Stars and Galaxies

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# Stars and Galaxies - PowerPoint PPT Presentation

Stars and Galaxies. The Evolution of Stars. Classifying Stars: The H-R Diagram. One of the most useful ways to classify and describe stars was discovered by Ejnar Hertzsprung and Henry Russell in the early 1900s.

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### Stars and Galaxies

The Evolution of Stars

Classifying Stars: The H-R Diagram
• One of the most useful ways to classify and describe stars was discovered by EjnarHertzsprung and Henry Russell in the early 1900s.
• They noticed that in general, stars with higher temperatures also have brighter absolute magnitudes.
• They developed a graph to show this relationship, now called the Hertzsprung-Russell diagram, or simply the H-R diagram for short.

The relationships among a star’s color, temperature, and brightness are shown in this H-R diagram.

• Stars in the upper left are hot, bright stars, and stars in the lower right are cool and faint.
• Our Sun is a “main sequence” star about halfway through its 10 billion year lifetime, and falls about in the middle of this diagram.
Classifying Stars: The H-R Diagram
• The Main Sequence
• Forms a diagonal band on the H-R diagram
• In the upper left are stars that are hot, blue, and bright.
• In the lower right are stars that are cool, red, and dim.
• The Sun is an average yellow star in the middle of the main sequence.
• About 90% of all stars fall in the main sequence.
• Dwarfs, Giants, and Supergiants
• The 10% of stars that don’t fall in the main sequence.
• They represent stars that are at later stages of their life cycle.
• White Dwarfs: hot, but not very bright: they’re small.
• Red Supergiants: cool, but extremely bright: they’re huge.
How Do Stars Shine?
• Generating Energy:
• In the 1930s, scientists discovered reactions between the nuclei of atoms: nuclear fissionand nuclear fusion.
• Scientists hypothesized that temperatures in the center of the Sun must be high enough to cause hydrogen to fuse to form helium.
• This nuclear reaction would release tremendous amounts of energy.
• In 1905, Albert Einstein had discovered the relationship between mass and energy: E =mc2
• Nuclear Fusion:
• Fusion occurs in the cores of stars.
• Here, temperatures are high enough (10 million K) for fusion to occur.
• As hydrogen nuclei move so fast that they collide and fuse, a small amount of mass is “lost” and converted to a large amount of energy.
Fission or Fusion?
• Nuclear Fission:
• A large atomic nucleus is split, forming lighter elements.
• This converts a small amount of mass into tremendous energy.
• This is nuclear energy: used in nuclear power plants and in nuclear weapons (atomic bombs).
• Nuclear Fusion:
• Atomic nuclei are fused together to form a heavier element.
• This converts a small amount of mass into tremendous energy. Fusion occurs in the cores of stars.
• This is not yet used to produce energy in a power plant. However, this process is used in thermonuclear weapons (hydrogen bombs).
Evolution of Stars
• The Birth of a Star:
• Stars begin as a large cloud of gas and dust called a nebula.
• As the nebula contracts due to gravity, temperatures in the center of the nebula increase.
• When temperatures reach 10 million K, fusion begins.
• The Main Sequence:
• While the star continues to fuse or “burn” hydrogen, it is a “main sequence” star.
• The Sun is about midway through its 10 billion year life span, gradually fusing hydrogen into helium in its core.
• Stars more massive than the Sun can use up their hydrogen in as little as 1 million years. Stars faint and less massive can last for billions of years on the main sequence.
• What happens when the hydrogen runs out?
Red Giants and White Dwarfs
• Red Giants and Supergiants:
• No longer a main sequence star, the Sun will become a red giant in about 5 billion years.
• As its core contracts and heats to 100 million degrees, helium nuclei will fuse into carbon.
• Its outer layers will expand as the sun swells to about 2 AU in diameter.
• Becoming a White Dwarf:
• A star like the Sun will use up its helium and its core will contract even more.
• The Sun’s outer layers will escape to space leaving behind a hot, dense core: a “white dwarf.”
• A white dwarf is only about the size of the Earth.
White Dwarfs and Planetary Nebulas
• A star like the Sun may lose half its mass in forming a white dwarf and “planetary nebula” like the Helix Nebula shown here.
• In this way, stars return much of their material to space, to form new stars from new nebulas.
Evolution of Massive Stars
• Supergiants:
• Stars more than 10 times the mass of the Sun do not become white dwarfs, and will fuse all their hydrogen into helium in only a few million years.
• In stages, they will then produce heavier and heavier elements as their cores contract and heat to 1 billion K or more.
• The star becomes a “red supergiant.” The star Betelguesein the constellation Orion is an example.
Evolution of Massive Stars
• Supernovas:
• When a red supergiant begins to produce iron in its core, the energy produced by fusion can no longer balance the collapse due to gravity.
• The core of the star collapses violently, sending shock waves outward.
• The outer portion of the star explodes, producing a supernova explosion.
• All other elements in the universe (up to uranium) are produced by fusion in this shock wave.
• This is one of the most violent events in the universe: in one second, a supernova explosion can release more energy than the Sun produces during its entire 10 billion year lifetime.

Supernova 1987a

Crab Nebula