The Evolution and Death of Stars
Stars spend most of their life cycles
on the Main Sequence
• Main Sequence stars are in hydrostatic equilibrium
because nuclear fusion is turning hydrogen into
helium and producing enough outward pressure to
balance gravitational collapse.
• 90% of all stars are found on the Main Sequence
• 90% of the whole life of all stars is spent on the
Main Sequence
• BUT – What happens when the hydrogen runs out?
Stars Leave the Main Sequence
• The hydrogen atoms in the core of the star that
fuse together to create helium, start to run out
and fusion begins to slow down
• The system becomes out of balance
• Something has to happen to keep the star from
collapsing in on itself
Out of balance
• Start running out of hydrogen in the core, now the
outward pressure is less than the gravitational
collapse
Out of balance
• What will happen to the core?
When core hydrogen fusion ceases, a
main-sequence star becomes a giant
• When hydrogen fusion ceases in the core, the star
will collapse inward – this causes the layer just
outside the core to become so hot and dense that
hydrogen fusion will begin in this outer layer.
• The energy produced by hydrogen fusion in this
layer just outside the core causes the rest of the
star to expand into a giant star.
• Stellar burp!
Helium fusion begins at the core
of a giant
• While the exterior layers expand, the helium core
continues to contract and eventually becomes hot
enough (100 million Kelvin) for helium to begin
to fuse into carbon and oxygen
– core helium fusion
– 3 He  C + energy and C + He  O + energy
Main Sequence Stars become Red Giants
Hydrogen fusion
Helium fusion
As stars evolve,
stars move from
being main
sequence stars to
Red Giants where
they
increase in
luminosity and
brightness and
decrease in
temperature
The Life of a Star
Interstellar
Cloud (gas
and dust)
Main
Sequence
Star
Red
Giant
Main Sequence Stars become Red Giants
Hydrogen fusion
Helium fusion
What happens after core helium fusion
stops?
The shell and core equilibrium game
continues!
Depending on the mass of the star,
heavier elements are produced: carbon,
oxygen, neon, silicon, the heaviest
element being iron.
We are all made of Star Stuff!!
So what happens after the giant phase?
It depends on the mass of the star!
Low Mass stars (< 8 M ) have a
different fate from
High Mass stars (> 8 M )
Low Mass stars (< 8 M )
• The core runs out of fuel!
• Shell fusion begins outside the core.
Example of a low-mass giant:
its outer layers and core
Low Mass stars (< 8 M )
• The core runs out of fuel!
• Shell fusion begins outside the core.
• Eventually the process shell fusion creates too
much outward pressure and energy which
explosively pushes out the outer layers of the star
and produce a planetary nebula.
Low mass stars (< 8 M )
Interstellar
Cloud (gas
and dust)
Main
Sequence
Star
Red
Giant
Planetary
Nebula
Ring Nebula
The burned-out core of a low-mass
star becomes a white dwarf
• Surrounding planetary nebula disperses
leaving behind just the remaining WHITE
DWARF
White Dwarf
• A core with remaining mass less than 1.4 M.
• These tiny star remnants are approximately the
size of planet Earth
• One cubic centimeter (like a sugar cube) of a
White Dwarf star would weigh several tons.
White Dwarf
Ring Nebula
Low mass stars (< 8 M )
Interstellar
Cloud (gas
and dust)
Main
Sequence
Star
Red
Giant
Planetary
Nebula
White
Dwarf
What happens to white dwarfs?
Do they just sit there??
If the white dwarfs are isolated, yes.
They will cool down and become
BLACK DWARFS.
Sirius and its White Dwarf companion
white dwarf
BUT: White dwarfs are not always left alone. Sometimes they can
have a companion star! As its companion evolves and gets bigger,
the white dwarf can steal mass from it. The stolen matter forms an
external layer which can quickly ignite and shine brightly creating a
Nova.
What’s a Nova?
• A nova occurs in binary
systems where a white dwarf
is pulling mass from its
companion.
• A nova is a relatively gentle
explosion of hydrogen gas on
the surface of a white dwarf in
a binary star system.
• This process does not damage
the white dwarf and it can
repeat.
Sometimes the mass transfer can be excessive. So
excessive that the white dwarf will not be able to support
the mass it gains. So, what would have been a nova
becomes a SUPERNova!
Low mass stars (< 8 M )
Interstellar
Cloud (gas and
dust)
Main
Sequence
Star
Red
Giant
Nova
Pulling material
off of a
companion star
Supernova Ia
Planetary
Nebula
White
Dwarf
White
Dwarf
Leaves
no
remnant!
So what is the fate of out Sun?
• Since the Sun has a mass less than 8
M and since it is alone without a
companion, it will become a White
Dwarf and then slowly cool into a
Black Dwarf
High Mass Giant Stars (> 8 M )
Have a Different Story
• Fusion in the core continues through many more
stages than for low mass stars
• Heavier elements are produced:
–
–
–
–
–
carbon,
oxygen,
neon,
silicon,
and so on up to iron
• We’re all made of star stuff!!
A series of different types of fusion
reactions occur in high-mass stars
Core runs out of fuel!
Gravity (
) wants to collapse the star!
High-Mass Stars (> 8 M )
• The core and outer layers run out of fuel.
• The star then collapses, due to gravity.
• The mass, however, is high enough that
nothing can balance the gravitational
collapse and…..
Supernovae -Type II
• The collapsing outer layers of the star will
collapse against and bounce outward off the
compact collapsed core in an explosive event
sending out a shockwave. This explosive
event is called a Type II Supernova!!!
• During the Supernova, heavier elements are
created from fusion events, like magnesium,
lead, or gold.
A Supernova Type II
occurred here before we did.
• The atoms that created our world and solar
system come from nuclear fusion in stars and
from Supernovae events!
• We are all made of star stuff!
High-Mass Stars (> 8 M )
Gravity (
) wants to collapse the star
No outward pressure = implosion
Rebound of outer
layers against the
core = supernova
After
Supernovae can be as bright as a whole galaxy!
Before
High-Mass Stars (> 8 M )
Interstellar Big Main
Cloud (gas Sequence Star
and dust)
Red
Giant
Type II
Supernova
A101 Slide Set:
From Supernovae to Planets
Drafted by Manning for the SOFIA Team
Topic: Supernovase.
Concepts: Supernovae, planet
formation, infrared observations
Missionb: SOFIA
Coordinated by: the NASA
Astrophysics Forum
An Instructor’s Guide for using the
slide sets is available at the ASP
website
https://www.astrosociety.org/educatio
n/resources-for-the-higher-education42
audience/
The Discovery
SOFIA data reveal warm dust (shown as white contour
lines) surviving inside a supernova remnant (SNR) near
the center of our galaxy. The SNR Sagittarius A East
cloud is traced in X-rays (blue). Radio emission (red)
shows expanding shock waves colliding with
surrounding interstellar clouds (green). Credits:
NASA/CXO/Herschel/VLA/SOFIA-FORCAST/Lau et al.
•
Astronomers using data gathered by the
Stratospheric Observatory for Infrared
Astronomy (SOFIA) have found a
massive dust cloud within the supernova
remnant Sagittarius A East.
•
The dust was created in the supernova
explosion about 10,000 years ago that
left behind the expanding, hot remains of
the original star (the remnant).
•
Astronomers estimate that the dust cloud
contains enough material to create about
7,000 Earths.
•
The discovery confirms that supernovae
are capable of producing the material
needed to form planets, and may be
responsible for most of the dust found in
43
young galaxies.
How was the Discovery Made?
• Astronomers made detailed infrared
observations of the Sagittarius A
East supernova remnant using
instruments aboard the SOFIA
aircraft—measuring the long
infrared wavelengths that can travel
through intervening interstellar dust
clouds to reveal activity in the center
of the supernova remnant.
Supernova remnant dust detected by SOFIA (yellow
contour lines) survives away from the hottest X-ray gas
(purple). The red ellipse outlines the supernova shock
wave. The inset shows a magnified image of the dust
(orange) and gas emission (cyan). Credits:
NASA/CXO/Herschel/VLA/SOFIA-FORCAST/Lau et al.
• They found infrared emission
coming from a dust cloud, and
measured the mass of the cloud
based on the intensity of the
emissions.
• The “contour map” of the dust cloud
traces lines of equal intensity, with
greater intensity lines in the center 44
The Big Picture
• When massive stars end their lives in titanic
explosions called supernovae, the chemical
elements forged in the stars’ interiors-and
created in the heat and pressure of the
explosion--are released into space as a
debris cloud of hot gas and dust.
• Scientists had evidence of such dust
formation, but couldn’t be sure that the dust
wasn’t destroyed in the “rebound” shock
wave when the expanding supernova
remnant collided with the interstellar
medium of thinly scattered material,
creating another shock wave traveling
inward toward the source of the explosion.
• The new finding demonstrates that
supernova-formed dust can survive rebound
shock waves and spread into space to form
The Crab Nebula supernova remnant, created
by an exploding massive star in AD 1054.
Credit: HST, NASA, ESA, J. Hester and A. Loll
(Arizona State University)
45
What are the Implications?
Artist’s concept of a stellar system in formation,
in which leftover gas and dust in the disk
surrounding a newborn star clump together to
form planets. Credit: NASA/JPL-Caltech..
• The SOFIA finding demonstrates that supernovas not only produce dust,
but that the dust can survive the explosion to become raw material for the
formation of other stars—and planets.
• This result supports the notion that most of the dust observed in distant
young galaxies may have been made by supernova explosions of early
massive stars, since no other known mechanism could have produced
nearly as much dust.
• This result may provide the “missing link” between supernovas and planet 46
What happens to the core
after a supernova?
– the whole story depends on mass!
• neutron star
– the really big ones: remaining mass of 1.4 M
to about 3 M
• black hole
– the really really big ones: remaining mass
greater than 3 M
Neutron Stars
• A core with remaining mass of 1.4 to 3 M,
composed of tightly packed neutrons.
• These tiny stars are much smaller than
planet Earth -- in fact, they are about the
diameter of a large city (~20 km).
• One cubic centimeter (like a sugar cube) of
a neutron star, would have a mass of about
1011 kg! (hundreds of billions of pounds!)
Neutron Star
Neutron Star
Supernova
Neutron Star Spins Down
Topic:
Neutron stars
Concepts:
Pulsars, magnetic fields, pulsar
timing
Missions:
Swift, Fermi
Coordinated by
the NASA Astrophysics Forum
An Instructor’s Guide for using
the slide sets is available at the
ASP website
https://www.astrosociety.org/e
ducation/resources-for-the-51
higher-education-audience/
The Discovery
• X-ray observations by
NASA’s Swift satellite
showed a sudden slowdown in the spin of a highly
magnetized neutron star
known as 1E 2259+586 .
Neutron
star
• The discovery creates a
puzzle for astronomers
because usually they see
neutron stars (the collapsed
cores of massive stars that end
their lives as supernovae)
suddenly speed up – this is
the first time they’ve seen
one slow down.
Supernova
Remnant
Neutron star 1E 2259+586 shines in blue-white in this
X-ray image of the CTB 109 supernova remnant.
(Credit: ESA/XMM-Newton/M. Sasaki et al.)
52
How Was the Discovery
Made?
• The Gamma-ray Burst Monitor on NASA’s Fermi Gamma-ray Space
Telescope observed a brief but intense outburst from IE 2259+586 on April
21, 2012.
• Data from the X-ray
Telescope on Swift showed
that the spin rate of IE
2259+586 decreased
abruptly on April 28, 2012 –
just a week after the
outburst.
• The spin rate of once every
seven seconds decreased
by 2.2 millionths of a second
– a tiny but significant
change for a neutron star.
An artist’s rendering of an outburst from a highly-magnetized pulsar,
which is a fast-spinning neutron star. (Credit: NASA/GSFC)
53
The Big Picture
•
Neutron stars contain twice the mass of our Sun
crushed into a sphere about 12 miles across.
•
Pulsars are rapidly rotating neutron stars that emit
a beam of radiation (directed along their magnetic
poles) and sweep it across our line-of-sight. Much
like a lighthouse, we observe a pulse every time
the beam sweeps past us.
•
Astronomers have observed “glitches” in many
neutron stars – these are sudden increases in the
rate of rotation of the pulsar.
•
This new observation showed an “anti-glitch” – or
a sudden decrease – in the rotational frequency
of 1E 2259+586.
Top: A neutron star has the mass of about two
suns in a sphere the size of Manhattan.
Credit: NASA's Goddard Space Flight Center
Bottom: A pulsar emits light beams (magenta)
that we see as pulses as it rotates. Pulsars
also have super-strong magnetic fields (blue
lines illustrate magnetic fields lies; the beams
are emitted along the magnetic poles). Credit:
NASA
54
How Does this Discovery
Change our View?
•
•
•
The internal structure of neutron stars is a long-standing
puzzle in astronomy. The current view is that a neutron
star has:
• an interior containing oddities that include a
neutron superfluid (which is a state of matter
without friction)
• a crust of electrons and ions
• a surface where streams of high-energy
particles are accelerated through the star’s
powerful magnetic field
Current theory holds that a glitch occurs when streaming
particles drain energy from the crust causing the crust to
spin down. The fluid inside the crust resists this change.
Under the strain, the crust fractures, which results in an
X-ray outburst and a speedup kick from the fasterspinning interior.
Processes that would lead to a sudden slow-down in the
spin pose a new theoretical challenge!
A composite optical (red) and Xray (blue) image of the
environment near the Crab
Nebula pulsar (center), a pulsar
from which multiple glitches
have been observed. (Credit:
NASA/HST/CXC/ASU/J. Hester
et al.)
55
Resources
Press releases
•
NASA press release
http://www.nasa.gov/mission_pages/swift/bursts/new-phenom.html
•
Penn Sate press release
news/Kennea5-2013
http://science.psu.edu/news-and-events/2013-
Scientific article
• Archibald, R. F., et al. 2013, Nature, 497, 591
http://www.nature.com/nature/journal/v497/n7451/full/nature12159.html
(subscription required)
Multimedia:
• Visit the Interactive Fermi Pulsar Explorer
http://www.nasa.gov/externalflash/fermipulsar/
Additional stories/visuals on pulsars available at NASA/Goddard Space
Flight Center’s Science visualization Studio:
http://svs.gsfc.nasa.gov/cgi-bin/search.cgi?sortby=relevance&value=pulsar
•
56
Pulsars – The discovery of rotating
neutron stars
• First detected in 1967 by Cambridge University
graduate student Jocelyn Bell.
• She found a radio source with a regular on-off-on
cycle of exactly 1.3373011 seconds.
• Some scientists speculated that this was evidence
of an alien civilization’s communication system
and dubbed the source LGM (Little Green
Men!!!)
• Today, we know pulsars are rapidly spinning
neutron stars.
Lighthouse Model
Black Holes
• A remaining core with a mass of more than
3 M, will continue to collapse into an
infinitely small location in space.
• We cannot observe what is left behind,
directly. We can only detect its presence if
it has a companion star, and it attracts
material in an accretion disk.
Black Holes
A black hole is a collapsed stellar core. It is a
location in space of enormous gravitational
attraction. The gravitational attraction is so
strong that photons of light can not even
escape (that’s why it’s black)!
Black Hole
To detect a black hole, we look for the x-rays given off
by material as it falls toward the black hole.
High-Mass Stars (> 8 M )
Neutron
Star
Interstellar
Cloud (gas
and dust)
Big Main
Sequence
Star
Black Hole
Red
Giant
Type II
Supernova
Tutorial: Stellar Evolution (p.83)
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer, ask
another group.
Black holes are formed by
1. a lack of any light in a
region of space.
2. supernovae from the
most massive stars.
3. supernovae from
binary stars.
4. collapsed dark nebulae.
0/0
Cross-Tab Label
Which of the following lists, in the
correct order, a possible evolutionary
path for a star?
1. Red Giant, Neutron Star, White
Dwarf, nothing
2. Red Giant, Type I Supernova, Black
Hole
3. Red Giant, Type II Supernova,
Planetary Nebula, Neutron Star
4. Red Giant, Planetary Nebula, White
Dwarf
5. Red Giant, Planetary Nebula, Black
Hole
0/0
Cross-Tab Label