Download Stellar Motions

10/17/2012
Stellar Motions
Lecture 7
EVERYTHING MOVES
AND IT IS
RELATIVE MOTION
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The Doppler Effect
• The Doppler Effect is a change in the observed
frequency of light due to relative motion.
• Only the motion along the line-of-sight
matters.
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Water wave example.
Waves
Boat
• When the boat is traveling into the waves, the
peaks hit the bow more rapidly than if the boat
were standing still.
Example (cont’d)
Waves

Boat
Likewise when the boat is traveling away from the
waves, the peaks hit the rear of the boat less
rapidly than if the boat were standing still.
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Sound waves
• The frequency of sound waves increases as a
source approaches the observer, and
decreases as it recedes.
• You may have noticed this happens when a car
or train passes by you.
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Electromagnetic Waves - Light
• The same thing happens with light.
• Because atoms emit light at discrete
frequencies we can detect their motion
(velocity of motion) by a “shift” in frequency
from the expected one.
Spectral Line reminder
Hot Gas
Emission Line Spectrum
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Stationary Source
Blue
Red
Blue
Red
Blue
Red
Approaching Source
Receding Source
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Blueshift and redshift
• Approaching sources
– Spectral lines shifted to higher frequencies
– => short wavelengths.
– Spectrum is blueshifted
• Receding sources
– Lines move to lower frequencies
– => longer wavelength
– Spectrum is redshifted
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Doppler shift example
• In a star, the Balmer line H is observed at a
wavelength of 6565 A. What is the star’s radial
velocity? (H rest  = 6563 A.)
• Star is receding from us. (longer )
vr 



c
2A
3  105 km / s
6563A
So vr = 91 km/sec
Importance of Doppler Effect
• The Doppler effect is very important because
it gives us our only way of measuring the
motions of distant objects.
• As we shall see later, the Doppler effect
allowed Edwin Hubble to deduce the universe
was expanding.
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Proper Motion
• In 1718, Edmond Halley found that Sirius,
Aldebaran and Arcturus had moved nearly a
degree from where Hipparchus charted them
1850 years earlier!
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Barnard’s Star
• Fourth-closest star to the Sun
• Highest proper motion observed
– 10 arcsec/yr!
Signs of the Zodiac in Trouble!
• Ursa Major (Big Dipper)
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Space Velocity
Vr = Radial Velocity (Doppler Effect)
VT = Tangential Velocity, across the line of sight
v = Real space velocity, magnitude and direction
D = Distance
Local Standard of Rest
• LSR: Average motion of stars in our vicinity.
– The Local Frame
• The Sun's motion relative to the LSR is 20
km/sec towards the constellation Hercules.
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Stellar Peculiar Motion
• Motion of a star relative to the LSR.
– About a few tens km/sec.
• High velocity stars, a few hundred km/sec.
• LSR is in motion around the galactic center at
~ 250 km/sec.
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