Chapter 11: Milky Way; Not Just a Delicious Candy Anymore… See that band of light stretching across this 360o panoramic image? That’s us. Key Points: 1. Our Sun is a part of the Milky Way galaxy. 2. Our Sun is just one of approximately 325 Billion stars that make the Milky Way galaxy (MW). 3. Every one of those 325 Billion stars orbit around the center of the MW. 4. MW is about 100,000 light years wide; we are about 35,000 ly from the center. 5. There is a supermassive BH at the center of the MW; about 4 Million Mo. 6. It wasn’t till 1925 that we realized the Universe was more than our MW! Chapter 11 P a g e | 1 So, did you ever look up on a clear dark night and see a long streak of fuzzy cloudy looking stuff that stretched all the way from horizon to horizon? That is our MW galaxy. Seen from here on Earth, we are looking into the dense central disc of the MW. First, what the heck is a “galaxy” anyway? Galaxies in General These are simply large groups of stars that have all been formed at the same time and are held together by their mutual gravity. One of the current unsolved mysteries in cosmology is just how galaxies formed. Yes, we have two basic theories on the origin of galaxies. Here’s the main theory: Start with a hugely huge gas cloud of almost all hydrogen and helium similar to the one that started our own little solar system without the heavy elements, but really REALLY REALLY huge; a Million light years wide perhaps! This gas cloud clumps together to form many stars that are then all attracted to each other via gravity. This forms a small galaxy; maybe 50 or so Billion stars. So, how do large galaxies like ours form? Cannibalism. Our MW is thought to have started as a small-ish galaxy that merged with 4 or 5 other small-ish galaxies that all pulled together due to their mutual gravity. We have specific evidence of this in the leftover artifacts of several galaxies that seem to have been stripped of their main stars, but are still looped around the MW. Take a look: Notice that we recently (2009) found this long stream of stars called the Sagittarius Star Stream. Looking closely, you can imagine how the Sagittarius System USED to be its own galaxy, but the MW has ripped it apart and Chapter 11 P a g e | 2 consumed most of it. Also, Seque and Ursa Major II are dwarf galaxies that may have been larger Billions of years ago. Following is a computer image of all combined observed stars that surround the MW. Chapter 11 P a g e | 3 The general view of galactic formation: Individual huge dust clouds merged and began rotating. Gravity pulled more stuff into the center and created the central bulge. Recently, however, a new theory has been gaining steam; BHs caused galaxies! Wait. What? Yep. All large galaxies have a supermassive BH at the center. The MW has one of 4 Million solar masses, Andromeda Galaxy, our closest large galaxy neighbor, has a BH of 200 Million solar masses! So, which came first; BH or galaxy? These are competing theories and both make sense. If BH was first, its huge gravity pulled in all the surrounding stars and formed a group called a galaxy. If the galaxy was first, the central bulge where gravity pulls lots of stars gets so crowded and so jammed with stars, these stars squish into each other and, via gravity, form a BH. So, take your pick. Speaking of Andromeda! It is on a collision course straight for us! Chapter 11 P a g e | 4 Andromeda is a monster galaxy 2.5 Million light years away from our own little MW. It has a mass of 12 approximately 1x10 Mo, roughly the same mass as MW, with over a TRILLION stars! Yes, a monster! Remember the MW is 100,000 ly wide? Andromeda is 220,000! Yep, a MONSTER! Andromeda used to be called the Great Andromeda Nebula until 1925 when Edwin Hubble realized it was just another galaxy sorta like our MW. Below is an image from 1899! Chapter 11 P a g e | 5 Notice how it does look like a fuzzy cloudy thing? That’s why, till 1925, it was called the Great Andromeda Nebula. So, what will the Earth sky look like when Andromeda gets here in 200 Million years? I can’t wait to see it! Chapter 11 P a g e | 6 Anyhow, MW & Andromeda is just one of the 50 or so galaxies in our Local Group. Notice the MW is at the center of the above 3-D-ish image and we are surrounded by a batch of other galaxies all traveling in the same basic direction in space. The gravitational center of the Local Group is between the MW and Andromeda; the two largest members. This Local Group has a diameter of about 10 Million light years and is just a part of the Virgo Supercluster of galaxies. Chapter 11 P a g e | 7 This “supercluster” of galaxies contains our Local Group of 54 galaxies (shown dead center above) and at least 100 other groups and clusters of galaxies and spans at least 110 Million light years in diameter. The total number of galaxies in the Vigo Supercluster is upwards of 100,000! Each dot in the image above represents a group or a cluster of galaxies. WOWZERS! Chapter 11 P a g e | 8 Milky Way Specific Our galaxy is a “barred spiral” type that is 100,000 light years wide. It contains at least 400 Billion stars and is 13.5 Billion years old. Image courtesy NASA.gov Chapter 11 P a g e | 9 A new Image of MW courtesy ObamaCare.gov Courtesy Space.com Chapter 11 P a g e | 10 As with all spiral galaxies, the MW has a central bulge. It had always been assumed the bulge was simnply a symmetrical circular clump of densely populated stars. However, just recently, 2010, it was found that the central bulge is more complex than that. Notice the “peanut” shapped central bulge. Why is it shaped like this? No clue. Another Peanut… Chapter 11 P a g e | 11 Following is an image of the basic structure of the MW: (GNP & GSP measn Galactic North Pole and …) Besides the main disc where the Poulation I stars (young, blue) hang out and the Central Bulge where the Population II stars (older, red) are, note there is a halo (solid sphere of scattered stars surrounding the entire galaxy). Within this halo are hundreds of star clusters. Clusters are simply large groups of stars bound together by their own gravity, but orbit the “center” of the cluster in haphazard ways; like a 3-D demolition derby. There are two types of star clusters; open and globular. Open clusters contain a few hundred stars and span a diameter of up to 20 ly. The most “famous” open cluster is the Pleiades, or Seven Sisters. Chapter 11 P a g e | 12 The Plaiedes Star Cluster Chapter 11 P a g e | 13 Wiki: Subaru is the Japanese name for the Pleiades star cluster (M45, or "The Seven Sisters") Chapter 11 P a g e | 14 Globular clusters contain a few thousand to a few million stars jammed into about 100 ly. A few images and comparisons: Open Cluster Image Globular Cluster Image Plaeides Messier 80 NGC 3572 NGC 7006 NGC 265 NGC 2808 Omega Centauri NGC 346 (Brightest in MW) Chapter 11 P a g e | 15 Types of Galaxies There are four main categories of galaxies: spiral, barred spiral, irregular, and elliptical. Of course, there are many galaxies that fit into more than one category; these are sometimes called “Peculiar”. Let’s just stick to the 4 main categories. There is a new catalogue of nearby galaxies made by NASA’s Spitzer satellite using Hubble’s old “tuning fork” model: Hubble’s Galactic Classification “Tuning Fork” diagram Chapter 11 P a g e | 16 Slightly more complex version… I think it is best to show image examples of each. Here. Elliptical galaxies: Unlike spiral galaxies, elliptical galaxies have very little gas or dust and no stars have recently formed within them. They lack a specific shape and can range from spherical to “football” shaped. Approximately 60% of all known galaxies are elliptical. Chapter 11 P a g e | 17 Enormous Elliptic Galaxy NGC 2937 & Porpoise Galaxy it disrupted (NGC 2936). Even though I think it looks more like a hummingbird… APOD.com Chapter 11 P a g e | 18 Giant elliptical galaxy M60 and spiral galaxy NGC 4647. APOD.com Chapter 11 P a g e | 19 Elliptical Galaxy M83: APOD.com Chapter 11 P a g e | 20 Spiral Galaxies Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxy NGC 4921. APOD.com Chapter 11 P a g e | 21 M74. APOD.com Chapter 11 P a g e | 22 M51: The Whirlpool Galaxy. APOD.com Chapter 11 P a g e | 23 Barred Spiral Galaxies These are spiral galaxies with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies.