Galactic morphology

We can see several types of galaxies, differring for example for their form, dimensions, brightness, mass, stellar contents and, in the end, for the energy emission distribution in the different bands of the electromagnetic spectrum.
The principal classification, called Hubble Sequency, is based on the form and it divides the galaxies between elliptical, spiral and irregular ones.
Elliptical galaxies
They show regular systems, approximately with a spherical form, with just few dust and interstellar gas, fitted with a really dense nucleus, whose superficial brightness decreases from the centre towards the periphery. Their structure may change from the circular form, called E0, to the extremely crushed one, described like E7. The elliptical galaxies are made, above all, by red stars (or Population II) that, according to the theory of the stellar evolution, are very ancient.
Stars are in fact used to change their color becoming old. In the first part of their life they show a blue color, becoming then more yellow-red.
Spiral galaxies
They appear like systems full of interstellar gas and dusts, based on a central bulge surrounded by a disc, from where run bright spiral filaments, called arms, site of an intense stellar formation.
We can also divide these spiral galaxies into two classes: normal ones (S), with a central and at least perfectly spherical central nucleus and spiral arms, and barred spirals (SB), different from the normal ones because of a central structure placed through the nucleus, a so-called bar-shaped structure.
Irregular galaxies
They appear like systems full of interstellar gas and dusts, usually being inferior, for their mass, to the spiral and elliptical galaxies. They are usually called “irregular” because their aspect has no simmetry. They typically host young stars, or stars from the population I.

Special reports

From the Multimedia section

  • space

    Virgo cluster

    Look

    space

    The Hercules Globular Cluster (M13)

    Look

    space

    Mice Galaxies

    Look
  • space

    The Pleiades

    Look

    space

    Quasar

    Look

    space

    Cosmic radiation

    Look
  • space

    Recession velocity and distance

    Look

    space

    Black hole

    Look

    space

    The flux of cosmic ray particles

    Look
  • space

    Virgo cluster

    Look
  • space

    The Hercules Globular Cluster (M13)

    Look
  • space

    Mice Galaxies

    Look

Facts

  • 17 May 2011

    Neutrinos

    Neutrinos are extremely light neutral particles...

    The comet’s tail

    Comets are “dirty snowballs” that spend most of their life at the edges of the Solar System...

  • The definition of a dwarf planet

    Astronomy, like all scientific disciplines, is continuously evolving...

    The PP cycle

    Nuclear fusion within the stars begins with the different passages...

  • Children of the stars

    We are made of star dust. The iron in our blood, the oxygen we breathe...

    The Olbers paradox

    Why is the sky dark at night? What seems like an obvious question...