Lecture 39

Stellar Lives (continued). Galaxies.

• Last Stages of Low-Mass Stars

• Lives of High-Mass Stars

• Galaxies: Types and Structure

Chapter 17.14 17.16, 18.1 18.5

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 20003000 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 helium- and a hydrogen-burning shell.

At this point the paths of intermediate- and high-mass stars diverge.Intermediate-mass stars blow their outer layers away and become white dwarfs.

Massive Stars after Main-SequenceIn 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.

SupernovaThe 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

~7585% 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 low-density, 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).