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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 Ejnar Hertzsprung and Henry Russell in the early 1900s.

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Stars and galaxies

Stars and Galaxies

The Evolution of Stars


Classifying stars the h r diagram
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.


Here’s a simpler version. Where’s our Sun? brightness are shown in this H-R diagram.


Classifying stars the h r diagram1
Classifying Stars: brightness are shown in this H-R diagram.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
How Do Stars Shine? brightness are shown in this H-R diagram.

  • 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
Fission or Fusion? brightness are shown in this H-R diagram.

  • 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
Evolution of Stars brightness are shown in this H-R diagram.

  • 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 White Dwarfs brightness are shown in this H-R diagram.

  • 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
White Dwarfs and Planetary Nebulas brightness are shown in this H-R diagram.

  • 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
Evolution of Massive Stars brightness are shown in this H-R diagram.

  • 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 stars1
Evolution of Massive Stars brightness are shown in this H-R diagram.

  • 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


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