Transcript Galaxies
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 nonstellar 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 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 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 Galaxies Spiral Galaxies Irregular 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 Galaxies Hubble 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 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 Galaxies