Astro 201: Sept. 30, 2010 Pick up Midterm #1 from piles along
the wall. Correct answers are printed on the scantrons, I will post
keys also and correct mistake on Green Key New Homework 4 on web
site, due next Thursday Next on-line quiz will be posted after
Tuesdays lecture Read Hester, Chapter 15 Today: STARS: Ages of
Clusters Redux, Star-formation After this: Evolution of the Sun,
Stellar endpoints: white dwarfs, neutron stars and black holes
Slide 2
H-R Diagram Hertzsprung- Russell Diagram Plot Luminosity versus
Surface Temperature (or equivalently, Luminosity versus spectral
classification)
Slide 3
Stellar Lifetimes on the Main Sequence: More Massive Stars are
more luminous, and are burning hydrogen more efficiently. They
therefore have shorter lifetimes on the Main Sequence before they
burn up the Hydrogen in their core Mass of the Star (M sun )Main
Sequence Lifetime 110 billion years 5100 million years 1010 million
years Shine bright, die young
Slide 4
After the hydrogen fuel in the core of the main sequence star
is used up, There is no longer enough thermal pressure in the core
to balance gravitational collapse. No more hydrostatic equilibrium
What happens next? Star rearranges itself outer layers expand and
cool Star becomes a red giant or supergiant Eventually more
processes happen and the red giant becomes a supernova or planetary
nebula, and then a white Dwarf, neutron star or black hole more on
this later
Slide 5
STAR CLUSTERS All the stars in a cluster are (1) at the same
distance, and (2) were formed together, so are the same age. Open
Clusters: Young (Less than a billion years old) found in the disk
of the Milky Way typically 100's - 1000's of stars often have gas
and dust Globular Clusters: Contain oldest stars in the Milky Way
-- 12-13 billion years old stars in orbit around center of cluster,
gravitationally bound Typically 100,000 - million stars never have
gas and dust
Slide 6
PLEIADES Open Cluster
Slide 7
Omega Cen: Globular Cluster
Slide 8
Ages of Star Clusters and the HR Diagram In Old clusters, some
of the stars have "left" the Main Sequence and become Red Giants,
white dwarfs, etc. The age of the cluster = the lifetime of the
stars at the "main sequence turnoff" in the H-R diagram.
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Homework #4: Step 1: Find the Turn-off MASS, ignore Blue
Stragglers Step 2: Use the plot on the right to figure out the main
sequence lifetime (GYRS) Of the Turn-off Mass star. This is the age
of the cluster.
Period of pulsation is correlated with absolute magnitude.
Thus, given the apparent magnitude and period of pulsation, you can
derive the DISTANCE to the star.
Slide 16
STAR-FORMATION Key Concepts: * Stars form by the gravitational
collapse of dense clumps of interstellar gas, molecules and dust. *
The collapsing protostar forms a disk and jet. * The collapse stops
when nuclear fusion begins in the protostar core. * When a giant
molecular cloud of interstellar gas forms stars it fragments into
several protostars, and eventually a cluster of stars forms. * If
the clump is too massive it splits into a binary star system. *
Eventually the protostellar disk turns into a system of planets
orbiting the star.
Slide 17
Schematic of Star-Formation
Slide 18
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A real protostellar disk and jet, with dark dust lane running
through the disk:
Slide 20
Two important physical principles which govern what happens
when gas clouds collapse to form stars 1.ADIABATIC Contraction and
Expansion (no source of sink of energy) Gas contracts HEATS up Gas
expands COOLS 2.CONSERVATION OF ANGULAR MOMENTUM When gas cloud
collapses, conservation of angular momentum makes the cloud spin
faster and faster in one plane, and collapse in the other
dimensions
Slide 21
Conservation of Angular Momentum caused the gas to form a
spinning disk.
Slide 22
The Spinning Skater
Slide 23
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Images of Star-Formation in the Milky Way Galaxy
Slide 26
Bow shock by a young stellar object in Orion
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Disk in Orion
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Big star-formation region in the Large Magellanic Cloud: 30
Doradus When a cluster of stars form, the massive stars (which are
short-lived) produce UV photons which blast the surrounding gas and
dust. The UV photons excite the electrons to high energy orbitals
In the hydrogen atoms --> H-alpha emission, red in these
pictures. Black areas are clouds of dust. The green is another
emission line of oxygen.
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Spitzer Space Telescope IR image
Slide 49
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Pillars of Creation HST Picture of part Of the Eagle
Nebula
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Protostars in the H-R diagram:
Slide 55
How long does it take a protostar to "reach" the main-sequence,
i.e. start nuclear fusion? It depends on its mass:
Slide 56
What do stars look like inside when they just finish the
proto-star stage and start their main-sequence phase?
Slide 57
Failed stars: If mass < 0.08 solar masses, nuclear fusion
never starts, and we call the object a "brown dwarf". Gravitational
contraction in brown dwarfs is halted by Electron DEGENERACY
PRESSURE Electrons cannot be smushed so hard that two electrons
occupy the same place at the same time. (Pauli Exclusion
Principle)