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 • 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
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
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
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
Every square meter of a hot thing emits much more light that a square meter of a cold thing
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
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
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 fuelcore 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