Stars and Galaxies – Notes
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The three types of galaxies are spiral, elliptical, and irregular.
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Our sun is in the spiral- shaped Milky Way galaxy. Spiral galaxies are the most
common type of galaxy found and are oval shaped. They have a dense center
with spiral arms and are bright and easy to see.
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Elliptical galaxies vary in shape from nearly spherical to flat disks. They contain
very little dust and gas. Stars found here are generally older than those in other
types of galaxies.
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Irregular galaxies have no specific shape and are hard to see.
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Nebulae are huge clouds of dust and gas from which new stars are born.
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Steps for the “birth” of stars:
o Large clouds of gas and dust, called nebulae, collapse due to gravity.
o The collapsing cloud becomes very dense.
o Nuclear reactions involving hydrogen and helium begin.
o These nuclear reactions power the star.
o A star is born.
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Many stars are found in multiple-star systems. Alpha Centauri is in a multiple star
system. It is made up of three stars called a triple star system. Over half of the
stars in the sky have at least one companion star. Most of these stars are doublestar systems in which two stars revolve around each other. Double-star systems
are called binary stars.
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A spectrum is used when describing the movement of stars. A spectrum is a band
of colors formed when white light passes through a prism.
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Every distant galaxy shows a red shift, indicating that the universe or star is
expanding or moving away from Earth. A blue shift would indicate that the
universe or star is moving closer.
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Most astronomers agree that the universe began with the big bang. The Big Bang
Theory states that the universe began to expand with the explosion of
concentrated matter and energy and has been expanding ever since.
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Stars differ in many features, including size, mass, color, temperature, and
brightness.
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The color of a star is related to its temperature.
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The main factor that shapes the life and eventual death of a star is its mass.
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Supergiant stars are the largest of all stars. They have a diameter up to 1000 times
the diameter of the sun. They tend to tie off quickly and are the shortest-lived
stars in the universe.
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Giant stars have a diameter of about 10 to 100 times as large as the sun.
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Medium-sized stars, like our sun, make up the majority of the stars. Our sun has a
diameter of about 1,392,000 kilometers or about 109 times the diameter of Earth.
They very in size from about one-tenth the size of the sun to about ten times its
size. These stars tend to be very bright
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White dwarfs stars are smaller than the Earth.
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Nova stars are stars that suddenly increase in brightness and then get dim.
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Neutron stars are the smallest stars of all. A typical neutron star has a diameter of
only about 16 kilometers.
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Surface temperature of a star can be determined by its color. Blue being very hot
35,000oC and above and Red being cool 3,000oC. Our sun is classified in yellow.
Its average surface temperature is around 6,000oC.
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Most stars are made up primarily of hydrogen and helium gases.
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A star’s brightness as observed from Earth is its apparent magnitude. A star’s true
brightness is its absolute magnitude.
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The Hertzsprung-Russell diagram shows the relationship between a star’s
absolute magnitude and its temperature this also includes color.
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The layers of the sun are the corona, chromosphere, photosphere, and core.
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The main factor that affects the evolution of a star is its starting mass.