ASTR 1210 (O'Connell) Study Guide
2. COSMIC HISTORY
Distant galaxies seen in an 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 so far
about the evolution of the universe and its contents. This story is
by no means complete, but we have a solid foundation to build upon.
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 scientific study of the origin,
evolution, and fate of the universe is
A. Billions and Billions
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, 1,000 x 1,000,000 = 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
- A trillion is one thousand billion or one million million, or
1012 in powers of ten notation.
- See Supplement I for more information
on powers of ten notation and units we will use in this course.
B. Light as a Distance Standard
The scale of even the local universe is so huge that we would
have to quote 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). The speed of light has been measured in physics
labs to a precision of about 1 part in a billion.
Furthermore, according to
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/it 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
Scale diagram of the Milky Way Galaxy (edge-on)
(Click for enlargement)
C. Our Galaxy and Beyond
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 believed before the invention of the
Here is a pictorial "zoom
out" from the Earth to the distance of Alpha Centauri.
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 we call our galaxy.
It is an enormous disk-like
structure about 75,000 light years (450 million billion or
4.5x1017 miles) across. (See the diagram above or
click here.) It
contains about 100 billion (1011) stars.
The Sun is a perfectly ordinary star. It does not stand out
among the myriad of stars in our galaxy.
The Sun is not located near the center of our galaxy --- it is
very much in the "suburbs." See the drawing above.
Recall our scale model from the last lecture: if stars near the Sun
are modeled as oranges, the oranges would be 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---which we call
the "Milky Way".
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. (But these are the only
things you can see which are not in our galaxy.)
The Andromeda Galaxy in 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
The Andromeda Galaxy is 2.5 million light years away, or 15 billion
billion (15 x 1018) miles.
D. The Lookback Effect
"What seest thou else in the dark
backward and abysm of time?"
--- from "The Tempest," Shakespeare
The fact that we can detect cosmic objects at such enormous distances
has one tremendously 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
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
This animation shows how light propagates through the expanding
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 to viewable parts of the "early universe"
(over several billions years ago) are so large that only very bright
objects can be detected, even by our largest telescopes. So, we can
sample the universe at early epochs but only in distant regions where
we can detect only rare and
luminous objects. 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 in orbit
Extract from the "ERS GOODS" deep field survey,
with the HST in 2009.
Click for a panorama of the full
survey (6.8 MB).
E. 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, we have found that
there is nothing special about the physical properties or location of
Here is a supercomputer simulation of a trip outside our Galaxy to
the distance of the largest concentration of galaxies in the nearby
universe (the Virgo Cluster), about 50 million light years away (38 MB
mpeg file from the National Center for Supercomputing
Here is another nice simulation of a trip from Earth to the
farthest observable reaches of the universe, far beyond the Virgo
Cluster. Both these videos are based on actual astronomical data
sets. (American Museum of Natural History).
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 Hubble Space Telescope has been used to make a number of deep
imaging surveys of the distant universe. Click on the image above
right for a panorama of one of these.
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
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 over 13 billion years ago!
One of the basic conclusions from studying these distant objects is
that they are different from local galaxies in many ways. For
instance, the distorted shapes you can see in the picture at the top
of the page are rare among local galaxies. In other words, the
deep images provide direct evidence that the universe
has evolved with time.
For more details on the deep fields, 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 is of order 2x1022.
This is a fascinating number. On one hand, it is amazing that we can
calculate it at all and have some confidence that it's correct to
within a couple of orders of magnitude. On the other, it is so huge
that it gives a good sense of the scale of the observable universe.
Needless to say, most of those stars are not detectable
The radius of the "observable universe" is given by the distance a
light ray can travel in the time since the origin of the universe
(i.e. the "Big Bang"). That is about 13.7 billion light years. But
this is only a tiny part of the whole universe. If, as we now
think, the whole universe is infinite in spatial extent, what fraction
of the total volume of the universe lies within our observable
Star-forming region in a nearby galaxy
F. 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,
- 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
expansion of space and 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
Sun (in the H-alpha atomic emission 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. It
is middle-aged: it 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. All the heavy elements that make up the
Earth, and us, originated inside stars now long dead.
- Planetary systems are a normal byproduct of star
formation. We now know of over 700 other planetary systems, some
including Earth-sized planets. The present estimate is that almost
all stars host planets, and most of them probably host Earth-sized
- 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
are confident 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.
If you're interested in exploring the astronomical hazards facing
the Earth, have a look at
From the Skies, by UVa PhD Philip Plait (cover shown at
G. 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 or the value of refined petroleum as an energetically
dense, portable fuel?
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 Maya were powerful and
Fundamentally, it seems to be because no earlier culture had the right
mind-set to pursue nature---the right combination of a deep desire to
understand the world, empiricism, skepticism, independence, and mental
toughness---as well as freedom from everyday drudgery.
As we will discuss over the next couple of weeks, most early cultures had
never moved very far toward these conclusions. With the striking
exception of the ancient Greeks, they may have collected a great deal
of information about the motions and appearance of astronomical
objects, but they failed to interpret it critically. The idea
that human beings would one day walk across the face of one of
those godlike, glowing lights in the sky would have been inconceivable
to most early cultures.
It must also be admitted that our scientific understanding 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
Optional: browse the material on the structure &
evolution of the universe in the Bennett textbook Chs
22 and 23
Reading for the next lecture:
September 2013 by rwo
Text copyright © 1998-2013 Robert W. O'Connell. All
These notes are intended for the private, noncommercial use of
students enrolled in Astronomy 1210 at the University of Virginia.