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Unformatted text preview: Doppler Effect 1. Stationary source 2. Source is moving Source receding So rce roa hin
Ii 3PI“ 6 g Observer sees redshift Observer sees blueshift Both observe same wavelength (7») Hy HB 'HO'.‘ 1 source ' i i . 5:31
source I , 7 l _ shorter 7»
2A approaching i l
2B source redshift
receding longer A 3. Greater velocity greater produces greater shift A i . ..
4340A 4861A ' 6563A What astronomers observe as a result of radial motion appears in the spectrum of a star.
The pattern of spectral lines with known (laboratory measured) wavelengths is either red shifted (moving away) or blue shifted (moving toward). Direction of radial motion is determined by red (away) or blueshift (toward).
From amount of shift, the velocity can be determined. Stellar Properties (Part 2) 5. RADIAL MOTION—How fast is it moving?
This is motion toward or away from the observer. 1. Take spectrum.
2. Identify the spectral lines with known wavelengths. 3. Measure the amount of red shiﬂ or blue shiﬁ.
4. Calculate the approach velocity or velocity of recession. Stars moving toward us have spectral lines shiﬁed to shorter wavelength: blue shiﬂ.
Stars moving away from us have spectral lines shifted to longer wavelength: red shiﬂ.
The greater the shiﬂ the greater the velocity. Results: Stars typically are moving at velocities of a few
up to 1000 [an/sec. due to their motion around the center of the galaxy. Spectroscopic binaries show shifting spe
“Stationary” positions of H-alpha, beta,
very far away. Algol’s changes in brightness
are due to eclipses Orbit is seen edge
on from Earth Brightness and time
= > Light Curve From observations
get masses and radii 7. RADIUS: How large in extent? Identify eclipsing binary stars: 1. Observe a series of eclipses. 4 2. Plot a graph known as a light curve. 'This shows how
the brightness changes over time. 3. Measure the duration of eclipses (prirnary’large drop
in brightness, secondary—smaller drop in brightness). 4. Take the spectrum and use the Doppler eﬁect to ﬁnd
orbital Velocity. 5. Calculate the size of the star causing the eclipse (this is
just velocity times the duration of eclipse). (A larger star
would cause a longer eclipse ifthe orbital velocity were the same.) Results: Stars basically range in size from 1/100 of a solar
radius (~size of Earth) to several astronomical units. . 6. MASS: How much matter?
Identify stars with companions:
Visual—movement over decades I e-r" Spectroscopic—Doppler shifted lines regularly repeat 1: Determine orbital period and separation (may be
difﬁcult if distance and tilt of orbit are unknown)
2. *Use Newton’s version of Kepler’s third laW' MA + MB : a3 / P2 I
If you know distance and orbit tilt can get individual
stellar masses. Results: Stars range in mass for a few tenths of a solar
mass to about 100 solar masses. \/ ctral lines that are double except when there is no radial motion.
and gamma are shown. Earth is located at t he bottom of the page, we“? '
no HQQB CD 9‘"
., t x
x 4 l ...
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- Spring '07