Galaxies & Star Systems Astronomy 2

Star Systems Our solar system only has one star (our sun); however, most are grouped together to groups of two or more – called star systems Star systems with 2 stars are called double stars or binary stars – 3 stars are called triple stars Sometimes binary stars cannot be seen from Earth – only one star can be seen

Star Systems When one star hides another star it is called an eclipsing star Astronomers know there are actually 2 stars by looking at the effects of gravity Our solar system is not the only solar system with planets revolving around a star In 2000, astronomers discovered a solar system about 10.5 light-years away with planets similar to our solar system

Star Systems Scientists hypothesize that this solar system might contain life Since this discovery scientists have been sending out radio waves and looking for radio waves from unnatural sources hoping to discover extra- terrestrial life However, this could take many years because radio waves travel much slower than light waves

Galaxies Galaxy: A group of millions or billions of stars, gas, and dust held together by gravity Our sun and all visible stars are members of our galaxy, the Milky Way

Galaxies There are 3 main categories of galaxies  Spiral galaxies  Elliptical galaxies  Irregular galaxies

Our Galaxy The Milky Way is visible as a band of light crossing the sky during summer and winter Represents the concentrated light of the billions of stars in our galaxy At night, we witness the view from within our galaxy

Milky Way Galaxy A disk of stars that contains billion stars 100,000 light years in diameter, about 3000 light years thick The center of the Milky Way (nuclear bulge), swells to about 10,000 LY thick

Milky Way Diagram

Milky Way

Infrared Milky Way (COBE) The view from within, in IR light

Sun and Galaxy Sun located about 2/3 distance away from the center (30,000 light years) The entire Milky Way rotates around its center Sun orbits the center of the galaxy at 563,000 mph One galactic revolution takes 220 million years

Sun’s Revolution

Milky Way, a Spiral Galaxy Spiral galaxy, most are concentrated in a central nucleus Spiral arms of stars wind outward from the nucleus M101

Milky Way Map

Formation of the Milky Way Milky Way appears to be about 13 billion years old MW likely represents one of the original galaxies created after the Big Bang Sun and planets are 4.5 billion years old Big Bang origin of universe 13.7 bya

Elliptical Galaxies  Look like flattened balls  Contains billions of stars, but little dust and gas  Due to the lack of dust and gas new stars rarely form  Most elliptical galaxies contain only old stars

Irregular galaxies  Irregular galaxies Does not have a regular shape One of the closest neighboring galaxies to the Milky Way is an irregular galaxy It is about 160,000 light years away

Star Clusters: Open Clusters A few dozen to thousands of stars loosely bound together by gravity Found mainly in the galaxy’s disk and spiral arms More than 1000 have been discovered in the Milky Way Young stars that recently formed from nebulosity Jewel Box open cluster (NGC 4755)

Star Clusters: Globular Clusters Huge, concentrated balls of thousands to millions of stars Found in galactic halo, a spherical region centered on the nucleus Contain the oldest known stars About 150 globular clusters have been discovered Omega Centauri, globular cluster (NGC 5139)

Cluster Summary TypeOpen ClusterGlobular Cluster StarsDozens to thousands (Population I) Hundreds to millions (Population II) LocationWithin galactic disk Orbit galaxy in halo AgeYoung, thousands to millions of years Among oldest known stars (billions of years)

Open Clusters Pleiades—M45 M67 Which cluster is oldest? Note the star colors of the main stars in each cluster

Globular Cluster M55 Colors indicate temperatures, red (cool) to blue (hot) The “turn off” area on the main sequence represents the cluster’s age

Between the Stars Space between the stars in a galaxy is a vacuum, or empty space Also termed interstellar medium Matter between the stars is of gas and dust

Between the Stars Interstellar matter 99% gas, 1% dust Gas consists of 75% hydrogen and 25% helium Interstellar dust similar in size to cigarette smoke In spiral galaxies, gas and dust is concentrated in the disk and spiral arms

Pillars of Creation (M16) In this iconic photo from the Hubble Space Telescope, a small portion near the center of M16, the Eagle Nebula, is revealed Note the pink, newly formed stars within the nebula

Barnard’s S Nebula A dark nebula represents interstellar dust that blocks the light of stars from behind

Horsehead Nebula Famous dark nebula (B33) located near the Belt of Orion

Messier Catalog All of the Messier objects are shown in this montage M1 starts the upper left, M110 ends the lower right

Other Galaxies Until 1924, Milky Way was the “universe” Edwin Hubble proved that galaxies other than our Milky Way exist Hubble used Cepheid variable stars to measure the distances to galaxies Hubble also photographed stars in the Andromeda “Nebula” The universe contains an estimated 100 billion galaxies. Each of these galaxies contains about 100 billion stars

Andromeda Galaxy Our nearest large neighbor Similar in size and shape to Milky Way Andromeda Galaxy is visible to the naked eye as a glowing spot in the constellation of Andromeda 2.3 million light years distant (wave!)

Andromeda Galaxy (M31, M32, M110)

Andromeda Constellation Locate M31 and M33

Triangulum Galaxy (M33)

M33 The pink glow of emission nebulas can be seen in this photo

Center of Virgo Cluster Virgo Cluster near M84, M86 M86

Sombrero Galaxy (M104) from HST

Large-Scale Structure of the Universe Sheets & Voids: c.nasa.gov/d ocs/cosmic/s heets_voids. html

Galactic Cannibalism Galaxies can merge together to form larger galaxies Elliptical galaxies may be the result of multiple galaxy collisions NGC 2207 (left) and IC 2163 (HST)