ASTR 1210 (O'Connell) Study Guide



Distant galaxies seen in an extract from the Hubble Ultra Deep Field,
which records the faintest astronomical objects ever observed.

"To me it is remarkable that this astonishing discovery, the vastness and the emptiness of space, has not attracted the imaginative attention of poets and religious thinkers...Naively one might have thought that both poets and priests would be so utterly astonished by these scientific revelations that they would be working with a white-hot fury to try to embody them in the foundations of our culture." --- Francis Crick

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 outlook 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:

B. Light as a Distance Standard

The scale of even our local star system 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 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/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 distance:

Milky Way Diagram

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 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 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.

Other Galaxies

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.
Note: the white dots are all foreground stars in our galaxy.
Andromeda is far beyond them.

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.

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:


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 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.

Naturally, there's a catch to exploiting this feature of the universe:

The Hubble Space Telescope (HST) in orbit

Distant galaxies seen in an extract from the "ERS GOODS"
deep field survey, obtained with the HST in 2009.
Click for more information and images from the full survey.

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 from extensive surveys of galaxies that there is nothing special about the physical properties or location of our galaxy.

The depths of the universe are plumbed by instruments like the Hubble Space Telescope and the James Webb Space Telescope, our premier orbiting observatories, and the many huge ground-based telescopes built since 1990.

The Hubble and Webb telescopes have been used to make a number of deep imaging surveys of the distant universe. Click on the image above right for more survey images. 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.

The radius (or "horizon") 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.

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 in volume.

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 infinite universe.

Consider just a couple of the implications:

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:

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 in this Wikipedia article.

30 Dor

Star-forming region in a nearby galaxy (Hubble Space Telescope)

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.

  1. 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.

  2. 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 of gravity.

  3. 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.

  4. 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.

    The Sun (in the H-alpha atomic emission line) showing
    active regions and a flame-like "prominence."
    The Earth is about 1/8 the size of the elongated bright region.

  5. 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 century.

      "Across the sea of space, the stars are other suns."
      --- Christiaan Huygens (1692)

  6. 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 billion years.

  7. 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 philosophy.

      Here's a video featuring Neal Tyson discussing this "most astounding fact."

  8. 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-sized planets.

  9. 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.

  10. 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 explosions.

      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 Death From the Skies, by UVa PhD Philip Plait (cover shown at right).

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Last modified September 2023 by rwo

Text copyright © 1998-2023 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.