The Milky Way

• Appears as a band of light stretching across the sky

• There are dark regions along the band, giving the appearance of a lack of stars

• This is caused by dust and gas obscuring the light of stars behind

The Milky Way

• The gas and dust limits our view to about 6000 light years

• Much of this gas and dust can be observed in the form of nebulae

• A nebula is a cloud of gas seen in visible light

• How the gas is seen depends on the stars around it

The Milky Way• Types of Nebulae• 1. Reflection Nebula:

– this type of nebula reflects the light of the stars in and around it

– it appears blue because the nearby stars are luminous, young main-sequence stars

– Ex: the Pleides

The Milky Way• Types of Nebulae

• 2. Emission Nebula– This type of nebula is

caused by high energy radiation (UV) from nearby stars exciting the gas into emitting its own light

– These regions are known as HII regions (hydrogen has been ionized

The Milky Way• Types of Nebulae• 3. Absorption Nebula

– this type of nebula is made of cooler gas and dust

– it doesn’t glow; it obscures our view of things behind

– it appears as a dark silhouette

The Nuclear Bulge

• Radius: 16,000 light years

• Contains Population II stars– Older stars, K type stars

• Generally free of gas and dust

• Nucleus: 10 light years across– Studies indicate the presence of a massive non-

stellar black hole

The Disk

• Extends 50,000 light years beyond the central bulge

• Forms spiral arms that contain a lot of gas and dust

• Population I stars are found in the spiral arms– these are young O and B main-sequence stars– they are often found in open clusters

• The Sun is about 2/3 of the way out from the center

The Disk

• The disk is very thin

• Its “thickness” is only about 2% of its width

• Spiral structure has been determined through radio observations– Radio observations have shown how the spiral

arms move around the center of the galaxy

• It takes 250 million years for the Sun to make one orbit around the galactic center

The Halo

• The halo completely surrounds the disk of the galaxy

• It contains Population II stars

• Most of the stars are found in globular clusters

The Galactic Corona

• 200,000 - 300,000 ly beyond

• Contains 5 - 10 times as much mass as the rest of the galaxy

• Can only detect it through gravitational effects

Elliptical GalaxiesElliptical Galaxies• Most common type of

galaxy, trillion solar masses;

• Largest: 10 - 100 kpc across; these are very rare

• Dwarf ellipticals are the most common– 2 - 3 million solar masses;

2000 pc across

• Shape is based on optical appearance:– circular: E0

– most elongated: E7

Spiral GalaxiesSpiral Galaxies• Brighter & larger than

ellipticals

• 25,000 - 800,000 pc across

• 109 - 1012 solar masses

• 1/3 of all spirals are barred

• Classified based on the size of nucleus & how tightly arms are wound

Spiral GalaxiesSpiral Galaxies

Spiral GalaxiesSpiral Galaxies

Irregular and Peculiar GalaxiesIrregular and Peculiar Galaxies

• Irregular galaxies show no regular shape

• Could be remnants of a collision

• Ex: Large magellanic clouds

• Peculiar galaxies are very rare

• may look like a regular galaxy exploded

• May be a result of a collision

Irregular and Peculiar GalaxiesIrregular and Peculiar Galaxies

Hubble ClassificationHubble Classification

• Amount of interstellar gas and dust differs in each type of galaxy

• Ellipticals have very little gas or dust; spirals have a lot

• In spirals, amount of gas increases from Sa to Sc

• In spirals, amount of star formation increases from Sa to Sc

• O and B stars formed mostly in Sc, SBc, and Irr galaxies

Clusters of GalaxiesClusters of Galaxies• The Local Group:

– Contains about 2 dozen of the nearest galaxies– 1 megaparsec in diameter

• Distant Clusters– Contain 100’s to 1000’ of galaxies

• Virgo Cluster: – Closest cluster to us; 20 Mpc distance, 2 Mpc across

• Superclusters– Many clusters form a supercluster– There are 16 superclusters within 2 billion ly

Clusters of GalaxiesClusters of Galaxies