Formation of Galaxies Types of Galaxies

Learning Goals Students will: Learn to classify types of galaxies Explain the formation of galaxies

Success Criteria Students will show their understanding of learning goals by: Explaining how galaxies are classified based upon shape (and structure) Explaining how galaxies were formed How the classification of galaxies strongly indicates the age (stage of development) of the galaxy)

http://www.youtube.com/watch?v=ENiqpA2yCfc (Introduction to galaxies) This is the Andromeda Galaxy (M31), our closest neighbouring large galaxy. It is one of the farthest things you can see with the naked eye in the night sky. It is 2.4 million light years away.

Hubble’s Tuning Fork Created in 1925 by Edwin Hubble. A classification used to classify galaxies that are elliptical (E) in shape or are spiral with (SB) or without bars (S). Classified based on bulge strength and arm characteristics for spirals. This system is only based on photographed evidence. Astronomers in the past few decades have found that many galaxies do not fit into this system very well. One problem is that galaxies often interact with each other with the result that gravity pulls the two galaxies into irregular shapes.

Hubble’s Tuning Fork Hubble’s Tuning Fork is the method of classifying galaxies – note that the Spiral galaxies are broken into two groups – unbarred and barred. Note the sub-classifications (Astronomers will use sub-classifications such as Sab, which is an intermediate of Sa and Sb).

Elliptical galaxies are broken into seven division based on shape, E0 galaxies are circular while E7 galaxies are elliptical. Spiral galaxies are broken into two classes, the Spirals and Barred Spirals

Types of Galaxies 1. Elliptical Galaxy (E0 - E7) Elliptical galaxies are shaped like a spheriod, or elongated sphere. In the sky, where we can only see two of their three dimensions, these galaxies look like elliptical, or oval, shaped disks. The sub-classifications (E0 to E7) are based on the axial ratio of the elliptical shape. E0 galaxies are circular in shape and E7 are very flat. One third of the galaxies are elliptical. Range in size form 6000 ly across to 300 000 ly.

Types of Galaxies 2. Spiral (Sa-Sc or SBa-SBc) Spiral galaxies have three main components: a bulge, disk, and halo (see right). The bulge is a spherical structure found in the center of the galaxy. This feature mostly contains older stars. The disk is made up of dust, gas, and younger stars. The arm structures extend directly from the central bulge. Our Sun is located in an arm of our galaxy, the Milky Way. Sa - Sc : Unbarred Spirals - these galaxies have a large nuclear bulge and tightly wound, smooth arms. SBa - SBc: Barred Spirals - this group has a small bulge, loosely wrapped, textured arms.

Types of Galaxies 2. Spiral Galaxies This visual classification chart shows examples of each type of galaxy. Note that the S0 galaxies are an intermediate between elliptical and spiral galaxies called lenticular galaxies. Lenticular galaxies contain a central bulge like a spiral galaxy, but there are no well-defined arms.

Types of Galaxies Spiral Galaxies with no bars (S) the Sombrero galaxy, M104 Whirlpool Galaxy (M51)

Types of Galaxies Barred Spiral (SB) have a straight bar of gas extending from either side of the nucleus. NGC 1365 Barred Spiral Galaxy NGC 1300

Types of Galaxies 3. Irregular Galaxies These have no regular shape and are composed of mostly gas and no dust. They do not fit the Hubble classification system. They are really faint, so they are hard to see. Barnard's Galaxy or NGC 6822

NGC 2363 Cigar Galaxy (M82)

Types of Galaxies 4. Peculiar Galaxies These are galaxies that look regular, but have one major difference. Many peculiar galaxies are the result of the interaction of two galaxies – the gravitational effects disturb the spiral shape. Oddly, as two galaxies pass through each other, there are very few star collisions. It is thought that our own galaxy will eventually collide with its larger neighbour, the Andromeda galaxy. Centaurus A shows evidence of a large explosion in the middle.

Some peculiar Galaxies interact NGC3808A & NGC3808 NASA/ESA

Newer Classification Systems Morgan classification - This system is based on the shape of the galaxy as well as its spectrum, rotational symmetry, and central concentration. 2. de Vaucouleurs-Sandage classification – This system is a more detailed version of the Hubble system and includes irregular galaxies.

Other Types of Classification DDO (or van den Bergh) classification of galaxies This system is based on shaped and luminosity.

Relating Age to the Structure of Galaxies The shape of galaxies evolve as they get older. They start off as a disk containing a number of bright clumps in which stars are forming but with little star formation elsewhere in the disk. As a galaxy ages, the central bulge of star formation emerges. Clumps of stars at the edges of the galaxy begin to spread out, forming indistinct arms. Only later, do the arms become more distinct and take on the familiar spiral shape. At first, spiral galaxies have only two arms, but later they evolve to have more arms. Hence, the position a galaxy has on the Hubble Tuning fork is suggestive of the age of galaxies.

Local Groups Once thought to be isolated, galaxies actually exist in groups and they can interact and/or collide. The Milky Way is one of the largest galaxies in our local group of about 40 known galaxies. Our group also includes the Andromeda galaxy (the largest in the Local Group and the closest) as well as the Large Magellanic Cloud (LMC) – a dwarf galaxy.

The Milky Way

The Structure of the Milky Way In general our galaxy looks like a pancake with a bulge in the middle. There are four parts to our galaxy: The nucleus Located 25,000 light-years from us. It is hidden from view by dust, but it can be observed by infrared and radio waves. At the centre is a large black hole that has a mass of millions of times that of our sun.

The Structure of the Milky Way The nuclear bulge Looks like a flattened sphere that is twice as long as it is wide. Contains a lot of dust and densely packed old stars. 10, 000 light-years in radius.

The Structure of the Milky Way The galactic disk The part of the “pancake” that is outside the bulge. It extends for 70, 000 light-years from the middle. Our sun is located 28, 000 light-years from the middle. The galactic disk is broken down into two parts Thin disk – this contains 95% of the stars of our galaxy Thick disk – Contain older stars and is 3 times as tall.

The Structure of the Milky Way The Halo Composed of very old stars, dust, gases, and Globular clusters. Globular clusters are compact stellar systems that contain lots of stars. These object do not rotated around the nucleus.

Homework Questions P. 838#1,2 & 3 p. 846 #1,2,4, & 5