Maggie McKee, New Scientist (translation: Dikla Oren)
Direct link to this page:
https://www.hayadan.org.il/distantmeasers.html
A new measurement of the distance to the "Seven Sisters" star cluster casts doubt on the reliability of one of the most reliable databases in astronomy. The measurement contributes to the settlement of a seven-year dispute regarding the method of determining cosmic distances.
This is a matter of principle, says Valerie Makrob of the California Institute of Technology: "All our knowledge regarding distances to astronomical objects is based on the distances to the nearest open clusters." Open clusters such as the "Seven Sisters" are loose clusters of stars.
Measuring astronomical distances is not an easy task at all. For example, distant bright objects may appear as nearby but dim objects. Therefore, astronomers have put together a detailed method, based on accurate measurements of the distances to several nearby objects by several independent methods. These objects are the open clusters. These measurements are then used to define a "cosmic ruler" for measuring greater distances.
Traditional measurements place the "Seven Sisters" at a distance of 430 light years with an accuracy of about a dozen light years. These measurements were produced by comparing curves describing the color and brightness of some of the cluster's stars with similar curves of a closer cluster, the Hades Cluster, whose distance is known from simpler geometric measurements.
However, things got complicated in 1997. In the spring of 1997, a huge database of distances to more than 10,000 stars was published. The database was based on information from the "Hypparcos" satellite of the European Space Agency. He calculated the distance using the parallax method, that is, looking at some target from two separate points along our orbit around the sun and measuring the angle between the two measurements to an accuracy of one thousandth of an arc second.
Astronomers immediately realized that the distances to about a dozen open clusters differed from the known distances sometimes by about 20%. The distance determined by Hypercus to the "Seven Sisters" was about 385 light years, ten percent less than the distance that was known until then.
"The issue has been controversial from the beginning, with Hypercus's claim that systematic errors could not possibly be that large," says Bohdan Paczynski of Princeton University, New Jersey. "Others argued that if Hypercus was right, there would be a need for a significant correction in our understanding of the stars and star clusters. The best way to settle the dispute is to measure the distance with high precision but with the help of a different method, which is exactly what this study did."
The lengthy study was conducted by Cassiope Pan, an astronomer at NASA's Jet Propulsion Laboratory in California, and two of his colleagues. They used two linked telescopes, called interferometers, to track a pair of bright stars in the "Seven Sisters" for a decade.
They determined how often the stars revolve around each other and how far apart they appear in the sky. Their brightness provided an estimate of a valve, and the researchers processed all this information into a simple equation, from which they arrived at the distance to the star: about 440 light years.
"This article does not completely resolve the controversy but provides solid arguments against Hypercus," Paczinski says. He also says that another team may provide conclusive evidence by the end of 2004, evidence that would confirm Pan's results.
The study confirms that traditional methods for estimating distances are good, Pan told New Scientist: "And Hypercos is also good, but only up to a thousandth of an arc second."
Paczynski agrees with him: "In this case, they tried to push the device beyond its capabilities." This is because all the clusters, whose distance was disputed, were too far away for the satellite to accurately measure the distance with a single star. An average was made, therefore, of the distances to several dozen stars in each cluster. Usually this method will work, Paczynski says, but some unknown errors have appeared.
"I don't think this is an error in the device," says Makarov, who was involved in compiling the database from Hypercos. He believes that the error stems from the equations used to reduce the information, and he published a possible solution to the problem.
The solution may turn out to be of great importance, as it is likely that Hypercos will remain the best source of information available regarding distances, until next-generation astronomy missions such as the European Space Agency's "Gaia" and NASA's interferometry mission are launched in the next decade. They will be 1000 times more accurate than Hypercos.
Long distance measurement
A new measurement method calls into question the astronomical distances we know
Dr. Noah Brosh
The basic and direct method for measuring distances in space is up to the test. The criticism is mainly directed against the fact that the method indicates distances that are 11% greater than those previously measured. If so, all the astronomical distances known to us so far must change.
The direct measurement method is called "parallax". according to which the relative position of a nearby celestial body will be measured with a time difference of about half a year compared to the more distant celestial bodies. If the measurement is accurate enough, the nearby body appears to be moving against the background of the more distant celestial bodies, because the Earth has completed half of its revolution around the Sun during this period of time.
With the Earth our perspective also changes. A period of time of six months between the two measurements translates into a gap of 300 million km. The small displacement can be measured if the object of the study is a nearby body that is hundreds of light years away from us. The closer the star is, the more accurate the measurement of the distance to it will be.
In contrast, another measurement method calculates a relative distance to the target star by comparing the measured brightness of the source body to the brightness of a similar star, whose distance from us is measured directly. The more distant star is measured when it is paler than the similar one. Based on this, the distance to it is calculated.
It was reported in the science weekly Nature that the criticism stems from a distance test made towards the star cluster "Kima" (Pleiades). The new measurements revealed that the distance to this cluster is about 440 light-years, compared to previous measurements made by satellite and showed that the distance is about 385 light-years. The measurements from space, which are considered extremely accurate, were used to calibrate the cosmic distance scale, which, as mentioned, measures relative distance. The new measurements examined the distance between a pair of stars orbiting each other in the Kima cluster.
Link to the original article in New Scientist
https://www.hayadan.org.il/BuildaGate4/general2/data_card.php?Cat=~~~748844266~~~60&SiteName=hayadan
