Sept 28 This set of notes is turning out to be lengthy... I hope all such notes are not as lengthy. I warn you that I often add little ditties to these notes, so they are not by any means a transcript of my lectures. they cover the same amterial, and sometimes I skip things and sometimes I add things. Remember, it's the sum total of the lectures, the texts, these notes, and the homeworks and labs, that makes up the course. No one part of the course stands alone. We finished up scaling. See HW 1 solution, and notes to Sept 26. CHECK OUT THIS WEBSITE: http://www.anzwers.org/free/universe/index.html IT'S A VERY GOOD WEBSITE We then talked about motions in the night sky and how they come about. First, we gave some crude definitions (we'll refine them as needed) of SOLAR SYSTEM, MILKY WAY (ie, GALAXY), and, in passing, BINARY STAR, STAR CLUSTER, CLUSTER OF GALAXIES. Then, we talked about the words APPARENT, INTRINSIC, and THEORY, since colloquial English gives different connotations to the first and third words than does scientific English. SOLAR SYSTEM- a) the Sun and its planets (that's pretty good except that it leaves out things like moons and comets, for instance) b) Objects under the gravitational influence of the Sun c) basically same as b- the region halfway to the nearest star, though we showed in the homework that the major planets only extend 40/260000 of the way to the nearest star, which is a whole lot less than halfway. MILKY WAY (GALAXY)- The Milky Way is the Galaxy in which the Sun lives. It's a pretty typical big galaxy, as is our sister galaxy, M31. A typical big galaxy contains about a trillion (10**12, where "**" means "to the power") stars. The Sun is a pretty typical star, not the least massive by a factor of 20 or so, not the most massive by a factor 100 or so. A galaxy is a collection of a trillion stars under each other's gravitational influence. The orbit of the Sun around the center of the Milky Way, in a quarter of a billion years, is derived from the gravitational attractions of all the stars in the Milky Way. So the Sun feels the gravity of the Milky Way substantially more than it feels the gravity of M31. M31 hardly counts at all in determining the orbit of the Sun, we can ignore it. BINARY STAR- Unlike the Sun, more than half of the stars in the sky are actually two or more stars orbiting each other. STAR CLUSTER- there are conglomerations of hundreds or thousands (or hundreds of thousands) stars in the sky. The gravity of these star clusters is strong enough so that the stars orbit the cluster. So in the previous 2 cases, the first-order effect is the local gravity. That unit, be it a binary star or a star cluster, then acts as a single entity acting under the gravity of the host galaxy. Think of a swarm of bees moving together through the desert. The bees move around with respect to each other, but seen from afar, they are acting as a swarm, a single group. GALAXY CLUSTER- Galaxies, too, can be found alone, or in small groups, or in large clusters of 1000s of "Milky Ways". Again, gravity is what matters here. LOCAL GROUP- The Milky Way and M31 are the two largest and most massive members of the LOCAL GROUP. The Local Group consists of 2 big galaxies, 2-3 medium-sized galaxies, and 20-40 puny galaxies. VIRGO CLUSTER- the Virgo Cluster is the nearest big cluster of galaxies to the Local Group. The Local group is a couple of million light years across, while Virgo is 50 million light years away. Roughly speaking, it's 10-15 million light years across, and contains several thousand galaxies I'm not providing links here, but just the URLs, to pictures VIRGO CLUSTER CENTER- http://antwrp.gsfc.nasa.gov/apod/ap030804.html http://antwrp.gsfc.nasa.gov/apod/ap010126.html COMA CLUSTER OF GALAXIES- http://antwrp.gsfc.nasa.gov/apod/ap000806.html DISTANT GALAXY CLUSTER- http://antwrp.gsfc.nasa.gov/apod/ap010610.html Here are some pictures of individual galaxies in the LOCAL GROUP NGC 6822, one of the puny galaxies - http://antwrp.gsfc.nasa.gov/apod/ap020123.html NGC 205, puny satellite galaxy of M31- http://antwrp.gsfc.nasa.gov/apod/ap001023.html One of the REALLY PUNY satellites of M31- http://antwrp.gsfc.nasa.gov/apod/ap990122.html Hickson Compact Group 87- a small group of galaxies (kindof like the Local Group, except more concentrated)- http://antwrp.gsfc.nasa.gov/apod/ap990906.html CLUSTER OF STARS emerging from a cloud of gas and dust- http://antwrp.gsfc.nasa.gov/apod/ap951101.html Cluster M38- http://antwrp.gsfc.nasa.gov/apod/ap030107.html The PLEIADES star cluster- http://antwrp.gsfc.nasa.gov/apod/ap021201.html GLOBULAR STAR CLUSTER M15- http://antwrp.gsfc.nasa.gov/apod/ap011210.html OK, NOW THEORY- Science tries to make testable models of what we see. These models need to be consistent with what we know (unless they're pointing out or fixing an inconsistency), and needs to make predictions about new tests (experiments, observations). Models come in many flavors, the strongest and most general of which are elevated to the status of THEORY (or LAW). While we recognize that all of our models are approximations to nature, some of them embrace so much of what we're trying to explain and have so much predictive power, that they are accorded special status. In other words, a theory is a model that explains a lot and has been tested a lot. In all cases, finding a situation in which a model fails invalidates the model or requires that the model be fixed up. These falsifiable and self-correcting characteristics of science are its strength. When you find that a model is starting to fail, you soon find the even-better model that explains what we see (measure) in Nature. In everyday speech, theory is almost used as an insult. The complete illogic of "it's only a theory, therefore it's wrong, and my idea is right" is often seen in the newspapers. The other thing to say about models/science is that the Universe/Nature is apparently amenable to modelling and testing. It didn't have to be that way! Furthermore, the rules we discover locally are testably the same as the rules far away in the Universe. This is good, it makes the Universe explainable, and it didn't have to be that way. Finally, the Universe behaves in ways explainable by mathematical models. APPARENT property- An apparent property is something measured by an observer/ measurer. So an apparent property is what we see. Apparent properties don't have the same strength as instrinsic properties. A simple pair of examples will suffice: a) Stars are faint, the Sun is bright. From a planet orbiting another star, that star would be bright, the Sun would be faint. So the apparent brightness depends on the position of the observer. b) Driving down a dark road in the rain, you see some headlights are faint and some bright. You immediately turn these apparent properties into intrinsic properties and say "the fainter headlights are obviously farther away then the brighter ones." INTRINSIC PROPERTIES- properties connected to the objects themselves, that would be agreed upon by independent observers. So in the case of the car headlights, you are implicitly saying "I know how bright a car headlight really is, so if they appear to be of different brightnesses they much be at different distances from me." So the intrinsic property might be "how many optical photons per second leave the car headlight," NOT "how bright does this light placed at a random distance appear to my eyes." So one of the goals of astronomy is to find uot ways to deduce instrinsic properties from apparent properties, because then we can figure out how stars work. Parallax, discussed on August 28, is one way to do that. OK, finally, we come to motions in the sky. Our goal here was not, as in "Astro 100" (NATS 102), to give you the 2000-year history of how we realized the Earth was a moving platform. If your curiosity is piqued, please look in an Astro 100 textbook. Our goal was to remind you that there are motions that occur on a variety of timescales, and to tell you what real motion gives rise to those apprent motions. The STARRY NIGHT lab will reinforce some of these ideas. 1) the Earth is a spinning top, it spins on its axis once per day. The axis is a line passing through the earth from the north pole to the south pole. If you extend this line "up to the sky", you'll find the north and south celestial poles, about which all objects in the sky rotate once per day. So the REAL MOTION of the spinning earth gives rise to an apparent motion of a spinning sky (sun, moon, planets, stars). THIS IS A DAILY MOTION, the west-to-east spin gives rise to an east-to-west apparent motion of the sun, moon, planets, stars. I note in passing that we can PROVE that it's the Earth doing the movement. The simplest proof is that we can send spacecraft to other planets, which we couldn't do if we had the wrong model. Other proofs of the many motions of the Earth come from things like aberration of starlight, which you don't need to know for this class. A third proof, which you do need to understand, is parallax, see below. 2) The Earth orbits the Sun once per year. This second motion adds a second apparent motion to things in the sky. This apparent motion is the change of the stars seen in the night sky as the year progresses. It also changes the time a star rises or sets during the year. 3) then there are real motions of the moon and planets themselves. The moon orbits the Earth, so its real motion is added to its two apparent motions described above. While the moon travels east to west each night because of the spin of the earth, it travels west to east all month, causing it to rise "1 hour later" each night. Similar things happen to the other planets. see the figures from Aug 28 lecture for this and other ideas. 4) We mentioned briefly that since the earth is a tilted, spinning top, it actually wobbles or precesses on a 26000 year period. This wobble changes the position of the north celestial pole, for example, changing which star is the "pole star". 5) Since the earth is a moving reference frame, nearby objects will be seen to move their apparent position with respect to distant objects in a yearly cycle. This PARALLAX, see the figure in the Aug 28 figure section, is the basis of surveying, and of figuring out how far away the stars are. We did a simple experiment with our thumb, holding it right up to our nose and alternating which eye was open. This is a huge parallax or apparent change of position caused by the observer herself changing position. We then moved our thumb out to arm's length and discovered that the parallax got smaller. So the measured parallax depends on the distnace to the object in question. Aristotle couldn't see stellar parallax, so he concluded that the earth wasn't moving. Little did he know that the stars are so far away (remember homework 1, the nearest star is 260000 AU's away) that it took until 1840 to have good enough equipment to actually measure stellar parallax. The other thing that parallax depends on is the 2 different observing places of the observer. The farther away these 2 places are, the bigger the apparent parallax. So parallax depends on the SIZE OF THE OBSERVER's BASELINE, and inversely on the DISTANCE TO THE OBJECT. In astronomy, we make measurements using the Earth's orbit as the baseline, so the baseline is 2AU. Without telescopes in orbit elsewhere, or telescopes on a moon of Jupiter, say, that's the biggest baseline we the observer get. PARALLAX is SUPER IMPORTANT, it's the most fundamental way of figuring out how far away stars are. If we know the distance to a star, we can turn its apparent properties into intrinsic properties. We'll stress this idea a lot in this class. 6) Stsrs ahve real motions of their own. Astronomers call these real motions PROPER MOTIONS. With the naked eye, you could see these motions of a few stars on timescales of a thousand years. We showed a picture of Barnard's Star (see the figures) taken 22 years apart, in which the real motion of that star is quite apparent. We also showed what happens to the Big Dipper on timescales of 50,000 years (again, see the figures). There are observatories dedicated to measuring parallax and proper motions. This field of astronomy is called ASTROMETRY. One of the most famous such observatories is the US Naval Observatory in Flagstaff, Arizona. Yes, it's run by the NAVY (the reason for this goes back to timekeeping on the high seas, and now involves things like making missiles go to where they're intended... the astrometry itself is really really important for figuring out the properties o stars). Finally, one last idea that we talked about... It's this notion that the Universe behaves by the same laws of physics everywhere. Well, how do we know that? ONE WAY is that we can TOUCH some things from far away. While indeed, in general, in astronomy we can't touch or design experiments (how can we make two otherwise identical stars, change one thing, and see what happens? In biology, we CAN do this with fruit flies!), there are some things we can touch. We have brought back Moon rocks. We have found rocks on the Earth that came from Mars! We have brought back solar wind particles from experiments on board spacecraft. We have brought sophisticated analysis machinery to Mars. We have found rocks from outer space, mostly from the asteroid belt. We have captured dust particles from comets and from other stars. We have captured the so-called COSMIC Rays from very very far away. So we haven't tested this idea, at least as i'm describing it here, as completely as one might like, but we have made tests that agree with the notion that the laws of physics are the same everywhere. From the properties of light, we can make this test throughout the Universe.