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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