Variable stars change their brightness over time, giving them their name. Now, a new type of variable star has been found 7,000 light years from Earth. Slightly brighter and hotter than our Sun, they were discovered in NGC 3766, an open star cluster, or loosely-packed grouping of stars, in the constellation Centaurus (The Centaur).
Swiss researchers spent seven years studying 3,000 stars in the cluster, finding 36 members which varied their brightness in cycles varying between two and 20 hours long. These periods of variance in luminosity were not pronounced, however - they amounted to a change of just one part in a thousand of the star's brightness. That's about how much our own Sun varies over an 11-year cycle of sunspot activity. Not only is this cycle short, but current theories of stellar formation can't explain their behavior.
"The very existence of this new class of variable stars is a challenge to astrophysicists. Current theoretical models predict that their light is not supposed to vary periodically at all, so our current efforts are focused on finding out more about the behavior of this strange new type of star," Sophie Saesen, from the Geneva Observatory, said.
If you plot stars on a graph with luminosity on the vertical axis and surface temperature on the horizontal axis, you produce what is known as a Hertzsprung-Russell (H-R) diagram. As stars live out their lives, changes in their brightness and temperature make stars of different ages appear in different areas on the diagram. Astronomers recognize four areas on this chart where variable stars are found. This new type of variable, which so far does not have a name, would represent a fifth region of variability.
Each of the new-found class of stars discovered so far has another quality in common with each other, apart from their variability. They all rotate very quickly - over half the critical speed at which they would tear themselves apart. This may have something to do with the cause of their light cycle.
"In those conditions, the fast spin will have an important impact on their internal properties, but we are not able yet to adequately model their light variations. We hope our discovery will encourage specialists to address the issue in the hope of understanding the origin of these mysterious variations," Nami Mowlavi, research team leader, said.
The new image was taken by one of the smaller telescopes that make up the European Southern Observatory in Chile. This collection of observatories sits almost 7,900 feet above sea level. The largest of these observatories is the four-telescope network of the Very Large Telescope array, each 315 inches across. The telescope used by Mowlavi and her team to observe NGC 3766 had a modest diameter of just 47 inches - but the researchers needed an instrument where they could observe for long periods. NGC 3766 is thought to be about 20 million years old.
The journal Astronomy and Astrophysics published the results of the finding on June 12.
