Transcript To the Stars and Beyond - University of Wisconsin

To the Stars and Beyond
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University of Wisconsin – Eau Claire
Continuing Education
Dr. Nathan Miller
Department of Physics & Astronomy
WELCOME BACK!
Main topics of Course
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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
The Stars
How bright?
How big?
How massive?
How hot?
How old?
What are they made of?
What causes them to shine?
How far away?
First Question: How Bright?
• Hipparchus – 2nd cent. BC. Put many
stars in 6 brightness categories
• 1st magnitude = brightest
•
6th magnitude = dimmest seen
• 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
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
To find Distance, use Parallax
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
Absolute magnitude:
How bright would the star be if it
were at 10 parsecs?
A star with a brighter absolute
magnitude is really putting out
more light than a star with a
dimmer absolute magnitude.
• Apparent Brightness
• Absolute Brightness (“luminosity”,”Absolute magnitude”)
• Distance
• Give me any two and I will tell you the third
To study color better, use a prisim
to spread out starlight into colors
Star’s colors are caused by
“blackbody radiation”
•
http://phet.colorado.edu/en/simulation/blackbody-spectrum
The Hertsprung-Russell Diagram
- The Rosetta Stone for Stellar
Astrophysics
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
The basic Hertsprung Russel Diagram:
Plotted on the graph, most stars
are on the Main Sequence
Every square meter of a hot thing
emits much more light that a
square meter of a cold thing
So the main sequence stars are all
roughly the same size.
All the nearest stars plotted:
Some stars do not fall on the Main
Sequence: Giants and White
Dwarfs
• 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
So we can find the sizes of stars:
Draw lines of equal radius on the HR diagram:
Which of the directions in the following HR
diagram correspond to an object which is
contracting?
•
•
•
•
•
A.
B.
C.
D.
E.
A.
B.
C.
D.
More than one of the above
Star Clusters
• 2 kinds –
• Open Clusters – young, in galactic
plane
• Globular Clusters – old, swarm
around galaxy
Pleiades
Open Cluster
Open Cluster
Near Galaxy Center
Open
Cluster
M38
Globular
Cluster M2
Globular
Cluster M15
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
Zero-Age Main Sequence (ZAMS)
–
Position on HR diagram where
stars begin H fusion in core
Core slowly depletes H fuel
core shrinks
 core heats up
 higher fusion rate
 star gets slightly brighter
Cluster Main Seq.Turnoff
• Bright, high mass stars evolve first
• In older clusters, these stars have started
to “turn off” the main sequence
Which is Older?
A. M41
B. NGC 752
Evolution of Individual Stars
Brown Dwarfs
Not enough mass to start fusion, so never
really a true star
Still glow through gravitational contraction.
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
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
Sun will become huge
 gravity less strong on outer
layers
Outer layers drift off to become
“Planetary Nebula”
Core left behind is “White Dwarf”
As they cool, white dwarfs get:
• A. Quite a bit bigger
• B. Quite a bit smaller
• C. They remain about the same size
Evolution of Sun
(click on image)
The
Ring
Nebula
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Will concentrate on Type II – Explosion of a
massive Star
Type Ia – involves a white dwarf in a binary
system
After core is fused to iron, star can get no
further energy from fusion
The last Supernovae Observed in our Own Galaxy
Kepler’s SN
-- 1604
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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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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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Core of massive star after supernova
Protons and electrons squeezed together, only
neutrons remain
Radius of about 12 kilometers
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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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A. Yes
B. No
C. Maybe – it depends
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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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Rs = 3M (Rs in km, M in Solar Masses)
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Schwarzschild radius locates “event horizon”
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Anything crossing the event horizon will never
be seen from again.
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Matter or gas rotating fast around a small point
indicates mass must be extremely concentrated
Where does the material come from? Often a binary companion.
Which star initially had more mass?
A. Black hole
B. Companion Star
C. It could be either – no way to tell
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A. It would rapidly spiral into the black hole
B. It would continue merrily along its orbit