stellarproperties - Stellar Properties I. COMPOSITION: What...

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Unformatted text preview: Stellar Properties I. COMPOSITION: What elements are present? 1. Take spectrum. 2. Compare spectral lines to lines of wavelengths measured for difierent elements in laboratory. Results: Stars are mainly hydrogen and helium, but contain other elements in small quantities. 2. TEMPERATURE: How hot? 1. Take spectrum. 2. Scan for most intense wavelength (peak 3.). 3. Use Wien’s law (T a. 1/7») to calculate. Results: Stats range in surface temperature fi'om 3000 K to 100,000K. Astronomers use a classification system for stars based on temperature—spectral type. The temperature of a star greatly influences the dominance of certain spectral lines. Stars with surface temperature of l0,000 K have strong (visible) hydrogen lines. For stars like the Sun (5800 K) ionized calcium lines appear stronger than the hydrogen lines—this doesn’t mean that there is more calcium than hydrogen in the Sun. The brightness of spectral lines depends on temperature. Cool stars (3 000 K) have spectral features associated with molecules present. . . Spectra reveal temperature " and composition 2 Analysis of spectral lines gives _ composition (H, He) but temperature influences line dominance (1) Wien's law—“peak” wavelength of emission indicates temperature (2) ; Spectral classification system was _ developed in the early 1900s 7 3. LUMINOSITY: How much energy is radiated? The energy emitted every second by a star depends on its temperature (Stefan’s law [E a T4] gives the energy radiated per square centimeter) and its size (firom radius you can calculate the number of square centimeters of surface area). This isn’t quite so siraight forward; since what we observe on Earth depends on the distance to the star. Here what astronomers do: 1. Take spectrum. 2. Classify the star (OBAFGKM) and from the width of the spectral lines—estimate its luminosity class (indicates whether this is a large or small star). This analysis will allow you to estimate the star’s absolute magnitude (M)——-how bright the star would appear at a distance of 10 parsecs (32.5 LY). 3. This analysis allows astronomers to compare the amount of energy radiated so that they can be compared. Results: There are stars emitting one millionth the Sun’s energy to a few million times the Sun’s energy. Peak u'sible #2 Visible peak is wavelength is red 1 yellow 500 mm rm now she m 1500 we mm ma huh." Us. From the peak wavelength, the temperature is determined \‘ isible peak is blue l P i .i/ 40—25—5145 —io —s ii iii». is 29 as' Appmmmngmmmv) . cram“! Pin-4mg cm.“ u Apparent magnitude is how bright an object appears from Earth (observation) A smaller magnitude indicates a brighter object. i Sirius looks bright because it is relatively close, 8 LY away a Spica looks bright because it is a powerhouse—260 LY away but 100 times more luminous than Sirius There is a relationship between distance luminosity (spectrum), and how bright star appears(observation) from Earth. Annie Cannon 8: others scrutinized 350,000 spectra Systematic progression in ‘ appearance of spectra (Lab #8) due to surface temperature OBAFGKM—Oh, be a fine guy/gal kiss me a Sun is a 62 star a (F9, GO, G1, 62, ...GS, K0, K1, etc.) 4. DISTANCE—How far away is it? 4A. DISTANCE—Stellar Parallax 1, Observe the positions of stars over a year. 2. Measure parallax angle (p). 3. Use formula to solve for distance, d = 1/p. d in par-secs, p in arcseconds Results: This method only works for stars <1000 parsecs (Hipparcos satellite). Actually from the ground it is more like < 50 parsecs/due to atmos heric blurrin . 4B. DISTANCE—Spectroscopic Parallax ii Take spectrum. 2. Classify spectral type and luminosity class. This is most easily expressed as absolute magnitude, M. 3. Measure the apparent magnitude (m) using a telescope. The apparent magnitude is how bright a star appears from the Earth. Very easy to measure! 4. Use the equation relating apparent and absolute magnitudes to distance. Remember: the smaller the magnitude value, the brighter the star. m — M =5 log d— 5 solve for distance, d m — M is called the distance modulus Results: A distance can be estimated if the star is bright enough to get good spectral information, This method is used for their/est majority of stars in our galaxy HR Diagram Absolute Magnitude £2 @30 A0 _F, I Spectral type lo; 000 V- IIgawiL . I - 5200K Surface temperature ’ Km 3300K Using trigonometry to determine distancis is used by surveyou on Earth. 1 (4a) The most accurate distances nre determined using stellar parallax. (Direct observations) u mum ) (4b) m — M = 5 log d - 5 v; m = apparent magnitude, observe ; M = absolute magnitude (3 measure of luminosity), it is the apparent magnitude 3 star has at the standard distance of 10 EC. from spectrum spectral type (Lab 8) (m-M) is called the distance modulus 54% H would the Sun be at a distance of 10 pc? .41 Only 1 AU away, the Sun’s apparent g magnitude is ~26] 7 ' z 1 Solar luminosity = 2 x 102‘ Watts 3 But at a distance of 10 pc (32.5 LY) the Sun’s apparent magnitude would only be +4.8 FAINT! ' "-3 Brightest stars have negative absolute magnitude values How do stars compare? 5 1910 Hertzsprung & Russell independently plottted similar graphs 1 A graph is a comparison—here stellar luminosity & temperature (absolute magnitude & spectral type) Stars are not randomly é scattered on HR Diagram a Sun’s coordinates: G2, 1 L9 (5800 Kelvins; M=+4.8) 3 Most stars lie on the main sequence 3 Also find red giants, white dwarfs, and supergiants ...
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This note was uploaded on 05/18/2008 for the course ASTR 101 taught by Professor Deming during the Spring '07 term at Maryland.

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stellarproperties - Stellar Properties I. COMPOSITION: What...

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