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Astronomy 1020
Stellar Astronomy
Course Announcements
• Mar. 31 (Fri) – Last day to drop a class with W, F, FA
SW chapter 15 – Due Monday 4/3
SW chapter 16 – Due Monday 4/10
SW chapter 17 – Due Friday 4/14
SW chapter 18 – Due Wednesday 4/19
SW chapter 19 – Due Wednesday 4/26
SW chapter 20 & 21 – Due Thursday 5/4 – 10:30 am – no late accepted
The LAST 1st Quarter night is Monday 4/3 at 7:30pm -- On campus
• Reports are due at Class Time on Monday April 24.
• EXAM-3: Wed. 4/5 – Chapters 13, 14, 15, 16
 Main-sequence
stars fuse
hydrogen to helium
in their cores.
 Eventually, much of
the core H is
converted to He.
 A core of He ash is
built up (does not
fuse at this point).
Helium Core Is Degenerate
 H fusion only takes place in a shell around the 100
percent He core: hydrogen shell burning.
 If H fusion is not happening in the core, the star is
no longer main sequence.
 Since the He is not fusing, gravity begins to win
over the pressure, crushing the He.
 The core becomes more dense, and becomes
 This means pressure is not from moving atoms,
but from a quantum mechanical effect: There’s a
limit to how tightly electrons can be packed
 When the fuel runs out
of the core, the
luminosity increases.
 When the core shrinks,
its gravitational pull gets
 Weight of the outer
layers increases.
 This results in increased
pressure: Fusion in the
shell goes faster.
 Faster nuclear reactions
release more energy.
 This leaves the star’s
surface at a higher rate
(higher luminosity).
 Increase in pressure and luminosity results in
increased size and decreased surface
temperature: red giant.
 H-R diagram: Star moves up and to the right.
iClicker Questions
Stellar Evolution:
Main Sequence Lifetimes
Core Stages 1 – option 1
 He core is small, dense, electron-degenerate.
 Outer envelope is greatly expanded, cooler.
 Fusion of H in shell creates more He, making
He nuclei in core denser and hotter.
 Once hot enough, fusion of He begins in the
degenerate core.
 He fuses to carbon (C) via the triple-alpha
process  starts suddenly in the helium
 Star shrinks and heats up.
 After the helium flash, the star is on the
horizontal branch of the H-R diagram.
 At first, He  C in the core, H  He in a shell
around the core.
 Star is smaller and hotter.
 Helium is then used up in the core.
 He fusion in an inner shell and H fusion in an
outer shell all surrounding a C core.
 Star gets more luminous and cool, and enters
the asymptotic giant branch (AGB).
 As an AGB star, the star expands even more
than as a red giant, and cools.
 H-R diagram: moves up and to the right
 Dense, electron-degenerate carbon core.
iClicker Questions
Stellar Evolution:
Core Stages 1 – option 4
After the AGB: Planetary Nebula
 The star is very thinly spread.
 Cannot hold on to the outer layers easily.
 Outer layers are ejected into space, due to
instabilities in the interior.
After the AGB: Planetary Nebula
 The ejected material creates a planetary
 The core shrinks and first gets very hot, but
eventually cools into a compact white dwarf.
 If the conditions are right, the star will ionize
the gas in the expanding outer layers.
 Will last for about 50,000 years before the
gas expands too far and disperses.
iClicker Questions
Stellar Evolution:
Sun Stages
Planetary Nebula Metals
Planetary Nebula Spectra
 Leftover core of
star remains as
white dwarf.
 They are hot,
but not very
 Masses 0.6–1.4
M, size like
 Density: a ton
per teaspoonful!
 Core is mainly
carbon and
is electrondegenerate.
 Cools off in
millions of years
(not making new
 All low-mass
stars become
white dwarfs.
As the star burns its
hydrogen, it accumulates a
helium ash
Because energy flow in the
core of the star is by radiation,
the helium ash isn’t being
stirred out.
Eventually a dead
helium core starts
to form at the
center of the star
The central helium core is not fusing.
It’s just being squeezed by gravity
and added to by the hydrogen fusing
above it
Because the helium ash gets in
the way, the star gets brighter
To continue to burn hydrogen with all that helium in the way,
the core of the star gets a little hotter, the surface gets a
little bigger and the star gets a little brighter .