to Earth. The easy way to find it is to draw an imaginary line through the line of stars in Orion's belt extending to the west, you will see a fuzzy cluster of stars (see chart to right). The Pleiades cluster is about 400 light years away and contains over 3000 stars. The cluster is about 13 light years across, The Pleiades, M45 picture Derek P. Blake and is moving towards the star Betelgeuse in Orion [ See above]. Surrounding the brightest stars is a blue 'reflection nebulae' caused by reflected light from many small carbon grains. These reflection nebulae look blue because the dust grains scatter blue

6 light more efficiently than red light. The grains form part of a molecular cloud through which the cluster is currently passing. (Or, to be more precise, did 400 years ago!) Dr. Derek P. Blake With thanks to Jodrell Bank Centre for Astrophysics