StarsHow Do We Know the Distance to the Stars?How Do We Know Anything About Stars?LuminositySlide 5The Inverse-Square LawStellar SpectraSpectral ClassificationSpectral ClassesMeasuring a Star’s CompositionSlide 11Example: Measuring the Radius of SiriusBinary StarsTypes of Binary StarsEclipsing BinariesSummary of Measurement MethodsThe Hertzprung-Russell Diagram or The H-R DiagramMain Sequence, Giants and DwarfsThe Mass of Stars on the Main SequenceThe Mass-Luminosity RelationLuminosity ClassesSlide 22Slide 23Luminosity Class as Distance EstimatorSummary of the HR DiagramSummary1Stars•Stars are very far away.•The nearest star is over 270,000 AU away! (Pluto is 39 AU from the Sun)•That is equal to 25 trillion miles!•At this distance it takes light 4.3 years to travel from this star. In other words the star is 4.3 light years away.•The space shuttle travels 17,500 miles/hour, at this speed it would take over 160,000 years to get to the nearest star! The Pleiades Star Clusterabout 400 light years away2How Do We Know the Distance to the Stars?Using the size of the shift due to parallax and knowing the distancefrom the Earth to the Sun we canfind the distance to nearby stars.•The ancient Greeks realized that if the Earth moved we should see shifts in the positions of the stars over one year.•We do see these shifts in star positions but they are very,very small.•This shift in an object’s position due to the motion of the observer is called parallax.•The farther an object the smaller the shift and the harder it is to find its distance.3How Do We Know Anything About Stars?•Since stars are so far away they almost always appear just as points of light.•But as we already know we can learn a lot from light! •Light can tell us about a star’s:–surface temperature–distance–motion–rotation–composition•and if the star is part of binary star system we can determine its mass.The Trapezium Star Clusterin the Orion Nebulaabout 1500 light years away4Luminosity•Luminosity is the amount of energy a body radiates each second.•Stars appear brighter or dimmer to us for two reasons:–they are at various distances from us and–some stars are naturally more luminous than other stars•If we know a stars distance and we measure its apparent brightness we can determine its luminosity.•A stars luminosity is related to both its temperature and its radius.•So if we also know a star’s temperature we can determine its radius.Large stars have a higherluminosity than small stars.Hot stars have a higher luminosity than cool stars.5LuminosityFor stars with the same temperature,larger stars are more luminous.For stars with the same radius, hotter stars are more luminous.6•The inverse-square law (IS) is:•B is the brightness at a distance d from a source of luminosity L•This relationship is called the inverse-square law because the distance appears in the denominator as a squareThe Inverse-Square Law24LBdp=7Stellar Spectra•The temperature of a star can be determined two ways.–from Wien’s law–from the presence or absence of certain spectral lines.•Strong Hydrogen lines are only seen in stars that have surface temperatures between 8,000 and 15,000 Kelvin. •The Sun does not show strong Hydrogen lines even though it is more than 70% Hydrogen!A) A spectrum from a star hotter than the Sun with strong Hydrogen lines.B) A spectrum from a star like our Sun.C) A spectrum from a star cooler than our Sun.8Spectral Classification•To understand the properties of stars astronomers gathered hundreds of thousands of stellar spectra. •To understand the patterns they saw they developed spectral classification systems in order to help understand the nature of stars.•The first system was developed in 1866 by Pietro Angelo Secchi an Italian priest and scientist. He grouped stars by their color.•The system used today was developed by Annie Jump Cannon who ordered stars by temperature.Annie Jump Cannon 1863 - 19419Spectral Classes•In 1901 Cannon developed a system where letters were assigned to stars of different temperature.•In the 1920’s another astronomer Cecilia Payne explained why spectral lines change with temperature and confirmed the system that Cannon developed.In order from hottest to coldest starsthe letter classification is O, B, A, F, G, K, MIn this system our Sun is a G star.10Measuring a Star’s Composition•To find the quantity of a given atom in the star, we use the darkness of the absorption line•This technique of determining composition and abundance can be tricky!11Measuring a Star’s Composition•Possible overlap of absorption lines from several varieties of atoms being present•Temperature can also affect how strong (dark) an absorption line is12Example: Measuring the Radius of Sirius•Solving for a star’s radius can be simplified if we apply L = 4R2T4 to both the star and the Sun, divide the two equations, and solve for radius:•Where s refers to the star and  refers to the Sun•Given for Sirius Ls = 25L, Ts = 10,000 K, and for the Sun T= 6000 K, one finds Rs = 1.8R122s s sR L TR L T� � �� � � �=� � � �� � � �13Binary Stars•Most stars in the sky actually exist as part of a group of 2 or more stars.•Two stars gravitationally bound are called binary stars.•Binary stars provide a means of determining the masses of stars.•Other properties can also sometimes be determined from binary stars.The two stars of a binary starsystem orbit around theircommon center of mass.Using Newton’s modified formof Kepler’s 3rd Law the massof the stars can be found.14•If both stars in a binary star system are visible this is a visual binary. The motion of the two stars can be observed directly.•Sometimes only one point of light is seen but the spectra of this star shows two sets of lines. This is a spectroscopic binary. The motion of the two stars can found from the Doppler shift of the lines.Example of a spectroscopic binaryTypes of Binary StarsExampleof a visualbinary15Eclipsing Binaries•When the orbit of a binary is viewed edge-on from Earth the stars may actually eclipse one another. This is an eclipsing binary.•Eclipsing binaries can give information on the radius, mass and shape of the stars.16Summary of Measurement Methods17The Hertzprung-Russell Diagramor The H-R Diagram•The H-R diagram puts into one figure many of the properties of stars already