Download Stages in the Life of a Star

Stages in the Life of a
Star
Stellar Evolution
Timescales
• Stages and timescales depend on mass (little
bit of composition dependence). Massive
stars evolve more quickly than light stars.
• Main sequence lifetime = fuel / consumption
rate ~ Mass / Luminosity.
Stages
• The basic scheme is:
• Gas Cloud → Main Sequence →
Red Giant → (Planetary Nebula or
Supernova) → Remnant.
Stages
Giant Molecular Cloud
• Giant Molecular Cloud--large dense gas
cloud (with dust) that is cold!
• 100,000's to few million solar masses of
material
• Has fragments of 10's to 100's solar masses
that start collapsing
• Reason(s) - shock waves, cool enough for
gravity to take over, etc..
Stages
Protostar
• Protostar - gas clump collapsing and heating
up in center as it collapses.
• Gravitational energy being converted to heat.
• Lots of Infrared and Microwave radiation
produced.
• Gets hot enough to glow red (2000-3000 K),
but gas/dust cocoon blocks visible light.
• IR and Microwave can pass through dust.
False-color infrared and radio maps of protostars.
(Courtesy Karen Strom, Mark Heyer, Ron Snell, FCAD and FCRAO.)
False-color images of a protostar and jet. Notice the
Herbig-Haro objects (the blobs) in nearby gas.
(Courtesy Patrick Hartigan, NOAO, and STSCI.)
Bipolar flow of gas
ejected from a
young star. This is a
radio picture in
which the gas
receding from us is
shown in red,
whereas the gas
approaching us is
shown in blue.
(Courtesy Ronald Snell, FCRAO.)
Stages
T-Tauri
• T-Tauri -- star like object visible to
outside.
• Strong winds eject lots of material from
young star (preferentially along
rotational axes).
• Cocoon gas/dust blown away.
• Star starts fusion (H converted to He).
Stages
Main Sequence
• Main Sequence--star is stable because
of Hydrostatic Equilibrium.
• Fusing Hydrogen to Helium in core.
• Stars spends about 90% lifetime as
main sequence.
Stages
Sub-giant, Red Giant, Supergiant
• Sub-giant, Red Giant, Supergiant--Run out
of core fusion fuel. Hydrostatic equilibrium
upset.
• The Core shrinks. Fusion in shell around
core starts. This Fusion is very rapid.
• The Luminosity (energy output) increases
so gas envelope surrounding the core puffs
out.
Stages
Sub-giant, Red Giant, Supergiant
• At the bloated-out surface, the energy is
spread out over a larger area so each
square centimeter will be cooler giving the
light a red color.
• Red giants can eject a lot of mass through
“winds”’. Note: A Red Giant may be large in
terms of linear size, but it is less massive
than the main sequence star it came from!
(A)The core of a star begins to shrink as a star
uses up the hydrogen in its core. This
compresses and heats the core.
(B)The heated core ignites the surrounding gas
to make a shell source, and the outer layers
of the star expand, turning it into a red giant.
Stages
Core Fusion
• Core Fusion -- core has shrunk enough to create
high enough temperatures to start Helium (or
heavier element) fusion (100 million K).
• In low mass stars the onset of Helium fusion can
be very rapid, producing a burst of energy helium flash.
• Eventually it settles down.
• Core Fusion is releasing more energy than main
sequence stage, so star is bigger, but stable!
Stages
Red Giant, Supergiant
• Red Giant, Supergiant -- core fuel runs out
again. If massive enough, repeat core
fusion (stage 5). Number of times to go
through these stages depends on mass.
• Stellar nucleosynthesis of heavy elements
occurs. Interplay of gravity and nuclear
fusion.
Stages
Planetary Nebula or Supernova
•
•
•
Planetary Nebula or Supernova -- outer
layers ejected as core shrinks to most
compact state.
Low mass stars (0.08 - 5 Msun) will go the
planetary nebula route;
High mass stars (5 - 50Msun) will go the
explosive supernova route. Supernova
explosion: the core has formed a very stiff
neutron star and the in-falling outer layers
rebound off it
Stages
Remnant
• Low mass core (< 1.4Msun) shrinks to white
dwarf. Electrons prevent further collapse.
Size about that of Earth. Outer layers are
planetary nebula.
• Higher mass core (1.4Msun - 3Msun) shrinks
to neutron star. Supernova happens when
neutron star is created. Neutrons prevent
further collapse. Size about that of a large
city.
Photographs of several planetary nebulas. (A) The Helix nebula. (Courtesy AngloAustralian Observatory/photograph by David Malin.) (B) The Ring nebula. (Courtesy
Hubble Heritage Team (AURA, STScI/NASA).) (C) The Butterfly Nebula (Bruce Balick,
University of Washington. Vincent Icke, Leiden University (The Netherlands). Garrelt
Mellema, Stockholm University/NASA.). Notice the central star in each. Other stars that
look as if they are inside the shell are foreground or background stars.
(A)
(B)
(C)
Light and Atoms
Stages
Remnant (continued)
• Highest mass core (> 3Msun ) shrinks to a
point. On the way to total collapse it may
momentarily create a neutron star and the
resulting supernova rebound explosion.
• Gravity finally wins. Nothing holds it up.
Space so warped around the object that it
effectively leaves our space. Black hole!
Stellar Nucleosynthesis
•
•
•
•
Stellar Nucleosynthesis--creating heavier
elements (heavier than Helium) from lighter
elements in stars.
Lowest mass stars can only synthesize
Helium. Stars around the mass of our Sun
can synthesize Helium and Carbon.
Massive stars with M > 5Msun can
synthesize Helium, Carbon, Oxygen, etc; all
the way to Iron.
Elements heavier than Iron are made in
supernova explosion.
Main Sequence Turnoff
•
•
•
•
A star cluster’s H-R diagram changes with
age.
Main Sequence Turnoff-mass at that point
tells you age of cluster.
Assume that all stars in cluster form at
about the same time.
Stars slightly heavier than turnoff have
already evolved away from main sequence.
Stellar Remnants
Degenerate matter
• Degenerate matter: very dense matter in a
state where the pressure no longer depends
on temperature; due to quantum mechanical
effects.
• Resist compression. Degenerate particles
have no “elbow room”
• Gas like hardened steel!
Stellar Remnants
White Dwarfs
• White Dwarfs--if core mass < 1.4 solar
masses. Electrons are degenerate.
• Mass of Sun compressed to size of Earth.
• The density is about 1,000,000 g/cm3 (one
sugar cube > 1 car!).
• White dwarf cools off from initial formation,
temperature of about 100,000 K.
Stellar Remnants
Neutron Stars
• Neutron Stars: core mass is between 1.4
and 3 solar masses.
• Compression so great that protons fuse with
electrons to form neutrons. Neutrons are
degenerate.
• About 30 km across! One sugar cube =
mass of humanity!. Formed in supernova
explosion.
Stellar Remnants
Pulsars
• Pulsars--rapidly rotating neutron stars with
STRONG magnetic fields (many times Sun's).
• Light flashes with period of milliseconds at start
and lengthening over time.
• Lighthouse model-strong magnetic field creates
electric field making charged particles flow out of
the magnetic poles, producing a beam at the
magnetic poles.
• If the beam sweeps past Earth, we see a flash of
“light”.
Stellar Remnants
Black Holes
•
•
•
Black Holes: core mass > 3 solar masses.
Gravity finally wins, compressing core to
mathematical point at center. Formed in
supernova explosion.
Surface gravity so strong that nothing can
escape (not even light!) within a certain
distance from mass point.
Stellar Remnants
Black Holes
• Boundary is called the event horizon
(or Schwarzchild radius)-no messages
of events happening within radius
• To find the Schwarzchild radius of an
object:
3 x core mass [in solar masses] km
Stellar Remnants
Black Hole Detection
Mass of Companion in Binary:
• For binary, observe how the black hole
moves visible companion around. Use
Kepler's 3rd law to find masses.
X-rays from Accretion Disk
• For binary, look for X-rays produced in hot
accretion disk--material pulled off visible
companion spirals onto black hole.
Comparison of Stellar Remnants
Size
Density (1 tsp)
White Dwarf
Earth
5 tons
Neutron Star
10 – 20 km
100 million
tons
Black Hole
Point
Infinite
Comparison of Stellar Remnants
Size
Density (1 tsp)
White Dwarf
Earth
5 tons
Neutron Star
10 – 20 km
100 million
tons
Black Hole
Point
Infinite