PHL 4710 Class Notes. Copyright © 2015 by William B. Irvine.

Introduction: A Tour of the Universe


 1.   The Solar System (= the sun and planets)

      a.   Artists’ (mis)conceptions:

             i.    Neither relative size nor relative distance is right:        

             ii.   Relative size is right, but not relative distance:

             iii.  Another depiction:

             iv.  Conclusion: It is possible to show relative size or relative distance, but very difficult to show both at the same time in the same picture.

             v.   Building a scale model of the solar system:

                   (1) Notice that if we make the sun 100 inches in diameter, the earth will be merely one inch in diameter and will be located nearly 900 feet (three football fields) away from the sun!

                   (2) Notice, too, that Proxima Centauri (the closest star, other than the sun)would be 46,000 miles away!

                   (3) Why are the solar system pictures shown in books so misleading? Because there is no way to picture the solar system on a single page of a book, and still have the planets be visible!

                   (4) To fit on the same 8-inch wide page, the sun would have to be 0.07 inches wide and the earth would end up .0006 inches wide. Mars, Jupiter, Saturn, and Uranus wouldn’t even make it onto the page. (Jupiter, for example, would be off the page by 3 feet.)

      b.   Actual photographs (including cosmic “selfies”):

             i.    The earth and moon as seen (with a telescope!) from the Messenger spacecraft near Mercury: (The earth and moon are not as close together as they look.)

             ii.   Several of the planets, seen from near Mercury:

             iii.  The earth seen from the surface of the moon:

             iv.  The earth seen from the surface of mars:

             v.   The earth seen from the spacecraft Juno, which is currently headed toward Jupiter:

             vi.  The sun, seen from the edge of the solar system (photo by Voyager spacecraft):

             vii. The earth, seen from the edge of the solar system:

      c.   A video that puts planet and star sizes into perspective:

      d.   The Solar System is mostly nothing. It took 33 years for the Voyager spacecraft, launched in 1977, to reach the edge of the solar system. If it continued to travel at its current speed, it would take 93,000 years for it to reach Proxima Centauri.

 2.   Re-thinking the constellations:


      b.   To understand the point of this article, go to the black rectangles showing stars, left click on the little white box and WHILE STILL HOLDING DOWN YOUR CLICK, drag the white box to the right; it will show you what a constellation would look like from different points of view.

 3.   The Milky Way (= our galaxy):

      a.   A galaxy is a collection of stars.

             i.    The Milky way is estimated to have 200 to 400 billion stars. It is an estimation because we can’t see them all.

                   (1) With the naked eye, we can see only perhaps 8,000 stars, and fewer than half of these will be visible at any one time, since the others will be blocked from our view by the earth.

                          (a)  In Dayton, thanks to the light pollution, you won’t be able to see many stars at all.

                   (2) Even with a powerful telescope, we can’t see all the stars in our galaxy. Some we can’t see because our view is blocked by dust and gas. Others we can’t see because other stars block them from our view. Yet others are sufficiently dim and distant that we can’t see them.

                          (a)  Proxima centuri is the closest star to us (other than the sun), but it can’t be seen with the naked eye since it is rather dim. The nearest star that can be seen with the naked eye is Alpha centuri, but in seeing it you are actually cheating a bit, since what you see is in fact two stars (Proxima centuri A and B) instead of one. The stars in question are a binary pair, and revolve around each other the way Jupiter revolves around the sun. Not only that, but Proxima centuri is thought to revolve around these two stars, making it a triple star system.

                          (b) Although our sun isn’t part of a binary system, it is thought that most stars are. Polaris (the north star), for example, is not a unitary star, but is instead part of a five star system!

                   (3) If you look at the sun with a telescope (or even with the naked eye), it is a disk with angular diameter. There are only a few other stars, though, with measurable angular diameters. The rest appear as points of light, even when seen through the most powerful telescopes. This is because they are so very far away.

                   (4) It is thought that most of the stars in our galaxy—indeed, most of the stars in the universe—have planets orbiting them. The Kepler space telescope is routinely discovering them.

                          (a)  It usually can’t see them individually, the way we might see Jupiter in a backyard telescope. But their effect on the star they are orbiting can be detected. The star in question might wobble because of the planet’s pull, for example. Or the star might grow slightly dimmer when the planet passes between us and it.

      b.   The Milky Way as seen from earth:

             i.    Any star you can see with your naked eye will be within our galaxy—indeed, it will be fairly close by in our galaxy. There will be many stars within our galaxy that you can’t see individually; instead, the region they are in will look “milky.” Another galaxy, Andromeda, can be seen with the naked eye if you are in a very dark area and know just where to look; it will look like a lighter patch.

             ii.   There are two exceptions to the rule that you can see, with your naked eye, only stars within the Milky Way:

                   (1) One exception is if there is a supernova explosion in another galaxy. This can result in a star being visible from the earth—more precisely, result in its remnants being visible. Thus, in 1987, there was a naked-eye supernova in the Large Magellanic Cloud, which is a dwarf galaxy that, although separate from the Milky Way, is gravitationally bound to it.

                          (a)  It is interesting that three hours before the light from the 1987 supernova became visible, neutrino detectors recorded an influx of neutrinos. This is because the process of stellar collapse that produces a supernova releases neutrinos before the final explosion.

                          (b) This was the first naked-eye supernova since 1604, when Johannes Kepler saw one. That supernova, unlike the 1987 supernova, was within our own galaxy.

                          (c)  Another famous naked-eye supernova took place in 1054 (apparently on the 4th of July). Chinese and Japanese astronomers recorded it. The remnant of this supernova can still be seen. It is known as the Crab Nebula: Astronomer’s measured how fast the gas in this nebula is expanding outward, and when they used this data to determine when it “exploded,” they got results compatible with the 1054 date. The Crab Nebula is also in the same part of the sky as the 1054 supernova was recorded as having taken place.

                   (2) The second exception is gamma-ray bursts. Seven-and-a-half billion years ago, a collapsing star emitted such a burst, and if you had been looking at the right point in the sky on March 19, 2008, for the right 30-second period, you would have seen a “star” from a very distant galaxy flare into existence and then disappear. See

             iii.  Stars in other galaxies can be seen with powerful telescopes, but it wasn’t until the early 1920s that astronomer’s had telescopes powerful enough to perform this feat. Indeed, it was only in 1925 that astronomers became confident of the existence of galaxies other than our own!

      c.   The Milky Way as seen from outside our galaxy:

             i.    Notice that in the same way as we are not at the center of the solar system, we are not at the center of our galaxy.

                   (1) Question: if we have never been outside our galaxy, how can we know what it looks lik e from outside? (A student could draw a map of the classroom without leaving his seat.)

             ii.   It is actually a good thing that the sun is not at the middle of our galaxy, since there is evidence that a supermassive black hole, 4 million times the mass of the sun, resides there and would make our existence rather miserable.

             iii.  The insignificance of the sun: If we were 56 light years away from the sun, it would be so dim that we could no longer see it with our naked eye (but could still find it with a telescope). The Milky Way is itself 100,000 light years across. Suppose, then, that we made a poster showing the Milky Way and that on this poster, the Milky Way was 14 inches across. Suppose that we indicated the position of the sun within the Milky Way by a dot the size of a (1/64th of an inch wide) period. The circumference of this period would also represent the distance from which the sun would be observable with the naked eye!

      d.   The Andromeda Galaxy (the nearest neighbor to the Milky Way):

             i.    Realize that in the same way as the Solar System is mostly nothing, and the Milky Way is mostly nothing, the space between the Milky Way and Andromeda is remarkably empty. Indeed, if we could travel at the speed of light, we would spend 2.5 million years traveling through empty space before we reached Andromeda. Conclusion: the universe is truly huge, and it is filled with mostly nothing.

             ii.   The Milky Way and Andromeda Galaxy, by the way, are on a collision course:

      e.   Some other galaxies:

      f.    Galaxies in collision:

             i.    When galaxies collide, though, the stars within them are unlikely to collide as well; there is simply too much space between stars for this to happen.

      g.   The universe is full of galaxies: The Hubble Ultra Deep-Field view.

             i.    During 400 orbits between September 24, 2003, and January 16, 2004, the Hubble telescope was pointed at the same tiny patch of sky. It made two exposures per orbit, and in the 800 exposures that resulted, the total exposure time was 11.3 days, or nearly a million seconds. This allowed lots of light to accumulate, the way keeping an old-fashioned camera lens open for a very long time would.

             ii.   This long exposure time allowed the telescope to see extremely dim objects. Indeed, some of the objects were so dim that the telescope collected only one photon of light per minute from them. By way of contrast, it collects photons from nearby galaxies at the rate of millions per minute.

             iii.  The area of sky in the photograph was chosen because it contained no stars that are visible to the naked eye or even to a moderately powerful telescope. It is an area of the sky as big as 1/50 of the full moon. And just so you are clear about how big this area is, hold your hand at arm’s length with the back of your hand facing you. Look at the nail of your little finger. That nail is big enough to cover the full moon! The area shown in the Hubble Ultra Deep-Field view is 1/50th that large!

                   (1) Data taken from: ]

             iv.  For the Ultra Deep-Field image:


                   (2) See also:

             v.   Hubble, Extreme-Deep-Field image, released in 2011, shows the same region in even greater detail. It adds 5,000 galaxies, some of which are 13.2 billion years old.

                   (1) Click on this link for the image, then click on the image itself for the high-res version. Almost ever speck you see is a galaxy:

                   (2) For more information on this picture, see this link:

      h.   One important thing to realize about the Hubble Ultra-Deep and Extreme-Deep Views is that no matter where they pointed the telescope, they would see the same thing. (Although point it in some directions, and dust or nearby stars would impede the viewing.) The scientific term for this is that the universe, seen from our point of view, is isotropic: it looks the same in every direction. (At least it does when you look at very distant objects; look at closer objects, and it is not isotropic. There is, for example, more of the Milky Way on one side of us than on the other.)

             i.    This is a significant fact. If I had students draw a map of the classroom and locate themselves on that map, those who were near the walls would see more people on one side of them than the other: their view wouldn’t be isotropic. Only those in the middle would see about the same number of people no matter which direction they looked.

      i.    Someone might conclude that from the fact that the universe looks isotropic, we must be in the middle of it. (This is some consolation after finding out that we are in the middle of neither our solar system nor our galaxy.)

             i.    This conclusion can be challenged, though. Scientists are convinced that besides being isotropic, our universe is homogeneous. (According to the cosmological principle, the universe is both isotropic and homogeneous.) Wherever you are, you will see the same isotropic distribution of galaxies. How can this be?

                   (1) One way is if the universe is infinite.

                   (2) Another way is if space is curved. To understand this, imagine the predicament of a sailor on a planet that was completely submerged by water. Things would look the same in every direction—it would be isotropic—and no matter where he traveled, things would still be isotropic—meaning that it would be homogenous as well.

 4.   The galaxies are themselves organized into structures:

      a.   The Local Group (of galaxies, of which the Milky Way is a member):

      b.   And bigger than groups are clusters and superclusters:

             i.    Notice the “voids” in this image. The universe if full of areas full of nothing. The space between the sun and planets is full of nothing, the space between stars is full of nothing, and the space between galaxies is full of nothing. Some of these spaces between galaxies are quite large and are known as voids. The largest known void is the Eridanus Supervoid which is a billion light years across.

      c.   These in turn form themselves into the filaments of the “cosmic web”:

      d.   Going up in scale from stars to the universe. Use the ZOOM feature; be sure to look at the captions under the pictures:

 5.   The most distant galaxy ever seen:

 6.   Your very complete current address is: Dayton, Ohio, USA, North America, Earth, Solar System, Milky Way, Local Group, Virgo Cluster, Local SuperCluster, Universe

 7.   It is possible that there are universes other than our own—that our universe is but one of many multiverses—in which case the last location in your address should be “This Universe” rather than simply “Universe.”