Transcript Lecture 5 - United International College

Space, time & Cosmos
Lecture 5:
Galaxies, expanding universe and
Relativity
Dr. Ken Tsang
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The Sun is currently traveling through the Local Interstellar Cloud in the low-density
Local Bubble zone of diffuse high-temperature gas, in the inner rim of the Orion Arm of
the Milky Way
galaxy.
Galaxy, between the larger Perseus and Sagittarius arms of the
The Orion Arm is a minor spiral arm of the Milky
Way galaxy. It is also referred to as the Local Arm,
the Local Spur or the Orion Spur.
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Data of the Milky Way Galaxy
Diameter
Thickness
Number of stars
Oldest known star
Mass
100,000 light years
1,000 light years (stars)
200 to 400 billion
13.2 billion years
5.8×1011 M☉
26,000 ± 1,400 light-years
220 million years
Sun's distance to galactic center
Sun's galactic rotation period
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The Milky Way as seen from Death Valley,
2007, a panoramic picture
360° panorama of Racetrack Playa in Death Valley at night. The Milky Way is visible as
the arc in the center.
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360-degree photographic
panorama of the galaxy
This image is mosaic of multiple shots on large-format film. It comprises all 360 degrees
of the galaxy from our vantage. Photography was done in Ft. Davis, Texas for the Northern
hemisphere shots and from Broken Hill, New South Wales, Australia, for the southern
portions. Note the dust lanes, which obscure our view of some features beyond them.
Infrared imaging reaches into these regions, and radio astronomy can look all the way
through with less detail. The very center, however, shows a window to the farther side. In
the center, stars are mostly very old and this causes the more yellow color.
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This dazzling infrared image from NASA's Spitzer Space Telescope shows hundreds of
thousands of stars crowded into the swirling core of our spiral Milky Way galaxy. In
visible-light pictures, this region cannot be seen at all because dust lying between Earth
and the galactic center blocks our view.
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Wide-angle view of
Magellan Clouds and
Milky Way (fromCosmos5
NOAO
image gallery)
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The Milky Way is thought to be a barred spiral
galaxy. Messier 109 is one possible analog.
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History of The Milky Way & galaxies
The Greek philosophers Anaxagoras (ca. 500–428 BC) and Democritus (450–
370 B.C.) proposed that the bright band on the night sky known as the Milky
Way might consist of distant stars. Aristotle (384-322 B.C.), however, believed
the Milky Way to be caused by "the ignition of the fiery exhalation of some
stars which were large, numerous and close together" and that the "ignition
takes place in the upper part of the atmosphere." The Arabian astronomer,
Alhazen (965-1037 A.D.), refuted this by making the first attempt at observing
and measuring the Milky Way's parallax, and he thus "determined that because
the Milky Way had no parallax, it was very remote from the earth and did not
belong to the atmosphere.“
Actual proof of the Milky Way consisting of many stars came in 1610 when
Galileo Galilei used a telescope to study the Milky Way and discovered that it
was composed of a huge number of faint stars. In 1750 Thomas Wright, an
English astronomer, in his “An original theory or new hypothesis of the
universe”, speculated (correctly) that the Galaxy might be a rotating body of a
huge number of stars held together by gravitational forces, akin to the solar
system but on a much larger scale. The resulting disk of stars can be seen as a
band on the sky from our perspective inside the disk.
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In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about
the structure of the Milky Way. Kant also conjectured that some of the
nebulae visible in the night sky might be separate "galaxies" themselves,
similar to our own.
The first attempt to describe the shape of the Milky Way and the position
of the Sun within it was carried out by William Herschel (1738 –1822,
British astronomer famous for discovering Uranus) in 1785 by carefully
counting the number of stars in different regions of the sky. He produced a
diagram of the shape of the Galaxy with the Solar System close to the
center.
In 1845, Lord Rosse (1800 –1867, British astronomer) constructed a new
telescope (the world's largest) and was able to distinguish between
elliptical and spiral-shaped nebulae. He also managed to make out
individual point sources in some of these nebulae, lending credence to
Kant's earlier conjecture.
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The sketch made by Lord Rosse of the Whirlpool
Galaxy in 1845.
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Rosse named the Crab Nebula, based on an earlier drawing made with his older
36-inch (91cm) telescope in which it resembled a crab.
In the 1840s, Lord Rosse built the Leviathan of Parsonstown, a 72-inch (183-cm),
the world’s largest at that time. When the 72-inch telescope was in service, he
produced an improved drawing of considerably different appearance, but the
original name stuck.
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Kapteyn, a Dutch astronomer, in 1920 arrived at the picture of a small
(diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the
center. A different method by Harlow Shapley, an American astronomer,
based on the cataloguing of globular clusters led to a radically different
picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun
far from the center.
Both analyses failed to take into account the absorption of light by
interstellar dust present in the galactic plane, but after Robert Julius
Trumpler, an American astronomer, quantified this effect in 1930 by
studying open clusters, the present picture of our galaxy, the Milky Way,
emerged.
The parsec ("parallax of one arcsecond", symbol pc) is a unit of length,
equal to just under 31 trillion kilometres (about 19 trillion miles), or about
3.26 light-years.
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Other galaxies
In the 10th century, the Persian astronomer, Abd al-Rahman al-Sufi made the
earliest recorded observation of the Andromeda Galaxy, describing it as a "small
cloud". Al-Sufi also identified the Large Magellanic Cloud; it was not seen by
Europeans until Magellan's voyage in the 16th century. These were the first galaxies
other than the Milky Way to be observed from Earth. Al-Sufi published his findings
in his Book of Fixed Stars in 964.
In 1054, the creation of the Crab Nebula resulting from the SN 1054 supernova was
observed by Chinese and Arab/Persian astronomers. The Crab Nebula itself was
observed centuries later by John Bevis in 1731, followed by Charles Messier in 1758
and then by the Earl of Rosse in the 1840s.
Toward the end of the 18th century, Charles Messier compiled a catalog containing
the 109 brightest nebulae (celestial objects with a nebulous appearance), later
followed by a larger catalog of 5,000 nebulae assembled by William Herschel.
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In 1917, Heber Curtis had observed the nova S Andromedae within the "Great
Andromeda Nebula" (Messier object M31). Searching the photographic record, he
found 11 more novae. Curtis noticed that these novae were, on average, 10
magnitudes fainter than those that occurred within our galaxy. As a result he was
able to come up with a distance estimate of 150,000 parsecs. He became a
proponent of the so-called "island universes" hypothesis, which holds that spiral
nebulae are actually independent galaxies.
The matter was conclusively settled by Edwin Hubble in the early 1920s using a
new telescope. He was able to resolve the outer parts of some spiral nebulae as
collections of individual stars and identified some Cepheid variables, thus allowing
him to estimate the distance to the nebulae: they were far too distant to be part
of the Milky Way. In 1936 Hubble produced a classification system for galaxies that
is used to this day, the Hubble sequence.
Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between
their period of variability and absolute luminosity.
Because of this correlation, a Cepheid variable can be used as a standard candle to determine the
distance to its host cluster or galaxy. Since the period-luminosity relation can be calibrated with great
precision using the nearest Cepheid stars, the distances found with this method are among the most
accurate available.
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Photograph of the "Great Andromeda Nebula" from 1899, later
identified as the Andromeda Galaxy
A visible light image of the
Andromeda Galaxy recently.
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North America Nebula, including Pelican Nebula
The remarkable shape of the emission nebula resembles that
of the continent of North America, complete with a prominent
Gulf of Mexico.
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Pillars of Creation in the
Eagle Nebula (M16): Stars
are being born here.
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Galaxy and nebula
A galaxy is a massive, gravitationally bound system that consists of stars and
stellar remnants, an interstellar medium of gas and dust, and possibly a
hypothetical substance known as dark matter.
A nebula is an interstellar cloud of dust, hydrogen gas and plasma. Originally
nebula was a general name for any extended astronomical object, including
galaxies beyond the Milky Way.
Nebulae often form star-forming regions, such as in the Eagle Nebula. This nebula is
depicted in one of NASA's most famous images, the "Pillars of Creation". In these
regions the formations of gas, dust and other materials 'clump' together to form
larger masses, which attract further matter, and eventually will become big enough to
form stars.
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The Great
Debate
whether distant nebulae were relatively small and
lay within our own galaxy or whether they were
large, independent galaxies.
The Great Debate, also called the Shapley - Curtis Debate was an
influential debate between the astronomers Harlow Shapley
and Heber Curtis which concerned the nature of spiral
nebulae and the size of the universe. The debate took place
on 26 April 1920.
Shapley was arguing in favor of the Milky Way as the entirety of
the universe. He believed galaxies such as Andromeda and the
Spiral Nebulae were simply part of the Milky Way.
Curtis on the other side contended that Andromeda and other
such nebulae were separate galaxies, or "Island universes". He
showed that there were more novae in Andromeda than in
the Milky Way. From this he could ask why there were more
novae in one small section of the galaxy than the others.
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Edwin Hubble
(November 20, 1889 – September 28, 1953)
an American astronomer, who profoundly changed astronomers' understanding of the
demonstrating the existence of
other galaxies besides the Milky Way.
nature of the universe by
He also discovered that the degree of
redshift observed in light coming from
a galaxy increased in proportion to the
distance of that galaxy from the Milky
Way. This became known as Hubble's
law, and would help establish that the
universe is expanding.
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Hubble's arrival at Mount Wilson in 1919 coincided roughly with the
completion of the 100-inch Hooker Telescope, then the world's largest
telescope. At that time, the prevailing view of the cosmos was that the
universe consisted entirely of the Milky Way. Using the Hooker Telescope,
Hubble identified Cepheid variables (a kind of star) in several spiral nebulae,
including the Andromeda Galaxy. Hubble's observations, made in
1922–1923, proved conclusively that these nebulae were much
too distant to be part of the Milky Way and were, in fact, entire
galaxies outside our own. This idea had been opposed by many in the
astronomy establishment of the time, in particular by Harvard-based Harlow
Shapley. His discovery, announced on January 1, 1925, fundamentally
changed the view of the universe.
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Some binaries, like Algol, can vary in brightness because of eclipses of one
star by the other or gravitational interactions between the two companions.
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Brightness curves for four variable Cepheids over several weeks
A Cepheid variable (or
Cepheid) is a member of a
particular class of variable
stars, notable for a fairly tight
correlation between their
period of variability and
absolute luminosity.
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仙王座
The origin of the name and
prototype of these variables is the
star Delta Cephei, discovered to be
variable by John Goodricke in 1784.
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The relationship between a Cepheid variable's luminosity and variability period is
quite precise, securing Cepheids as a viable standard candle and the foundation of
the Extragalactic Distance Scale. This period / luminosity connection was
discovered in 1912 by Henrietta Swan Leavitt. She measured the brightness of
hundreds of Cepheid variables and discovered a distinct period-luminosity
relationship.
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Henrietta Swan Leavitt (1868 –1921) was an American astronomer, and the deaf
daughter of a Congregational minister. A graduate of Radcliffe College, Leavitt
went to work in 1893 at the Harvard College Observatory in a menial capacity as
a "computer", assigned to count images on photographic plates. Study of the
plates led Leavitt to propound a groundbreaking theory, worked out while she
labored as a $10.50-a-week assistant, that was the basis for the pivotal work of
astronomer Edwin Hubble and radically changed the theory of modern astronomy,
an accomplishment for which Leavitt received almost no credit during her lifetime.
Unaware of her death four years prior, the Swedish
mathematician Gösta Mittag-Leffler considered
nominating her for the 1926 Nobel prize in physics,
and wrote to Shapley requesting more information
on her work on Cepheid variables, offering to send
her his monograph on Sofia Kovalevskaya. Shapley
replied, suggesting that the true credit belonged to
his interpretation of her findings. She was never
nominated, and the Nobel Prize is not awarded
posthumously
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Mount Wilson Observatory, placed in
operation in 1904, was the second (after Lick) of
the great astronomical research observatories to
be established in the Far West. The observatory,
at 5,710 feet altitude, is located in the Angeles
National Forest on a 1,050-acre plateau at the
summit of Mount Wilson.
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In October 1923, Hubble located 3
novae in the Andromeda Nebula,
each marked with an ‘N’. One of
these turned out to be a Cepheid
vriable, so the ‘N’ was crossed out
and the star relabeled ‘VAR!’.
Cepheid can be used to measured
distance, so Hubble could now
measure the distance to the
Andromeda Nebula. The result
was staggering: Andromeda
Nebula is roughly 900,000 light
year from the Earth.
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Hubble also developed a classification scheme for galaxies which,
with minor revisions remains in use today.
The Hubble sequence: classification of galaxies
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Hubble and Milton L. Humason discovered a rough proportionality of the
objects' distances with their redshifts. Though there was considerable scatter
(now known to be due to peculiar velocities), Hubble and Humason were able to
plot a trend line from the 46 galaxies they studied and obtained a value for the
Hubble-Humason constant of 500 km/s/Mpc, which is much higher than the
currently accepted value due to errors in their distance calibrations.
In 1929 Hubble and Humason formulated the empirical Redshift Distance Law of
galaxies, nowadays termed simply Hubble's law, which, if the redshift is interpreted
as a measure of recession speed, is consistent with the solutions of Einstein’s
equations of general relativity for a homogeneous, isotropic expanding space.
Although concepts underlying an expanding universe were well understood earlier,
this statement by Hubble and Humason led to wider scale acceptance for this view.
The law states that the greater the distance between any two galaxies, the greater
their relative speed of separation.
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Historically, spectroscopy referred
to the use of visible light
dispersed according to its
wavelength, e.g. by a prism.
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Emission spectrum of Hydrogen
Emission spectrum of Iron
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Solar spectrum with Fraunhofer lines as it appears visually.
Spectrum of a blue sky somewhat close
to the horizon pointing east at around 3 or
4 pm on a clear day.
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Absorption lines in the optical spectrum of a supercluster of distant
galaxies (right), as compared to absorption lines in the optical spectrum of
the Sun (left). Arrows indicate redshift. Wavelength increases up
towards the red and beyond (frequency decreases).
Redshift (increase in wavelength) and
blue-shift (decrease in wavelength)
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Hubble's observations of
galaxies with the redshift in
their spectral lines.
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Difficulties in Using Cepheids to measure distance
There have been a number of difficulties associated with using Cepheids as
distance indicators. Until recently, astronomers used photographic plates to
measure the fluxes from stars. The plates were highly non-linear and often
produced faulty flux measurements. Since massive stars are short lived, they
are always located near their dusty birthplaces. Dust absorbs light, particularly
at blue wavelengths where most photographic images were taken, and if not
properly corrected for, this dust absorption can lead to erroneous luminosity
determinations. Finally, it has been very difficult to detect Cepheids in distant
galaxies from the ground: Earth's fluctuating atmosphere makes it impossible to
separate these stars from the diffuse light of their host galaxies.
Another historic difficulty with using Cepheids as distance indicators has been
the problem of determining the distance to a sample of nearby Cepheids. In
recent years, astronomers have developed several very reliable and
independent methods of determining the distances to the Large Magellanic
Cloud (LMC) and Small Magellanic Cloud (SMC), two of the nearby satellite
galaxies of our own Milky Way Galaxy. Since the LMC and SMC contain large
number of Cepheids, they can be used to calibrate the distance scale.
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He inferred that galaxies were similar to each other in size so those that appeared
smaller must be further away. By plotting the velocity of the galaxies against their
distance he came across an interesting relationship. This is now known as
Hubble's law and is shown in the following plot.
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Hubble's law
If you study the above plot you will see that the more distant a galaxy is, on
average, the faster it is receding from us.
In fact Hubble realised he could fit a linear relationship to his data, as shown by
the pale blue line of best fit. The slope of this line is a constant and is now known
as the Hubble constant,
H0. This relationship is expressed mathematically as:
v∝d
where v is the recession velocity and d is the distance.
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Hubble's law is the statement that the redshift
in light coming from distant galaxies is proportional
to their distance.
The law was first formulated by Edwin Hubble in
1929 after nearly a decade of observations. It is
considered the first observational basis for an
expanding universe and today serves as one of
the pieces of evidence most often cited in
support of the Big Bang.
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How the Hubble’s law can be explained by an expanding
Universe?
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What is the Big Bang?
The "Big Bang" is the term given to what is currently the most
widely accepted scientific model for the origin and evolution of
the Universe.
This model has supplanted other models such as the Steady State theory proposed
by Hoyle, Bondi and Gold in the 1940s. Indeed it was Fred Hoyle who coined the
term "big bang" as a derisory one in an interview in the 1960s.
In the Big Bang theory the Universe comes into existence, creating time and space.
Initially the Universe would have been extremely hot and dense. It expanded from a
primordial hot and dense initial condition and cooled gradually. Some of the energy
involved was turned into matter. Current observations suggest an age for the
Universe of about 13.7 billion years.
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According to the Big Bang model, the universe expanded
from an extremely dense and hot state and continues to
expand today.
A common and useful
analogy explains that
space itself is
expanding, carrying
galaxies with it, like
raisins in a rising loaf of
bread.
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Tests of Big Bang Cosmology
The Big Bang Model is supported by a number of important observations, each of
which are described in more detail on separate pages:
The expansion of the universe
Edwin Hubble's 1929 observation that galaxies were generally receding from
us provided the first clue that the Big Bang theory might be right.
The abundance of the light elements H, He, Li
The Big Bang theory predicts that these light elements should have been
fused from protons and neutrons in the first few minutes after the Big Bang.
The cosmic microwave background (CMB) radiation
The early universe should have been very hot. The cosmic microwave
background radiation is the remnant heat leftover from the Big Bang.
These three measurable signatures strongly support the notion that the universe
evolved from a dense, nearly featureless hot gas, just as the Big Bang model
predicts.
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The expansion of the universe
The Big Bang model was a natural outcome of Einstein's
General Relativity as applied to a homogeneous universe.
However, in 1917, the idea that the universe was expanding was thought to be
absurd. So Einstein invented the cosmological constant as a term in his General
Relativity theory that allowed for a static universe.
In 1929, Edwin Hubble announced that his observations of galaxies outside our
own Milky Way showed that they were systematically moving away from us with a
speed that was proportional to their distance from us. The more distant the
galaxy, the faster it was receding from us. The universe was expanding after all,
just as General Relativity originally predicted! Hubble observed that the light from
a given galaxy was shifted further toward the red end of the light spectrum the
further that galaxy was from our galaxy.
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The expanding raisin bread model below illustrates why the specific
form of Hubble's expansion law (speed of recession is proportional to distance ) is
important. If every portion of the bread expands by the same amount in a given
interval of time, then the raisins would recede from each other with exactly a
Hubble type expansion law. In a given time interval, a nearby raisin would move
relatively little, but a distant raisin would move relatively farther - and the same
behavior would be seen from any raisin in the loaf.
In other words,
the Hubble law is just what
one would expect for a homogeneous
expanding universe, as predicted by
the Big Bang theory. Moreover no raisin, or
galaxy, occupies a special place in this universe unless you get too close to the edge of the loaf
where the analogy breaks down.
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Tests of Big Bang: The Light Elements
The term nucleosynthesis refers to the formation of heavier elements, atomic
nuclei with many protons and neutrons, from the fusion of lighter elements.
The Big Bang theory predicts that the early universe was a very hot place. One
second after the Big Bang, the temperature of the universe was roughly 10 billion
degrees and was filled with a sea of neutrons, protons, electrons, anti-electrons
(positrons), photons and neutrinos. As the universe cooled, the neutrons either
decayed into protons and electrons or combined with protons to make deuterium
(an isotope of hydrogen). During the first three minutes of the universe, most of
the deuterium combined to make helium.
Trace amounts of lithium were also produced at this time. This process of light
element formation in the early universe is called “Big Bang nucleosynthesis”
(BBN).
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George Gamow, a Ukrainian-born, US-based physicist and former
student of Friedmann's, made his mark early by applying quantum theory to explain
how alpha particles can be ejected from nuclei in alpha decay. Moving from the
USSR in 1931 he settled in the US and continued his work on stellar evolution and
beta decay. He was particularly interested in trying to solve the problem about the
origin of the elements.
Hans Bethe had already shown in the 1930s how helium could
be synthesised inside stars through fusion of hydrogen nuclei.
He had also explained how protons and neutrons added to
carbon nuclei could form heavier elements.
Gamow had realised from Hubble's work that the early Universe
must have been much smaller, hotter and denser than it is now.
In the late 1940s with his students Ralph Alpher and Robert
Herman he calculated that helium could form from the fusion of
protons (that is, hydrogen nuclei) and neutrons. This
nucleosynthesis would cease once the available neutrons were
used up and the Universe had expanded and cooled sufficiently.
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They also realised that the Universe should be filled with background microwave
radiation, the remnant of the original big bang now cooled to about 50 Kelvin. This
radiation would have the spectral characteristics of a blackbody.
Gamow's theory of the nucleosynthesis of primordial helium accounted for the
observed abundance of helium compared with hydrogen in the Universe whereas
stellar nucleosynthesis could not.
His prediction of remnant radiation was neglected by others until the 1960s but was
to provide the key evidence in support of the big bang model for the Universe.
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Evidence for the Big Bang Model:
Cosmic Microwave Background Radiation
In 1965 two scientists working for Bell Telephone Laboratories, Arno Penzias and
Robert Wilson were adapting a horn-shaped antenna near New York for use in radio
astronomy. They encountered noise in the system and despite repeated and
thorough attempts were unable to remove it or find its cause. They eventually
realized that this "noise" was in fact remnant
radiation from the big
bang.
Such radiation had been predicted by Gamow in the late 1940s. As the Universe
expanded it cooled so that today the background radiation corresponds to a
temperature of 2.725 K and has a black body spectrum.
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This plot shows the black-body nature of the cosmic microwave background
radiation. The spectrum corresponds to background radiation with a temperature of
2.725 K.
These measurements were
made by the FIRAS
instrument on the COBE
satellite. The error bars for
each measurement are
smaller than the width of
the red line.
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Black-body/Thermal radiation is electromagnetic radiation emitted from the
surface of an object due to the object's temperature. Infrared radiation from a
common household radiator or electric heater is an example of thermal radiation,
as is the light emitted by a glowing incandescent light bulb.
This diagram shows how the
peak wavelength and total
radiated amount vary with
temperature. Although this
plot shows relatively high
temperatures, the same
relationships hold true for any
temperature down to absolute
zero. Visible light is between
380 to 750 nm.
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Example of Black-body/Thermal radiation
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Vidoe to watch (and report):
Welcome to the Universe: Nebula & Galaxies: A Cosmic Journey
http://www.youtube.com/watch?v=X5zVlEywGZg&NR=1
Stephen Hawking's Universe - EP1:Seeing Is Believing (1/ 5)
http://www.youtube.com/watch?v=jd1tgLQg4ZU&feature=related
Stephen Hawking's Universe - EP1:Seeing Is Believing (2/ 5)
http://www.youtube.com/watch?v=PJamA_ulJ50&feature=related
Stephen Hawking's Universe - EP1:Seeing Is Believing (3/ 5)
http://www.youtube.com/watch?v=HFe1BL3gvo0&feature=related
Stephen Hawking's Universe - EP1:Seeing Is Believing (4/ 5)
http://www.youtube.com/watch?v=MxTCFkP-snI&feature=related
Stephen Hawking's Universe - EP1:Seeing Is Believing (5/ 5)
http://www.youtube.com/watch?v=sSfrbYyQIww&feature=related
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Stephen Hawking's Universe - EP2: The Big Bang (1/ 5)
http://www.youtube.com/watch?v=MZa7px6NtFY&feature=related
Stephen Hawking's Universe - EP2: The Big Bang (2/ 5)
http://www.youtube.com/watch?v=Rc2hNHjC84Q&feature=related
Stephen Hawking's Universe - EP2: The Big Bang (3/ 5)
http://www.youtube.com/watch?v=iv48uVZ2vnk&feature=related
Stephen Hawking's Universe - EP2: The Big Bang (4/ 5)
http://www.youtube.com/watch?v=IFPzBMTOnxM&feature=related
Stephen Hawking's Universe - EP2: The Big Bang (5/ 5)
http://www.youtube.com/watch?v=sUyrnvmg6zU&feature=related
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