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?