the milky way appears as a band of light stretching across the sky there are dark regions along the...
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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
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
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
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