GALAXIES

ELLIPTICAL IRREGULAR

SPIRAL

GALAXY• a massive gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter, an important but poorly understood component

• contain varying numbers of planets, star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Supermassive black holes reside at the center of most galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy is known to harbor at least one such object.

ETYMOLOGY• The word galaxy derives

from the Greek term for our own galaxy, galaxias (γαλαξίας, "milky one"), or kyklos ("circle") galaktikos ("milky") for its appearance as a lighter colored band in the sky.

GREEK MYTHOLOGY• Zeus places his son born by a mortal

woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away and a jet of her milk sprays the night sky, producing the faint band of light known as the Milky Way.

SPIRALS

ELLIPTICALSSPIRALS-b

Examples of

Galaxies

ELLIPTICALShas an ellipse-shaped light profile

SPIRALSare disk-shaped with dusty, curving arms

SPIRALS-b

IRREGULARS

those with irregular or unusual shapes

IRREGULARS

are galactic formations that develop unusual properties due to tidal interactions with other galaxies

IRREGULARS

are galaxies that cannot be readily classified into an elliptical or spiral