1000 light year radius planet killer ASASSN-15lh


Lorenzo von Matterhorn
Jan 31, 2009
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As seen on a recent How the Universe Works "Stars That Kill" episode. As many as a billion planets may have been within the "kill zone" of ASASSN-15lh.



Occurred 3.8 billion light-years from Earth.

At its peak, the absolute magnitude of ASASSN-15lh in the AB magnitude system u band was -23.5. Its bolometric luminosity is twice that of the previous brightest type-I superluminous supernova, iPTF13ajg.[7] At its brightest, it was approximately 50 times more luminous than the whole Milky Way galaxy,[8] with an energy flux 570 billion times greater than the Sun.[9][10] The total energy radiated in the first 50 days exceeded 1.1×1045 joules.[2] According to Krzysztof Stanek of Ohio State University, one of the principal investigators at ASAS-SN, "If it was in our own galaxy, it would shine brighter than the full moon; there would be no night, and it would be easily seen during the day."[11]


A number of factors suggested that ASASSN-15lh was not a superluminous supernova. For instance, it occurred in a large, reddish galaxy, "a location where we should not find superluminous supernovae," said lead study author Giorgos Leloudas, an astrophysicist at the Weizmann Institute of Science in Rehovot, Israel, and the Dark Cosmology Centre in Denmark. This kind of galaxy lacks the young massive stars that previous research suggested give rise to superluminous supernovas, he said.

Moreover, after gazing at ASASSN-15lh for 10 months of follow-up observations, Leloudas and his colleagues suggest that the way the explosion evolved over time "is not consistent with an expanding ball of gas," Leloudas told Space.com. Instead of cooling down over time as supernovas are supposed to do, "this object, after 100 days, started warming up again, and stayed hot at a constant and very high temperature for a long time, and it continues to do so," he said.

Instead, the researchers suggest that ASASSN-15lh may have been caused by a star disintegrating under the gravitational pull of a black hole — a so-called tidal disruption event. The composition of the elements seen in the explosion are more consistent with a tidal disruption event of a low-mass star than with a superluminous supernova, Leloudas said.

In addition, using images from the Hubble Space Telescope, Leloudas and his colleagues found that ASASSN-15lh occurred at the center of its galaxy. Previous research found that supermassive black holes reside in the center of virtually all known large galaxies. The researchers suggest that the supermassive black hole that may have caused this explosive outburst ranged from 200 million to 3 billion times the sun's mass.

Prior work found that the supermassive black hole at the center of ASASSN-15lh's galaxy was so big that it would have swallowed a star whole instead of ripping it apart. Using new models, Leloudas and his colleagues have discovered that a rapidly whirling supermassive black hole could rend a star to pieces instead of gulping it down whole.

Only about 10 tidal disruption events have been discovered, the researchers said. This new finding may demonstrate that "tidal disruption events show a much larger diversity than what we knew before, and that they can reach extreme luminosities," Leloudas said.

Leloudas cautioned that they cannot say with complete certainty that ASASSN-15lh was a tidal disruption event, and that they cannot explain every detail of what they observed. Still, "this is how science advances — now, more clever people can look at the data we publish and come up with theories that can explain the missing pieces," he said.


One star more relevant to the well-being of future Earth. Its passage through the Oort cloud could result in a shower of comets into inner solar system:

Gliese 710 or HIP 89825 is an orange 0.6 solar mass star in the constellation Serpens Cauda.

The predicted date of the minimum distance pass from our sun is 1.281 million years from now, possibly approaching as close as 0.0676 parsecs, 0.221 light years or about 13,300 AU:[10] being about 20 times closer than the current distance of Proxima Centauri. It will then reach a similar brightness to the brightest planets, perhaps reaching an apparent visual magnitude of about -2.7 (brighter than Mars at opposition). Maximum total proper motion will peak around one arc minute per year, [11][12] whose apparent motion will be able to be noticed over a human lifespan.

Gliese 710 has the potential to perturb the hypothetical Oort cloud in the outer Solar System, exerting enough force to send showers of comets into the inner Solar System for millions of years, triggering visibility of about ten naked-eye comets per year,[12] and possibly causing an impact event. According to Filip Berski and Piotr Dybczynski, this event will be "the strongest disrupting encounter in the future and history of the solar system".[13] Earlier dynamic models indicated that the net increase in cratering rate due to the passage of Gliese 710 would be no more than 5%.[7] They had originally estimated that the closest approach would happen in 1,360,000 years when the star will approach within 0.337 ± 0.177 parsecs (1.100 ± 0.577 light years) of the Sun.[14] Gaia DR2 now finds the minimum perihelion distance is 0.0676±0.0157 parsecs or 13900±3200 AU about 1.281 million years from now.[10]

Bobylev in 2010 further suggested Gliese 710 has an 86% chance of passing through the Oort cloud, assuming the Oort cloud to be a spheroid around the Sun with semiminor and semimajor axes of 80,000 and 100,000 astronomical units.

Dr. Bailor-Jones also determined that of the 300,000 stars he observed, 97 of them would pass within 150 trillion km (93 trillion mi; 1 million AU) of our Solar System, while 16 would come within 60 trillion km. While this would be close enough to disturb the Oort Cloud, only one star would get particularly close. That star is Gliese 710, a K-type yellow dwarf located about 63 light years from Earth which is about half the size of our Sun.

Whereas the average relative velocity of stars is estimated to be around 100.000 km/h (62,000 mph) at their closest approach, Gliese 710 will will have a speed of 50,000 km/h (31,000 mph). As a result, the star will have plenty of time to exert its gravitational influence on the Oort Cloud, which could potentially send many, many comets towards Earth and the inner Solar System.

Over the past few decades, this star has been well-documented by astronomers, and they were already pretty certain that it would experience a close encounter with our Solar System in the future. However, previous calculations indicated that it would pass within 3.1 to 13.6 trillion km (1.9 to 8.45 trillion mi; 20,722 to 90,910 AU) from our star system – and with a 90% certainty. Thanks to this most recent study, these estimates have been refined to 1.5–3.2 trillion km, with 2.3 trillion km being the most likely.