stars & galaxies in motion 6:30 wednesday, october 15 next planetarium show: tonight, 6:30 pm
TRANSCRIPT
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Stars & Galaxies in MotionStars & Galaxies in Motion
Wednesday, October 15 Next Planetarium Show: tonight, 6:306:30 pm
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Newton Says:
•Objects move in straight lines at constant speed unlessunless a force acts on them. •The Moon moves on a curvedcurved path
at changingchanging speed. •Therefore a force force is acting on the Moon:
that force is gravitygravity.
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Newton: shapeshape of orbit depends on speedspeed of satellite at launch.
Low speed = closedclosed orbit (circle, ellipse).
High speed = openopen orbit (parabola, hyperbola).
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A satellite will have a circular orbit if its initial speed = circular speedcircular speed ( vcirc )
r
GMvcirc
Presented without proof (life is too short).
r = radius of circular orbit M = mass of object being orbited
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To stay in low Earth orbit, a satellite must have v = vcirc= 7.9 km/sec (18,000 mph).
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Gravity makes the Moon orbit the Earth. It makes planets orbit the Sun. What does it do on larger scales?What does it do on larger scales?
Solar Neighborhood: Stars within 13 light-years (4 parsecs) of
the Sun.
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The radial velocityradial velocity of a star is found from its Doppler shiftDoppler shift.
Radial velocity = how fast an object is moving toward you or away from you.
Flashback:
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Results: nearby stars are moving toward and away from the Sun in equal numbers.
More results: nearby stars have radial velocities 20 to 30 kilometers/second.
Comparison: Voyager 1 is moving away from the Sun at 17 km/sec.
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The transverse velocitytransverse velocity of a star is found from its proper motionproper motion.
Transverse velocity = how fast an object is moving from side
to side.
Not a Flashback!
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Proper motion = how fast a star is moving relative to background objects
in arcseconds per year.
A.D. 1950 A.D. 2000
Barnard’s Star
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Barnard’s star has highest proper motion of any star: 10.3 arcseconds per year
(1 degree every 350 years).
All other things being equal, nearernearer stars have higherhigher proper motion. (Barnard’s star is just 1.8 parsecs away.)
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Over tens of thousands of years, proper motionproper motion makes the shape of constellations change.
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Stars in the solar neighborhood move randomly at speeds of about 40 km/sec
relative to the Sunrelative to the Sun.
But… Is it useful useful to think of stars’ velocity relative to relative to the Sun?the Sun?
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PtolemyPtolemy looked at planetary motions relative to the EarthEarth & got a mess.
KeplerKepler looked at planetary motions relative to the Sun Sun
& got neat ellipses.
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The Milky Way Galaxy is a disk. Stars orbit the center on nearly circular orbits.
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How can we measure the speed with which the Sun (& neighboring stars) move around the Galaxy’s center?
Blue dot = Sun Red dot = Galaxy center
Distance to Galaxy center doesn’t change. Therefore,
no Doppler shift.
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If there were distant objects at rest with respect to Galaxy’s center, we could
measure theirtheir Doppler shifts!
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The Galaxy has an entourage of star clusters that (on average) are at rest with
respect to the Galaxy’s center.
They are called “globular clusters” because of their shape.
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A globular cluster contains >100,000 stars in a region <10 parsecs across.
Easy to see. Easy to measure Doppler shifts.
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Globular clusters are blueshifted in the
direction of the Sun’s motion; redshifted in opposite direction.
Size of Doppler shift indicates Sun is moving at 220 kilometers per second220 kilometers per second
around the Galaxy’s center.
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Bright galaxies tend to have one of two shapes.
1) SpiralSpiral galaxies, like the Andromeda Galaxy and the Whirlpool Galaxy.
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Stars in a spiral galaxy go around on neat (almost) circular orbits.
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2) Elliptical Elliptical galaxies, like the galaxy known as M87.
Spiral galaxies are more photogenic, so they appear in all the “coffee table” books.
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Stars in an elliptical galaxy are on disordered, randomly oriented orbits.
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Spiral galaxy:Spiral galaxy: stars are “good citizens”, traveling on orderly orbits,
all moving in the same direction.
Elliptical galaxy:Elliptical galaxy: stars are “individualists”, traveling on chaotic
orbits, all in different directions.
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Why are some galaxies orderly (spiral) & others chaotic (elliptical)?
Let’s see what happens when the Milky Way and
Andromeda galaxies collide.
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When 2 orderly spiral galaxies collide, they become a chaotic elliptical galaxy.
(When 2 orderly cars collide, they don’t become an orderly truck: they become
a chaotic heap of metal.)
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Spiral galaxies are mainly in lower-density regions (like the Local GroupLocal Group which
contains Andromeda, Milky Way Galaxies).
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Elliptical galaxies exist mainly in high-density clusters of galaxies (like the Coma
Cluster, shown here).
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Wednesday’s Lecture:
The Elusive Dark MatterDark Matter
Reading:
Chapter 4