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.
- 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 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
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.
In 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".
This interpretation was first proposed
Wright in 1750. See Wright's
drawing 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 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 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
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 how to find it.
The Andromeda Galaxy is 2.5 million light years away, 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.
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 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 biases.
- 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 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
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 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
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,
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 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
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 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.
This is one of the most important discoveries in astronomy.
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 Copernicus.
The case was clinched by comparative spectroscopy
(see Study Guide 10) of the Sun and
typical stars late in the 19th century.
"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, 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 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 originated inside stars now long dead.
That is also true of the
biologically important elements that constitute all living
things. Stars are an essential part of our ultimate human cosmic
heritage. They are not merely incidental celestial decoration, as
they were often considered in pre-scientific philosophy.
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 over 900 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, 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 debatable.
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'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
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.
May 2015 by rwo
Text copyright © 1998-2015 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.