Transcript A105 Stars and Galaxies

A105
Stars and Galaxies
Today’s APOD
 ROOFTOP TONIGHT AT 9 PM
 HAND IN HOMEWORK
 Exam coming on Nov. 2
Upcoming Events
• Orionid meteor shower peaks
Saturday night, view from 11:45
onward – if weather is clear,
watch for at least 20 minutes from
a dark site
• Transit of Mercury
–Nov. 8, 2:15 PM – Sunset
–From Sample Gate
Let’s talk about…
• Mid-Term Grades
• The Next Exam
Temperature,
Diameter,
and
Brightness
Star
formation
brings
stars to the
main
sequence
…What
happens
next?
Explaining
the HR
Diagram
•Energy
•Gravity
•Energy
Transport
Review:
• Why was the Sun’s energy source a
major mystery?
– Chemical and gravitational energy sources
could not explain how the Sun could
sustain its luminosity for more than about
25 million years
• Why does the Sun shine?
– The Sun shines because gravitational
equilibrium keeps its core hot and dense
enough to release energy through nuclear
fusion.
How does nuclear fusion occur in
the Sun?
• The core’s extreme temperature and density
are just right for nuclear fusion of hydrogen to
helium through the proton-proton chain
• Gravitational equilibrium acts as a thermostat
to regulate the core temperature because
fusion rate is very sensitive to temperature
Stellar Mass and Fusion
• The mass of a main sequence star determines
its core pressure and temperature
• Stars of higher mass have higher core
temperature and more rapid fusion, making
those stars both more luminous and shorter-lived
• Stars of lower mass have cooler cores and
slower fusion rates, giving them smaller
luminosities and longer lifetimes
Massive Stars
Sun-like Stars
Low Mass
Stars
Star Clusters and Stellar Lives
• Our knowledge of the life
stories of stars comes
from comparing
mathematical models of
stars with observations
• Star clusters are
particularly useful
because they contain
stars of different mass
that were born about the
same time
Evolution of a Very Low
Mass Star
(~0.3 solar masses)
๏
Lifetime: 300 Billion Years
• The entire star is
convective.
• As hydrogen is
consumed, the core
shrinks and heats, the
luminosity rises along
the main sequence.
• Since convection occurs
through the whole star,
all the star’s hydrogen is
burned.
• Leaves a helium
remnant
What are the life stages of a
Sun-like star?
A star remains on
the main sequence
as long as it can
fuse hydrogen into
helium in its core
What happens next?
Life Track after Main Sequence
• Observations of
star clusters show
that a star
becomes larger,
redder, and more
luminous after its
time on the main
sequence is over
Sun-like stars become red giants
When the helium core contracts, the surrounding
hydrogen puffs up and the star becomes a red giant.
Broken Thermostat
• As the core contracts, H
begins fusing to He in a
shell around the core
• Luminosity increases
because the core
thermostat is broken—
the increasing fusion
rate in the shell does not
stop the core from
contracting
Helium fusion does not begin right away because it requires
higher temperatures than hydrogen fusion—larger charge
leads to greater repulsion
Fusion of two helium nuclei doesn’t work, so helium fusion
must combine three He nuclei to make carbon
Once helium burning begins the “thermostat”
starts to work again. Helium burning stars
neither shrink nor grow because core
thermostat is temporarily fixed.
End of Fusion
• Fusion progresses no further in a Sun-like
star because the core temperature never
grows hot enough for fusion of heavier
elements
• Electron pressure from quantum mechanics
supports the core against further gravitational
contraction
The End of Solar-type Stars
Main
Sequence
Red
Giant
Planetary
Nebula
White
Dwarf
When the carbon core reaches a density that is
high enough, the star blows the rest of its
hydrogen into space.
The hot, dense, bare core is exposed!
Surface temperatures as hot as 100,000 degrees
The hot core heats the expelled gas and makes it glow
Planetary Nebulae
• Fusion ends with
a pulse that ejects
the H and He into
space as a
planetary nebula
• The core left
behind becomes
a “white dwarf”
Planetary
Nebulae!
Life Track of a Sun-Like Star
Earth’s
Fate
•
Sun’s luminosity will rise to 1,000 times
its current level—too hot for life on
Earth
Earth’s
Fate
•
Sun’s radius will grow to near current
radius of Earth’s orbit
Summary
• The life stages of a Sun-like star
– H fusion in core (main sequence)
– H fusion in shell around contracting core
(red giant)
– He fusion in core
• How does a Sun-like star end?
– Ejection of H and He in a planetary
nebula leaves behind an inert white dwarf
Life Stages of High-Mass Stars
• Late life stages of highmass stars are similar
to those of low-mass
stars:
– Hydrogen core
fusion (main
sequence)
– Hydrogen shell
burning (supergiant)
– Helium core fusion
(supergiant)
What about Massive Stars?
• Massive stars continue to
generate energy by
nuclear reactions until
they have converted all
the hydrogen and helium
in their cores into iron.
• Once the core is iron, no
more energy can be
generated
• The core collapses and
the star explodes
Iron builds up
in core until
degeneracy
pressure can
no longer
resist gravity
Core then
suddenly
collapses,
creating
supernova
explosion
A “Recent” Supernova in Our
Galaxy
• A new star in Taurus
observed by the Chinese
in 1054 A.D.
•Visible in the daytime
•Gradually faded; gone
after about two years
•The Crab Nebula is a
supernova remnant
The Crab Nebula Continues to
Expand
• The Crab Nebula is about 7000 LY away
• The Nebula is about 10 LY across
• Expanding at a speed of about 1,400 kilometers
per second
• The Crab Nebula - Then and Now
• Images taken in 1973 and recently
The Large
Magellanic
Cloud
•Distance: about 150,000 LY
•Part of the Local Group
•“Irregular” galaxy
•Lots of star formation
Supernova
1987a
•Feb. 1987
•Star previously known – 18 solar masses
•Study formation of supernova remnant
Rings around Supernova 1987A
•
The supernova’s flash of light caused
rings of gas around the supernova to
glow
Summary
• The life stages of a high-mass star are
similar to the life stages of a low-mass
star
• Higher masses produce higher core
temperatures that enable fusion of
heavier elements
• A high-mass star ends when the iron
core collapses, leading to a supernova
Is life on Earth safe from harm
caused by supernovae?
Earth is safe at the present time
because there are no massive stars
within 50 light years of the Sun.
Sun-like Star Summary
1.
Main Sequence: H fuses to He in
core
2. Red Giant: H fuses to He in shell
around He core
3. Helium Core Burning:
He fuses to C in core while H
fuses to He in shell
4. Planetary Nebula leaves white
dwarf behind
Not to scale!
Life Stages of High-Mass Star
1.
Main Sequence: H fuses to He in
core
2. Red Supergiant: H fuses to He in
shell around He core
3. Helium Core Burning:
He fuses to C in core while H
fuses to He in shell
4. Multiple Shell Burning:
Many elements fuse in shells
5. Supernova leaves neutron star
behind
Not to scale!
Role of Mass
• A star’s mass determines its entire life story
because it determines its core temperature
• High-mass stars with >8MSun have short lives,
eventually becoming hot enough to make
iron, and end in supernova explosions
• Sun-like stars with <2MSun have long lives,
never become hot enough to fuse carbon
nuclei, and end as white dwarfs
• Intermediate mass stars can make elements
heavier than carbon but end as white dwarfs
The Evolution of Stars
The Composition of Stars
90% hydrogen atoms
10% helium atoms
Less than 1%
everything else
(and everything
else is made in stars!)
everything
else
Abundance of Elements in the Galaxy
Goals:
• Know how chemical
elements are created
• in the Early
Universe
• in Stars
• in Supernovae
• Know how the
Galaxy is enriched in
chemical elements
The Origin of Elements
• The process by which elements (nuclei) are
created (synthesized) is called
nucleosynthesis
• Nucleosynthesis has occurred since the
creation of the universe and will essentially
go on forever
• The elements created come together to
form everything material we know, including
us
Primordial
Nucleosynthesis
Hydrogen and helium were created during the Big
Bang while the Universe was cooling from its initial
hot, dense state.
About 10% of the lithium in the Universe today was
also created in the Big Bang. We’re still not sure
where the rest comes from.
The first stars formed from this material.
Hydrogen
Burning
Stars burn hydrogen in their interiors to
produce helium.
Hydrogen burning also rearranges carbon,
nitrogen, and oxygen.
Helium
Burning
Three helium atoms
combine to form carbon
Light
Elements
The Iron
Peak Metals
In the cores of massive stars just before
supernova explosions, atomic nuclei
exchange protons and neutrons to form
the iron peak metals.
• Hydrogen – from big
bang nucleosynthesis.
• Helium – from big bang
and from hydrogen
burning via the p-p chain
and CNO cycle.
• Nitrogen – from CNO
cycle.
• Carbon, Oxygen – from
helium burning.
• Light elements (Neon,
Magnesium, Calcium –
from carbon and oxygen
burning.
• Iron metals – from the
final burning
Making
Elements Up
to Iron
Heavy Metals
All heavier elements are formed
when iron peak elements capture
neutrons
Elements Heavier than Iron …
• Once iron is formed, it is no longer possible to create
energy via fusion.
 Elements heavier than iron require a different process
(Iron is atomic number 26.)
• The heaviest naturally occurring nucleus is uranium
(atomic number 92). How do we get to uranium then?
•Elements heavier than iron are created by
neutron capture
•The neutron is converted into a proton and
added to the nucleus, increasing the atomic
number to make the next element in the
periodic table.
Making Heavy Metals in Stars
• In low mass stars like
the Sun, heavy
metals are created
when the star is a
giant
• Massive stars make
heavy metals when
they become
supernovae
Stellar Nucleosynthesis
• We know now that all chemical elements heavier
than atomic number 5 (Boron) were produced in
stars.
• The light elements are essentially ashes of
nuclear burning during the normal stellar
evolution process.
• The heavier elements are produced in the
envelopes of giants and during explosive
nucleosynthesis that occurs during supernovae.
Chemical Enrichment of the Universe
• We know now that massive stars act as
factories for creating heavy elements
– Massive stars end their lives in supernova
explosions
– The explosion scatters the new elements into
interstellar space
• Elements synthesized inside stars are also
brought to the surface and expelled via
stellar winds
• A new generation of stars recycle this
material, enriching it further
The Galaxy (and the universe) is
gradually enriched in heavy elements
Despite all the
nucleosynthesis that
has occurred since the
creation of the
universe, only 2% of
the ordinary matter in
the universe is now in
the form of heavy
elements. Most is still
hydrogen and helium
 Star Death – Units 67, 68, 69
 News Quiz on Tuesday
 Homework Due EACH THURS.
EXAM NOV. 2nd