Stellar Evolution

Stellar Evolution Chapter 30.2

Star Formation • Nebula a large cloud of gas and dust in interstellar space • a region in space where stars are born.

Compressing the nebula pushes all the gasses and dust particles together They become close enough to each other that gravity is able to pull them tighter together. • This creates regions of dense matter build up within the cloud.

Protostars 1ST Stage in the life of a star. As the dense regions become more compact, they spin and shrink forming a flattened disk. The disk has a central concentration of matter called a protostar.

The protostar continues to contractand increase in temperature for several million years. • Eventually the gas in the region becomes so hot, that the gas is then considered a separate state of matter called plasma.

The Birth of a Star • A protostar’s temperature continually increases until it reaches about 10,000,000°C. • At this temperature, nuclear fusion begins. • Nuclear fusion is the process by which two hydrogen molecules join together, or "fuse", to form one helium molecule • 2 lighter molecules coming together to form a single heavier molecule. • The process releases enormous amounts of energy. • Nuclear fusion marks the birth of a star. • this process can continue for billions of years.

A Delicate Balancing Act • As gravity increases the pressure on the matter within the star, the rate of fusion increase. • In turn, the energy radiated from fusion reactions heats the gas inside the star. • The outward pressures of the radiation and the hot gas resist the inward pull of gravity. • This equilibrium makes the star stable in size.

Note Check • How does the pressure from fusion and hot gas interact with the force of gravity to maintain a star’s stability? The forces balance each other and keep the star in equilibrium. As gravity increases the pressure on the matter within a star, the rate of fusion increases. This increase in fusion causes a rise in gas pressure. As a result, the energy from the increased fusion and gas pressure generates outward pressure that balances the force of gravity.

Classifying Stars • The H-R Diagram is one way scientists classify stars. • Compares their temperatureto the their brightness. • By plotting the surface temperatures of stars against their luminosity. • The H-R diagram is the graph that illustrates the resulting pattern. • Astronomers use the H-R diagram to describe the life cycles of stars. Most stars fall within a band that runs diagonally through the middle of the H-R diagram. These stars are main sequence stars.

Visual Concept

The Main-Sequence Stage • The second and longest stage in the life of a star is the main-sequence stage. • During this stage, energy continues to be generated in the core of the star as hydrogen fuses into helium. • A star that has a mass about the same as the sun’s mass stays on the main sequence for about 10 billion years. • Scientists estimate that over a period of almost 5 billion years, the sun has converted only 5% of its original hydrogen into helium.

Leaving the Main Sequence Giant Stars • Giants are very large and bright stars whose hot core has used most of its hydrogen. • The 3rd stage of a stars life is when almost all of the hydrogen atoms within its core have fused into helium atoms. • A star’s shell of gases grows cooler as it expands. As the gases in the outer shell become cooler, they begin to glow with a reddish color. These stars are known as giants.

Supergiants • Main-sequence stars that are more massive than the sun will become larger than giants in their third stage. • These highly luminous stars are called supergiants. These stars appear along the top of the H-R diagram.

Note Check • Where are giants and supergiants found on the H-R diagram? Giants and supergiants appear in the upper right part of the H-R diagram.

The Final Stages of a Sunlike Star Planetary Nebulas • As the star’s outer gases drift away, the remaining core heats these expanding gases. • The gases appear as a planetary nebula, a cloud of gas that forms around a sunlike star that is dying.

White Dwarfs • As a planetary nebula disperses, gravity causes the remaining matter in the star to collapse inward. • The matter collapses until it cannot be pressed further together. • A hot, extremely dense core of matter - a white dwarf - is left. White dwarfs shine for billions of years before they cool completely.

Dying Stars Nova: a star that suddenly becomes brighter • Some white dwarfs revolve around red giants. When this happens, the gravity of the white dwarf may capture gases from the red giant. • As these gases accumulate on the surface of the white dwarf, pressure begins to build up. • This pressure may cause large explosions. These explosions are called novas.

The Final Stages of Massive Stars Supernovas in Massive Stars • Massive stars become supernovas as part of their life cycle. • After the supergiant stage, the star collapses, producing such high temperatures that nuclear fusion begins again. • When nuclear fusion stops, the star’s core begins to collapse under its own gravity. This causes the outer layers to explode outward with tremendous force.

Note Check • What causes a supergiant star to explode as a supernova? Giants and supergiants appear in the upper right part of the H-R diagram.

Neutron Stars Neutron star: a star that has collapsed under gravity to the point that the electrons and protons have smashed together to form neutrons • Stars more massive than the sun do not become white dwarfs. • After a star explodes as a supernova, the core may contract into a neutron star.

Pulsars • Pulsar: a rapidly spinning neutron star that emits pulses of radio and optical energy • Some neutron stars emit a beam of radio waves that sweeps across space and are detectable here on Earth. • These stars are called pulsars. For each pulse detected on Earth, we know that the star has rotated within that period.

Black Holes • Black hole is an object so massive and dense that even light cannot escape its gravity • Some massive stars produce leftovers too massive to become a stable neutron star. • These stars contract, and the force of the contratction leaves a black hole.

Note Check • Describe how a protostar becomes a star. • Explain how a main-sequence star generates energy. • Describe the evolution of a star after its main-sequence stage.

Stellar Evolution