ÿþ<html> <title>Albert van der Sel: Large Scale Structure of the Universe.</title> <body bgcolor="black" link="White" alink="White" vlink="White" text="#F5F5DC"> <h1>A few notes on "the Large Scale Structure" of the Universe.</h1> Version : 0.1<br> Date : 10/02/2011<br> By : Albert van der Sel<br> Type of doc : Just an attempt to decribe the subject in a few simple words. Hopefully, it's any good.<br> For who : For anyone interrested. <br> <hr/> <br> Here, in a few simple pictures, we will try to depict the local- and large scale structure of the Universe.<br> <br> A few used parameters explained: <ul> <li> ly = lightyear = the distance light has travelled in one year (at 300000 km/s)</li> <li> pc = parsec = 3.26 lightyears = the distance light has travelled in 3.26 years (at 300000 km/s)</li> <li> kparsec = kilo parsec, Mparsec = Mega parsec, Gparsec= Giga parsec etc..</li> <li> z = measure for the "redshift". The further away an object is, the larger the "redshift" will be.</li> </ul> <br> <font face="arial" size=2 color="blue"> <h2>1. Dimensions of our local Galaxy (the "milkyway"):</h2> <font face="arial" size=2 color="white"> Figure 1.<br> <br> <img src="milkyway1.jpg" align="centre"/> <br> <br> Our Galaxy (milkyway) is a spiral arm Galaxy. It's diameter is about 100000 ly. The sun is positioned in one spiral arm<br> located at about 8.5 kpc (about 30000ly) from the center of our Galaxy.<br> The disk containing the arms is relatively flat, containing dust clouds and a high variety of stars (like younger blue stars, as well as<br> many other type of stars like yellow stars as our Sun, white dwarfs, red giants etc..). <br> The milkyway is estimated to contain about 100 billion to 400 billion (100G-400G) stars.<br> The milkyway has a densely populated Center, and it has a spherical "halo" which has a number of smaller spherical<br> clusters of stars known as "globular clusters".<br> <br> <br> <font face="arial" size=2 color="blue"> <h2>2. Dimensions of our "Local Group" of Galaxies:</h2> <font face="arial" size=2 color="white"> Figures 2 and 3.<br> <br> <img src="local1.jpg" align="centre"/> <img src="local2.jpg" align="centre"/> <br> <br> The above figures, try to give an impression of the socalled "Local Group".<br> Here, you see the <B>immediate</B> neighboorhood of our Milkyway. This neighboorhoud contains about<br> 30 galaxies, of which most galaxies are socalled (irregular) "dwarf" galaxies. A couple of galaxies, most notably our Milkyway,<br> the Andromeda nebula (M31), and M33, are fairly large spiral type of Galaxies.<br> <br> Note that a typical spiral galaxy is about 100000 ly accross. The Local group spans about 10.000.000 ly (10 million ly).<br> There are a couple of other dwarf systems, just outside that (arbitrary) volume of space, so, some folks seem<br> to extend the Local group somewhat, making it to contain 40-50 systems.<br> <br> However large this volume of space may seem, it's still very small compared to Cluster- and Supercluster sizes.<br> Note that the Local Group resembles just a (small) gathering of nearby Galaxies, which does not seem to have<br> any spatial structure at all.<br> <br> <br> <font face="arial" size=2 color="blue"> <h2>3. Dimensions of Local Clusters of Galaxies:</h2> <font face="arial" size=2 color="white"> Figure 4.<br> <br> <img src="cluster1.jpg" align="centre"/> <br> <br> In figure 4, you see a region of about 150.000.000 ly (150 million ly) accross. Galaxies have bundeled into "clusters".<br> The Local Group is at the centre of the figure. You see the 'Eridanus' cluster, the 'Virgo' cluster, and a few others.<br> The Virgo cluster, contains about 2000 galaxies.<br> Note that you still <B>don't see</B> the typical "filaments" and "sheets" of the Large Scale Structure.<br> Also note that, this whole domain of clusters is collectively called the 'Virgo supercluster'.<br> <br> <br> <font face="arial" size=2 color="blue"> <h2>4. Dimensions of Superclusters of Galaxies:</h2> <font face="arial" size=2 color="white"> Figure 5.<br> <br> <img src="supercluster1.jpg" align="centre"/> <br> <br> This time we have "zoomed out" in order to see a region of about 1.000.000.000 ly (one billion ly) across.<br> Now, we can cleary identify the filaments of clusters surrounding large voids, making the region to look<br> like a spunge or swiss cheese type of structure.<br> Note how strangely the walls and filaments are formed around large empty holes or voids.<br> In the centre of figure 5, the 'Virgo supercluster' is located, which corresponds to the whole 'area'<br> as show in figure 4.<br> <br> Note that this region still only represents a small area in the whole of the Universe.<br> <br> A peculiar region is the "Great Attractor", which is located somewhere at the Centaurus region,<br> about 150-250 million ly away from our Milkyway.<br> It seems to be a sort of a gravity "anomaly" in intergalactic space, with a mass estimated to be<br> tens of thousands of Galaxies (Milky Ways).<br> Astronomers have determined this by the motion (redshift) of the "neighbooring" superclusters,<br> and thus it exerts a large gravitational effect.<br> Also, some astronomers have determined that even our "Local Group" members, all collectively have a speed component<br> of about 600 km/s towards the Great Attractor.<br> Furher research makes it plausible that the Great Attractor is a Supercluster itself, but it is obscured<br> because "the line of sight" is in the plane of our own Milkyway.<br> <br> <br> <font face="arial" size=2 color="blue"> <h2>5. Dimensions of the observable Universe:</h2> <font face="arial" size=2 color="white"> Figure 6.<br> <br> <img src="largescale1.jpg" align="centre"/> <br> <br> Now we have zoomed out to several billions of lightyears. This is what scientists today, believe the<br> "large scale structure" of the Universe is. Filaments of millions of galaxy clusters, galaxy superclusters (the bright knots)<br> span around dark, nearly empty voids.<br> The sponge-like structure could be explained by a network of "Cosmic Strings". But only a few<br> astronomers still advocate that model nowadays.<br> <br> In the last decade or so, another model was deviced, since it became more and more clear that<br> stuff called "dark matter" is a very abundant type of matter in the universe.<br> This "dark matter" is believed to be responsible for at least 80% of the total mass in the Universe.<br> <br> So, most scientists today, believe that the galaxies formed on a "sort of" dark matter "backbone".<br> This is ofcourse still a subject of intensive research.<br> <br> <br> If you want to read a few simple notes on Dark Matter, you might want to take a look at:<br> <br> <a href="dm.htm">A few simple notes on Dark Matter</a><br> <br> <br> <br> <br> </body> </html>