How many stars can be viewed in the known universe? The age-old question of how to estimate such a vast number has found a new bidder.
An astronomical team led by Simon Driver of the Australian National University Research School of Astronomy and Astrophysics had access to some of the world's most powerful telescopes, and then started counting. Their survey is part of the largest galaxy survey called the Two Degrees Field Galaxy Redshift Survey. This Anglo-Australian survey is trying to measure the distances to 250,000 nearby galaxies, notably using advanced equipment located at the Siding Springs Observatory in Australia's New South Wales state. Another telescope in the northern hemisphere was also used and is located on the Canary Islands, off the coast of Spain.
The astronomers' task however depended on more than just accurate telescopes and the best estimate yet for the number of visible galaxies. To get an estimate for the stars in the known universe also requires some assumptions about the geometry or shape of, for lack of a better term, 'everything'. The team is quick to point out that the actual size of the universe is presently not known, if it ever can be, but that a star count can provide a gauge on progress in the building of modern telescopes. At this week's General Assembly of the International Astronomical Union in Sydney, the researchers put forward their big number: 70 sextillion, or 70,000,000,000,000,000,000,000 [seven followed by twenty-two zeros]. Previous estimates were approximately twenty-five percent smaller. "This is not the total number of stars in the universe, but it's the number within the range of our telescopes," said Driver.
|The Milky Way. Credit: Akira Fujii
To arrive at such a vast number, the team surveyed one strip of sky containing some 10,000 galaxies, or about four percent as a sample. To estimate the number of stars in those ten thousand galaxies was not a rote count but based instead on the galaxies' average brightness. In such a statistical survey, brightness is a measure of how many stars the galaxy may host. In their statistical report, that sample was then multiplied by the number of similar sized strips needed to cover the entire sky, and then multiplied again out to the edge of the visible universe. A pedestrian measure of this edge-to-edge distance is sometimes given as 234 sextillion miles.
How the geometry is laid out across the sky, in turn, depends on the enclosing space defined by both the power of the telescopes and key constants for the size and age of the universe. The most well-known such constant is called the Hubble constant, which is named after the same astronomer, Edwin Hubble, whose contributions to science are also commemorated in the large NASA orbiting telescope. One fascinating photo featured in the observing schedule for the Hubble Telescope is called 'the Deep Field', since astronomers attempt to capture the oldest light, or farthest reach, of what galaxies can be resolved from orbit.
One important reason this estimate is constrained to visible stars however is not just a measure of how powerful a set of telescopes can be, but also depends on the age of the universe. The universe as it is currently described is not considered old enough (around 14 billion years) for light from its farthest reaches to be visible from Earth. By most estimates, our galaxy is about 100,000 light years across, and the vast empty space between galaxies stretches the limits of imagination when considering 250,000 copies of the Milky Way.
When Dr. Driver was asked by the Australian newspaper, The Age, if he believed there was other intelligent life out there, he said: "Seventy thousand million million million is a big number... it's inevitable."
Such large numbers--seven times ten raised to twenty-twenty powers--are beyond what is meaningful without an analogy to compare with it. The number of stars in the visible universe, for instance, is now comparable to some terrestrial references borrowed from a combination of science and poetry:
- ten times more than the number of grains of sand on Earth
- eleven times the number of cups of water in all the Earth's oceans
- ten thousand times the number of wheat kernels that have ever been produced on Earth
- one hundred million times more than the number of ants in all the world
- one hundred million times the dollar value of all the market-priced assets in the world
- ten billion times the number of cells in a human being
- one hundred billion times the number of letters in the 14 million books in the Library of Congress
|Life is possible on Earth because it lies in what is called a habitable zone.
In the realm of astrobiology, it may be said that most meaningful terrestrial analogies to the number of stars in the known universe are biological: only a fertile biosphere can yield such large numbers. One may ask how many living things the Earth itself can accomodate in its volume. If one cubic inch can hold ten billion animal or plant cells, and if one stacked these cells across both the land and oceans to a thickness of fifteen feet, the planet would be a vast teeming mass of biology--literally, life as far as the eye could see. The thickness of fifteen feet, while extreme overpopulation on the land, is likely an underestimate given the depth of the more three-dimensional ocean biosphere or the realms of winged species.
In this way, the ceiling on the carrying capacity of Earth for cellular life is vast, since about ten million times the number of plant or animal cells could pack the planet than the number of stars in the visible universe. Compared to 70 sextillion, the cellular capacity terrestrially is estimated to be what can be called one undecillion, or ten raised to the power of 30.
Given that the earth's biodiversity currently has around 28,000 species with a backbone, the Earth can be considered a rich source for cellular life, and a relatively scarce source for advanced life--a conclusion that is likely to temper the future searches for clues to life elsewhere. For this reason, the chances for finding microbial life off the home planet are regarded as a more likely initial research goal. But as illustrated by sampling only four percent of the galactic sky to count stars, one can also consider that the Earth as a sample size of one for life as we know it, makes possible only informed guesses about a range of options. For instance, as one statistical postulate, the wider the base from which the sample is drawn. i.e., the larger the number of units in the pool of experience, the higher the expectation is that these statistics will give a reliable index to the future. This is known as the law of large numbers.
Or as Galileo wrote in 1638: "Let us remember that we are dealing with infinities and indivisibles, both of which transcend our finite understanding, the former on account of their magnitude, the latter because of their smallness. In spite of this, men cannot refrain from discussing them, even though it must be done in a roundabout way".
Related Web Pages
The Universe in Forty Leaps
Powers of Ten
How Old is the Universe?: WMAP
Deep Field South: 1998 Release
Galaxy Evolution Explorer: Goddard Spaceflight Center