to the stars and beyond
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University of Wisconsin – Eau Claire
Continuing Education
Dr. Nathan MillerDepartment of Physics & Astronomy
To the Stars and Beyond
WELCOME BACK!
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
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 StellarAstrophysics
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. A.• B. B.• C. C.• D. D.• E. 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)
TheRing
Nebula
THE EVENTUAL FATES OF HIGH-MASS STARS
2 types of Supernovae
Will concentrate on Type II – Explosion of a massive Star
Type Ia – involves a white dwarf in a binary system
Star burns H, then He, then heavier and heavier elements up to Iron
After core is fused to iron, star can get no further energy from fusion
No fusion in core core collapses material “bounces” outward
Tycho’s SN -- 1572
Kepler’s SN-- 1604
The last Supernovae Observed in our Own Galaxy
Supernova 1987A
Nearest supernova observed in modern times
Not in the Milky Way, but right next to us in the Large Magellanic Cloud
Cas A
S147
Veil Nebula
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)
Crab in X-rays
chandramovie_sm.mov
Neutron Stars:
Core of massive star after supernova
Protons and electrons squeezed together, only neutrons remain
Radius of about 12 kilometers
Pulsars
Point like objects that “pulsed” quickly and rapidly in radio light
Discovered by Joycelyn Bell and Antony Hewish in late 60’s
The Crab Pulsar
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
Black Holes:
The most massive remnants cannot support themselves even after crushing all their material into neutrons
They collapse into black holes
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)
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.
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
Where does the material come from? Often a binary companion.
Which star initially had more mass?A. Black holeB. Companion StarC. It could be either – no way to tell
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
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