Download Stellar Lives (continued). Galaxies.

Lecture 39
Stellar Lives (continued). Galaxies.
Chapter 17.14  17.16, 18.1  18.5
• Last Stages of Low-Mass Stars
• Lives of High-Mass Stars
• Galaxies: Types and Structure
Last Stages of Evolution
The core helium runs out in ~100 million years.
When the helium is gone, the fusion stops, and
gravity shrinks the core again.
Now helium ignites in a shell around a carbon core.
The hydrogen shell burns around the helium shell.
Both shells contract, driving temperatures higher.
The star grows more luminous, but not for a long
time (a few million years).
Last Stages of Evolution
Carbon fusion is possible only at ~ 600 million K.
But degeneracy pressure halts the collapse.
The star has a large size, no core fusion, and,
hence, low connection to the surface layers.
The stellar wind increases.
Carbon is driven from the core to the surface by
convection.
Red giants with carbon-rich atmospheres are
called carbon stars.
Last Stages of Evolution
Carbon stars have temperatures of 20003000 K.
Dust particles may be formed in their winds.
At the end of its life, a low-mass star ejects its
outer layers into space.
The exposed core is still hot and radiates UV
photons, which cause the ejected nebula to glow.
Such nebulae are called planetary nebulae.
The dead remnant becomes a white dwarf.
High-Mass Stars. CNO Cycle.
The CNO cycle is the chain of reactions that leads
to hydrogen fusion in high-mass stars.
The escalated fusion rate of the CNO cycle produces
many more photons than in low-mass stars.
The photons have no mass, but carry momentum.
They transfer the momentum to anything the run into.
The result is radiation pressure.
Radiation pressure is responsible for strong stellar
winds in massive stars.
Life after Main-Sequence
When the core hydrogen is exhausted, massive
stars follow the same path as low-mass stars.
However, all the processes go more quickly.
When the carbon core forms, there is also a heliumand a hydrogen-burning shell.
At this point the paths of intermediate- and highmass stars diverge.
Intermediate-mass stars blow their outer layers
away and become white dwarfs.
Massive Stars after Main-Sequence
In massive stars the core temperature can reach
the critical 600 million K to ignite carbon.
But carbon burns away in a few hundred years.
Each successive stage of nuclear burning proceeds
more rapidly than prior stages.
Many different reactions may act at the same time.
At the end of a massive star’s life, iron forms in the
silicon-burning core and it becomes a red supergiant.
Iron cannot be ignited.
Iron has the lowest mass per nuclear particle.
Supernova
The degeneracy pressure briefly supports the iron
core.
But when the limit is passed, electrons cannot
exist freely and convert protons into neutrons.
In a fraction of a second, an iron core collapses
into a ball of neutrons a few kilometers across.
The collapse stops as neutrons have their own
degeneracy pressure.
It releases a huge amount of energy and results
in an explosion – a supernova.
Supernova
The neutron core is called a neutron star.
If gravity overcomes neutron degenerative
pressure, the core continues to collapse into a black
hole.
Supernova explosion may be due to the neutrino
shock wave, propagating through the star’s outer
layers.
Supernovae shine as ~10 billion Suns for a few
weeks.
The Origin of Elements
How do we know that elements are produced inside
stars?
If massive stars do produce heavy elements and
disperse them in space, then the total amount of
heavy elements should gradually increase with time.
We should expect stars born recently to contain
more heavy elements than older stars.
Stars in globular clusters have 0.1% of their mass
in heavy elements, while young stars – 2-3%.
Star Clusters
Open clusters and globular clusters.
Open clusters contain a few thousands stars and
span ~30 light-years (10 pc). Pleiades
Globular clusters can contain more than a million
stars and span 60-150 light-years.
Stars in clusters are at the same distance from the
Sun and are formed at about the same time.
It is easy to determine clusters’ ages.
Star Clusters
Age of cluster = lifetime of stars at main-sequence
turnoff point.
Most open clusters are relatively young (<5 billion
years).
Globular clusters are typically old objects (12-16
billion years), the oldest objects in the galaxy.
They place a limit on the possible age of the
Universe.
Summary
Virtually all elements besides H and He were created
inside stars.
The battle between gravity and pressure determines
how stars behave during their lives.
Low-mass stars live longer than high-mass stars.
High-mass stars dramatically explode as supernovae.
They create the entire variety of elements that exist
in nature.
Chapter 18
Galaxies
• Types of Galaxies
• The Structure of Galaxies
Other Galaxies
There as many galaxies in the Universe as stars in
our galaxy.
It is harder to study galaxies than stars, because
galaxies are more complex and more distant.
We do not know yet how they are formed and
developed.
However, many galaxies and even individual stars
in them have been studied.
Galaxy Types
Spiral galaxies – flat with bulges and spiral arms.
Elliptical galaxies – redder and rounder.
Irregular galaxies – strange-shaped.
Sizes of galaxies: from dwarf (~100 million stars)
to giant (~1 trillion stars).
Spiral galaxies show the presence of cool gas,
while ellipticals seem to contain mostly hot gas.
Spiral Galaxies
Spiral galaxies are similar to the Milky Way.
They have disks, bulges, and halos.
Bulge and halo make the spheroidal component.
The disk component is the galaxy midplane.
Some spiral galaxies have straight bars in their
centers, with spiral arms beginning from the bar’s
edges (barred spiral galaxies).
Some galaxies have disks, but no spiral arms
(lenticular galaxies).
Spiral Galaxies
~7585% of large galaxies are spiral or lenticular.
There are 2 large spirals in the Local group: the
Milky Way and the Great Andromeda galaxy (M31).
Lenticular galaxies are common in clusters of
galaxies (groups of hundreds or even thousands of
galaxies extending over >10 million light years).
Elliptical Galaxies
Ellipticals lack a significant disk component.
Thus, they have only spheroidal components.
Most of their interstellar medium consists of lowdensity, hot, X-ray emitting gas.
They also have very little dust.
Some ellipticals have small rotating disks at their
centers (perhaps remnants of a collision with a
spiral galaxy).
Irregular Galaxies
A small percentage of large galaxies are neither
spiral nor elliptical.
The irregulars are mostly small and “peculiar”.
Their star systems are usually white and dusty, like
the disks of spirals.
Distant galaxies are more likely to be irregular
than those nearby (they were more common in
younger universe).