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.
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, in powers of ten
notation, 103 x 106 = 109.
A billion of anything is very difficult to visualize. Visualizing
a million is much easier: a million seconds elapse in only 11.6 days.
But a billion seconds take almost 32 years. We will give some other
examples in class.
The scientific shorthand for quantities measured in billions of a
given unit is "giga" --- so 5 billion years (the age of the Sun) is
usually referred to as 5 giga-years or 5 Gyr.
The scientific shorthand for quantities measured in one billionth
of a given unit is "nano" --- so, for instance, "nanotechnology"
refers to things that have characteristic sizes of one billionth of a
meter.
A trillion is one thousand billion or one million million, or
1012 in powers of ten notation.
[It used to be said that such numbers were
"astronomical"---but now they're merely "economical." The national
debt of the USA has doubled in the last 8 years to $10 trillion, or
$33,000 for each and every American.]
See Supplement I for more information
on powers of ten notation and units we will use in this course.
Light as a Distance Standard
The scale of even the local universe is so huge that we would
be quoting distances in trillions of ordinary units like miles
or kilometers. Instead, astronomers sought a more convenient and
more universal standard for measuring distances.
Light travels very fast but not infinitely fast. Its speed is
186,000 miles per second or 300,000 km per second (or about 1 foot per
nanosecond). It has been measured in physics labs to a precision of
about 1 part in a billion. Furthermore, according to Einstein's Special
Relativity the speed of light will always be found to have to same
value for any observer in the universe as long as he/she is not
accelerating through space. Also, in Relativity, no physical object
can travel faster than the speed of light. Therefore, light speed is
an excellent choice for a standard of velocity.
Accordingly, astronomers use the
light travel time to objects as a measure of their distance. They
characterize distances in terms of the time it would take a
light ray to traverse that distance:
For instance, we define a light second to be the
distance a light ray travels in one second of time, which is
186,000 miles. The distance to the Earth's Moon is such that it takes
light 1.26 second to cross it. Therefore, we can say the distance to
the Moon is either "238,000 miles" or, equivalently, "1.26 light
seconds". See illustration below:
The relative sizes and separation of the Earth-Moon system
are shown to scale above. The beam of light is depicted traveling
between the Earth and the Moon in the same time it actually takes
light to scale the real distance between them: 1.255 seconds at its
mean orbital distance (surface to surface). (From Wikipedia)
The distance to the Sun is 8.3 light minutes.
Even the nearest stars are much more distant. A convenient unit
for typical stellar distances is the light year. One light
year is the distance light travels in one year.
A light year is about 6 trillion miles or 10 trillion km.
That's 6,000,000,000,000 miles,
or 6x1012 in powers of ten notation.
For technical reasons, having to do with the practice of
determining the distances to stars using the parallax
method, astronomers more commonly use the parsec as
a distance unit. One parsec is 3.26 light-years, so if you see
"parsecs" quoted as a distance, just multiply by 3 to convert
roughly to light-years.
Our Local Stellar System: the Milky Way Galaxy
Alpha Centauri, the nearest star, is 4.2 light years distant.
It is over 250,000 times more distant than the Sun, vastly farther
than anyone would have easily believed before the invention of the
telescope.
There are 36 stars within 12.5 light years (about 4 parsecs)
of the Sun. Click
here for a perspective illustration.
The Sun and all these stars, in fact all stars you can see
in the sky, are members of the gigantic star system
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".
Other Galaxies
There are many other galaxies near ours in space. Here is a chart of
the galaxies clustered near our own, the so-called "Local Group" of
galaxies. Galaxies can be 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 most luminous member of the Local Group.
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 rays from distant stars or galaxies have
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, about 50 million light years
away (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 in the HUDF 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.
The total number of galaxies within the observable universe is
of order 200 billion. On average each of these contains about
100 billion stars. So the total number of stars in the observable
universe (most not individually detectable) is of
order 2x1022.
Star-forming region in nearby galaxy
D. EARTH IN THE CONTEXT OF COSMIC HISTORY: THE "TOP TEN"
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. The star formation rate was very
fast at earlier times but is much more modest now. Some galaxies
are quiescent now; ours forms stars at a rate of about 1 solar mass
per year.
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 will eventually burn out. 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 300 other planetary systems (though
our methods are not yet capable of detecting Earth-sized planets).
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.
On the other hand, our scientific understanding of the cosmos differs
drastically from those of pre-scientific cultures. 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 those other crucial scientific facts with more
practical ramifications---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 also 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---as well as 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. The idea
that human beings would one day walk across the face of one of
those glowing, godlike lights in the sky would have been inconceivable
to most early cultures.
It must also be admitted that the scientific picture of the universe,
however well-founded, 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.
As a contrast, consider one of the most fascinating pre-scientific
cosmologies: 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". Click on the
link for more information. Mesoamerican astronomy will be discussed
further in Study Guide 5.
Reading for this lecture:
Bennett textbook: Ch 1 and Secs 3.4, 3.5.
Study Guide 2
Supplement 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 Bennett textbook Chs
22 and 23
Here
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