Ch. 29 The Stars29.3 Stellar Evolution

Basic Structure of Stars

Mass effects

• The more massive a star is, the greater the gravity pressing inward, and the hotter and more dense the star must be inside to balance its own gravity.

• The temperature inside a star governs the rate of nuclear reactions, which in turn determinesthe star’s energy output—its luminosity.

Basic Structure of Stars

Mass effects• gravity squeezing inward

• outward pressure from heat due to nuclear reactions and compression.

• This balance, governed by the mass of the star, is called hydrostatic equilibrium, and it must hold for any stable star.

Basic Structure of Stars

Fusion

• The density and temperature increase toward the center of a star, where energy is generated by nuclear fusion.

• On Main Sequence: Hydrogen fuses into Helium

• Off Main Sequence: stars fuse other elements than H or not at all

Stellar Evolution

• As its nuclear fuel runs out, a star’s internal structure and mechanism for producing pressure must change to counteract gravity.

• The changes a star undergoes during its evolution begin with its formation.

Stellar EvolutionStar Formation

• The formation of a star begins with a cloud of interstellar gas and dust, called a nebula(plural, nebulae), which collapses on itself as a result of its own gravity.

• As the cloud contracts, its rotation forces it into a disk shape with a hot, condensed object at the center, called a protostar.

Stellar Evolution

Star Formation

• Friction from gravity continues to increase the temperature of the protostar, until the condensed object reaches the ignition temperature (13 million K) for nuclear reactions and becomes a new star.

Stellar Evolution

Fusion Begins

• The first nuclear fusion reaction to ignite in a protostar is always the conversion of hydrogen to helium.

• the star becomes stable; internal heat produces the pressure that balances gravity. The object is then truly a star.

Life Cycles of Stars Like the Sun

• It takes about 10 billion years to convert all of the H in its core into He (main-sequence lifetime)

• a red giant is the next step in the life cycle of a small mass star

Life Cycles of Stars Like the Sun

Red Giants

• Only in the core of a star do temps get hot enough to do fusion (only ~10% of total H)

• a He center and outer layers made of H-dominated gas.

• Some hydrogen continues to react in a thin layer at the outer edge of the helium core.

• The energy produced in this layer forces the outer layers of the star to expand and cool.

Life Cycles of Stars Like the Sun

Red Giant-Helium Core

Life Cycles of Stars Like the Sun

Red Giants• Low surface gravity; loses gas from its

outer layers

• the core of the star becomes hot enough, at 100 million K, for helium to react and form carbon.

• Contracts back to a more normal size; stable for a while

• When the helium is depleted, the star is left with a core made of carbon.

Life Cycles of Stars Like the SunThe Final Stages• A star with the same mass as the Sun

never becomes hot enough for carbon to fuse, so its energy production ends.

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

• The shell of gas is called a planetary nebula.

• In the center, the core of the star becomes exposed as a small, hot object known as a white dwarf

• made of carbon about the size of Earth.

Planetary nebulae

Life Cycles of Stars Like the Sun

Internal pressure in white dwarfs• NO nuclear reactions, but…• the resistance of electrons being

squeezed together counteracts gravity • mass of the remaining core must be less

than about 1.4 times the mass of the Sun

• Can exist indefinitely; cools and becomes an undetectable black dwarf

Life Cycles of Massive Stars

• massive stars begin the same, with hydrogen being converted to helium

• much higher on the main sequence

• star is very luminous and uses up its fuel quickly; m.s. lifetime is short

Life Cycles of Massive Stars

Super Giant• A massive star undergoes many more

reaction phases

• produces many elements in its interior

• becomes a red giant several times as it expands following the end of each reaction stage.

Life Cycles of Massive Stars

Super Giant• More shells are

formed by the fusion of different elements

• the star expands to a larger size and becomes a supergiant.

• These stars are the source of heavier elements in the universe.

Life Cycles of Massive Stars

Supernova Formation • Star mass between about 8 and 20 times

the Sun’s mass (Sun = 1 Solar Mass) • core too massive to be supported by

electron pressure• Fusion of Si into Fe in the core is the last

energy-producing reaction that can occur• the core of the star violently collapses in on

itself• Protons and electrons merge to form

neutrons, resulting pressure halts the collapse

Life Cycles of Massive Stars

Supernova Formation • A neutron star

• is a collapsed, dense core of a star that forms quickly while its outer layers are falling inward

• diameter of about 20 km • 1.4 to 3 Solar Masses, • contains mostly neutrons• 1 teaspoon of a neutron star = 106 tons!

• A pulsar is a spinning neutron star that exhibits a pulsing pattern.

Pulsar

Double Pulsar-discovered by GBT

Life Cycles of Massive Stars

Supernova Formation • the outer layers of a star collapse into the neutron

core

• pressure causes this mass to explode outward as a supernova, leaving a neutron star.

• Explosion creates all elements heavier than Fe

Life Cycles of Massive Stars

Black holes• A star must begin with 20+ solar mass • too massive to form a neutron star. • The core continues to collapse,

compacting matter into a smaller volume.• A black hole is a small, extremely dense

remnant of a star whose gravity is so immense that not even light can escape its gravity field.