ASTR 1230 (O'Connell) Lecture Notes


NGC 1232

Spiral galaxy NGC 1232 (ESO VLT)


Even a casual familiarity with the sky reveals that the stars are unevenly distributed. For instance, the region containing the "watery" Zodiacal constellations like Capricorn, Aquarius, and Pisces in the autumn sky, contains few bright stars compared to the area between Lyra and Scorpio in the summer sky or the region of the "Winter Hexagon."

This raises an obvious question: what is the spatial structure of the star system in which the Sun resides?

The fact that the sky does not look the same in all directions tells you immediately that the matter in the universe cannot be distributed in a uniform fashion about the Earth's location. Our star system cannot, for instance, be a uniform sphere with the Earth at its center.

The study of the structure of our star system revealed the spatial scale of the universe near the Earth, analogous to the way that the study of the physics of the stars (in Lecture 5) revealed the temporal scale of the universe. Just as in the case of the temporal scale, the spatial scale of our universe is vastly larger than anyone had expected.


A question about "the structure of our star system" would have made no sense to pre-Copernican astronomers because in the ancient geocentric cosmologies, the stars were thought to be small luminous bodies fixed to a crystalline sphere centered on the Earth and rotating about Earth once a day. In this model, the stars had no distribution in depth, and they had no relationship to the Sun. There were thought to be no more than a few thousand of them (those visible to the naked eye).

(1) Post-Copernican Structure

(2) Deep Telescopic Probes


Shapley's picture has been refined considerably. An edge-on sketch of our star system based on our current understanding is shown at right below:

MW Mosaic

Panoramic mosaic of Milky Way.
Click for explanation and orientation.



Our Galaxy is an astonishingly massive structure, and for several decades at the beginning of the 20th century most astronomers believed it constituted the entire universe. But it was soon realized that the Galaxy is only one of innumerable building blocks of comparable or larger scale in the universe.

Shapley had used RR Lyrae variable stars to determine distances to globular clusters within our Galaxy. Shortly afterwards, Hubble (1923) applied a similar technique, using intrinsically luminous Cepheid variables, to estimate the distance to the brightest of the many faint, diffuse "spiral nebulae" which had been first recorded about 125 years earlier. [Note: Cepheid variables are the subject of ASTR 1230 Lab No. 6.]

By this method, Hubble was able to demonstrate conclusively that Messier 31 (the "great nebula in Andromeda") is an independent star system outside our own, at a distance now estimated to be 2.5 million light years (see picture at right; click for a larger view).

Although the more evocative term "island universes" was used for a while, external star systems quickly became known as galaxies and our own star system as the Milky Way Galaxy. ("Galaxy" is derived from the Greek root for "milk.")

Two galaxies in the northern hemisphere are visible with the naked eye or binoculars: M31 in Andromeda and M33 in Triangulum. M33 is quite faint, but M31 is readily visible on a dark night. In the southern hemisphere the Large and Small Magellanic Clouds are conspicuous; they are small satellite galaxies of the Milky Way.

Since Hubble's discovery, astronomers have devoted tremendous effort to probing the extragalactic universe. The biggest concentrations of massive galaxies nearby us lie in the direction of the constellations Virgo and Coma. These regions are easiest to observe in the late winter sky, and they transit near midnight in March.

N7752-3 Hundreds of nearby galaxies are accessible to an 8-in telescope under dark sky conditions. The views possible with visual observing are, of course, much less detailed than the deep exposure shown at the top of this page, though with good conditions you would be able to distinguish shape, spiral structure, and large dust lanes. Your 8-in telescopes are capable of revealing the three primary galaxy morphologies (spiral, elliptical, irregular). A good source of background information on observing bright galaxies is The Messier Catalog home page.

The Lookback Effect

Given their large intrinsic brightnesses, galaxies can be detected at very great distances. Because of the finite speed of light, we therefore see the galaxies not as they are today but as they were millions of years ago. At M31's distance of 2.5 million light years, the photons you see from it now left the galaxy 2.5 million years ago (before modern humans evolved on the Earth). The brighter galaxies in Virgo are 50 million light years away, and we see them at a "lookback time" of 50 million years.

The Far Universe

We have found that there are over 1 billion galaxies within reach of our best telescopes. There are many types of galaxies, covering a wide range of morphologies, an enormous range of mass, and wide variations in star formation histories and chemical content. Just as in the case of our Sun in the context of other stars, our Galaxy is only average in properties.

The Hubble Ultra Deep Field

With the Hubble Space Telescope and large ground-based telescopes, we have detected many galaxies over 10 billion light years away(!) We therefore see them as they were 10 billion years in the past. This "lookback time machine" allows us to observe galaxy evolution in progress.

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Last modified December 2020 by rwo

M31-M33 map copyright © Hawaiian Astronomical Society. Image of Milky Way star clouds copyright © M. Shainblum. Image of M31 copyright © G. Greaney. Nick Strobel. Galaxy merger animation by John Dubinski. Text copyright © 2000-2020 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1230 at the University of Virginia.