Lecture 11 : Galaxies Robert Fisher
Items • Nathan Hearn guest lecture on dark matter on April 20th. Lunch in the loop (on me) with Nathan following the lecture at Frontera Fresco for anyone who wants to join us. • Adler Planetarium field trip on May 4th - $16/person. Waiver forms to be signed!! • Final projects due May 11th, along with a short (5 minute) presentation that day.
Final Project • Your final project is to construct a creative interpretation a scientific theme we encountered during the class. You will present your work in a five minute presentation in front of the entire class on May 11. • The project musthave both a scientific component and a creative one. • For instance, a Jackson Pollock-lookalike painting would fly, but ONLY if you said that it was your interpretation of the big bang cosmological model AND you could also demonstrate mastery of the basic astrophysics of the big bang while presenting your work. • Be prepared to be grilled! • Ideas : • Mount your camera on a tripod and shoot star trails. • Create a “harmony of the worlds” soundtrack for the Upsilon Andromeda system. • Paint the night sky as viewed from an observer about to fall behind the horizon of a black hole. • Write a short science fiction story about the discovery of intelligent life in the universe.
Review of Two Weeks Ago • Stellar Structure • Stellar Evolution • Evolution of a low-mass star • Evolution of a high-mass star • Supernovae
Review of Last week • Michelson - Morley • Special Relativity • General Relativity
Today • Black Holes, White Holes, Wormholes • Galaxies • Distances in the universe • Types of galaxies • Ellipticals • Spirals • Irregulars
More Exotica From Relativity Theory • Black holes are perhaps the most exotic objects in the known universe. • These solutions were originally discovered by Karl Schwarzschild. • Schwarzschild (1873 - 1916) discovered the solutions while serving in the German army on the Russian front in WWI, within a year after Einstein’s theory was published. • Tragically, he died on the front shortly afterward. He was, however, survived by his son Martin Schwarzschild, who made fundamental contributions to stellar structure.
Cygnus X-1 • The first strong case for the detection of a black hole was made in the Cygnus X-1 x-ray emitting system in the 1970s.
Black Hole Physics • In addition, as she nears the horizon, only those photons from Alexis moving nearly vertically have a chance to escape; the ones moving horizontally begin to fall into the black hole. • This means that Bettie sees the signal from Alexis become more and more highly-beamed as she moves further in. • Alexis, on the other hand, sees the sky overhead begin to darken to absolute black apart from a narrow cone above her. Radio waves
Beyond the Horizon • While Bettie will never see Alexis move behind the horizon, Alexis actually falls behind the horizon in a finite time. • What happens behind the horizon, and in particular what happens as one approaches the center of the black hole is a matter of intense speculation, but is not understood in the current framework of physics. • According to General Relativity, all of the mass of the black hole is concentrated in a single point of infinite density -- the singularity. This is in fact a breakdown of the theory itself, and so General Relativity cannot be used to understand what goes on at the location of the singularity.
White Holes • The full weirdness of Schwarzschild’s solution took many decades to sink in. • In particular, the most general solution contains not only a black hole, but also a mirror image on the other side which ejects matter instead of accreting it. • The mirror image is known as a white hole. • The reality of white holes has been debated over time -- no one has ever seen anything in nature which resembles a white hole.
Wormholes • By joining a black hole to a white hole, one can construct a “wormhole” solution to the equations of General Relativity. • Such a solution was first discovered by Einstein and Rosen in the 1930s. • The neck of the Einstein-Rosen solution, however, is unstable to collapse. • In 1988, Kip Thorne and his graduate student Mike Morris showed that it is possible to stabilize the Einstein-Rosen wormhole solution using “exotic energy” that exerts a negative gravitational influence.
Kip Thorne (1940 - ) • Kip Thorne is perhaps the leading figure in contemporary General Relativity research in the world today. • He has contributed to virtually every aspect of General Relativity theory and has supervised a whole generation of students at the California Institute of Technology. • He also has an amazingly soft-spoken and kind manner and is of the most genuinely nicest people you could ever hope to meet.
Science Fiction Begets Science • When Carl Sagan was writing his science fiction novel Contact, in the early 1980s, he spoke with Kip to try to come up with a plausible way to rapidly transport the novel’s characters over vast distances without violating the laws of physics. • Kip went to work on the problem and actually worked out the details using relativity theory. He suggested that wormholes might work. • Intringued, Thorne picked up the wormhole problem over the next several years and began pursuing it as an active research project. • Inspired by his bold lead on such a far-out topic, other well-known scientists like Stephen Hawking and Igor Novikov also published work on wormhole theory.
Wormholes as Time Machines Accelerated Motion • Thorne suggested that it may be possible to create a time machine from a wormhole. • The physics requires more explanation than we have time for, but as a result of accelerating one end of the wormhole, one has an effective time machine. • One can pose “grandparent” paradoxes in a very clearly-defined way in this context, for instance imagining billiard balls moving through the wormhole time machine.
Surfing Spacetime -- Detecting Gravitational Waves • Einstein’s Theory of General Relativity predicts that spacetime itself will form ripples which propagate at the speed of light. • Where are these gravitational waves? Because gravity is a weak force in comparison to electromagnetism, we have not yet directly detected any gravitational waves. • Physicists have searched for these gravitational waves both in fantastically-difficult direct detection experiments on the Earth, and in observations of the astrophysical objects.
The Remarkable Binary Pulsar System PSR 1913+16 • Very strong indirect evidence for the existence of gravitational waves was demonstrated by Taylor and Hulse, who were measuring the properties of the binary pulsar system PSR 1913+16. • Using the pulsed radio emission from the puslars themselves as incredibly-accurate clocks, Taylor and Hulse were able to demonstrate that the binary system is actually spinning down, at precisely the rate predicted if the loss is due to gravitational waves.
Direct Detection of Gravitational Wave -- The Laser Interferometer Gravitational Observatory (LIGO) • Using an interferometer very similar to the one which Michelson and Morley used in their classic experiment, scientists are attempting at this very moment to measure the spacetime distortion produced by gravitational radiation. • The strongest conceivable sources of gravitational radiation are coalescing binary black holes and neutron stars. • Even with these incredibly intense and rare events, the expected signal is minute -- about 1/100th of a proton diameter.
LIGO • Two interferometers are place at two sites (one in Washington, the other in Louisiana). • If a signal is detected, its position on the sky will be triangalized.
The Question of the “Nebulae” -- How Big is the Universe?? • For hundreds of years astronomers observed fuzzy “nebulae” (literally “clouds” from Latin) in their telescopes. • The precise nature of these nebulae was the subject of intense speculation and debate. • Since no one could see any individual stars in these using the smaller telescopes and less sensitive photographic plates of the 19th century, the consensus opinion was that all these nebulae were gas clouds in the larger distribution of stars of our own Milky Way. • Some of these nebulae are indeed known today to represent actual gaseous regions nearby to us in our own galaxy.
Will the Real Nebulae Please Stand Up ?? Andromeda Galaxy • Other “spiral nebulae” turned out to be entire galaxies like our own Milky Way, like Andromeda. • Viewed from a smaller telescope, however, these galaxies appear very blurred out and nebulous just like the real gaseous clouds in our own galaxy. • The issue reached a head in the Great Debate of 1920.
The Great Debate -- A Universe of Galaxies, or a Galaxy Universe? • The National Academy of Sciences sponsored a debate in 1920 on the scale of the universe, and invited astronomers Harlow Shapley and Heber Curtis. • Shapley held that the Milky Way was the entire Universe -- the “spiral nebulae” were actually clouds of gas within our own galaxy. He further held that our sun was off-center within that galaxy. • Curtis held that the Milky Way was only one of many galaxies in a vast universe, and that the “spiral nebulae” were enormously distant from us. He held that our sun was near the center of our own galaxy.
Not Seeing the Forest for the Trees -- The Problem of Finding our Place in the Galaxy • In understanding the problem of determining the shape of the galaxy, consider an analogy. • Imagine that we find ourselves lost in a misty forest and we attempted to find our location by mapping out the trees. • Because of the mist, we only see those trees nearby us. • Even if we were close to the edge of the forest, we would never know so from this method. Finding Ourselves in a Misty Forest of Trees
Not Seeing the Forest for the Trees -- The Problem of Finding our Place in the Galaxy • In determining the position of our sun within our galaxy, astronomers were long confused by the fact that simply counting stars, we appear to be at the center of the Milky Way. • The problem with this method is that it does not take into account the absorption and reddening of starlight by intervening interstellar gas and dust, so the sun appears to be smack in the center of the galaxy, regardless of its actual location. Herschel’s Universe (c. 1780)
The Shapley Model of the Universe • Shapley made a fundamental breakthrough in our understanding of the structure of the Milky Way by using globular clusters instead of individual stars. • Shapley observed that globular clusters are evenly distributed both above and below the plane of the Milky way, and therefore they are associated with the Milky way itself. • It follows that the globulars should be centered about the center of the Milky way.
Shapley’s Globular Cluster Distribution • Shapley’s results showed that the sun was far from the center of the galaxy. • The modern accepted distance is about 8.5 thousand parsecs (kpc) -- Shapley’s value is off because he did not properly account for reddening, but the basic conclusion is correct. • How did Shapley measure distances of thousands of light years?? He used a method which had been recently discovered by Henrietta Leavitt. Center of Milky Way
Henrietta Leavitt (1868 - 1921) • Leavitt made fundamental contributions to astronomy, and is one of the unsung heroes of modern science. • Leavitt overcame enormous barriers. Besides being a woman in an era when science was almost exclusively male, she was also deaf. • After graduating from Radcliffe in 1892, she was hired as a “computer” at the Harvard Observatory. • Despite her initial position, she persisted and made her own discoveries. Shortly before the time of her death she was the head of photometry at the observatory.
Variable Stars • Leavitt most important discovery dealt with variable stars. • While some stars have nearly constant luminosity (like our sun), others vary their output brightness dramatically. • In some cases (like explosive novae and supernovae) the brightness is not systematic, but in others it is highly regular. Brightness Period Time
Cepheid Variables • Leavitt studied one type of variable star in particular -- a certain kind of yellow giant called a Cepheid variable. • When the star contracts, its atmosphere becomes more opaque, absorbs more and transmits less light. • When it expands, its atmosphere becomes more transparent, absorbs less and transmits more light.
The Period-Luminosity Relationship for Cepheid Variables • Leavitt discovered that the intrinsic luminosity of Cepheid variables was directly related to its period. • One can easily measure the period of any visible Cepheid. • Using the period, and knowledge of the relationship Leavitt discovered, one can infer the intrinsic luminosity of the Cepheid. • Knowing its intrinsic luminosity and its observed apparent luminosity, one can determine the distance to the star !!
Where The Spiral Nebulae Are • On the more fundamental issue of the “spiral nebulae,” however, it was Curtis who was ultimately more correct. • Curtis presented a number of lines of evidence in favor of his idea. In particular, he • counted the number of novae arising in the Andromeda “spiral nebula” and found it to be larger than the rest of the Milky Way. • measured the distribution of spiral nebulae on the sky and found it to be concentrated away from the disk of the Milky Way. • observed that the spectra of the “spiral nebulae” were indistinguishable from other clusters of stars. • Shapley’s argument was partially based on observations which would later turn out to be incorrect (eg, that Andromeda was rotating rapidly enough to be seen in a telescope) and partially on biases. In particular, it was nearly impossible for astronomers of that time to accept that galaxies were separated distances of hundreds of millions of light years, even though this was precisely the case.
Hubble and the Conclusive Evidence • The conclusive evidence in favor of the Universe of Galaxies came a few years later when Hubble was able to resolve individual Cepheid variables in the Andromeda galaxy. • Using Leavitt’s period-luminosity relationship, calibrated by Cepheid variables in our own galaxy, he was able to measure the distance to Andromeda and conclusively demonstrate that it was far outside our own galaxy. • Practically overnight on the scale of history, our conception of the universe shifted dramatically. Where space before was just plain huge (tens of thousands of light years across the Milky Way, filled with a billion stars), now space was now nearly unfathomably enormous (billions of light years across the observable universe, filled with the light of millions of galaxies each with billions of stars)!!
The Great Debate in Retrospect • “The Shapley-Curtis debate makes interesting reading even today. It is important, not only as a historical document, but also as a glimpse into the reasoning processes of eminent scientists engaged in a great controversy for which the evidence on both sides is fragmentary and partly faulty. This debate illustrates forcefully how tricky it is to pick one's way through the treacherous ground that characterizes research at the frontiers of science." Frank Shu (contemporary astrophysicist) • "As to relativity, I must confess that I would rather have a subject in which there would be a half dozen members of the Academy competent enough to understand at least a few words of what the speakers were saying if we had a symposium upon it. I pray to God that the progress of science will send relativity to some region of space beyond the fourth dimension, from whence it may never return to plague us.” Abbot to Hale
Classification of Galaxies • Like the O-B-A-F-G-K-M classification scheme of stars, it is useful to classify galaxies. • Classification is a bit like butterfly-collecting; it may at first glance appear tedious, but in reality it is the first step towards knowledge, by beginning to observe broad classes and trends. • Once we have established classes and trends in galactic systems, we can begin to ask meaningful questions about how things got that way.
Spiral Galaxies • Spiral galaxies are one of the two major types of galaxies. • Spirals are distinguished by • Bluish-light indicative of massive hot young stars. • Current star formation. • Complex spiral structure ranging from simpler two-armed spirals to richly-complex “flocculent” spirals. • Lanes of dark -- indicative of dust absorption -- mixed in with lanes of starlight. • Generally, broken into three components -- relatively thin disk of stars and gas, a central “bulge” of stars, and a more weakly-defined spherical halo of stars and globular clusters.
Barred Spirals • Many spiral galaxies have a central “bar,” varying from a very weakly-defined bar to a very strongly-defined one. • In some cases one can observe a nested bar structure, where there is also an “inner bar”. • The problem of determination of the Milky Way highlighted by the Curtis-Shapley debate is complex enough that it took until the late 20th century before astronomers began to conclude that our own Milky Way probably is a weakly-barred spiral itself.
Elliptical Galaxies • Elliptical galaxies, along with spirals, are the second major class of galaxies. • Elliptical galaxies are distinguished by their • Reddish light indicative of older stars • Absence of current star formation • Smooth centrally-condensed distribution of light, and absence of other strongly-defined internal structure • Generally few dust features and little interstellar gas content • Frequently located in clusters of galaxies, particularly towards the cluster center
Irregular Galaxies • Some galaxies do not fall into either major category. These are the irregulars. • Quite often they are smaller galaxies. • In images of the distant (and therefore very young) universe, these types of irregular galaxies also become more common.