The Galaxy and Night Sky



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Chapter 6 : The Galaxy and Night Sky



The Galaxy arrow_upward


  • Earth and the other planets that orbit the Sun and together form our solar system.
  • The Sun is relatively a small star and only one of the billions.
  • The Milky Way is the galaxy that contains our Solar System.
  • The galaxy has been found to be made up of countless individual stars that seem relatively close together.

  • Types and Structure of the Galaxies arrow_upward



    Spiral Galaxies arrow_upward


  • The spiral is the main type of galaxy in the universe.
  • Spiral galaxies have arms that form a spiral pattern around a central bulge.
  • The arms form a disk around the nucleus as the nucleus spins, the arms follow behind it.
  • The youngest stars in the spiral galaxies are found in loosely packed arms.
  • Older stars lie mainly in the dense nucleus.
  • The oldest star of all resides in a sparsely populated spherical halo that surrounds the galactic disk.
  • The Andromeda galaxy is a spiral galaxy and is the furthest object visible to the naked eye on Earth.
  • The arms also contain much gas and dust that have yet to form into stars.

  • Barred-Spiral Galaxies arrow_upward


  • A Barred-spiral galaxy has an elongated, bar-shaped central bulge.
  • While the nucleus rotates, an arm seems to follow at each end.
  • Spiral and barred-spiral galaxies ranges in shape from those with large, central bulges circled by tightly bound arms, to those with small bulges and loose arms.

  • Irregular Galaxies arrow_upward


  • Irregular galaxies have no regular shape or structure.
  • They are typically less massive than other galaxies, and most of their stars are bright and young.
  • Although many irregular galaxies contain regions of luminous gas in which stars are being born.
  • Although irregular galaxies make up about 25 % of all known galaxies.
  • Only about 5 % of the 1000 brightest galaxies are irregular.

  • Elliptical Galaxies arrow_upward


  • Elliptical galaxies range in shape from ellipsoidal to spherical.
  • Unlike other galaxies, in which dust reflecting from hot young stars can be seen, elliptical galaxies appear yellow.
  • This is because the process of star formation in them has stopped, so that nearly all their light comes from old red-giant stars.
  • Both the least and the most massive galaxies observed so far are elliptical.

  • Interstellar Matter arrow_upward


  • The regions between stars in a galaxy are filled by the interstellar medium.
  • It consists mainly of hydrogen and helium gases, with traces of other gases and a tiny amount of dust.
  • This material is uneven in distribution and temperature.
  • Its density is many billions of times less than that of air.
  • Much of the interstellar medium (ISM) consists of clouds.
    • Some of which can be detected as nebulae if they emit or reflect light from stars.

    Night Sky arrow_upward


    The Celestial Sphere:
    • An imaginary sphere surrounding the Earth.
    • An axis through Earth’s North and South Pole also passes through the celestial North and South Pole.

    Celestial Equator

    Observer Coordinates:
    • Horizon – the plane you stand on.
    • Zenith – the point right above you.
    • Meridian – the line from North to Zenith and then to South.

    The Celestial Sphere arrow_upward


  • The sky is like a hollow sphere with the stars attached to it.
  • This sphere rotates once every 24 hours.
    • This imaginary sphere is called the Celestial Sphere.
  • It is useful to imagine the Earth as being stationary while the celestial sphere rotates around it.
  • The north celestial pole is directly above the North Pole on the Earth.
  • The south celestial pole is directly above the South Pole on the Earth.
  • The celestial equator is an extension of the Earth’s equator on the sky.
  • The zenith is the point, directly overhead.
    • The horizon is the circle, 90 degrees from the zenith.
  • The celestial poles and the celestial equator are the same for everyone.
  • The zenith and the horizon depend on   the point where we stand.

  • Constellations arrow_upward


  • A group of stars forming a pattern that is traditionally named after its apparent form or identified with a mythological figure.
  • People have long made up stories about groups of stars that appear close together in the sky. Such groupings are called Constellations.
  • The sky was officially divided up into 88 constellations so that a star is associated with only one constellation.
  • The modern constellations have strictly defined boundaries by international agreement.
  • The stars are usually not physically associated with each other.


  • Thank You from Kimavi arrow_upward


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