Physical Agent Inventory: Galaxies

Last modified: Wed 4/16/97 1600 PDT

Canon Terms	Object	Classes	Galaxy; EllipticalGalaxy; SpiralGalaxy; PeculiarGalaxy; GalacticNucleus; SpiralArm
		Instances	(none)
	Event	Classes	(none)
		Instances	(none)

A galaxy is a Physical Agent in which large numbers of stars (and presumably their undiscovered star systems) are aggregated. It is estimated that there are about 50 billion (5 x 10¹⁰) galaxies in the physical universe.

• You can learn about galaxies here:
• Hands-On Universe: Galaxy Features Online (Lawrence Berkeley Lab)
• Deep Sky Home Page
• Multiwavelength Atlas of Galaxies (Mackie)
• The Galaxy Catalog ()
• Understanding the Hubble Sequence of Galaxies (George Lake)

An elliptical galaxy is a galaxy in which the stars are concentrated in a central nucleus. Its overall shape may be spherical or elongated.

• You can learn about elliptical galaxies here:
• Elliptical Galaxies (Hartmut Frommert)
• How Elliptical is YOUR Galaxy? (Brian Martin)

A spiral galaxy is a galaxy which contains two or more spiral arms which extend out from the nucleus. They are regions in which new stars are being born.

The Milky Way Galaxy, in which our Solar System is located, is a spiral galaxy.

• You can learn about spiral galaxies here:
• Spiral Galaxies (and Other Disks) (Hartmut Frommert)
• Dwarf Spirals Homepage (Jim Schombert)

A peculiar galaxy is any galaxy which is neither an elliptical galaxy nor a spiral galaxy. Peculiar galaxies are sometimes called irregular galaxies.

• You can learn about peculiar galaxies here:
• Irregular Galaxies (Hartmut Frommert)
• Arp's Catalog of Peculiar Galaxies (Dennis Webb)
• Ring Galaxies: Formed in Cosmic Collisions (Richard Gerber/Curtis Struck)