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Astronomers discovered a star that went supernova in 1954...then again in 2014.


Lead author: "This supernova breaks everything we thought we knew about how they work."


In this artist’s concept of a supernova, an expanding shell of dust and gas is blown outward, away from the star’s dense, white-dwarf core. Most supernovae explode just once, which ends their lives for good. However, iPTF14hls appears to have gone supernova twice in just 60 years.


To quote the first line of a letter published today in the journal Nature, “Every supernova so far observed has been considered to be the terminal explosion of a star.” In other words, when a massive star blows itself up, it should remain dead. This is something astronomers have witnessed thousands of times before with absolutely no exceptions.


That is, until now.


For the first time, astronomers have discovered a star that has gone supernova more than once. This so-called “zombie star” — which exploded at least twice in the last 60 years alone — has baffled scientists by challenging many of the existing theories about how massive stars end their lives.


“This supernova breaks everything we thought we knew about how they work,” said the study’s lead author Iair Arcavi, a NASA Einstein Postdoctoral Fellow at the University of California-Santa Barbara and Las Cumbres Observatory, in a press release. “It’s the biggest puzzle I’ve encountered in almost a decade of studying stellar explosions.”

这项研究的通讯作者Iair Arcavi ——美国加州大学圣塔芭芭拉分校和拉斯昆布瑞天文台的美国国家航空和宇宙航行局爱因斯坦博士后研究员在一次新闻发布会上说:“这颗超新星打破了我们自认为掌握的关于它们如何运作的一切知识”。

The undying star, named iPTF14hls, was first discovered in September 2014 by the Palomar Transient Factory (PTF), a fully automated, wide-field survey designed to spot cosmic objects that vary in brightness over time — such as variable stars, transient objects, and, of course, supernovae.


Over the course of two years, iPTF14hls grew and dropped in brightness at least five times. Most supernovae remain bright for around 100 days before fading for good.


When the international team of astronomers first spotted the explosion, a spectral analysis indicated it was just a run-of-the-mill, Type II-P supernova whose brightness would likely fade after about 100 days. Supernova iPTF14hls, on the other hand, had a different plan.


Although Supernova iPTF14hls initially faded after its 2014 explosion, within a few months it began to mysteriously grow brighter again. Over the course of three years, iPTF14hls fluctuated between bright and dim at least five separate times.


“This [was] one of those head-scratcher type of events,” said co-author Peter Nugent, a senior scientist at Lawrence Berkeley National Laboratory. “At first we thought it was completely normal and boring. Then it just kept staying bright, and not changing, for month after month.”

劳伦斯伯克利国家实验室的一名资深科学家,也是这篇文章的合作作者Peter Nugent 说:“这是一种令人费解的现象,一开始我们认为这完全是稀疏平常的。但是它持续保持着亮度,而且没有变化,月复一月都是如此。”

When the astronomers realized iPTF14hls was not an average supernova, they decided to go back and search through archival data. The researchers were flabbergasted when they found that in 1954, another explosion was recorded in the exact same location as iPTF14hls. Somehow, the star survived its first explosion, waited 60 years, and then exploded again.


An image taken by the Palomar Observatory Sky Survey reveals a possible explosion in 1954 at the exact location of iPTF14hls (left). It is not seen in the later image taken in 1993 (right).


Although researchers are still uncertain what caused iPTF14hls to go supernova twice, one theory is that the “zombie star” is actually a “pulsation pair instability supernova.”


“According to this theory, it is possible that this was the result of a star so massive and hot that it generated antimatter in its core,” said co-author Daniel Kasen, a professor of physics and astronomy at the University of California-Berkeley. “That would cause the star to go violently unstable, and undergo repeated bright eruptions over periods of years.” The authors of the study calculated that before the first explosion, the star was at least 50 times as massive as the Sun (and likely larger).

加州大学伯克利分校的物理学和天文学教授,同时也是合作作者之一的Daniel Kasen 说:“按照这一理论,这种现象可能是一颗质量非常巨大、温度非常高的恒星在其核心产生反物质所导致的结果”。这项研究的作者们计算得出,在第一次爆炸之前,这颗恒星的质量至少有太阳的五十倍那么大(可能更大)。

“These explosions were only expected to be seen in the early universe and should be extinct today,” said co-author Andy Howell, leader of the LCO supernova group. “This is like finding a dinosaur still alive today. If you found one, you would question whether it truly was a dinosaur.”

LCO超新星团队的领导者,合作作者之一Andy Howell说:“这种类型的爆炸被认为只会出现在早期的宇宙当中,现在应该已经绝迹。这就像是在今天发现了活着的恐龙。如果你真的发现了一头恐龙,你将会质疑它是否真的是恐龙。”

Using LCO’s global telescope network, which is specifically designed for sustained observations, astronomers will continue to monitor iPTF14hls for changes in brightness over time.


“We could not have kept tabs on iPTF14hls for this long and collected data that challenges all existing supernova theories if it weren’t for the global telescope network,” said Arcavi. “I can’t wait to see what we’ll find by continuing to look at the sky in the new ways that such a setup allows.”

Arcavi 说:“如果没有全球望远镜网络,我们无法在这么长的时间里持续关注iPTF14hls并采集能够挑战所有现存超新星理论的数据。我等不及想看到通过这一步所允许的天空持续观测,我们能够发现什么。”