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Download Lecture 6: Properties of Stars The Constellations The Constellations
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Lecture 6: Properties of Stars o Overview: The Constellations o Ancient astronomers grouped stars into constellations representing people, animals, etc. These have no real significance. Orion Constellation o Constellations Betelgeuse o Stellar Brightness o Below are brightest stars that make up Ursa Major - also called ‘The Plough’ or the ‘Big Dipper’. Look north this evening. Grid lines indicate position on the celestial sphere; Right Ascension (RA) and Declination (Dec). o Stellar Distances o Colour and Temperature o The Hertzsprung-Russell Diagram NASA Solar Dynamics Observatory Rigel o Chapter 6 of Introduction to Astronomy and Cosmology The Constellations o Position on sky – the celestial sphere – is given by Right Ascension (RA) and Declination (Dec) - celestial longitude and latitude. Apparent Magnitudes o Greek astronomer Hipparchos made first known catalogue of stars in ~130-160 BC, which was added to by Ptolomy in ~150 AD. o Hipparcus grouped stars into six magnitude groups, with 1st magnitude being brightest and 6th the faintest. o In 19th century, it was shown that stars of a given magnitude are ~2.5 times brighter than those of next fainter magnitude. o 1st magnitude stars are ~100 times brighter than 6th magnitude stars. o Observed magnitude are called apparent magnitudes. Hipparchus of Rhodes (180-125 BC) Apparent Magnitudes o Norman Robert Pogson (1829-1891) put magnitude system on a precise scale. Realized eyes respond to light logarithmically. o If F1 is flux (Wm-2) from Star 1 and F2 is the flux from Star 2 then m2 – m1 = 2.5 log10( F1 / F2 ) Stellar Distances o Stellar distances are measured using parallax. o Distant stars used as reference points. Closer star appears to move relative to distant stars during Earth’s orbit about Sun. o Parallax angle: p ~ 1 AU / d => d = ~ 1 AU / p where m2 and m1 are magnitudes of Star 2 and 1. o The zero point of system is defined by the star Vega, which is magnitude zero. o Some stars like Sirius and Canopus are brighter than Vega, so have negative magnitudes. Stellar Distances o As angle and distance are inversely related, astronomers use a unit called parsec (pc): Absolute Magnitudes o If all stars were at a fixed distance, their brightness would be a true indication of their relative luminosity. d=1/p where d is the distance in parsec and p is the parallax in arcseconds. o Used this idea to set up absolute magnitude scale. o The absolute magnitude is defined as the apparent magnitude at a distance of 10 pc. o A star with a parallax of 1/10th of an arcsecond will be at a distance of 10 pc. o Note: 1pc = 3.26 light years o Using o Nearest star has a parallax of 0.772 arcsec => distance in light years: o As flux (F) varies as 1 / d2 => mabs – mapp = 2.5 log10( ( dabs / dapp )2 ) o Setting dabs = 10 pc => d = 1/ 0.772 pc = 1.295 pc = 1.295 x 3.26 light years = 4.22 light years mabs – mapp = 2.5 log10( Fapp / Fabs ) mabs = mapp + 2.5 log10( ( 10 / dapp )2 ) o Example: Rigel is at 237 pc and an apparent magnitude of 0.12. What is its absolute magnitude (at 10 pc)? Colour and Temperature The Sizes of Stars o Colour tells us about temperature. o Angular sizes of a few nearby stars can be measured directly. o Use Wein’s Law for black bodies: o Diameter = distance x angular width in radians o For Betelgeuse, diameter ~1 billion km or 700 times diameter of the Sun. T = 2.897 x 10-3 / λmax o o o o o Blue stars: White stars: Yellow stars: Orange stars: Red stars: λmax ~ 20,000 K ~ 10,000 K ~ 6,000 K ~ 4,500 K ~ 3,000 K o Temperature has a major effect on the spectral lines seen in atmosphere of a star The Sizes of Stars o Optical interferometer uses two or more mirrors separated by tens of metres to measure much smaller stellar diameters. o Two 8.2 m telescopes of Very Large Telescope at European Southern Observatory in Chile used to measure diameter of Promixa Centauri, nearest start to Earth ~ 1/7th that of Sun. Stellar Luminosity o Stars have a very wide range of intrinsic luminosity – their energy output across the whole electromagnetic spectrum. o Luminosity can be calculated using Stefan-Boltzmann Law: L = σ A T4 watts where A is area, T is temperature and σ is Stefan-Boltzmann constant (5.671 x 10-8 W m−2 K−4) o Luminosity of Sun, Lsun, is 3.86 x 1026 watts. See more at www.eso.org/public/images/eso0134a/ Stellar Spectra Stellar Spectra o In 1666, Isaac Newton, using a prism, showed that sunlight composed of all the colours of spectrum. o In 1804, William Wollaston observed dark lines in spectrum. In 1911, Joseph Fraunhofer mapped many of these “absorption lines” with reasonable accuracy - called Fraunhofer lines. ‘Na D lines’ In late 19th century, spectra of thousands of stars were photographed and classified into spectral types by a Harvard astronomers – mainly women. o O type stars are hottest (30,000 – 60,000 K). Contain ionised helium (He+ or He II). o M type stars are the coolest (<3,500 K). Molecular lines present. Slit position 60,000 K λ o o Temperature Kirchoff and Bunsen found wavelengths of absorption lines seen in Sun corresponded to emission lines observed when the atoms of a particular element are excited e.g., salt gives orange colour due to pair of sodium “D lines”. 3,500 K λ Hertzsprung-Russell Diagram Hertzsprung-Russell Diagram with Sizes o In early 1900s, Hertzprung and Russell independently plotted stellar luminosity against temperature – the H-R diagram. o Main Sequence contains 90% of stars. o Sun is on Main Sequence – G2 star. o Top right has bright cool stars – Red Giants. o Betelgeuse in Orion is a red supergiant. o Rigel in Orion is blue supergiant. o White dwarfs are small and hot. Sun Lecture 6 – Practical Task o Identify the constellations of Lyra and Cygnus in the night sky – they are almost overhead in the early night sky. Vega: 5th brightest star in sky. It is a bluewhite star having magnitude of 0.03, and lies 26 light years away. It weighs ~3MS is ~50 times brighter. Cygnus X-1: X-ray source ~14.8MS Thought to be a black hole. M57 - Ring Nebula