Transcript Death of sun

The Deaths of Sunlike Stars
Low Mass Dwarfs
Low mass red dwarf stars cannot achieve any advanced fusion
because they cannot get hot enough (Temp < 100 million K) to
begin the next reaction (helium to carbon)
Hydrogen => Helium fusion ends at core
Star shrinks to form a white dwarf
Formation of a Giant Star
When H  He fusion ceases, hydrostatic
equilibrium is disrupted and gravity forces
dominate. The core collapses. The collapse
heats the inner core enough so that helium
fusion may begin.
The new fusion reaction generates an even
larger fusion pressure, which pushes the star’s
outer layers outward.
As they expand, they cool, and turn redder in
color. This change in temperature is known as
adiabatic cooling.
Triple Alpha Process
If the core T > 100 Million K, three helium nuclei (alpha
particles) fuse to make a carbon nucleus. Some carbon
nuclei further combine with one more alpha particle to
make oxygen. The star swells to become a giant.
Inside a Giant
The star develops a
two-layered fusion:
He  C & O
in the core,
and H  He in a
shell above the core.
Giant Stars
– Stars like the sun
become giant stars
of 10 to 100 times
the Sun’s present
diameter.
– The most massive
stars become
supergiant stars as
much as 1,000
times larger than
the Sun.
Our Sun as a Red Giant
When our Sun becomes a red giant (in about
7.6 billion years from now), it will expand so
much that it will engulf the planets Mercury,
Venus, and Earth.
The Earth’s orbit will slowly degrade and
Earth will spiral farther into the Sun, if it has
not already vaporized.
The atoms that were on Earth (like yours!) will
be reclaimed by the star.
Giant and Supergiant Stars
Star Clusters: Evidence of Evolution
By comparing star
clusters of different
ages, you can visualize
how stars evolve—
almost as if you were
watching a film of a
star cluster evolving
over billions of years.
Star Clusters: Evidence of Evolution
Star clusters provide the evidence that the
evolution is visible on the H-R Diagram.
Giant Stars- Internal Layers
Medium mass stars will die when carbon and oxygen
build up in their cores.
High mass stars can form elements up to iron before
fusion stops.
Planetary Nebulae
A dying giant can expel its outer atmosphere in
repeated episodes to form a planetary nebula.
The first planetary nebulae
discovered looked like the
greenish-blue disk of a planet
such as Uranus or Neptune.
However, they have nothing to do
with planets. The colors come
from the large amounts of
ionized oxygen they expel.
Planetary Nebulae
The PN shells of gas are symmetrical because of
magnetic fields.
They are lit by ultraviolet light coming from the collapsing
star. When UV strikes the expelled gases, they fluoresce.
Planetary Nebulae
A white dwarf star forms at the center of a planetary
nebula. White dwarfs are very hot because they
have been condensed into a very small area, about
the size of the Earth. This heating process is
adiabatic heating.
A typical planetary nebula
will shine for 20,000 to
50,000 years, but the
white dwarf formed will
glow for billions of years.
White Dwarfs
So far, there has not been
enough time in the history of
the universe for any white
dwarf to cool off so much
that it does not glow (a black
dwarf). The coolest white
dwarfs in our galaxy are
about the temperature of the
Sun.
The planetary nebula designated NGC 2440,
contains one of the hottest white dwarf stars
known. The white dwarf can be seen as the
bright dot near the photo's center.
Credit: H. Bond (STScI), R. Ciardullo (PSU),
WFPC2, HST, NASA
Degenerate Matter
The contraction of a white dwarf compresses the
gases in its interior to such high densities that
the electrons in the gas are pushed as close
together as is permitted. Such a gas is termed
degenerate matter.
Degenerate matter is much more dense than
normal matter because all of the “empty space”
has been squeezed out of the atoms. A teaspoon
of “white dwarf” would weigh several tons.
White Dwarfs
White dwarfs are
composed primarily of
crystallized carbon (the
endpoint of fusion for stars
like the Sun), so they can
be thought of as the
biggest diamonds in the
universe!
Giant Diamonds
V886 Cen is about 50 LY away
from Earth. This white
dwarf star weighs 5 million
trillion trillion pounds. That
would equal a diamond of
10 billion trillion trillion
carats.
After it was discovered in
2004, astronomers
nicknamed the star “Lucy”
after the Beatles song Lucy
In The Sky With
Diamonds.
Illustration of “Lucy” by an artist at the
Harvard-Smithsonian Center for Astrophysics
White Dwarfs in Binary Systems
If a white dwarf is a member of a binary star
system, and if the two stars are close together,
they can transfer mass back and forth. This
may alter the evolution of the stars.
White Dwarfs in Binary Systems
Mass transferred from one star to another
forms a rapidly rotating whirlpool around
the called an accretion disk.
White Dwarfs in Binary Systems
The gas temperature can exceed a million
degrees, producing X rays.
In addition, the matter accumulating on the
white dwarf can eventually cause a violent
explosion called a nova.
Novae
A nova forms when star
material falls onto the
surface of a sizzling hot
white dwarf.
The material can flare off,
producing a temporary
brightening of the star.
Nova Cygni, 1992
Type I Supernovae
Sometimes there is so much material accreted
onto the white dwarf, it collapses and forms a
spectacular explosion known as a Type I
supernova.