Stars of the Orion Constellation By Peter Starr 7/04/2004 Orion is one of the most noticeable constellations in the night sky. When I was young I noticed it as the saucepan with a picture frame around it. It was the first constellation I new along with others in that part of the Sky, Taurus, the Pleiades, and Canis Major. Most of the stars looked similar except for the brightness and one was red and another was dim and fuzzy. This is where astronomy started for me. Orion is made up of several bright stars. The most noticeable are Rigel and Betelguese. Rigel is a bright white supergiant and is the 2nd brightest star in the constellation. The brightest is Betelguese and is reddish in colour. Betelguese, a supergiant near the end of its life, also varies in brightness over a 6 year period. [2] The other two prominent stars are Bellatrix and Saiph. Bellatrix is a supergiant and is 4000 times more luminous than the sun [4]. Three stars that look very similar Alnilam, Alnitak, and Mintaka. Through binoculars these appear as blue white stars. They make up the belt of Orion. They are young supergiant stars with relatively short lifetimes. They, along with Rigel and Bellatrix, are about 10 million years old [3]. Mintaka is surrounded by a nebula viewed through a telescope. The famous Horsehead Nebula is found here. Alnilam is also embedded in a nebula. The stars in Orion’s sword are fainter but one has a fuzzy appearance. This is The Great Orion Nebula containing a group of 4 young stars named the trapezium that light up the nebula. The nebula consists mainly of hydrogen [4] and is 10 light years across [9]. Much of Orion is made up of molecular gas clouds, the birthplace of stars. Rigel, Bellatrix, and the three belt stars were probably born from these nebulae [4]. Orion gives us a picture of stars at different stages in their lifecycle, from the new born stars in the trapezium to Betelguese which is close to the end of its life. When viewed in very dark skies like in outback Australia, hundreds of stars leap out that cannot be seen from the cities in Australia. These are stars from the spiral arm of our galaxy where Orion lies. From the observer the major stars of Orion appear as different colours and have different brightness. A keen observer may also notice that the brightness of some stars may vary over time. A common misconception is that the fainter the star the further away it must be. This is far from the truth as Betelguese is three times closer to us than Rigel, even though Betelguese is slightly brighter. In fact at a distance of 10 parsecs from Rigel it would appear as an extremely bright –7 magnitude star [4] and betelguese as –5.1 [9]. Betelguese is so large, it is one of the few stars that can be seen as a disc using the Hubble Space Telescope [10]. It may vary in size by up to 60% [11] and therefore its brightness. Betelgeuse [10] Colour and brightness can tell us a great deal about the stars. The colour of a star can tell us what the surface temperature is. The brightness of a star can tell us how large the star is if we know its distance from us. The distance can be determined by analysing the spectra of the light coming from the star. These properties also indicate what stage of life the star is in, what the lifetime of the star is and can tell us the mass of the star. The stars in Orion differ in luminosity, i.e. the amount of light over all wavelengths emitted per second. The hotter a star is the more luminous it is for main sequence stars. To determine the luminosity of a star we have to know how far away the star is. This can be calculated by measuring the parallax when the Earth is on either side of the sun during its orbit [7]. The star will appear to change position. The degree of change in the position, the distance of the star can be calculated. This works for stars up to 100 parsecs. The brightness a star would appear if we were 10 parsecs away is called the absolute brightness. This can be calculated by knowing how far away the star is and the apparent brightness using the inverse square law [1]. The absolute magnitude or luminosity of a star is determined by the amount of energy that is produced within the core of a star and this is dictated by how massive the star is. The energy produced in 90% of stars including our sun is caused by fusing hydrogen into helium. This reaction called the proton-proton chain, produces energy in the form of gamma rays. The gamma rays are quickly absorbed by atoms within the core and are re-emitted into many more photons but at a lower energy as they radiate from the core. The temperature in the core must be greater than 8 000 000 K for hydrogen burning to take place. This is the temperature required for 2 protons to overcome the electrostatic force. As mentioned above the colour of a star tells us what the surface temperature is of that star. Betelguese being red is relatively cool at 3500 degrees Kelvin (K). Rigel being white is 12 000K. Minatak, Alnilam, and Alnitak are blue white stars and have surface temperatures of 50 000 to 60 000K. Stars behave as blackbodies. Any object that behaves as a blackbody will radiate electromagnetic radiation. The temperature of the object tells us what wavelength the radiation is. The cooler an object is the less energy is radiated and therefore the longer the wavelength. The hotter the object is the more energy is radiated and the shorter the wavelength the radiation is. To achieve a red colour the temperature needs to be 3500K like the surface of Betelguese, yellow is 5800K like our sun, and blue 12 000K like Rigel. Bellatrix at 21 500K appears blue but is brighter in the UV range of the spectrum. Therefore a stars surface temperature can be determined by colour. The luminosity of a blackbody is related to its temperature. This relationship is the Stefan-boltzman Law [8]. The luminosity is proportional to the power of four of temperature. Therefore a small increase in temperature gives a huge increase in luminosity. Two stars of the same temperature or colour but have different luminosities means that the more luminous star is larger. From this we can determine the radius of a star. For example Rigel has a similar luminosity to Betelgeuse but Rigel gives off 10 000 times more energy per unit area. Therefore Betelguese must be 100 times larger in radius than Rigel. In fact if Betelguese was placed where the sun is, its radius would extend between the orbits of Mars and Jupiter. The bluer the star appears the more luminous the star is and therefore the more energy it gives off (the hotter it is). Therefore the flux of Bellatrix (blue) and Rigel (white) is much higher than Betelguese (red). Rigel is 58 000 times more luminous than the sun, Bellatrix is 4000 times, and Betelguese is 10 000 times. The bluer a star is, the hotter the star is and therefore the more massive it must be. More mass means more gravity acts on the core of the star. This higher compression leads to much higher temperatures and the faster hydrogen burning occurs. Therefore stars like Rigel are much more massive than our sun. By analysing the spectra of stars we can peer into the chemical composition of the upper layers of a star. This information can also tell us what stage in the lifecycle the star is, its spectral class and therefore its size, how far away the star is and if it is a binary star it can give clues on the masses of the 2 stars. Not bad for describing a star at a distance of many light years. Mintaka is a binary and was discovered by spectroscopic parallax [13]. The spectra of stars show up a series of dark lines called absorption lines. These are caused by atoms in the atmosphere absorbing certain wavelengths of light as electrons move up to higher energy levels in the atom. Different types of atoms absorb at different wavelengths, so we are able to determine what atoms are in the atmosphere of the star. Different types of atoms absorb light at certain temperatures. For example Hydrogen electrons jump from level 2 to higher levels at 9 000K [1], but very poorly at 500K.Hydrogen produces a series of lines and they are known as the Balmer series. Calcium absorbs well at 4000K but not at 10 000K. Assuming that all stars have similar compositions we can estimate the temperature of the star by the absorption lines observed. A system of spectral classes was developed in the 1800s. They are designated by letters, O, B, A, F, G, K, and M. These were further divided by numbers 0 to 9. eg Mintaka is B0, Bellatrix B5, Rigel is B8, our Sun is G2, Beteguese is M2. So O are the hottest stars and M are the cooler stars. The rest are in between Stars are further classified on a chart of Luminosity (absolute Magnitude) versus Surface Temperature (Spectral Type).[5]. This chart is called a Hertzsprung Russell diagram. [8] [8] The x axis increases with luminosity and the x axis drops in temperature to the right. 90% of stars fit on a narrow band from the top left to the bottom right. These stars are referred to as main sequence stars. These stars are all burning hydrogen to helium in their cores. The stars spend most of their lifetime here. The blue stars at the top left are very luminous and hot. These stars are very large and have short lifetimes (millions of years) as they burn their hydrogen very quickly. Rigel, Bellatrix, and the 3 stars in the belt of Orion fit into this category. Bellatrix has probably consumed its hydrogen and will start burning Helium in a few million years. It will change colour to orange as it expands and its outer layers cool. Its position in the HR diagram will change to move to the right [7]. This is what has happened to Betelguese which was once a smaller (still a giant) blue white main sequence star. It has now consumed its hydrogen and is burning helium into oxygen and carbon and its radius has expanded dramatically. Due to the expansion the outer layers of the star are less dense and therefore the temperature is cooler. This results now in its red colour. Yellow stars like our sun are in the middle of the main sequence, they are medium in size and therefore burn their hydrogen at a slower rate and live much longer, 10 billion years. The red dwarfs at the bottom of the main sequence are cool and not so luminous. They burn their hydrogen extremely slowly and may live for a 100 billion years. Two groups of stars are found on the upper right hand side of the chart. These are the giants (10 to 100 times larger than the sun) and the supergiants (1000 times larger than the sun). They are very luminous but have cool surface temperatures. These stars near the end of their life and have finished most of their supply of hydrogen and are converting helium to carbon and oxygen. The supergiants are converting carbon and oxygen to heavier elements such as neon and magnesium. Betelguese fits into this category. Another group lies in the bottom left corner. These are the white dwarfs. They are not very luminous but are very hot. These are stars at the end of their life. Protostars are found to the right of the main sequence. The time spent there is relatively short especially the ones destined to be A. O, and B stars. Many protostars exist in the Orion Nebula which formed from the gas and dust within the cloud. We can determine whether a star we see as orange is a dwarf or an orange giant by examining its spectra. In giants the lines are very thin and in the main sequence star the lines are broader. This is because the atmospheres in the giants are much more tenuous as the star has expanded to cover a larger volume of space. So by observing the colour of a star and determining whether it is a main sequence star or not, the luminosity or absolute magnitude can be calculated by reading off the HR Chart. From this the distance of the star can be calculated using the inverse square law. The radius of the star can be also calculated from the colour as the temperature can be derived and luminosity. Bibliography [1] Universe, 6th ed, Freedman, Kaufmann III [2] http://www.museum.vic.gov.au/planetarium/constellations/orion.html [3] http://www.gb.nrao.edu/~rmaddale/Education/OrionTourCenter/belt.html [4] http://www.cvilleastro.org/casnews/current/stars%20of%20orion.htm [5] http://www.slider.com/enc/24000/Hertzsprung-Russell_diagram.htm [6] http://www.computing.edu.au/~bvk/astronomy/HET603/atlas/html/writeUp.html [7] http://www.astro.uiuc.edu/~kaler/sow/bellatrix.html [8] http://members.ozemail.com.au/~swadhwa/chap4lec3.html [9] Extreme Stars, James Kaler, Cambrige University Press. [10] http://antwrp.gsfc.nasa.gov/apod/ap980419.html [11] http://einstein.stcloudstate.edu/Dome/constellns/betelgeuse.html [12] http://www.allthesky.com/constellations/orion/constell.html [13] www.astro.uiuc.edu/~kaler/sow/mintaka.html