Objectives

^ Define stellar evolution.

^ List the stages in the life cycle of a star like our Sun according to the modern theory of stellar evolution.

^ Explain the importance of the H-R diagram to theories of stellar evolution.

^ Explain the relation between a star's age and its position on the H-R diagram.

^ List the main steps in the birth of a star.

^ Describe the energy balance and pressure balance in main sequence stars.

^ Compare and contrast what happens in the advanced stages of evolution for stars of large and small mass: planetary nebulas, white dwarfs, supernovas, pulsars/neutron stars, and black holes.

^ Identify nebulas, main sequence, blue giant, red giant, and pulsating variable stars that can be observed in the sky.

^ Explain how supernovas and pulsars are observed.

^ Describe the origins of the different chemical elements and the importance of supernovas to new generations of stars.

^ Describe observational evidence for black holes.

To every thing there is a season, and a time to every purpose under the heaven. A time to be born, and a time to die.

Ecclesiastes 3:1-2

5.1 LIFE CYCLE OF STARS

No star shines forever. Stellar evolution refers to the changes that take place in stars as they age—the life cycle of stars. These changes cannot be observed directly, because they take place over millions or billions of years. Astronomers construct a theory of stellar evolution that is consistent with the laws of physics. Then they check their theory by observing real stars shining in the sky.

In checking theory against observations, astronomers make use of H-R diagrams. Theoretical predictions are made regarding a sequence of changes in luminosity and temperature for stars as they go from birth to death. These changes are plotted on an H-R diagram, forming theoretical tracks of evolution. Theoretical H-R diagrams are then compared with H-R diagrams for groups of real stars (Section 6.4).

The predictions of the modern theory of stellar evolution, described in this chapter, agree well with the data from observations of real stars.

What is stellar evolution?_

Answer: The changes that take place in stars as they age—the life cycle of stars.

5.2 BIRTHPLACES

Stars form out of matter that exists in space. The gigantic interstellar (between stars) clouds of gas and dust must be the birthplaces of stars.

You can see the nearest cloud in space where new stars are forming now. The famous Orion Nebula, located about 1500 light-years away in the constellation Orion, is a region of intense star formation (Figure 5.1).

Look for the Orion Nebula in the winter. It is marked on your winter skies map in the sword of Orion the Hunter. The Orion Nebula looks like a hazy patch to your eye. Through a telescope you will see it glow with a greenish color. Hot, newly formed stars in the region make the gases glow. A much larger associated dark molecular cloud is not visible.

Are new stars still being born today? Where?_

Answer: Yes. In gigantic clouds of gas and dust, as in the Orion Nebula.

Figure 5.1. Orion Nebula, in the constellation Orion.

5.3 BIRTH

A protostar is a star in its earliest observable phase of evolution. You can think of a protostar as a star that is being born.

Protostars form by chance at high-density clumps inside huge turbulent gas (mostly hydrogen) and dust clouds that exist in space. Perhaps a shock wave from an exploding star (supernova) triggers the process.

A protostar is held together by the force of gravity. Initially, the force of gravity pulls matter in toward the center of a dense clump, causing it to contract and become even denser. Matter continues to accrete onto the protostar as it contracts. Gravitational contraction of the cloud and protostar causes the temperature and pressure inside to rise greatly.

Heat flows from the protostar's hot center to its cooler surface. The protostar radiates this energy into space. It shines at infrared wavelengths.

In a rotating cloud, a disk of dust and gas may surround a protostar. This disk also reradiates the energy as infrared. Possibly particles in the disk accrete to form planets (Figure 12.2).

When the temperature in the protostar's center reaches 10 million K, nuclear fusion reactions start. These nuclear reactions release tremendous amounts of energy. Energy is generated in the center as fast as it is being radiated out into space. The very high internal temperatures and pressures are thus maintained.

The outward pressure of the very hot gases balances the inward pull of gravity (Figure 5.2). This balance is called hydrostatic equilibrium. The

Figure 5.2. The outward gas pressure balances gravity at every level in a star.

protostar stops contracting. It shines its own light steadily into space. The protostar becomes a newborn star. Most likely our Sun was born in this way about 5 billion years ago.

Recent observations support this theory of star birth. Protostars in dense cores of gaseous clouds are imaged at infrared wavelengths. Jets of gas are seen streaming away from young stars. They may be aligned by a planet-forming circumstellar disk.

List the three main steps in the birth of a star. (1)

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