to the stars and beyond
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To the Stars and Beyond. University of Wisconsin – Eau Claire Continuing Education Dr. Nathan Miller Department of Physics & Astronomy. WELCOME BACK!. Main topics of Course. Appearance and motions of night sky objects - PowerPoint PPT PresentationTRANSCRIPT
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University of Wisconsin – Eau Claire
Continuing Education
Dr. Nathan MillerDepartment of Physics & Astronomy
To the Stars and Beyond
WELCOME BACK!
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Appearance and motions of night sky
objects Visit to the planetarium to see sky motions
in 3D (we will walk over together) Telescopes: design and basic use The Lives of the Stars The Universe and the Big Bang Life in the universe and planets where it
may be found
Main topics of Course
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The Stars
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How bright?How big?
How massive?How hot?How old?
What are they made of?What causes them to shine?
How far away?
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First Question: How Bright?
• Hipparchus – 2nd cent. BC. Put many stars in 6 brightness categories
• 1st magnitude = brightest• 6th magnitude = dimmest seen
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• Magnitude 5 star is 100 times dimmer than Magnitude 1 star
• Sun = Mag -26• Brightest star = Mag -1• Dimmest star you can see = Mag 6• Amateur Telescope = Mag 12• Hubble Space Telescope = Mag 25
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But raw brightness doesn’t tell you much about stars themselves.
i.e. A 100-watt bulb held next to your eye appears much brighter than a street light. But which is the more powerful bulb?
You need the distance
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To find Distance, use Parallax
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Parallaxes are small.
• A star with a parallax of 1 arcsecond would be at a distance of 1 parsec (=“parallax second”)
• No stars are this close
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Absolute magnitude:How bright would the star be if it
were at 10 parsecs?
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A star with a brighter absolute magnitude is really putting out more
light than a star with a dimmer absolute magnitude.
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• Apparent Brightness• Absolute Brightness (“luminosity”,”Absolute magnitude”)• Distance
• Give me any two and I will tell you the third
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To study color better, use a prisim to spread out starlight into colors
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Star’s colors are caused by “blackbody radiation”
• http://phet.colorado.edu/en/simulation/blackbody-spectrum
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The Hertsprung-Russell Diagram
- The Rosetta Stone for StellarAstrophysics
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What Russell needed to know (1913):
Spectral types of the nearest stars (Spectra)
Distance of nearest stars (Parallax)
Brightness of nearest stars (photography)
Use Distance and Brightness to get
Intrinsic luminosity
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The basic Hertsprung Russel Diagram:
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Plotted on the graph, most stars are on the Main Sequence
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Every square meter of a hot thing emits much more light that a square meter of a cold thing
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So the main sequence stars are all roughly the same size.
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All the nearest stars plotted:
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Some stars do not fall on the Main Sequence: Giants and White
Dwarfs
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• If something is hot but dim, it must not have many square meters small
• If something is cool but bright, it must have many square meters huge
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So we can find the sizes of stars:
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Draw lines of equal radius on the HR diagram:
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Which of the directions in the following HR diagram correspond to an object which is
contracting?
• A. A.• B. B.• C. C.• D. D.• E. More than one of the above
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Star Clusters• 2 kinds –
• Open Clusters – young, in galactic plane
• Globular Clusters – old, swarm around galaxy
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Pleiades Open Cluster
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Open Cluster Near Galaxy Center
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Open Cluster
M38
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Globular Cluster M2
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Globular Cluster M15
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Clusters and Stellar Evolution
In each cluster:• Stars all made at nearly same time• Stars all the same distance from Earth • Stars in cluster that look brighter really are
brighter
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Zero-Age Main Sequence (ZAMS) –
Position on HR diagram where stars begin H fusion in core
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Core slowly depletes H fuel core shrinks
core heats up higher fusion rate
star gets slightly brighter
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Cluster Main Seq.Turnoff• Bright, high mass stars evolve first
• In older clusters, these stars have started to “turn off” the main sequence
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Which is Older?
A. M41
B. NGC 752
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Evolution of Individual Stars
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Brown Dwarfs
Not enough mass to start fusion, so never really a true star
Still glow through gravitational contraction.
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Very Low Mass Stars
• Universe not old enough for them to have evolved much are still on Main Sequence
• When they do evolve, they will move left on the HR diagram to be White Dwarfs
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Sun-like Stars
• Eventually, they run out of H fuel in their cores
• Core shrinks until it is supported only by “degeneracy pressure”
• H burning continues in shell around core
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Sun will become huge gravity less strong on outer
layers
Outer layers drift off to become “Planetary Nebula”
Core left behind is “White Dwarf”
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As they cool, white dwarfs get:• A. Quite a bit bigger• B. Quite a bit smaller• C. They remain about the same size
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Evolution of Sun (click on image)
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TheRing
Nebula
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THE EVENTUAL FATES OF HIGH-MASS STARS
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2 types of Supernovae
Will concentrate on Type II – Explosion of a massive Star
Type Ia – involves a white dwarf in a binary system
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Star burns H, then He, then heavier and heavier elements up to Iron
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After core is fused to iron, star can get no further energy from fusion
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No fusion in core core collapses material “bounces” outward
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Tycho’s SN -- 1572
Kepler’s SN-- 1604
The last Supernovae Observed in our Own Galaxy
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Supernova 1987A
Nearest supernova observed in modern times
Not in the Milky Way, but right next to us in the Large Magellanic Cloud
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Cas A
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S147
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Veil Nebula
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The Crab Nebula
Close – 2 Kpc away Supernova observed by Chinese
astronomers in 1054 AD (we know from expansion velocities)
In void in ISM – did not sweep up material (that’s why the edge is not well defined)
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Crab in X-rays
chandramovie_sm.mov
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Neutron Stars:
Core of massive star after supernova
Protons and electrons squeezed together, only neutrons remain
Radius of about 12 kilometers
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Pulsars
Point like objects that “pulsed” quickly and rapidly in radio light
Discovered by Joycelyn Bell and Antony Hewish in late 60’s
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The Crab Pulsar
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We observe a pulsar from earth. We contact an alien civilization outside the solar system.
Will they see the object as a pulsar?
A. Yes B. No C. Maybe – it depends
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Black Holes:
The most massive remnants cannot support themselves even after crushing all their material into neutrons
They collapse into black holes
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Black holes are black because their escape velocity is greater than the speed of light (300,000 km/s) – i.e. not even light can escape.
(For comparison the Earth’s escape velocity is 11 km/s)
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Schwarzschild Radius how “big” a black hole is
Rs = 3M (Rs in km, M in Solar Masses)
Schwarzschild radius locates “event horizon”
Anything crossing the event horizon will never be seen from again.
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How do you detect black holes if you cannot see them?
Matter or gas rotating fast around a small point indicates mass must be extremely concentrated
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Where does the material come from? Often a binary companion.
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Which star initially had more mass?A. Black holeB. Companion StarC. It could be either – no way to tell
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If the Sun were instantaneously replaced by a 1 solar mass black hole, what would
happen to the Earth? A. It would rapidly spiral into the black
hole B. It would continue merrily along its
orbit