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.
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.
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.
A. Billions and Billions
Yes, you really do need to use "billion-babble" in an astronomy class.
Here's a quick "powers of ten" reminder:
- One million is one thousand thousand, or, in
powers of ten notation, 1,000 x 1,000 = 103 x 103 = 106.
- 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 of measure 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)
"You Are Here" marks the location of the Sun
(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
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," about 25,000 light years from the
galaxy's nucleus. 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.
If we tried to construct a model at that scale for the whole galaxy,
it wouldn't even fit on the surface of the Earth! At that scale, 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".
This interpretation was first proposed
Wright in 1750. Wright's original drawing is
shown here. Wright
also conjectured that some of the faint, diffuse patches of light
found by telescopes were distant galaxies, something that would not be
proved until 170 years later.
It was not until the 1920's that astronomers realized that ours is
only one among a vast number of 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."
Galaxies can be tremendously bright systems intrinsically, and
therefore we can detect 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.
Like the Milky Way, the Andromeda Galaxy is a large disk galaxy,
comparable in size to the Milky Way. It is visible as a faint,
elongated patch of light on a dark, clear night.
Here is a
sky map showing how to find it.
The Andromeda Galaxy is 2.5 million light years distant, or 15 billion
billion (15 x 1018) miles.
Note: the white dots
are all foreground stars in our galaxy.
Andromeda is far beyond them.
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. (Of course, we
have no choice in this. We cannot, for example, see distant galaxies
as they are at the present time.)
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.
Naturally, there's a catch to exploiting this feature of the universe:
- The distances to viewable parts of the "early universe"
(over several billion light years away) are so large that
only very bright objects can be detected, even by our largest
telescopes. We must learn to compensate for the resulting
- Also, we can't usefully explore our own personal 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 (HST) in orbit
Distant galaxies seen in an extract from the "ERS
deep field survey, obtained 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. (American Museum of Natural History). Both this and the
previous video are based on actual astronomical data sets.
The depths of the 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 600
hours) that contains images of the faintest objects ever
detected. Click on the
image to see the entire Hubble Ultra Deep Field.
There are only a handful of stars in either of these deep
survey pictures. Those are the objects with the extended "spikes".
Every other thing visible in the images, down to the faintest tiny
blob, is a galaxy consisting of billions of individual stars.
There are about 10,000 galaxies over the whole HUDF field.
These pictures represent the present edge of the observable
The faintest images here are 10 billion times fainter than you
can see with your unaided eye.
The HUDF covers only a tiny patch on the sky, slightly more than 2
"minutes of arc" across, which is equivalent to the size of a quarter
seen at a distance of about 130 feet. (See the
next study guide for more information on angular measurement.)
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 HUDF are rare among
local galaxies. In other words, these deep survey images
provide direct evidence that the universe has evolved with
For more details on the deep fields, click here.
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. We expect that more distant objects exist, but light rays
from them have not had time to reach us yet.
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 next time you go to the beach, pick up a handful of sand and
contemplate this fact: there are more stars in the observable universe
than there are grains of sand on all the beaches on Earth.
F. The Infinite Universe
The structure of space and time on the largest scales is governed by
Einstein's theory of General Relativity. By combining that with
the plethora of recent data about the expansion of the
universe and the structure of the most distant regions we can measure,
astronomers have concluded that the universe is spatially infinite
Infinity may be the most difficult concept humans have ever
grappled with because it is completely alien to our everyday
experience, which, of course, transpires in a finite world. It is
impossible to visualize. The ancient Greeks and Indians had
discovered the concept of infinity in mathematics, and a number of
their thinkers were comfortable with the notion of an infinite
universe containing an infinite number of possibly inhabited worlds.
Other scientists have been horrified by the implications of an
Consider just a couple of the implications:
The volume of the observable universe is unimaginably large. Yet
it constitutes exactly zero percent of the volume of the
In an infinite universe, anything that is possible according to the
laws of physics must happen somewhere even if it is extremely
improbable. That means there is another ASTR 1210 class out there
that is exactly like this one. And there is not just one of
these but an infinite number of them!
If you try hard enough to explore examples like this, you may get a faint
glimmer of understanding of what infinity means---and you will almost
certainly find this disturbing. Blaise Pascal (d. 1662), a leading
mathematician and physicist, famously recoiled from the concept
of an infinite universe:
"...engulfed in the infinite
immensity of spaces whereof I know nothing,
But the universe is not infinite only in space. Because of its
ever-increasing expansion, it will be infinite in future time
as well. You can find informed speculation about the ultimate fate of
the familiar world around us
and which know nothing of
me, I am terrified.
The eternal silence of these infinite spaces
fills me with dread."
Star-forming region in a nearby galaxy
G. 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 and energy
during the Big Bang which have been "amplified" over the
intervening time by the expansion of space and the force
- 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
at least 12 billion years old. But it 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 high
in earlier times, peaking about 10 billion years ago, but is much more
modest now. Some galaxies are quiescent now; ours forms new stars at
a rate of about 1 solar mass per year.
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.
This was one of the most important discoveries in science. However,
it cannot be credited to a single individual, because it involved a
long chain of incrementally improving evidence and speculation by many
astronomers since the time of the Greek philosopher Democritus
(ca. 420 BC). The case was clinched by
comparative spectroscopy (see Study
Guide 10) of the Sun and typical stars late in the 19th
"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 now: it formed about 5 billion years ago, about 60%
through the age of our galaxy, and its remaining lifetime is about 5
- Other than hydrogen and helium, the chemical elements are
synthesized during fusion reactions in stars. They are
recycled to the interstellar medium when stars lose their outer layers
or explode at the end of their lives.
All the heavy elements
that make up the Earth originated inside stars now long dead.
That is also true of the
biologically important elements (carbon, oxygen, nitrogen, etc)
that constitute all living things. Stars are an essential part of the
ultimate human cosmic heritage. They are not merely incidental
celestial decoration, as they were often considered in pre-scientific
Here's a video
featuring Neal Tyson discussing this "most astounding fact."
- Planetary systems are a normal byproduct of star
formation. We now know of thousands of other planetary
systems, some including Earth-sized planets. We believe that
almost all stars host planets, and most of them probably host
- 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, which has been present for at least 3 billion
years. Most astronomers are confident that there are millions of
planets like the Earth in our galaxy, but the extent to which those
support advanced lifeforms is unknown without better data.
Human beings are definitely latecomers on Earth: Homo sapiens
has been present only for about 200,000 years---just 0.004% of
the age of the Earth. The fact that this single species
has already begun to alter the Earth's atmosphere and oceans is
dramatic testimony to the power of human technology.
- Earth's biosphere is highly vulnerable to certain
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
Here is a video of violent magnetic activity
on the Sun. It shows vividly how material is flung off the Sun's
surface during eruption events.
In Study Guide 22 we will consider
the threat from asteroid impacts.
If you're interested in exploring all the astronomical hazards facing
the Earth, have a look at
From the Skies, by UVa PhD Philip Plait (cover shown at
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
Optional observing: After you've done the Constellation Quiz (see the
next guide) and become familiar with the
sky, you might want to go to a good dark location on a clear, Moonless
night and try finding:
Reading for the next lecture:
The Milky Way, the plane of our galaxy seen edge-on. The best
views in the evening from the northern hemisphere are in July through October,
when it stretches from the northern to the southern horizon. A deep,
wide-angle exposure is shown here.
The Andromeda Galaxy, the most distant
thing (2.5 million light years) you can see with the naked eye.
See the finding chart
here. The Andromeda region is
visible in the evening sky August through February.
The Scutum Star Cloud. A concentration in
the northern Milky Way composed of about 1 billion stars. See
the finding chart
here. The Scutum region is
visible in the evening sky July through October.
June 2018 by rwo
Text copyright © 1998-2018 Robert W. O'Connell. All rights
reserved. These notes are intended for the private, noncommercial use
of students enrolled in Astronomy 1210 at the University of Virginia.