Date: January 17, 2010

Title: What is the Universe?


Podcaster: Stuart Clark

Organization: Dr. Clark’s website:

Description: Just what is the Universe? What does it hold and how do astronomers go about studying it?

Bio: Dr. Stuart Clark is an award-winning astronomy author and journalist. His books include The Sun Kings, and the highly illustrated Deep Space, and Galaxy. His next book is Big Questions: Universe, from which this podcast is adapted. Stuart is a Fellow of the Royal Astronomical Society, a Visiting Fellow of the University of Hertfordshire, UK, and senior editor for space science at the European Space Agency. He is also a frequent contributor to newspapers, magazines, radio and television programmes. His website is and his Twitter account is @DrStuClark.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Rick Stiles in appreciation of Dr. Pamela Gay and Fraser Cain for providing the fantastic resource “Astronomy Cast”, and their dedication to popularizing astronomy and science. Thank You!



Hello I’m Dr Stuart Clark, astronomy author and journalist. Today I want to explore the question: what is the universe?

The ‘Universe’ is what we call everything: every planet, every star, and every distant galaxy. It is vast beyond human comprehension, yet that has not stopped us trying to make sense of it all.

The Greek word kosmos means ‘orderly arrangement’, and from it we derive today’s words cosmos and cosmology, the branch of astronomy that goes about studying the way the Universe behaves, how it began, and how it will all eventually end.

As a true science, cosmology only really started in 1916, when Albert Einstein published his General Theory Of Relativity. Before this, astronomers lacked the mathematical framework in which to describe the behaviour of the whole Universe. Nevertheless. the urge to understand the Universe developed early. Babylonian stone tablets dating back to around 3500 BC have been found that record the variable length of day throughout the year and the Chinese have written records of eclipses since about 2000 BC.

These earliest astronomical observations were almost certainly used to set the calendar. The phases of the Moon defined the passing of a month, and the passage of the Sun through the sky defined both the length of a day and of a year.

Early civilizations told stories inspired by the patterns of stars in the night sky. They imagined lines joining stars to form pictures of familiar or mythical characters. The first century AD Greek astronomer Claudius Ptolemy compiled a list of 48 constellations, but since not all the sky could be seen from Greece, the regions around the South Pole remained uncharted until intrepid astronomers ventured far from Europe during the 16th and 17th centuries.

In 1922, things were finally put on a firm footing when the International Astronomical Union ratified 88 constellations with defined boundaries. The constellations were not the only aspects of Greek astronomy to pass into modern usage.

More than two millennia ago, the astronomer Hipparchus meticulously compiled a catalogue of 850 stars, recording each star’s position and ranking its brightness. He had no equipment to measure the brightness; he simply made estimates by eye. The brightest stars he called ‘first magnitude’, the faintest ‘sixth magnitude’, and the rest he ranked in categories between. Amazingly, astronomers still use this system today, although modern measuring devices have extended Hipparchus’s original six classifications.

But we must not forget that the brightness of a luminous object is affected by its distance from the observer, as well as by the actual amount of light it gives out. Thus a nearby dim star may well appear brighter than a highly luminous star far away. To acknowledge this, magnitudes measured without any correction for distance are known as ‘apparent’ magnitudes. The ‘absolute’ magnitude is the brightness value that has been corrected for distance. Because it is so close, the Sun has an enormous apparent magnitude of –26.7, the brightest object in the sky. However, when corrected for its proximity, its absolute magnitude is just 4.8. In other words, our Sun, for all its glory and importance in driving life on Earth, is nothing but a thoroughly average star.

The Ancient Greek astronomers, called five particular stars, plenetes, meaning ‘wanderers’, because of their individual movement across the sky from one night to the next. These are our nearest five planets, Mercury, Venus, Mars, Jupiter and Saturn. Mercury and Venus follow orbits inside Earth’s and so always appear to stay close to the Sun and are only ever seen in the twilight sky. Mars, Jupiter and Saturn orbit the Sun further out than the Earth and can be clearly seen making their slow paths through the night sky. In the 17th century Johannes Kepler distilled the movement of the planets into three mathematical laws. This was a watershed in human history, because it proved that the Universe could not only be measured, but also understood by mortal brains. Before Kepler, even educated men were likely to assume the movement of the heavens was guided by God, and therefore unknowable to any mere human.

At the same time as Kepler, in Germany, Galileo Galilei in Italy, was making discoveries that sparked our fascination with the wider Universe. In 1609, he raised his telescope and pointed it at the misty band of light that stretches across the night sky, known as the Milky Way. Through his basic telescope, tiny by today’s standards, Galileo could see that the Milky Way was composed of a multitude of faint stars. His work started a centuries-long fascination, with each generation of astronomers developing larger and larger telescopes to see fainter and fainter objects. Today, the largest optical telescopes are fully 10 metres across, some 500 times larger than Galileo’s original telescope.

Today we know that the Sun is one star in a giant collection known as the Galaxy, which contains at least 100 billion stars arranged in a spiral pattern in a flat disc and orbiting a bulbous hub of even more stars. From our position in one of the spiral arms, we see the disc as a haze of myriad stars − the Milky Way. The centre of the Galaxy is towards the south, in the constellation of Sagittarius. If you could observe from a dark site in the southern hemisphere, you might be able to see the Milky Way widen into the vast star clouds of the Galaxy’s central bulge.

In the Sun’s immediate neighbourhood there are 33 stars. The majority are smaller, dimmer stars than our own Sun. Known as red dwarfs, these celestial minnows make up the largest population of stars in the Universe. Just two stars in our neighbourhood are similar in size to the Sun, and only one is greater: Procyon in the constellation Canis Minor.

Yet, as vast as it seems, the Galaxy is not the whole Universe; in the grand scheme of things, it is little more than a small island in an expansive ocean. And there are innumerable other islands. Galaxies come in three basic types: spirals, barred-spirals and ellipticals. The spiral galaxies are particularly beautiful, with their sweeping arms of bright young stars surrounding a central bulge of older stars. The barred-spiral galaxies, of which our own Milky Way is an example, are similar but with a pronounced elongation that connects the central bulge to the spiral arms. The ellipticals are totally different in appearance; they can be much larger than the spiral or barred-spiral galaxies and anything from cigar-shaped to perfectly spherical. There are some oddballs too, known as the irregular galaxies, some of which may once have been spirals, others are just truly disordered systems. Supercomputer estimates currently suggest that there may be 500 billion (500,000 million) galaxies spread throughout the Universe.

To investigate the origins of these galaxies and thus uncover the evolution of the Universe, cosmologists exploit the fact that light does not travel instantaneously across space. As fast as the speed of light is by our everyday standards − and light could circle Earth’s equator seven times in a second − it still takes many years for light to traverse the vast tracts of space between celestial objects. If a star is 100 light years away, its light takes 100 years to cross space to reach us and, as a consequence of this, we see it not as it is today but as it looked 100 years ago when the light began its journey. Cosmologists call this the ‘look-back time’ and it is invaluable to their work.

The further an astronomer looks into space, the more ancient the celestial objects appear. It’s like an archaeologist digging down into deeper and more ancient strata of rocks. With current telescope technology we can see celestial objects as they looked billions of years ago, and that allows us to trace the way the universe has developed into the grandeur we see around us today.

Cosmology is the oldest science; it may even be the noblest one. Welcome to the world of the cosmologist.

End of podcast:

365 Days of Astronomy
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