AST 115 1st Edition Lecture 23 Outline of Last Lecture I. Brightness of Starsa. Magnitudes and examplesII. Definitionsa. Apparent magnitudesb. Absolute magnitudesc. ExamplesIII. More about magnitudesIV. Color Indexa. “Standard” colorsV. Inverse Square Lawa. ExampleVI. Stellar Spectra IntroductionOutline of Current Lecture I. Spectral Classificationa. OBAFGKMII. Types of Starsa. H – R DiagramIII. Binary Starsa. William Herschelb. “Optical Double”c. Categories of Binariesi. Visualii. Astrometriciii. Spectroscopiciv. EclipsingCurrent Lecture Spectral Classificationo Astronomers began sorting stellar spectra using capital letters in alphabetical order: A – O o Later it was learned that the surface temperature determines the appearance of the spectrum.o Many letters were deleted and the remaining seven were re-ordered:These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.O B A F G K M(HOTTEST → COOLEST)(Oh Be A Fine Girl, Kiss Me)o Decimal subdivisions are also used:…F7, F8, F9, G0, G1, G2…G7, G8, G9, K0…o Classes “L” and “T” have been added to the sequence for objects with very low surface temperatures (Brown Dwarfs).o The spectral class of the Sun in G2. This corresponds to a surface temperature of about 6,000 ˚K.o Spectral classification is done by comparing unknown spectra to standard ones.o The goal of spectral classification is the determination of the surface temperatures of stars. Types of Starso H – R Diagram- Henry Noris Russell and Ejnar Hertzsprung discovered a correlation between luminosity and surface temperature of stars.- The underlying physical properties of the H – R Diagram are: Luminosity and Surface Temperature.- The H – R Diagram is an important tool used to keep track of stellar evolution. Binary Starso Binary stars were discovered by William Herschel, who was able to show that stars are gravitationally bound to each other.o Multiple star systems can have 2, 3, or more members, but the term “binary” is still used.o “Optical Double” = two stars aligned by chance; not a binary. - Example: Mizar. The middle star in the handle of the Big Dipper. Adjacent to it is a fainter star “Alcor”.o Recent estimates indicate that 2/3 of stars exist as singles. But slightly more than ½ are members of binary systems.o Categories of binaries (“VASE”):- Visual = 2 stars orbiting a common gravitational center, visible as 2 stars ina telescope.- Astrometric = presence of second star shown by “wavy” proper motion.- Spectroscopic = presence of 2 stars is shown by doubled and/or moving spectral lines.- Eclipsing = 2 stars get in each other’s way because the plane of their orbit is in our line-of-sight.*Note that these categories are NOT mutually
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