Archive for September, 2014

New distance estimate to the Pleiades

There has been a debate about the distance to the Pleiades cluster that has been going for decades. Although it appears to have been resolved, well this is the claim of a team of radio astronomers in the US, who concluded that the cluster is approximately as far away as originally estimated. This contradicts analyses made form data from the Hipparcos, which suggested that the cluster is 13 parsecs closer than astronomical models predict.

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The Pleiades is the star cluster most obvious to the naked eye in the night sky, and has been known since antiquity. In modern astronomy. It often appears low in the horizon in the Southern hemisphere. The distance to the Pleiades is used to calibrate the cosmic-distance ladder, allowing astronomers to calculate distances to other star clusters and galaxies that are further away. And so, it is important to know the distance to Pleiades precisely, and multiple calculations of it have been made using various methods by various researchers over the decades.

Currently the generally accepted distance was about 134 parsecs (about 437 light-years). Yet in 1999, Floor van Leeuwen of the Institute of Astronomy in Cambridge, UK, used data from the European Space Agency’s Hipparcos satellite to produce what was the most precise calculation to date. This estimate was obtained using trigonometric parallax. Van Leeuven arrived at a distance of about 120 parsecs, and in 2009 he refined his analysis but reached a similar conclusion.

In new research by Carl Melis of the University of California, San Diego and colleagues at several other US institutions, they did their own trigonometric-parallax measurement of five selected stars in the Pleiades cluster using very long baseline radio interferometry. In this technique, measurements are made by linked radio antennas spread across the world, giving the total resolution of a telescope the size of the Earth. The researchers found that the distances of all five stars were in broad agreement with the original figure, with the lowest value being 134.8 parsecs and the highest being 138.4.

However in 2013 ESA launched Gaia, a successor to Hipparcos with much higher specifications, such as higher-sensitivity cameras, that will measure the parallaxes of thousands of stars in the Pleiades cluster. The design principles are conceptually similar, which leads Melis and colleagues to suggest that the unidentified error they believe distorted the Hipparcos measurements of the Pleiades could also affect Gaia. Nevertheless, Melis suspects that “the Gaia measurement is not going to be the same as the Hipparcos measurement. Hopefully then the Hipparcos community is going to have to face the fact that Hipparcos did not produce the correct result.”

See the paper at Science: http://www.sciencemag.org/content/345/6200/1029

The Milky Way’s new home: Laniakea

Analysis of galaxies shows local supercluster to be 100 times larger than previously thought.

 

The supercluster of galaxies that includes the Milky Way has been found to be 100 times bigger in volume and mass than previously thought. The enormous region near the Milky Way has been mapped with newer precion and found that the supercluster that the Milky Way resides in is so much bigger than anyone believe and they the named it Laniakea, which is Hawaiian for ‘immeasurable heaven’.

The new study, which was published in Nature, describes a novel way to define where one supercluster ends and another begins. A team led by Brent Tully, an astronomer at the University of Hawaii in Honolulu, charted the motions of galaxies to infer the gravitational landscape of the local Universe, and redraw its map.

See Nature for the full story:

http://www.nature.com/news/earth-s-new-address-solar-system-milky-way-laniakea-1.15819

Intermediate-mass black hole found in Messier 82 (M82).

Astronomers in the US have used X-rays to pin down the mass of a black hole in the nearby galaxy M82, has been thought to be an intermediate-mass black hole (100 to 10,000 solar masses). This means it is of the rarest, mid-sized black-hole type.

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Image of the starburst galaxy, Messier 82 (M82). (Courtesy: NASA, ESA and The Hubble Heritage Team (STScI/AURA))

 

Mass is a fundamental property of any black hole, which has so much gravity that nothing can escape its grip. Not even light can escape, when it strays close enough. Black holes usually come in two main types: stellar-mass black holes that are roughly 10 times as massive as the Sun, such as Cygnus X-1, and supermassive black holes, which are typically millions or billions of times as massive as the Sun and inhabit the centres of large galaxies. The largest supermassive black hole is believed to reside in  the quasar APM 08279+5255, with a mass of 21 billion solar masses.

On the other hand, intermediate-mass black holes are much less studied compared with stellar and supermassive black holes, and that is because intermediate-mass black holes are rare.

With this confirmed candidate: the black holeM82 X-1. Previous mass estimates for this object ranged from just 20 solar masses to more than 1000, so astronomers did not know whether it was an ordinary stellar-mass black hole or a rare intermediate-mass black hole. Indeed, it was already suggested in 2006 that the black hole was an intermediate mass one, but this was yet to be confirmed. The black hole lies in M82, a “starburst” galaxy only 12 million light-years away, which spawns lots of new stars. M82 orbits M81, a giant spiral whose gravity stirs it up and triggers the starburst.

Stars are often caught by a black hole’s immense gravitational force and lose material to the objects. Before plunging into the black hole though, the trapped stellar material gets so hot it emits X-rays. The team analysed six years of X-ray observations and discovered two oscillations every 0.2 and 0.3 seconds. These periods indicate how long the hottest material takes to orbit the black hole, and far exceed the periods of similar oscillations seen around stellar-mass black holes. The longer period suggests a much greater mass, because the more massive a black hole, the larger it is and the longer material takes to revolve around it. Using two different methods, the researchers conclude that M82’s black hole is 428±105 and 415±63 times as massive as the Sun.

Supermassive black holes grow to millions or billions of times the Sun’s mass because they occupy galactic centres that attract stars and gas. But M82 X-1 is not at the centre of its galaxy. Astronomers have suggested that one way in which this black hole could have grown to such an abnormal size is thanks to a cluster of stars near its location, which could have fed the object, before a massive star’s approach ejected the black hole from the cluster.

The research is published in Nature http://www.nature.com/nature/journal/v513/n7516/full/nature13710.html