AST 115 1st Edition Lecture 22 Outline of Last Lecture I. Interior of the Sun (Theory)II. Interior of the Sun (Observations)a. Two ways of making observations of the interior of the SunIII. Introduction to Ch. 10: Characterizing Starsa. Measuring the Distances to StarsOutline of Current 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 IntroductionCurrent Lecture Brightness of Starso Magnitudes- Hipparchus invented the magnitude system about 2,000 years ago1st → 6th magnitude(Brightest → Dimmest)o A difference of 5 magnitude corresponds to a ratio of energy received of 100.0o The number zero and negative numbers are also used.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.- Zero or negative numbers are used for objects that are brighter than the 1st magnitude. Positive numbers bigger than +6 are used for objects too faint to see with the eye alone.…-2, -1, 0, 1, 2, 3, 4, 5, +6, +7, +8, +9…o Examples:Object MagnitudesSun -26.5Full Moon -12Venus -4Jupiter -2.5Sirius -1.5 (brightest star in the night sky)Vega 0 (0th magnitude)Polaris +2Naked eye limit +6 (for most people)Binoculars +9Image with Keck 10-m telescope +30 Definitionso m = apparent magnitude- (a measure of the brightness a star has at its current distance from the Earth)o M = absolute magnitude- (a measure of the brightness a star would have if seen from a standard distance of 10 pc = 32.6 LY)o Examples:Star m M m – MSun -26.5 +5 -31.5Vega 0 +0.5 -0.5Sirius -1.5 +1.5 -3.0Antares 1 -5 +6.0Deneb 1.3 -8.7 +10.0- Distance modulus = m – M(a measure of the distance to a star. The more positive the (m – M) is, the farther the star is)(Deneb is the farthest star from us) More about Magnitudeso The magnitudes listed about are actually known as “visual” magnitudes.o This is because they were determined by using special filters to limit what part ofthe spectrum contributes to the brightness measurement.o It is also possible to use red, blue, etc filters. These are symbolized as…my = apparent visual magnitude = V mB = apparent blue magnitude = Band…MV = absolute visual magnitudeMR = absolute red magnitude Color Indexo It is possible to measure a star’s brightness through different filters and use the results to estimate the star’s surface temperature.o The color of a star depends on its surface temperature: the redder star appears, the cooler its surface is.o Astronomers use filters to measure the brightness of stars in “standard” colors:U = UltravioletB = BlueV = Visual (UBV)o The difference between Blue and Visual magnitude of a star is called the (B – V) “color index” or “color” and is a measure of the star’s surface temperature.o The more positive (B – V) is, the cooler the surface temperature of the star (the redder the star is). Inverse Square Law (of E – M Radiation)o How much energy passes through a unit area at a distance (d) from a source?- For a given detector (eye, CCD, etc) the brightness is the energy received per unit area.o Example:- Two stars, A and B, have the same luminosity (wattage). How much dimmer will star B appear if it is 9 times farther than star A? B 1/d2, so B will appear 192 = 1/81 as bright as star A. Stellar Spectrao In addition to telling what chemicals are present, the spectrum of a star also contains information about surface temperature.o This is NOT learned from the patterns of the spectral lines, but from the darkness(or “strength”) of these lines that tell temperature information.o Each element compound shows different amounts in line strength (darkness) over the normal temperature range: 2,500 to 50,000
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