Stellar EvolutionUnit 1: SpaceThe Study of the Universe

Basic Structure of Stars Mass governs a star’s temperature,

luminosity, and diameter.Mass Effects:

The more massive the star, the greater the force of gravity towards its center of mass (the core).

As a result, a star needs to be hotter and denser to counteract its own gravity.

The balance between gravity squeezing inward and outward pressure is maintained by heat due to nuclear reactions and compression.

Stellar Evolution Star formation

The formation of a star begins with a cloud of interstellar gas and dust called a nebula.

Provided the cloud is big enough, it will begin collapsing in on itself as a result of gravity.

As it continues to contract, its rotational forces it into a disk shape with a hot and dense center.

This is called a protostar.

Stellar Evolution Fusion Begins.

When the temperature at the core of the protostar becomes hot enough, nuclear fusion reactions begin.

The first fusion reactions always begin with the conversion of hydrogen into helium.

Once this happens, the star becomes stable and it takes its place along the main sequence according to its mass.

Stellar Evolution

Stellar Evolution Life Cycles of medium-

low mass stars. A star like the Sun will

gradually become more luminous because the core density and temperature rise slowly and increase the reaction rate.

It takes about 10 billion years for a star like the Sun to convert all of the hydrogen in its core to helium.

Stellar Evolution Red Giant Phase

Once a star begins burning helium in its core, it grows to become a red giant.

Red giants are so large because hydrogen continues to react in a thin layer at the edge of the helium core. The energy produced in this layer forces the outer layers of the star to expand.

Stellar Evolution Red giants are so large that

their cores are a great distance from the outer layers. As a result, surface gravity is low and some of the outer layers can be released by small expansions of the star due to instability.

Meanwhile, the core of the star becomes hot enough (100 million K) for helium to react and begin forming carbon.

At this point, the star contracts again and becomes more stable.

Stellar Evolution The final stages

Stars that are about the same mass as the Sun never become hot enough to fuse carbon and so energy production ends.

The outer layers expand again and are expelled by pulsations that develop in the outer layers.

This shell of gas is known as a planetary nebula.

Stellar Evolution White Dwarfs

A white dwarf is made of carbon and it is stable despite its lack of nuclear reactions.

It counteracts the effects of gravity with the resistance of electrons being squeezed so closely together.

This electron pressure does not require ongoing reactions so it can last indefinitely.

Eventually, the white dwarf cools and loses its luminosity: it has become a black dwarf.

Stellar EvolutionNot this kind of black dwarf.

Stellar Evolution Life Cycles of Massive

Stars Stars that are much

larger than our Sun have a different life cycle.

These star may begin in the same way but, because its initial mass is greater, it is more luminous and its main sequence lifetime is much shorter.

Stellar Evolution Supergiant

Supergiants can undergo many more reaction phases and, therefore, they can produce far more elements in their interior.

These stars can become red giants several times as they expand following the end of each reaction phase.

Supernova Formation A star that begins with a

mass that is 8-20 times greater than the Sun’s mass will end up with a core that is too massive to be supported by electron pressure.

Once core reactions have produced iron, no further energy producing reactions can occur.

Stellar Evolution

Stellar Evolution Neutron Star

▪ The iron core collapses in on itself and protons and electrons merge to form neutrons.

▪ The neutrons resist being so near to each other and this creates a tremendous amount of pressure.

▪ The core becomes a collapsed star remnant – a neutron star.

▪ A neutron star can have a mass of 1.5 to 3 times the Sun’s mass but with a radius of only about 10km.

▪ Some neutron stars are unique in that they have a pulsating pattern of light. These pulsating stars are known as pulsars.

Stellar Evolution Supernova

A neutron star forms quickly while the outer portions of the star are still falling inward.

The falling gas rebounds quickly after it strikes the hard surface of the neutron star.

The entire outer portion of the star is blown off in a massive explosion called a supernova.

This explosion is the only way that elements heavier than iron are produced in the universe.

Stellar Evolution Black Holes

Some stars are too massive to form neutrons stars.

The resistance of neutrons being squeezed is not great enough to counteract the force of gravity (collapse).

Matter gets compacted into an increasingly small volume.

The small, extremely dense object is known as a black hole.

In a black hole, gravity is so immense that nothing, not even light, can escape it.

Stellar Evolution

Homework Answer questions 4 and 8 on page 349 Complete the Inquiry Investigation (8-B)

on page 352:▪ Answer questions 1-5 and 7 on page 353.