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Physics news from November 25th, 2015
With ESO's Very Large Telescope (VLT), astronomers have captured the most detailed image of the hypergiant star VY Canis Majoris to date.
The observations show how unexpectedly large dust particles surrounding the star allow it to lose a tremendous amount of mass once it begins to die. This process, which has now finally been understood, is necessary for such huge stars to prepare them for their explosive supernovae doom. VY Canis Majoris is a stellar Goliath, a red hypergiant, one of the largest known stars in the Milky Way. It has 30 to 40 times the mass of the sun and is 300,000 times more luminous. In its current state, the star would span Jupiter's orbit as it expanded enormously in the latter stages of its life.
P. Scicluna et al.
Large dust grains in the wind of VY Canis Majoris
Astronomy & Astrophysics 2015
DOI: 10.1051 / 0004-6361 / 201527563
The new observations of this star were made with the SPHERE instrument at the VLT. The instrument's adaptive optics system corrects images much better than previous adaptive optics systems. This enables structures that are very close to the bright light source to be observed in detail . SPHERE clearly demonstrated how the bright light from VY Canis Majoris illuminates clouds of matter surrounding it.
With SPHERE's ZIMPOL mode, astronomers could not only peer deeper into the center of the cloud of gas and dust around the star, but also observe how the starlight was scattered and polarized by the surrounding matter. These measurements were critical to the difficult determination of the properties of the dust particles.
Thorough evaluations of the polarization measurement results showed that these dust grains with a diameter of 0.5 micrometers correspond to comparatively large particles, which may appear tiny, but grains of this size are about 50 times larger than the dust particles that were otherwise found in interstellar space.
As they expand, massive stars lose large amounts of matter - each year VY Canis Majoris emits 30 times the mass of Earth from its surface in the form of dust and gas. This cloud of matter is pushed further out before the star finally explodes and some of the dust is destroyed while the rest is hurled into interstellar space. This matter, together with heavier elements that were formed during the supernova explosion, can then be used by the next generation of stars to form planets.
How matter in the upper atmosphere is pushed into space before the star explodes has long remained a mystery - until now. The most likely explanation seemed to be the radiation pressure, i.e. the force exerted by starlight. Since this pressure is very weak, the large dust grains are essential for this process, otherwise the surface area is insufficient to produce a noticeable effect .
"Massive stars live a short life," explains Peter Scicluna from the Academia Sinica Institute for Astronomy and Astrophysics in Taiwan, the first author of the article. “When their last days come, they lose a lot of mass. In the past we have only been able to guess at how this actually happens. With the new SPHERE data, however, we have now found large specks of dust around the hypergiant. They are big enough to be pushed away by the star's strong radiation pressure, which explains the star's rapid mass loss. "
The presence of such large grains of dust that could be observed so close to the star means that the cloud can actually scatter the star's visible light and be pushed away from the star by the radiation pressure. Due to the size of the dust grains, some of them are likely to survive the radiation caused by the dramatic death of Canis Majoris as a supernova . The dust then mixes with the interstellar matter in the area, which encourages the formation of future generations of stars, and animates these stars to form planets.
This news report was created with material from the Information Service of Science (idw)
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