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Basic Properties of Stars
Astronomy
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Kirchhoff’s Three Kinds of Spectra
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A Model of a Hydrogen Atom
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Spectral Lines A. Electrons have a definite binding energy.
B. Each element has its own set of energy levels C. If an electron absorbs enough energy, it jumps to a higher energy level. D. When an electron falls, it releases energy in the form of light. E. wavelength inversely proportional to frequency F. Dark lines are produced when a cooler gas absorbs light. G. An emission spectrum shows the chemical element that produced those lines.
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Electron Distances and Energy Levels
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Possible Absorption and Emission Lines for the Hydrogen Atom
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An Emission Spectrum of Hydrogen
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Stellar Spectra A. Predominantly patterns of dark lines on a continuous band of colors. B. Star’s bright visible surface is called the photosphere. C. As light travels through the star’s outer atmosphere, the cooler gases absorb some colors/wavelengths.
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Chemical Composition A. Our sun was the first absorption spectrum analyzed in 1814 by Fraunhofer 1. Fraunhofer lines--strongest dark lines from the sun B. By comparing the dark lines with spectral lines from other elements, we find what’s in the sun.
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Spectral Classes A. Absorption spectra are used to classify stars into 7 types. B. If hydrogen lines are stronger… 1. It’s not because of more hydrogen…ALL stars have hydrogen. Stars are classified in the following order: O, B, A, F, G, K, M “oh, be a fine girl/guy, kiss me !”
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Spectral Classes E. So what’s the difference?
1. Stars at different temperatures display certain lines better than others. The temperature is the difference ! class O stars are hottest….class M stars are coolest.
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The Spectra of Radiation Emitted with Temperatures of 4500 K, 6000 K, and 7500 K
Things will become “bluer” when they are hotter. Stars will become “redder” when they are cooler. If we can find the brightest part of the spectrum of a star, we can find its temperature.
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Temperature B. Every chemical element has a characteristic temperature and density at which its most effective in producing certain lines. C. At extremely high temps.--Helium atoms are ionized; bluer stars (class O) D. Temps. Around 5800 K--metal atoms E. Temps. Below 3500 K--titanium oxide molecules; redder stars (class M)
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Spectra of the Spectral Classes
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The Relative Number of Hydrogen Atoms in the Second Energy Level for
Various Temperatures
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The Number of Hydrogen Atoms with
Their Electrons in the Second Energy Level Compared with the Total Amount of Hydrogen, Whether in Atomic or Ionized Form
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The Relative Numbers of Atoms of Different Elements on a Typical Star
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Other information from Spectral Lines
A. Other info is gathered from spectral lines. B. Collisional broadening--broader lines might show a denser star C. Rotational broadening--broader lines can show how fast a star rotates/spins D. Zeeman effect--split lines show magnetic fields E. Redshift--lines shifted toward the red show a star moving away (blueshift—means star is moving towards you)
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The Spectra of a Rapidly Rotating Star
and a Slowly Rotating Star
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The Doppler Shifts of a Rotating Star
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The Parallax of a Nearby Star
A parsec is a unit of distance such that a star that exhibits a shift of 1” (1 second or 1/3600 of a degree) of arc. This is only an apparent shift of the star in the sky (and it’s very small) as a result of the real motion of the earth around the sun. We are looking at the star at different angles. The distance of a star can be found by observing its parallax angle. The equation is: distance (in pc) = 1 / parallax angle(“) Example: Alpha Centauri has a parallax angle of 0.742”. So its distance from Earth is 1/0.742” = 1.35 parsecs. To convert this to light years (1pc = 3.26 ly): pc x 3.26ly = 4.4 ly
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The Proper Motions of a Nearby and Distant Stars
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Propagation of light Remember that light falls off according to the inverse square law An object 3x farther away will appear 1/32 = 1/9 as bright
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Apparent magnitude (m)
Definition: a measure of how bright a star appears The general rule: the lower the number, the brighter it appears
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Apparent magnitude (m)
The modern magnitude scale is set up so that a difference in magnitudes goes up as an exponential function 2.512(x) Where x is the difference in apparent magnitudes of A and B
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Absolute magnitude (M)
Definition: a measure of how much light a star is putting out into space (its luminosity) The general rule: the lower the number, the more luminous it is Note: you can’t just say, “that star is brighter”…do you mean it appears brighter, or do you mean that it’s giving off more light? Question: Why would it matter? Answer: a really luminous star might appear fainter simply because it’s very, very far away
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Absolute magnitude (M)
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