STARS JUST GOT BIGGER: A 300 Solar Mass Star Uncovered
July 21, 2010. A team of astronomers led by Paul Crowther, Professor of Astrophysics at the University of Sheffield, has used ESO’s Very Large Telescope (VLT), as well as archival data from the NASA/ESA Hubble Space Telescope, to study two young clusters of stars, NGC 3603 and RMC 136a in detail. NGC 3603 is a cosmic factory where stars form frantically from the nebula’s extended clouds of gas and dust, located 22,000 light-years away from the Sun. RMC 136a (more often known as R136) is another cluster of young, massive and hot stars, which is located inside the Tarantula Nebula, in one of our neighbouring galaxies, the Large Magellanic Cloud, 165,000 light-years away.
The team found several stars with surface temperatures over 40,000 degrees, more than seven times hotter than our Sun, and a few tens of times larger and several million times brighter. Comparisons with models imply that several of these stars were born with masses in excess of 150 solar masses. The star R136a1, found in the R136 cluster, is the most massive star ever found, with a current mass of about 265 solar masses and with a birthweight of as much as 320 times that of the Sun.
In NGC 3603, the astronomers could also directly measure the masses of two stars that belong to a double star system, as a validation of the models used. The stars A1, B and C in this cluster have estimated masses at birth above or close to 150 solar masses.
Very massive stars produce very powerful outflows. “Unlike humans, these stars are born heavy and lose weight as they age,” says Paul Crowther. “Being a little over a million years old, the most extreme star R136a1 is already ‘middle-aged’ and has undergone an intense weight loss programme, shedding a fifth of its initial mass over that time, or more than fifty solar masses.”
If R136a1 replaced the Sun in our Solar System, it would outshine the Sun by as much as the Sun currently outshines the full Moon. “Its high mass would reduce the length of the Earth's year to three weeks, and it would bathe the Earth in incredibly intense ultraviolet radiation, rendering life on our planet impossible,” says Raphael Hirschi from Keele University, who belongs to the team.
These super heavyweight stars are extremely rare, forming solely within the densest star clusters. Distinguishing the individual stars — which has now been achieved for the first time — requires the exquisite resolving power of the VLT’s infrared instruments.
The team also estimated the maximum possible mass for the stars within these clusters and the relative number of the most massive ones. “The smallest stars are limited to more than about eighty times more than Jupiter, below which they are ‘failed stars’ or brown dwarfs,” says team member Olivier Schnurr from the Astrophysikalisches Institut Potsdam. “Our new finding supports the previous view that there is also an upper limit to how big stars can get, although it raises the limit by a factor of two, to about 300 solar masses.”
Within R136, only four stars weighed more than 150 solar masses at birth, yet they account for nearly half of the wind and radiation power of the entire cluster, comprising approximately 100 000 stars in total. R136a1 alone energises its surroundings by more than a factor of fifty compared to the Orion Nebula cluster, the closest region of massive star formation to Earth.
Understanding how high mass stars form is puzzling enough, due to their very short lives and powerful winds, so that the identification of such extreme cases as R136a1 raises the challenge to theorists still further. “Either they were born so big or smaller stars merged together to produce them,” explains Crowther.
Stars between about 8 and 150 solar masses explode at the end of their short lives as supernovae, leaving behind exotic remnants, either neutron stars or black holes. Having now established the existence of stars weighing between 150 and 300 solar masses, the astronomers’ findings raise the prospect of the existence of exceptionally bright, “pair instability supernovae” that completely blow themselves apart, failing to leave behind any remnant and dispersing up to ten solar masses of iron into their surroundings. A few candidates for such explosions have already been proposed in recent years.
Not only is R136a1 the most massive star ever found, but it also has the highest luminosity too, close to 10 million times greater than the Sun. “Owing to the rarity of these monsters, I think it is unlikely that this new record will be broken any time soon,” concludes Crowther.
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It was twenty years ago next month that I met Shawn. I was all of nineteen at the time, he was twelve. We were assigned together for a summer camp held at Nolin Lake in Kentucky. It was, actually, a serendipitous meeting as I was a last-minute staff addition (thanks to an out of the blue wrangling in by a friend of mine), and he was a last-minute camper (thanks to a last minute application by his parents and opening that became available). The thing I was immediately struck with about Shawn was his wise-beyond-his-years and dry sense of humor, which fit in really well with my cynical wit (which was completely unrestrained at the time). He was preparing for seventh grade and he lived, surprisingly, only a few blocks from us in Louisville. He told me that he wanted to join the Navy when he got older, so he could learn to be a pilot, and eventually an astronaut. Shawn Shawn and I hit it off right away and had a great time that week: hiking, swimming, crafts, repelling, a hot air balloon ride. More than enough to tire the hardiest of 12-year olds, and Shawn braved it all out. But Shawn was no ordinary twelve year old. And this was no ordinary summer camp. This was a camp for kids with terminal illnesses.
Early in that summer of 1990, Shawn had been diagnosed with cancer of the liver. He was scared. Shawn, unlike many of the kids at the camp, was old enough to know the gravity of his condition. He knew about his own mortality. When the opportunity for him to attend camp came about, there were a lot of questions and fear from his parents and family, especially (and understandably) from his mother. Somehow, and for some reason, though, she was intuitively comforted by me and the fact that I would be the companion for her son, less than two months after his diagnosis and less than two weeks after his initial release from the hospital.
I, on the other hand, had entered that summer as a typical, invincible nineteen year old. My entire existence was one of selfishness. Yet, that summer signaled the beginning of a sea change in my attitudes toward life and the world. A friend from high school had suffered a ruptured aneurysm while on a camping trip and was now re-learning how to walk and talk. Another friend suffered a drug-induced nervous breakdown, a situation aggravated by his mother casting sole blame on my best friend. I was struggling with slack and indecisiveness in almost every aspect of my life.
Shawn and I, as I mentioned, had a great time at camp. He wrote me at school once or twice that Fall, and we saw each other at a camp reunion over Christmas (a reunion he almost missed because of his deteriorating health). He regained his strength and eventually made it to high school, attending my alma mater (and becoming a classmate of my youngest brother). He even got well enough to ride his bike to our house one summer afternoon. Although not in remission, he still talked about his goal of becoming an astronaut (he had even been granted the opportunity to meet with the Navy’s Blue Angels). A goal, a dream, a hope, to keep him going.
Shawn called me one evening in 1993. From the hospital. His cancer had spread throughout his body. He was dying. Weakly, he asked me to come visit him. Weakly, I said I’d try. A lie, an escape, a selfish decision, a regret.
Shawn died in November 1993. He was fifteen.
I still think about Shawn sometimes. I know I meant a lot to him and his family. He means a lot to me, too. It just took a long time, and in little pieces, for me to comprehend how much meeting him, knowing him, missing him, means. Whenever some big astronomical discovery is made, I think about him. He’d be thirty-two now, and well on his way to attaining his dream of becoming an astronaut, and I’m pretty sure he’d be completely geeked about the discovery of R136a1.…
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This mix began as a collection of some things I’ve been listening to lately, as some songs I’ve re-come across since transferring my library to my new computer. What it has become, with the coincidence of the discovery of a new giant star, the forty-first anniversary of the first moon landing, and the spurred memories of a very dear friend, is, at least to me, more than the sum of its parts. NW012.
01. Magnetic Fields, Different Voice/from THE CONET PROJECT
02. 1969/BOARDS OF CANADA
03. Reefer Spin In The Galaxy/THE ORB
04. Involution/PATRICK VAN DE VEN
05. The Private Psychedelic Reel/THE CHEMICAL BROTHERS
06. Lanx 3/AUTECHRE
07. Ecstasy Symphony/SPACEMEN 3
08. Blue Milk/STEREOLAB
09. Laughing Gas/JUNO REACTOR
10. Foster (deep space dub)/BREAKS THE BLANK DAY
11. The Beautiful Unknown/NUNC STANS
12. Deep Blue Day/BRIAN ENO
