Spiral Galaxies

Ron Buta

University of Alabama

What are spiral galaxies?

Flattened systems of stars, gas, and dust stars seem to concentrate in spiral “arms” all bound by gravity into a single unit typical size:50,000-100,000 light years typical mass: 10billion-500billion suns often there is also a prominent bulge

Face-on spiral galaxy

Highly inclined spiral galaxy

Edge-on spiral galaxy

Edge-on Spiral Galaxy

Why are spirals interesting?

An intriguing morphology difficult to explain common and widespread thoughout the

Universe new stars tend to form in spirals but not in

other types of galaxies part of process of galaxy evolution and

interaction our Galaxy is a spiral

Discovery of Spirals

1845 William Parsons, Third Earl of Rosse, Ireland

72-inch reflector visually detected spiral arms in the

Whirlpool Nebula M51 many other “nebulae” found to be spirals

William Parsons, “Lord Rosse” 1800-1867

Discovery of spiral structure in M51

What causes spiral structure?

Natural phenomenon in flat, rotating stellar disks

possibly triggered by an interaction with another galaxy

or generated by bars “density waves”

Spiral galaxy research at UA

morphology sense of winding of spiral arms gravitational torques due to bars and spiral

arms star formation in galactic rings

The de Vaucouleurs Atlas of GalaxiesGerard de Vaucouleurs 1918-1995

The de Vaucouleurs Atlas: what is it?

Authors: R. J. Buta (U. Alabama) H. G. Corwin, Jr. (Caltech) S. C. Odewahn (U. Arizona) publisher: Cambridge Univ. Press Purpose: to illustrate the de Vaucouleurs

revised Hubble classification system with modern digital images

NGC 7457 SA(rs)0-

NGC 1553 SA( r )0

NGC 1291 (R)SB(s)0/a

NGC 2713 (R’)SB(rs)ab

NGC 4736 (R)SA( r )ab

NGC 5850 SB( r )b

NGC 4501 (M88) SA(rs)b

NGC 4321 (M100) SAB(s)bc

NGC 1566 SAB(s)bc

NGC 6643 SA(rs)c

NGC 7479 SB(s)c

NGC 6946 SAB(rs)cd

NGC 5585 SAB(s)d

NGC 4618 SB(rs)m

Sense of winding of spiral arms

Trailing arms: follow direction of rotation Leading arms: oppose direction of rotation most spiral arms trail but one galaxy has leading arms!

Trailing arm spiral M81

Leading arm spiral galaxy NGC 4622

Barred spiral galaxies

A bar-like pattern of old stars crosses the center

arms break from the ends of this bar 70% of spirals have a bar, including the

Milky Way

Barred spiral galaxy NGC 1365

Barred spiral NGC 1300

How stars move in presence of a bar

In solar system, planets follow elliptical paths with Sun at one focus

no torque on planet as it orbits in presence of a bar, star may follow a

centered elliptical orbit there is a torque on star as it orbits amount of torque is a measure of “bar

strength”

Rings of Star Formation

Real bar orbit “lit up” by new stars often not circular

Gallery of spirals from the Hubble Space Telescope

Typical spiral galaxy: NGC 4414

Starburst spiral galaxy NGC 3310

Ringed spiral galaxy NGC 7742

Superposed spiral galaxies NGC 3314

Edge-on spiral galaxy ESO 510-13

Edge-on spiral galaxy NGC 4013

Peculiar ring galaxy: Hoag’s Object

Barred spiral galaxy NGC 4319 with quasar

Polar ring galaxy NGC 4650A

Compact galaxy group

Colliding spiral galaxies: NGC 6745

The Spiral Menagerie: Visual Observations of Spirals from McDonald and Siding Spring

Observatories

1977-1984 photoelectric galaxy photometry runs McDonald 30, 36, 82, and 107-inch

telescopes Siding Spring 40-inch telescope

NGC 5194-5 (M51) - McDonald 30-inch

NGC 1566, SSO 40-inch

NGC 2403, McD 30-inch

NGC 3034 (M82), McD 36-inch

NGC 3893(36McD), 4449(36McD), 4676(107McD), 3646(36McD), and 5921(36McD)

NGC 4631 (left) and NGC 4565(right) (both 30McD)

M99, M100, M88, and M61 (all 30McD)

M77(36McD), NGC4535(30McD), NGC5394-5(36McD), NGC3395-6(36McD), NGC5426-7(36McD), NGC5247(36McD)

Thank you!