Download Motions of the Stars

The Motions of the Stars
Doppler Effect
Shift in the observed wavelength when the source
is moving relative to the observer.
Examples:
Sound Waves (Siren or Train Horn)
Light Waves
Amount of the shift and its sign depends on
relative speed of the source & observer
direction (towards or away)
Stationary Source
Moving Source
High Pitch
(short waves)
Same Pitch
Same Pitch
Doppler Effect in Sound
High Pitch
(short waves)
Doppler effect in light
Low Pitch
(long waves)
Low Pitch
(long waves)
The Doppler Effect in Light
Amount of Shift depends upon the emitted
wavelength (lem) and the relative speed v:
If the motion is away from observer
–Wavelength gets longer = REDSHIFT
If the motion is towards the observer
–Wavelength gets shorter = BLUESHIFT
Doppler Effect in Practice
Used by astronomers to measure the speeds of
objects towards or away from the Earth.
Other Uses:
Traffic Radar Guns:
• Bounce microwaves or laser light of known
wavelength off of cars, measure reflected
wavelength: Doppler shift gives the car’s speed.
Doppler Weather Radar:
• Bounce microwaves off of clouds, measure speed
and direction of motion. Strength of the reflected
signal gives the amount of rain or snow.
The “Fixed” Stars
Way to Measure Speeds
Observe the wavelength (lobs) of a source with a
known emitted wavelength (lem)
The difference is directly proportional to the speed
of the source, v:
l ( observed)   l ( emitted )
V

l ( emitted )
c
Q: The observed change in wavelength due
to Doppler effect occurs:
A) Only when the temperature of an object
changes.
B) Whenever the light source is moving with
respect to the observer (independent of
the direction)
C) Only when the source moves across the
line of sight
D) Only when the source has radial velocity
towards or away from the observer.
Radial Velocities
To the naked eye, the stars appear “fixed”
to the sky.
No Shift
In reality, the stars are in constant motion.
Great distances make the amount of
motion small on a human lifetime.
Blueshift
Sun
Redshift
Proper motion = how fast a star is
moving relative to background objects
(in arcseconds per year).
Radial Velocity
Motion along the line of sight.
360 degrees in a circle,
60 arcminutes in a degree,
60 arcseconds in an arcminute.
(An arcsecond is a tiny angle.)
Measure using the Doppler Shift of the
star’s spectrum:
• Moving towards Earth: Blueshift
• Moving away from Earth: Redshift
• Moving across line of sight: No Shift
Proper Motions
Barnard’s star has the highest proper motion of any
star: 10.3 arcseconds per year (1 degree per 350 years).
1902
Angular motion across the sky of nearby stars
with respect to distant stars.
• Typical proper motion: ~0.1 arcsec/year.
1952
Reflects true motion of the stars through space
(not parallax)
2002
Discovery of Proper Motions
Proper motions are cumulative
Edmund Halley (1718):
Effects build up over time…
Noticed that the positions of 3 bright stars,
Sirius, Aldebaran, & Arcturus
had moved by about 20 arcminutes from
the positions given by Hipparchus of
Rhodes in 300BC.
• The longer you wait, the greater the
angular motion is.
Measuring proper motions:
• Compare photos of the sky taken at two
separate times
• Measure how much stars have moved.
Example
Case Study: The Big Dipper
Star with a proper motion of 0.1 arcsec/year:
• After 1 year: it moves 0.1 arcsec
• After 10 years, it moves 100.1=1 arcsec
• After 100 years, it moves 1000.1=10 arcsec
It takes many millennia for the constellations
to noticeably change shape.
Today
50,000 BC
More distant stars tend to have smaller Proper
Motions for same transverse velocity:
Today
Can usually measure accurate proper motions
out to distances of ~1000 parsecs.
50,000 BC
50,000 AD
Sun
Space Motions
Want to know the true motion of a star
through space.
Observed motions:
• proper motions: angular motion
• Transverse or tangential velocity: need to
know the Distance and Proper Motion
• radial velocity: motion along line-of-sight
True space motion
• Combine Transverse & Radial Motions
Sun
Why measure space motions?
Most useful when measured for many stars.
Use statistics of the motions to find:
• Motion of the Sun through nearby space
(towards the constellation of Hercules)
• Local rotation of the Milky Way galaxy
Important tool for studying the structure of
the Milky Way galaxy.
To measure a star’s tangential velocity, we
need to measure its:
A)
B)
C)
D)
Proper motion and parallax
Proper motion and radial velocity
Doppler shift and spectral type
Parallax and radial velocity
Parallax or Proper Motion
(1) If observed at the same time of the year,
every year, parallax is zero but proper
motion would be visible.
(2) Parallax is only along the direction of
earth’s motion. Star’s movement in other
directions would imply proper motion.
(3) Parallax has to go back and forth--- but
proper motion is cumulative, i.e. the effect
builds up with time.
A spectrum of a star gives no information
on the star’s
A)
B)
C)
D)
E)
Radial velocity
Tangential velocity
Composition
Luminosity
Temperature