Extract from the Hubble Ultra Deep Field, which
records the faintest astronomical objects ever observed.
In the last lecture, as a way to provide context for the rest of the
course, we had constructed a scale model giving a sense of the vast
distances between the Sun and even its nearest neighbor stars. In
this lecture, we extend our discussion to the largest measurable
scales of time and space.
Then we give a broad-brush overview of what we have learned about the
evolution of the universe and its contents. Astronomy is the only
science that attempts to understand the nature of the universe as a
whole (in empirical, not religious or mythological, terms). The study
of the origin, evolution, and fate of the universe is called
cosmology.
Finally, we contrast our scientific understanding of the universe with
pre-scientific cosmologies.
A. BILLIONS AND BILLIONS: OUR GALAXY AND BEYOND
Yes, you really do need to use "billion-babble" in an astronomy class.
One billion is one thousand million, or 103 x 106
= 109 in powers of ten notation.
Even the nearest stars are so distant that it is convenient
to measure their distances in light years rather than miles
or kilometers.
One light year is the distance light travels
in one year.
We use the speed of light (which is 186,000 miles per second or
300,000 km per second) as a standard here because it is
the highest speed attainable by any physical object.
A light year is 6 trillion miles or 10 trillion km, where
one trillion is one thousand billion. That's 6,000,000,000,000 miles,
or 6x1012 in powers of ten notation.
[It used to be
that such numbers were called "astronomical"---but now they're
merely "economical." You could remember the light year's length
that by noting that it's about the same in miles as is the national
debt in dollars. The debt is now about $5 trillion or $17,000
for each and every American.]
Alpha
Centauri, the nearest star, is 4.2 light years distant.
Here is a pictorial "zoom
out" from the Earth to the distance of Alpha Centauri.
The gigantic star system in which the Sun resides is called our
galaxy. It is an enormous disk-like structure about 75,000 light years
(450 million billion or 4.5x1017 miles) across. It
contains about 100 billion (1011)stars. The Sun does not
stand out among all these other stars.
Recall our scale model from last lecture: if stars near the Sun are
modeled as oranges, the oranges are separated by distances of over 1000
miles! The density of matter near us in the galaxy is very
low.
On that scale model, the center of our galaxy would be 10,000,000 miles
away (40 times farther than the Moon).
From the Sun's vantage point, we see the disk-like distribution
of stars in our galaxy projected on the sky at night as a faint band
of light---the "Milky Way".
There are many other galaxies near ours in space.
Galaxies are tremendously bright systems intrinsically, and therefore
we can see them across enormous gulfs of space. Even with the naked
eye, you could see four galaxies. (These are the only things you can
see which are not in our galaxy.)
The Andromeda Galaxy (Click for a long exposure image.)
The most distant object you can see without a telescope is the
Andromeda Galaxy. The Andromeda Galaxy is comparable in size to our
own Galaxy. It is visible as a faint, elongated patch of light on a
dark, clear night. Here is a sky map showing its location in the
constellation Andromeda (see the "M31" label).
The Andromeda Galaxy is 2.5 million light years away, or 15 billion
billion (15 x 1018) miles.
B. THE LOOKBACK EFFECT
The fact that we can detect cosmic objects at such enormous distances
has one very important consequence. Light travels fast but not
infinitely fast. Light rays from distant stars or galaxies have
therefore been traveling for long periods of time before they reach
us: in fact, they have traveled one year of time for every light year
of distance.
Therefore, by looking out in space we are looking back in
time. Because of this "lookback effect," we are able to see other
parts of the universe as they were at earlier times.
For instance, the light you could see tonight coming from the Andromeda
Galaxy left its stars 2.5 million years ago, before the modern human species
even existed!
This animation shows how light propagates through the expanding
universe.
Astronomy is unique in this regard: in no other human endeavor are we
actually able to SEE THE PAST. In effect, astronomers have a kind of
time machine at their disposal. They are able to directly
study the evolution of the universe as it happened.
Of course, there's a catch:
The distances are so large that only very bright objects can
be detected, even by our largest telescopes. So we sample the
universe at early times but only rare and luminous objects then.
We must learn to compensate for the resulting biases,
Also, we can't usefully explore our own past in this way.
We do see nearby objects as they were in the past, but the lookback
is only on the order of nanoseconds.
The Hubble Space Telescope on
orbit
C. THE DEEP UNIVERSE
The universe is filled with galaxies, both smaller and larger than our own.
As in the case of the Earth and the Sun, there is nothing special about
our galaxy.
Here is a supercomputer simulation of a trip
outside our Galaxy to the distance of the largest concentration of
galaxies in the nearby universe (38 MB mpeg file).
Despite the great distances of these galaxies, large telescopes can
see well beyond them. The depths of the observable universe are
plumbed by instruments like the Hubble
Space Telescope, our premier orbiting observatory, and the many
huge ground-based telescopes built over the last 15 years.
The picture at the top of the page is an
extract from the "Hubble Ultra Deep Field," a super-long exposure
(over 350 hours) that contains images of the faintest objects ever
detected. Click on the image to see the entire Hubble Ultra Deep
Field.
This image represents the present edge of the observable universe.
The faintest images here are 10 billion times fainter than you
can see with your unaided eye. There are only a few stars in
this picture. Everything else you can see is a galaxy, about
10,000 of them in the whole HUDF field.
The objects visible here are so distant their light has taken
billions of years to reach us. Some
of these galaxies are seen as they were 10 billion years ago!
One of the basic conclusions from studying these distant objects is
that they are different from local galaxies in many ways. In
other words, the deep images provide direct evidence that the universe
has evolved with time.
For more details and other downloadable images, click
here.
Star-forming region in nearby galaxy
D. EARTH IN THE CONTEXT OF COSMIC HISTORY
We now think we have a good understanding of the broad outline of
cosmic history. I list the "top-ten" elements of that outline below,
roughly in order of their sequence in cosmic time. Some were already
highlighted in Guide 01. For a
narrative description of the history of the universe, click here.
The universe began about 14 billion years ago in an ultrahot and
ultradense state called the "Big Bang" and has been expanding ever
since. The spatial volume of the universe is now, and has always been,
infinite.
Physical structure in the present-day universe originated in tiny
irregularities in the distribution of matter during the Big Bang which
have been "amplified" over the intervening time by the force of
gravity.
The easily observable matter in the universe is organized into
galaxies, huge star systems with typical sizes of 10's of thousands
of light years containing billions of stars. Our galaxy is not
special in any way.
Stars form continuously out of the diffuse "interstellar"
gas in our own and other galaxies.
Sun (in the H-alpha atomic line) showing
active regions and a flame-like "prominence."
The Sun is a star, with average properties
"Average" means that the Sun is not distinguished from billions of
other stars in our Galaxy. This recognition resolves thousands of
years of religious, philosophical, and scientific debate.
"Across the sea of space, the stars are other suns."
--- Christiaan Huygens (1692)
Stars generate their energy by burning hydrogen in nuclear
fusion reactions. The hydrogen supply is large but
nonetheless finite, so this implies that stars must evolve as
they begin to run out of fuel. The Sun formed about 5 billion years
ago, and its remaining lifetime is about 5 billion years.
Other than hydrogen and helium, the chemical elements are
synthesized during fusion reactions in stars. They are
recycled outside stars when they lose their outer layers or explode at
the end of their lives.
Planetary systems are a normal byproduct of star
formation. We now know of over 200 other planetary systems.
Perhaps 30% of all stars have planets.
Earth is a planet in orbit around the Sun. It is unique
among the presently-known planets for its oxygen-rich atmosphere and
surface oceans and for harboring life. But most astronomers
expect that there are millions of planets like it in our galaxy.
Earth's biosphere is highly vulnerable to astronomical phenomena.
especially asteroid impacts, solar evolution, magnetically-induced
activity on the Sun (because the Earth is inside the Sun's
extended atmosphere), and stellar explosions.
Here is a video of magnetically-induced activity
on the Sun. It shows vividly how material is flung off the Sun's
surface during eruption events.
E. CULTURE AND SCIENTIFIC DISCOVERY
It took about 500 years of scientific effort to put together the
picture of the structure and evolution of the universe we just
described. A vast amount of evidence underpins the elements of this
understanding (and the details make up the bulk of the textbook). We
believe that this picture is right in its essentials---so, for
instance, when science is taught 300 years from now, it will still be
a valid first-cut description.
But this raises a fundamental question about human societies: Why
didn't we know all this thousands of years ago? More importantly,
why didn't we know other crucial scientific facts---like the role
of microorganisms in causing disease?
It's not because of the evolution of the human brain---as far as
we can tell, human beings were just as smart in 2000 BC as they are
today. It's not because we had to wait for sophisticated instruments
or electronics to be invented--- these didn't exist in 1500 AD, when
science began, either. It's not because all earlier societies were too
impoverished to worry about studying nature---ancient Egypt, Greece,
Rome, China, the Islamic Caliphates, and the Mayas were powerful and
wealthy cultures.
Fundamentally, it seems to be because no earlier culture had the right
mind-set---the right combination of a deep desire to understand nature,
skepticism, independence, mental toughness---AND the relative
freedom from everyday drudgery to pursue nature.
As we will discuss over the next couple of weeks, earlier cultures had
never moved very far toward these conclusions. With the striking
exception of the Greeks, they may have collected a great deal
of information about the motions and appearance of astronomical
objects, but they failed to interpret it carefully.
It must also be admitted that the scientific picture of the universe,
however well-founded it may be, is not congenial to everyone. The
human race, the Earth, even our galaxy, have no special place in it.
From a human point of view, the universe as revealed by science may
seem cold, dangerous, and purposeless. It is certainly not the
universe most people had hoped to find.
F. PRE-SCIENTIFIC COSMOLOGIES
The intense, if often unconscious, desire to find human meaning in the
universe combined with the absence of stringent standards for evidence
are the reasons that the cosmologies---i.e. attempts to explain the
origin of the world--- of pre-scientific cultures are so
different from ours.
They assume mankind to be the focus and purpose of the universe
(even if that universe is harsh or cruel, as in the case of Mesoamerican
cultures).
They are limited to ordinary human perceptual horizons of space,
time, and phenomena because no instruments expanding those horizons
were available
They usually feature strong allegorical, mythological, or supernatural
elements.
They have a strong "projective" tendency: human characteristics,
inflated to supernatural proportions, are imposed outwards on the
cosmos.
Direct, persistent, supernatural control of natural phenomena is
usually assumed.
They feature idiosyncratic elements not shared with other cultures
(in sharp contrast with scientific interpretations, which are pan-cultural)
The "provenance" of ideas and the empirical evidence supporting them are
considered unimportant.
One of the most fascinating pre-scientific cosmologies is that of
the Mesoamerican cultures that flourished in Mexico and Guatemala
between about 500 BC and 1500 AD. Their vibrant, if violent,
view of the world is beautifully captured in the so-called
"Aztec Calendar Stone," an extract of which is shown below.
Click on the image for a more complete description.
Reading for this lecture:
Seeds textbook: Ch 1
Study Guide 2
The Aztec Calendar StoneSupplement I (PDF file). Skim and then refer to
this later as needed.
Optional: Cosmic History: A
Brief Narrative
Optional: browse the material on the structure &
evolution of the universe in the Seeds textbook Ch
16 and 18
Here
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