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How Does Salmonella Get Inside Chicken Eggs?

Salmonella enteritidis is a bacterium that causes flu-like symptoms in humans. It usually enters the human body through undercooked food that we eat, such as chicken eggs. Symptoms develop 12-24 hours after the infected food has been eaten and last anywhere from 4 to 7 days. The bacterium actually lives inside infected chicken eggs and how it got ...

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Biology

Why Can't We Really Clone Dinosaurs?

You might think, if you saw the movie Jurassic Park, or read the book, that a real live cloned dinosaur would be on the TV evening news any day now. Not very likely! In the fictional version, the ... Continue reading

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Astronomy

Catch A Shooting Star

A meteor, sometimes called a 'shooting star,' can be the brightest object in the night sky, yet meteoroids are the smallest bodies in the solar system that can be observed by eye. Wandering through ... Continue reading

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Mathematics

Prime Numbers

A prime number is a number that is divisible only by one and by itself. Factors are numbers that can be divided into a number with no remainder. The factors of 18 are the numbers 1, 2, 3, 6, 9, and ... Continue reading

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Engineering

GPS (Global Positioning System)

The GPS, or Global Positioning System, is the high-tech application of one of the most fundamental principles of geometry. Surveyors routinely use geometry and triangulation to map and lay out areas ... Continue reading

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How Far Are The Seven Sisters?

HowFarAreTheSevenSistersThe Pleiades cluster, named by the ancient Greeks, is easily seen as a small grouping of stars lying near the shoulder of Taurus, the Bull, in the winter sky. Although it might be expected that the distance to this well-studied cluster would be well established, there has been an ongoing controversy among astronomers about its distance for the past seven years. The mystery began in 1997, when the European Space Agency's satellite Hipparcos measured the distance to the Pleiades and found it is 10 percent closer to Earth than traditional estimates, which were based on comparing the Pleiades to nearby stars. If the Hipparcos measurements were correct, then the stars in the Pleiades are peculiar because they are fainter than Sun-like stars would be at that distance. This finding, if substantiated, would challenge our basic understanding of the structure of stars.

But measurements made by the Hubble telescope's Fine Guidance Sensors show that the distance to the Pleiades is about 440 light-years from Earth, essentially the same as past distance estimates and differing from the Hipparcos results by more than 40 light-years. The new results agree with recent measurements made by astronomers at the California Institute of Technology and NASA's Jet Propulsion Laboratory, both in Pasadena, Calif. Those astronomers used interferometer measurements from Mt. Wilson and Palomar observatories in California, reporting that the star cluster is between 434 and 446 light-years from Earth.

The discrepancy in the distance to the Pleiades is more than an arcane argument over details. Astronomers have only one direct means for gauging distances to stars, called the parallax method. With current telescopes, this method gives accurate results only for distances up to about 500 light-years. Distances beyond that limit must be determined by indirect methods, based on comparing the brightness of distant stars with those of nearer ones of the same type, and making the assumption that both objects have the same intrinsic, or true, brightness. Astronomers can thus build up a distance ladder, based on ever more-distant objects, ultimately leading to the use of supernovae as 'standard candles' for the most distant reaches of the universe.