Galaxies

What is a Galaxy? A galaxy is a collection of billions of stars, dust, and gas all held together by gravity. Galaxies are scattered throughout the universe. They vary greatly in size and shape. Not all galaxies look alike.

A bit of History In 1995, the Hubble Space telescope observed a patch of sky in the Big Dipper for 10 days Many galaxies, each one an island of stars like our own Milky Way, were detected By counting them and correcting for the entire sky, we estimate that there are over 80 billion galaxies in the observable Universe

A bit of History We study the life cycle of the galaxies in the same way we do stars We piece together a life story by observing many different galaxies at various stages of their lives Studying life cycles of galaxies is more difficult than that of stars Study using the Electromagnetic spectrum

Absorption line spectrum                    dark lines in a continuous spectrum => light absorbed at certain wavelengths source: cooler gas with a continuous source behind it Emission line spectrum                                                               bright lines at certain wavelengths (no continuous spectrum)

Three major components: A thin disk consisting of young and intermediate age stars this disk also contains gas and is actively forming new stars. Dust in the disk makes it appear orange in the picture.

Three major components: (cont.) Dust absorbs blue light more than red light and thus makes stars appear reddish. Our Galaxy has spiral arms in its disk - these spiral arms are regions of active star formation.

Three major components: 2. a bar of older stars (white in the COBE picture).

Three major components: An extended dark halo whose composition is unknown. Since the matter in the halo does not consist of luminous stars, it does not show up in the COBE image. The existence of the dark halo is inferred from its gravitational pull on the visible matter.

Galaxy Size The sizes of all three types span a wide range, from Dwarf galaxies (contain 100 million stars) Giant galaxies (contain 1 trillion stars)

Classification of Galaxies Edwin Hubble looked at many galaxy pictures and decided to group, or classify them. He could have use size, color, shape or any other feature that he noticed but decided on shape or form as the deciding factor He used letters to represent the different types

Three Shapes: 1. Elliptical: oval shaped

Elliptical galaxy- “E” The word elliptical refers to its degree of “roundness”. Most galaxies are elliptical The shape can go from nearly circular to flattened or elongated. These variations are denoted by number {E0 – E8}

Lenticular Galaxies Lenticular galaxies are sometimes called "armless spiral galaxies." Lenticular galaxies have a central bulge, but no spiral arms. If the central bulge is not very bright, it can be very difficult to tell the difference between a lenticular galaxy and an E0 galaxy - where the tines meet the handle.

2. Spiral: looks like it has arms. The Milky Way is a spiral galaxy.

Spiral Galaxy Draw this diagram

Spiral Galaxy “S” Named so because it reminded Hubble of a pin wheel or whirlpool Those with spiral arms where divided into 3 sub-groups depending on how tightly the arms were wound around the nucleus Tightly wound around the nucleus are type “Sa” and loosest type “Sc”

Andromeda Galaxy (M31) The Andromeda Galaxy (M31) is the nearest spiral galaxy to our own, the Milky Way. It lies approximately 2,500,000 light-years away in the constellation Andromeda.

Barred- Spiral Galaxy “SB” Named so because it reminded Hubble of a spiral with a solid bar across the center In about 1/3 of spiral galaxies, the arms spiral out not at the center but from a straight bar of stars, gas, and dust that extends from both sides of the bright nucleus

Barred- Spiral Galaxy “SB” The sub-groups “a” – “c” are in order of increasing openness of the arms but with a “B” for barred inserted.

Barred - Spiral Galaxy Draw this diagram

3. Irregular: A big blob of stars

Irregular Galaxy These are galaxies that feature neither spiral nor elliptical morphology. Hubble defined two classes of irregular galaxy

Irregular Galaxy Irr I galaxies- have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars Irr II galaxies- have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters

The Large and Small Magellanic Clouds are irregular galaxies close to the Milky Way Galaxy

For the “E” or Elliptical galaxies, he used the amount of flattening or elongation, from E0 (E zero) to E7, to put the galaxies into smaller groups. For the “S” or spiral galaxies, he used how tightly the arms were wound around the bright core, to further classify these galaxies Sa, Sb, or Sc. For the “SB” or barred spiral galaxies, he used the increased openness of the arms top group these galaxies as SBa, SBb, SBc.

Galaxy Groups Galaxies are not spread uniformly throughout the universe. They tend to group together and form clusters. The Milky Way Galaxy belongs to a small cluster called the Local Group

Galaxy Groups The diameter of the Local Group is about 2 million light years The Milky Way and Andromeda Galaxies are the largest Some clusters are much larger ranging 5 to 30 million light years

Galaxy Groups Galaxies in clusters often collide and merge to form strangely shaped ones. It is believed that the Milky Way and Andromeda Galaxies are destined to collide and merge some time in the future

Galaxy Groups When galaxies collide stars are not destroyed; more stars are actually created by the new gravity motions

http://hubblesite.org/hubble_discoveries/hubble_deep_field/hdf.swf