Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Science 3210 001 : Introduction to Astronomy
Lecture 11 : Galaxies
Robert Fisher
Items
! Homework and first observational project due next week.
! Second midterm will be on April 11th. Covers the material in
between the last midterm and the end of the next lecture.
! We plan to have a guest speaker sometime shortly after the
second midterm. Lunch in the loop (on me) with the guest
speaker following the lecture at Frontera Fresco for anyone who
wants to join us.
! Short proposals (1 paragraph) for final projects will be due on
April 18th. We will have a peer review session of the proposals
that day.
! Final projects due on the last day of class (May 9th), along with a
short (5 minute) presentation that day.
Final Project
! Your final project is to construct a creative interpretation a scientifictheme we encountered during the class. You will present your work in afive minute presentation in front of the entire class on May 11.
! The project must have both a scientific component and a creative one.
! For instance, a Jackson Pollock-lookalike painting would fly, but ONLY ifyou said that it was your interpretation of the big bang cosmologicalmodel AND you could also demonstrate mastery of the basicastrophysics 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 Andromedasystem.
! Paint the night sky as viewed from an observer about to fall behind thehorizon of a black hole.
! Write a short science fiction story about the discovery of intelligent life in theuniverse.
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 mostexotic objects in the known universe.
! These solutions were originallydiscovered by Karl Schwarzschild.
! Schwarzschild (1873 - 1916)discovered the solutions whileserving in the German army on theRussian front in WWI, within a yearafter Einstein’s theory was published.
! Tragically, he died on the frontshortly afterward. He was, however,survived by his son MartinSchwarzschild, who madefundamental contributions to stellarstructure.
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 fromAlexis moving nearly vertically have a chance to escape; the onesmoving horizontally begin to fall into the black hole.
! This means that Bettie sees the signal from Alexis become moreand more highly-beamed as she moves further in.
! Alexis, on the other hand, sees the sky overhead begin to darkento absolute black apart from a narrow cone above her.
Radio waves
Beyond the Horizon
! While Bettie will never see Alexis move behind the horizon, Alexisactually falls behind the horizon in a finite time.
! What happens behind the horizon, and in particular whathappens as one approaches the center of the black hole is amatter of intense speculation, but is not understood in the currentframework of physics.
! According to General Relativity, all of the mass of the black holeis concentrated in a single point of infinite density -- thesingularity. This is in fact a breakdown of the theory itself, and soGeneral Relativity cannot be used to understand what goes on atthe 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 theleading figure in contemporaryGeneral Relativity research inthe world today.
! He has contributed to virtuallyevery aspect of GeneralRelativity theory and hassupervised a whole generation ofstudents at the CaliforniaInstitute of Technology.
! He also has an amazingly soft-spoken and kind manner and isof the most genuinely nicestpeople you could ever hope tomeet.
Science Fiction Begets Science
! When Carl Sagan was writing hisscience fiction novel Contact, in theearly 1980s, he spoke with Kip to tryto come up with a plausible way torapidly transport the novel’scharacters over vast distanceswithout violating the laws of physics.
! Kip went to work on the problem andactually worked out the details usingrelativity theory. He suggested thatwormholes might work.
! Intringued, Thorne picked up thewormhole problem over the nextseveral years and began pursuing itas an active research project.
! Inspired by his bold lead on such afar-out topic, other well-knownscientists like Stephen Hawking andIgor Novikov also published work onwormhole theory.
Wormholes as Time Machines
! Thorne suggested that it may bepossible to create a timemachine from a wormhole.
! The physics requires moreexplanation than we have timefor, but as a result ofaccelerating one end of thewormhole, one has an effectivetime machine.
! One can pose “grandparent”paradoxes in a very clearly-defined way in this context, forinstance imagining billiard ballsmoving through the wormholetime machine.
Accelerated Motion
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.
Galaxies
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.
M57 - The Ring Nebula
M42 - The Orion Nebula
Will the Real Nebulae Please Stand Up ??
! 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.
Andromeda Galaxy
The Great Debate --
A Universe of Galaxies, or a Galaxy Universe?
! The National Academy of Sciencessponsored a debate in 1920 on thescale of the universe, and invitedastronomers Harlow Shapley andHeber Curtis.
! Shapley held that the Milky Way wasthe entire Universe -- the “spiralnebulae” were actually clouds of gaswithin our own galaxy. He furtherheld that our sun was off-centerwithin that galaxy.
! Curtis held that the Milky Way wasonly one of many galaxies in a vastuniverse, and that the “spiralnebulae” were enormously distantfrom us. He held that our sun wasnear 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 fundamentalcontributions to astronomy, and isone of the unsung heroes of modernscience.
! Leavitt overcame enormous barriers.Besides being a woman in an erawhen science was almost exclusivelymale, she was also deaf.
! After graduating from Radcliffe in1892, she was hired as a “computer”at the Harvard Observatory.
! Despite her initial position, shepersisted and made her owndiscoveries. Shortly before the timeof her death she was the head ofphotometry at the observatory.
Standard Candles
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
Time
Period
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 wasCurtis who was ultimately more correct.
! Curtis presented a number of lines of evidence in favor of his idea. Inparticular, he
! counted the number of novae arising in the Andromeda “spiral nebula” andfound it to be larger than the rest of the Milky Way.
! measured the distribution of spiral nebulae on the sky and found it to beconcentrated away from the disk of the Milky Way.
! observed that the spectra of the “spiral nebulae” were indistinguishable fromother clusters of stars.
! Shapley’s argument was partially based on observations which wouldlater turn out to be incorrect (eg, that Andromeda was rotating rapidlyenough to be seen in a telescope) and partially on biases. In particular, itwas nearly impossible for astronomers of that time to accept thatgalaxies 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 theUniverse of Galaxies came a few yearslater when Hubble was able to resolveindividual Cepheid variables in theAndromeda galaxy.
! Using Leavitt’s period-luminosityrelationship, calibrated by Cepheidvariables in our own galaxy, he was ableto measure the distance to Andromedaand conclusively demonstrate that it wasfar outside our own galaxy.
! Practically overnight on the scale ofhistory, our conception of the universeshifted dramatically. Where space beforewas just plain huge (tens of thousandsof light years across the Milky Way,filled with a billion stars), now spacewas now nearly unfathomablyenormous (billions of light yearsacross the observable universe, filledwith the light of millions of galaxieseach 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 ofstarlight.
! Generally, broken into three components -- relatively thin disk of stars andgas, a central “bulge” of stars, and a more weakly-defined spherical halo ofstars and globular clusters.
M51 Spiral Galaxy
Black-Eye or Sleeping Beauty Galaxy M64
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 ofgalaxies.
! 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 otherstrongly-defined internal structure
! Generally few dust features and little interstellar gas content
! Frequently located in clusters of galaxies, particularly towards the clustercenter
NGC 1316
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.
Small Maganellic Cloud
Large Maganellic Cloud
Hubble’s Tuning Fork Diagram
Ellipticals into Spirals?
Or Spirals into Ellipticals?
! Hubble’s classification scheme is disfavored today as an
evolutionary scenario.
! The more likely evolutionary scenario is that elliptical galaxies are
the products of the collision of two (or sometimes more) spiral
galaxies.
! This scenario has been supported by computer simulations of
colliding galaxies.
Galaxy Collision Movie
But do Galaxies Actually Collide?
Arp 188 and Tidal Tails
! Halton Arp, a critic of the BigBang model, constructed acatalog of “unusual” galaxies inthe 1960s.
! This catalog is now understoodto be an excellent source ofgalaxies which have undergonecollisions in recent cosmichistory.
! The tidal tails seen in Arp 188(located four hundred millionlight years from the Earth) kinthis Hubble Space telescopeimage are several hundredthousand light years across.
Next Week : More Black Holes and Galaxies
! What would happen if two regions of spacetime were tied
together in a “wormhole”?
! What do we think happens at the very smallest scales in which
gravity and quantum effects both become important?
! And is there a black hole at the center of the Milky Way?