Galaxy Types

Hubble Tuning Fork

Elliptical

Elliptical

• On the left of the diagram are the Elliptical galaxies

• Elliptical galaxies contain very little gas and dust, and the gas that is present is very hot and diffuse.

• Consequently, there is no current star formation in elliptical galaxies.

• The stars in elliptical galaxies are old Population II stars.

• Unlike the disks of spiral and lenticular galaxies, elliptical galaxies are NOT rotating rapidly.

• Elliptical galaxies are subclassified according to how flattened they appear.

• Let ``a'' be the diameter of an elliptical galaxy along its longest dimension (its major axis, in the language of mathematicians).

• Let ``b'' be the diameter along the shortest dimension. The elliptical galaxy is then given the label ``En'', where ``n'' is a number given by the formula:n = 10(a-b)/a

• For instance, a galaxy which appears circular has a=b, and hence n=0. Galaxies which appear circular are thus given the label ``E0''. Slightly more flattened galaxies are labeled ``E1'', and so forth, up to the most flattened elliptical galaxies, which are called ``E7''.

• Since elliptical galaxies appear elliptical in the sky, they must be ellipsoidal in three dimensions. Just as an ellipse is a distorted circle, an ellipsoid is a distorted sphere. There are three types of ellipsoid:

• 1 - a stretched sphere (like a hot dog) • 2 - a squashed sphere (like a hamburger) • 3 - a sphere squashed in one direction, stretched in

another (like a baking potato)

Hubble Tuning Fork

Lenticular

Lenticular

• After these simple types of galaxies the diagram splits into two. On the upper branch are the S0 galaxies first - called lenticular galaxies because they are shaped like a lens in a magnfiying glass.

• The description is made up of the "S", meaning lenticular, the "0", meaning no arms, and the subscript number indicates how heavily a stripe is absorbed out of the image of the galaxy by dust in the galactic disc.

• flat, rotating disks

• central bulges

• VERY LITTLE gas and dust in the disk

• NO spiral arms

Lenticular galaxies have

• The fact that lenticular galaxies lack both gas and spiral arms has led to the hypothesis that interstellar gas is necessary for spiral arms to form.

Hubble Tuning Fork

• Continuing along that branch the next 3 types are all have spiral arms, and they are grouped by how tightly those arms are wound and how large the central bulge is - the two happen to be closely related. The name is defined by the "S" and the lower case letter after which indicates how wound up the arms are: from "a" to "d".– Sa galaxies have big central bulges, tightly wound spiral

arms, and a relatively small amount of interstellar gas.

– Sc galaxies have small bulges, loosely wound spiral arms, and a relatively large amount of gas.

– Sb galaxies are intermediate between Sa and Sc. Our own galaxy might be an Sb galaxy

All spiral galaxies have

• flat, rotating disks

• central bulges

• gas and dust in the disk

• star formation in spiral arms

Hubble Tuning Fork

Barred

Barred

• The lower branch of the tuning fork diagram is largely a copy of the upper branch, but its occupants all have a line of stars through the center - a bar. The first two pictures shown on the diagram are the SB0 galaxies. The B stands for barred, and a subscript number indicates how heavily defined the bar is.

• In a barred spiral galaxy, the spiral arms wind away from an elongated central bar rather than from a spherical central bulge.

• subspecies'' of spiral galaxy

Summation

• Spiral galaxies have rotationally flattened disks, and contain moderate amounts of gas and dust.

• Elliptical galaxies are slowly rotating ellipsoids, and contain little gas and dust.

Irregular

Irregular

• Irregular galaxies tend to contain lots of gas and dust. As a consequence, irregular galaxies contain copious star formation.

• The star formation is patchy (tending to occur in clusters), as is the distribution of dust.

• Therefore, irregular galaxies are given their characteristic irregular, patchy, raggedy appearance.

• The Large and Small Magellanic Clouds, about 50,000 parsecs away from our own galaxy, are examples of irregular galaxies

• The Hubble tuning fork does not show an evolution of Galaxy formation.

• By this I mean that it doesn’t start out with one type and evolve into another type.

• On a piece of paper. Label the following ten galaxies by type using the Hubble tuning fork

#1

#2

#3

#4

#5

#6

#7

#8

#9

#10