The Night Sky – Past, Present, and Future
© Bob Field 2007-0720
Look at all the stars in the photograph taken by the Hubble Space Telescope. You can’t see any stars, just galaxies containing billions of stars each. Did I fool you? The visible universe has approximately 100 billion galaxies each with about 100 billion stars. The light from distant stars may travel for billions of years to reach us and the stars may have evolved dramatically since the light left their hot surfaces. The visible universe is only a small fraction of the universe but dark matter and energy are hard to observe.
For many purposes, a space telescope is superior to a larger telescope on a high mountain because it is totally beyond the reach of the Earth’s atmosphere which scatters, absorbs, and distorts light from distant objects. Above the blue sky which scatters sunlight, the stars are always visible from space. The vacuum of space does not filter out any portions of the spectrum of light. Big telescopes not only gather more light but can produce sharper images if nothing interferes with the light they collect and if their optical components are high quality. No matter how big the telescope on Earth or how high it is located, atmospheric turbulence bends light in ways that make stars twinkle and that reduce the image sharpness.
We know that our ancestors benefited greatly from their day time and night time observations of the sky for at least several hundred centuries and perhaps for thousands of centuries, perhaps going back to our earlier hominid predecessors. The phases of the Moon have influenced the period of our biological cycles as has the length of day. Perhaps bipeds gazed at the stars while resting on their backs at night and their search for meaning and utility in the patterns in the sky contributed to the evolution of their brains. Modern people have gained independence from many celestial cycles, but lost touch with nature. Many animals continue to be influenced by celestial phenomena and seasonal change and some even navigate while tracking celestial features.
Ancient people were known to chart the sky and modern star charts are a continuation of this tradition. Random patterns of stars are hard to memorize, but if you have imagination, you can visualize them grouped in memorable patterns resembling animals or imaginary deities. These patterns are usually called constellations.
The most important fact that people need to know about constellations is that they don’t really exist. They are optical illusions or artifacts resulting from our position in space. They disappear from other viewing angles and they have no physical significance.
I think the key idea is the following: a planetary map shows the locations of objects that lie on a surface, whereas a star chart shows objects that do not lie on a surface but are in the same general direction in space as viewed from Earth. Three stars that appear to be in line on a star chart are not necessarily in line in space. Brighter stars may be more luminous or simply closer to us. All of the stars in the sky appear to move as a group on a single celestial spherical surface. They are so far away that our depth perception does not betray the fact that they fill a vast three dimensional space. We know better, but ancient people could not have imagined this possibility.
Modern people find constellations entertaining and maybe challenging to find. Educators encourage sky watching as an introduction to more serious scientific interests and as an opportunity to get people away from their electronic toys and back to nature.
Constellations were not invented as evening entertainment before television, mp3, and internet. In fact, their recognizable and memorable patterns were vital to help people track the months (based on the phases of the Moon) in the calendar in order to know when to hunt and gather or prepare for seasonal weather threats, flooding, etc. and to assist agricultural activities. Pacific islanders especially used them for navigation on the open ocean (http://pvs.kcc.hawaii.edu/navigate/navigate.html). Big advantages for survival in primitive times! Even today, you may hear a rare report of a lost traveler whose survival in the wilderness was aided by his knowledge of the sky.
The best constellations divide the sky into a dozen equal intervals because the Moon goes through its highly recognizable phases approximately twelve times a year. The location of the constellations tells you what month it is and the phase of the Moon tells you the day of the month.
This division led to the development of the zodiac shown in the figure. The power possessed by those who understood the “movements” of the stars gave them power over people’s lives and probably led to astrology, the pseudo-science that incorrectly links people’s lives to distant stars whose gravity and luminosity have little effect on Earth. The apparent movements of stars in the non-existent celestial sphere that we perceive from Earth are actually due to the rotation of the Earth and its orbit around the Sun. The stars have nothing to do with their apparent movements.
What are the three most important factors to understand about constellations? location, location, and location.
1 – the Sun’s location in our corner of the galaxy
2 – the observer’s location on the surface of our planet and the orientation in space resulting from the daily rotation about the Earth’s axis
3 – the Earth’s location in its annual journey around the Sun
There are also three factors that affect the luminosity of a star - its mass, composition, and age. In addition to its actual properties, the apparent brightness of a star in the sky depends on a fourth factor – the star’s location or distance from Earth. The Sun, an ordinary star, dominates our neighborhood because of its location. The Sun has grown 40% brighter as its composition evolved by nucleosynthesis due to thermonuclear fusion in the core.
Your local location is also important – a state park can be an ideal location if the sky is clear and dark – which is an opportunity to talk about Rayleigh scattering and why the blue sky is a million times brighter than all the stars combined which makes it difficult to see stars during the day – one of many atmospheric effects that are minimized by telescopes on high mountains or in space.
To summarize, the theme of our discussion of constellations could be – Sky watching is like prime real estate - location is everything!
You have just barely begun to learn about the evolving stars and planets. When you finish your sky watching activities, you are ready to dig into some real science. What are you looking at, where did everything come from? When did the stars form? How do stars function and how do they evolve? Why has the Sun grown 40% brighter in the last 4.6 billion years? The only plausible natural explanation of what we see in the sky involves billions of years of time and billions of light years of space. To answer some of these questions, go to a good library, attend a public program, take a science course, or Google key phrases.
Edited quotes from wikipedia definitions of astronomy and astrology:
“Astronomy is the scientific study of celestial objects such as stars, planets, comets, and galaxies and phenomena that originate outside the Earth's atmosphere. It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.
“Astronomy is one of the oldest sciences. Astronomers of early civilizations performed methodical observations of the night sky. However, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena.
“Modern astronomy is not to be confused with astrology, the belief system that claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, most thinkers in both fields believe they are now distinct. The scientific community considers astrology to be a pseudoscience or superstition as numerous Western astrologers have failed empirical tests in controlled studies”.
This should be no surprise to anyone who understands that stars are distant giant balls of hydrogen and helium whose positions are based on the location of our planet in space. In plain English astrological claims have been debunked by scientific tests.
Here are three authoritative websites with selected quotes about constellations:
The stars in a constellation or asterism rarely have any astrophysical relationship to each other; they just happen to appear close together in the sky as viewed from Earth and typically lie many light-years apart in space. However, one exception to this is the Ursa Major moving group.
The grouping of stars into constellations is essentially arbitrary, and different cultures have had different constellations, although a few of the more obvious ones tend to recur frequently, e.g., Orion and Scorpius.
What's a constellation?
A constellation is a group of stars that appears to form a pattern or picture like Orion the Great Hunter, Leo the Lion, or Taurus the Bull. Constellations are easily recognizable patterns that help people orient themselves using the night sky. There are 88 “official” constellations.
Are the stars in a constellation near each other?
Not necessarily. Each constellation is a collection of stars that are distributed in space in three dimensions – the stars are all different distances from Earth. The stars in a constellation appear to be in the same plane because we are viewing them from very, very, far away. Stars vary greatly in size, distance from Earth, and temperature. Dimmer stars may be smaller, farther away, or cooler than brighter stars. By the same token, the brightest stars are not necessarily the closest. Of the stars in Cygnus, the swan, the faintest star is the closest and the brightest star is the farthest!
Most constellations are made up of stars that are actually nowhere near each other in space, but appear to us to be near each other because they are in the same line of sight. But the three bright stars in a horizontal row for the belt of Orion and the fainter stars in a vertical row beneath the belt are all related. They are giant young stars that have all formed from the same nebulae. The actual birthplace of the belt stars is in the vertical row below. If you have binoculars, look at the middle of these stars. You will see a fairly bright hazy cloud. This is the great Orion nebula. A small telescope will show a breathtaking sight. From these clouds, stars are "born." The ones in the belt are even younger than the Pleiades mentioned in an earlier article.
It is somewhat ironic, that just as in the myth, Orion had to pay a price for all that boasting, the giant stars that make up the constellation will also pay a price for all that size. The bigger a star is, the faster it burns, and the shorter it’s lifespan. Our Sun is medium size, and will burn steadily for another 5 billion years or so. But stars as big as Betelgeuse, measure their lives in terms of millions of years. And while our Sun will meet a quiet and slow end, for Betelgeuse and many of the Orion stars, the end will be explosive. These stars will eventually explode as what is called a supernova. Betelgeuse will undoubtedly be the first of the Orion stars to go, being already elderly in star terms.