Galaxies

1. Galaxies 18th century: Charles Messier accidentally discovered many galaxies while searching for comets. He noticed a large number of faint, diffuse patches of light. To avoid confusing them with comets, he assigned them numbers and made a catalog of their positions Galaxies are given the name of M## Example: Andromeda Galaxy known as M31

2. Galaxies • 19th century: Sir William Herschel systematically mapped numerous astronomical objects, including galaxies • Herschel’s catalog known as New General Catalog (NGC) • Many galaxies appear in both catalogs and, therefore, have two names • Example: Messier’s M82 is the same galaxy as NGC 3034

3. Galaxies • The stars that make up galaxies fall into two classes: • Pop I: Young, blue or yellow stars • Pop II: Old, red stars • The type of star that a galaxy contains depends on the amount and type of gas and dust it contains • Astronomers still uncertain about what causes the differences in the different types of galaxies.

4. Spiral Galaxies • Spirals account for about 30% of all galaxies • Two types of spiral galaxies • Normal Spiral • Barred Spiral • Spirals can have several “arms;” most have two to four.

5. Spiral Galaxies • Very “flat” with a prominent central bulge • Central bulge made up of Pop II stars and much gas and dust. • Spirals exhibit a high amount of star production. • Many Pop I stars populate the arms of spirals.

6. Spiral Galaxies • Barred spirals are characterized by a large “bar” through their central region • Milky Way, once thought to be a normal spiral, now believed to be a barred spiral • All spirals surrounded by a “halo,” or a swarm of globular clusters, each having up to a million Pop I stars • Gas and stars move in circular orbits around galactic center

7. Elliptical Galaxies • Ellipticals account for about 60% of galaxies • Very dense central nucleus • Very odd-shaped, much like a football or rugby ball. • Can be very circular/spherical or extremely flat and elongated • Stars have random three-dimensional orbits • No significant star formation; No Pop I stars • Contain little or no gas or dust

8. Lenticular Galaxies • Hybrids – Lenticulars have characteristics of both spiral and elliptical galaxies • Lenticulars have a disk with a central bulge • No signs of a spiral structure (no arms) • No Pop I stars • Made up of Pop II stars • Often misclassified as ellipticals

9. Irregular Galaxies • Irregulars do not fit into any of the other classes • Type I: Share several characteristics with spirals • Flat • High gas & dust content • High star production (Many Pop I stars) • Type II: Extremely misshapen • No discernable order whatsoever • Often have “explosive” appearance • Stars and gas have very irregular orbits

10. Classes of Galaxies • Elliptical • Class E0 - E7 • Disk • Spirals • Class Sa, Sb, Sc, Sd • Barred Spirals • Class SBa, SBb, SBc, SBd • Lenticular • Class S0

11. Classes of Galaxies • Classification of galaxy types • Irregular • Type I, Type II • Dwarf Irregular • Class dIrr • Dwarf Elliptical • Class dE

12. Classes of Galaxies

13. Exotic Galaxies • Radio galaxies • Generally elliptical • Emit large amounts of energy in the radio part of the spectrum. • Radio emissions come from the core and fringes of the galaxy. (Normal galaxies emit radio waves from the body) • Radio emissions can be millions of times stronger than normal galaxies. • Radio emissions caused by jets of electrons streaming from the galaxy’s core

14. Exotic Galaxies • Seyfert Galaxies • Spiral galaxies that are abnormally luminous • The core luminosity of a Seyfert can equal that of the entire Milky Way • Luminosity comes from a region of galaxy less than a light-year across • The radiation emitted spans across many wavelengths: optical, infrared, ultraviolet, and X-ray. • Also contain gas clouds moving at speeds up to 10,000 km/sec.

15. Exotic Galaxies • Quasars • Extremely luminous, distant active galaxies • Some emit powerful radio waves • Some emit hot gas from their centers • Name contracted from quasi-stellar radio source • Some quasars exhibit a peculiar property: • The jets of hot gas appear to contain clumps that seem to move faster than the speed of light.