4 August 2005AST 2010: Chapter 211 Stars: From Adolescence to Old Age.

4 August 2005 AST 2010: Chapter 21 1

StarsStars::

From From Adolescence Adolescence

to Old to Old AgeAge


Mass Determines Life StagesMass Determines Life StagesThe mass of a star determines the stages it goes through and how long it lasts in each stage Massive stars evolve faster than small stars

Higher mass requires higher pressure to balance it, so that hydrostatic equilibrium is maintainedHigher pressure in turn is produced by higher temperatureThe higher the temperature inside a star, the faster it uses up its hydrogen fuelAlthough massive stars have more fuel, they burn it so quickly that their lifetimes are much shorter than those of low-mass starsThis also explains why the most massive main-sequence stars are the most luminous


Lifetime of Main-Sequence Lifetime of Main-Sequence StarsStars


Stellar CollapseStellar CollapseOn the main sequence, a star’s inward gravity is balanced by the outward pressure

The pressure is due to the nuclear fusion in the core All the hydrogen in the core eventually gets used up in the fusion into helium

In other words, the fusion of hydrogen inside the core eventually stopsThe core now contains only heliumThe star is no longer on the main-sequence

Then gravity takes over and the (helium) core shrinksThe energy of the inward-falling material in the core is converted to heatThe heat flows outward, raising the temperature of the hydrogen just outside the core


Shell BurningShell BurningThe shell layer outside the core becomes hot enough for hydrogen fusion to startThis fusion in the layer just outside the core is called shell burningThe helium core continues to contract, creating more heat in the shell around it, leading to more fusionThis causes the star’s luminosity to increase beyond its main sequence valueWith all the new energy pouring outward, the star’s size expands significantlyThis causes the star’s outer layers to cool down Thus the star becomes a red giant

Its surface gets redder as it gets cooler It is very luminous because of its huge surface area and increased luminosity


Comparing the sizes of the Comparing the sizes of the SunSun, the giant star , the giant star Delta BoDelta BoÖÖtistis (orange sphere), and the supergiant (orange sphere), and the supergiant

Xi CygniXi Cygni (red sphere) (red sphere)


Comparing the Sun to a Comparing the Sun to a SupergiantSupergiant


End of Life on Earth …End of Life on Earth …When the Sun becomes a red giant, it will swallow Mercury, Venus, and perhaps the Earth, too

Or conditions on the Earth’s surface will become impossible for life to exist


Time to Reach Giant StageTime to Reach Giant StageTheoretical calculations suggest that the time for a main-sequence star to reach the giant stage is

short for a high-mass star

as low as 10 million (=107) years

long for a low-mass star

up to 10 billion (=1010) years


Characteristics of Star Characteristics of Star ClustersClusters

We saw that stars tend to form in clustersThe stars in a cluster have different masses but about the same ageThe different stars in a cluster provide a test for theories of stellar evolution

Three types of clusters areglobular clusters, containing very old starsopen clusters, containing young to middle-aged starsstellar associations, containing very young stars


Testing Theory: Relatively Young Testing Theory: Relatively Young StarsStars

A comparison of the prediction for the stars of a hypothetical 3-million-year-old cluster (left) with measurements of the stars in cluster NGC 2264 (right) The theory is roughly consistent with observation


Testing Theory: An Older ClusterTesting Theory: An Older Cluster

A comparison of the prediction for a hypothetical 4.24-billion-year-old cluster (left) with measurements of stars in the globular cluster 47 Tucanae (right)The theory appears to be roughly consistent with observation


Further Aging: Helium Fusion Further Aging: Helium Fusion As a star becomes a red giant, its (helium) core continues to shrink, causing its temperature to continue increasingWhen the core temperature reaches 100 million K, the helium nuclei can fuse to form carbon nuclei through a process called the triple-alpha process

In this reaction, three helium nuclei are fused into a single carbon nucleus

As the triple-alpha process begins, the entire core is ignited in a quick burst called the helium flashAfter this, the star becomes stable, its surface temperature increases, and its luminosity and size decreases

At this stage, carbon nuclei sometimes fuse with helium nuclei to form oxygen nuclei

But this new period of stability does not last very long As the helium is quickly used up in the fusion into carbon and oxygen, gravity will once more take over

The situation is analogous to the end of the main sequence


Near Death of Near Death of Stars Like the Stars Like the

SunSunThen the star becomes a red giant again, but only briefly The core now contains only carbon and oxygenAt this stage, a star similar in mass to the Sun has exhausted its inner resources and will soon begin to die

The star’s luminosity may pulsate for a time due to its pressure and gravity being out of sync


Planetary NebulaePlanetary NebulaeWhen stars become giants, they begin to shed their outer layers

exposing hot inner layerslosing a substantial fraction of their mass into space

The shells of gas ejected by such stars are called planetary nebulae

They looked like planets in early telescopes, but have nothing to do with planets

The nebulae are glowing because the gas is heated by the ultraviolet radiation of the dying central stars


Images Images of of

PlanetarPlanetary y

NebulaeNebulae


Dying Process of Massive Dying Process of Massive StarsStarsAt the end of the helium-fusion stage, a star with

a mass greater than about 8 solar masses has not yet exhausted its inner resourcesSuch a star is massive enough to cause more contraction that can trigger other kinds of fusion in its center

Carbon can fuse into still more oxygen as well as neon, sodium, magnesium, and finally silicon

After each of the possible sources of nuclear fuel is exhausted, the star contracts until it reaches a temperature high enough to lead to the fusion of still heavier nucleiDepending on the star’s mass, this continues until the star has used up all of its energy supplies


Creation of Chemical ElementsCreation of Chemical ElementsThe creation of heavier elements from lighter ones by nuclear fusion is called nucleosynthesisTheoretical calculations suggest that all elements up to iron can be built up by nucleosynthesis

Stars like our Sun produce elements up to carbon and oxygenVery massive stars can produce elements up to iron

Elements heavier than iron are believed to be produced in the supernova explosions of very massive stars

4 August 2005AST 2010: Chapter 211 Stars: From Adolescence to Old Age.

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