Objectives Determine the effect of mass on a star’s evolution. Identify the features of massive and regular star life cycles. Explain how the universe

ObjectivesDetermine the effect of mass on a star’s evolution.

Identify the features of massive and regular star life cycles.

Explain how the universe is affected by the life cycles of stars.

Stellar EvolutionSection 29.3


The Sun and other stars follow similar life cycles, leaving the galaxy enriched with heavy elements.

Review Vocabulary

evolution: a radical change in composition over a star’s lifetime

New Vocabulary


nebula

protostar

neutron star

pulsar

supernova

black hole


Basic Structure of Stars

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 determines the star’s energy output—its luminosity.

Mass effects



The balance between gravity squeezing inward and outward pressure is maintained by 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.

Mass effects



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

Fusion

Stellar Evolution


Eventually, when 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 Evolution

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.

Star formation


Stellar Evolution

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

Star formation

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Stellar Evolution

The first nuclear fusion reaction to ignite in a protostar is always the conversion of hydrogen to helium. Once this reaction begins, the star becomes stable because it then has sufficient internal heat to produce the pressure needed to balance gravity. The object is then truly a star.

Fusion begins, star gets the on Main sequence

Life Cycles of Stars Like the Sun


It takes about 10 billion years for a star with the mass of the Sun to convert all of the hydrogen in its core into helium. Thus, such a star has a main-sequence lifetime of 10 billion years. From here, the next step in the life cycle of a small mass star is to become a red giant- moves out of the main sequence, fusing helium into carbon.



When the hydrogen in a star’s core is gone, it has a helium center and outer layers made of hydrogen-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.

Red giant



While a star is a red giant, it loses gas from its outer layers. Meanwhile, the core of the star becomes hot enough, at 100 million K, for helium to react and form carbon. When the helium in the core is depleted, the star is left with a core made of carbon.

Red giant


The 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.



The final stages

In the center of a planetary nebula, the core of the star becomes exposed as a small, hot object about the size of Earth. The star is then a white dwarf made of carbon.


Life Cycles of Stars Like the SunInternal pressure in white dwarfs

A white dwarf is stable despite its lack of nuclear reactions because it is supported by the resistance of electrons being squeezed together. This pressure counteracts gravity and can support the core as long as the mass of the remaining core is less than about 1.4 times the mass of the Sun.


Life Cycles of Massive Stars


A more massive star begins its life with hydrogen being converted to helium, but it is much higher on the main sequence. The star’s lifetime in this phase is short because the star is very luminous and uses up its fuel quickly. When the white dwarf cools and loses its luminosity, it becomes an undetectable black dwarf.



Supergiant

A massive star undergoes many more reaction phases and thus produces a rich stew of many elements in its interior. The star becomes a red giant several times as it expands following the end of each reaction stage.

Life Cycles of Massive StarsSupergiant

As more shells are formed by the fusion of different elements in a massive star, 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 StarsSupernova formation

A star that begins with a mass between about 8 and 20 times the Sun’s mass will end up with a core that is too massive to be supported by electron pressure. Once reactions in the core of the star have created iron, no further energy-producing reactions can occur, and the core of the star violently collapses in on itself.



Supernova formation

A neutron star is a collapsed, dense core of a star that forms quickly while its outer layers are falling inward. It has a radius of about 10 km and a mass 1.5 to 3 times that of the Sun, and it contains mostly neutrons.



Supernova formation

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


Life Cycles of Massive StarsSupernova formation

When the outer layers of a star collapse into the neutron core, the central mass of neutrons creates a pressure that causes this mass to explode outward as a supernova, leaving a neutron star.


Life Cycles of Massive StarsBlack holes

A star that begins with more than 20 times the Sun’s mass will be too massive to form a neutron star. The resistance of neutrons to being squeezed is not great enough to stop the collapse. The core of the star continues to collapse, compacting matter into a smaller volume.



Black holes

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.

Evolution of stars- as they get off the main sequence.( M – is the mass of Sun)

Mass up to 8M Mass 8-20 M Mass greater than 20M

1.Red giants 1.Super giants 1.Super giants

2.Planetary nebula 2.Super nova 2.Super nova

3.White dwarf 3.Neutron Star 3.Black hole

(about Earth size, mass up to1.4M)

(city size, mass 1.5 – 3M)

( point size, very dense , even light cannot escape its gravity)

CH

The Sun and other stars follow similar life cycles, leaving the galaxy enriched with heavy elements.

The mass of a star determines its internal structure and its other properties.

Gravity and pressure balance each other in a star.

Section 29.3 Stellar Evolution

Study Guide Key Concepts

CH

If the temperature in the core of a star becomes high enough, elements heavier than hydrogen can fuse together.

A supernova occurs when the outer layers of the star bounce off the neutron star core, and explode outward.

Section 29.3 Stellar Evolution

Study Guide Key Concepts

CH

The diagram depicts a star that is stable and will not expand or contract. What is this balance called?

a. electrostatic equilibrium

b. hydrostatic equilibrium

c. gravitational equilibrium

d. luminosity equilibrium

29.3 Section Questions

Stars

CH

The density of a neutron star is comparable to that of an atomic nucleus.

a. true

b. false


Stars

CH

If light cannot escape a black hole, how do astronomers locate black holes?

Answer: Because light cannot escape, a black hole is invisible. However, gases spiraling into a black hole emit X rays. Astronomers can locate the black hole by looking for those X-ray emissions.

Stars


CH

The diagram shows a star with a helium core. At which stage of its life cycle is this star?

a. main sequence

b. red giant

c. white dwarf

d. helium-carbon

Chapter Assessment Questions

Stars

CH


Which is the outermost layer of the Sun?

a. corona

b. prominence

c. chromosphere

d. photosphere

Stars

CH

What is the difference between absolute magnitude and apparent magnitude?

Possible answer: Apparent magnitude is how bright a star appears to be from Earth. Absolute magnitude takes the star’s distance into account.


Stars

CH

How is parallax used to determine the distance from Earth to a star?


Stars

CH

Answer: As Earth orbits the Sun, nearby stars appear to shift position in the sky when compared with more distant stars. The closer the star, the greater the shift. By measuring the angle of the change, astronomers can estimate the distance to the star.


Stars

CH

What causes the dark bands in a star’s spectrum?

Answer: The various chemical elements that make up the star absorb light at specific wavelengths. This causes dark bands to appear in the star’s spectrum.


Stars

CH

a. radioactive decay

b. X-ray emissions

c. fusion reactions

d. nuclear fission

Where does the Sun’s energy come from?

Standardized Test Practice

Stars

CH

What do astronomers measure to determine a star’s motion relative to Earth’s?

a. wavelength shift

b. absolute magnitude

c. angle of parallax

d. apparent magnitude


Stars

CH

a. age and size

b. position

c. color and size

d. spectral type

Which do astronomers use to classify a star?


Stars

CH

a. protostar

b. main sequence

c. beginning stages

d. final stages

At which part of its life cycle is a Sun-sized star with a carbon core?


Stars

CH


Stars

a. apparent magnitude

b. absorption spectra

c. absolute magnitude

d. emission spectra

Which property takes a star’s distance into account?

Documents

Objectives Determine the effect of mass on a star’s evolution. Identify the features of massive and regular star life cycles. Explain how the universe