Massive stars – I mean really massive, with 20 times or more the mass of the Sun – are terrifyingly powerful.
The energy they produce scales strongly with mass, so at the upper end of this range, stars can emit so much light that they can be seen in other galaxies with small telescopes, and would bake all the planets they have. They can light up entire nebulae, and when they explode at the end of their short, violent lives, they can eclipse entire galaxies. Galaxyare plural.
But how massive can stars be? This is an important question in astronomy. We have a good understanding of how stars like the Sun behave, but as you add more mass to them, their behavior can get wonky. They can become unstable, pulsating and even experience explosions that are just this side of a catastrophic supernova.
Also, when massive stars explode they seed the galaxy around them with heavy elements like the iron they make during their lifetime. These elements are needed to make planets and life. We literally owe our existence to massive stars that have gone supernova.
And theoretically speaking, we’d like to know what the upper mass limit is, because that helps astronomers understand how stars are born and how they live their lives.
Until recently, this limit was estimated at more than 300 times the mass of the Sun, which is enormous. But now, new observations of the most massive star known show that it could be an overshoot. The new results imply that the mass limit is more like 200 solar masses.
The star is called R136a1. It is part of a tight cluster of very young stars called NGC 2070, which sits at the center of the Tarantula Nebula, a vast, sprawling cloud of star-forming gas in a Milky Way satellite galaxy called the Great Cloud. from Magellan. It’s about 160,000 light years away.
Observing R136a1 at all is hard. The stars at the core of NGC 2070 are very close together, and even with Hubble it’s hard to separate them. And this is essential; you cannot observe a star and understand what it is doing if its light mixes with another nearby star.
To do better, a team of astronomers used the Gemini South 8.1-meter telescope in Chile with a special camera called Zorro. [link to paper]. What makes it special is that it can do speckle imaging.
Earth’s atmosphere is a pain for astronomers. It is constantly in motion, with small air packets coming and going. Each of them acts as a lens, distorting incoming starlight. Over time, even a fraction of a second, the image can shift so much that it blurs in a circle called, oddly, the vision disc. Two or more stars close together in the sky can be mixed together in a single drop.
One solution is to take extremely short exposures, freezing this movement. That’s what Zorro can do, taking images as short as 60 milliseconds. On Halloween 2021, astronomers got 40,000 such short exposures by using several different filters to pick out certain star colors. They then moved and combined the observations of each filter to negate the atmospheric dance that muddled them.
The larger the telescope, the better the resolution; that is, the closer two objects can be and you can still separate them. Because Gemini is so large, using speckle imagery can yield observations with as good or better resolution than Hubble, which has a 2.4-meter mirror but doesn’t have to face an atmosphere.
In the new images – the highest resolution visible light observations of the star ever taken – R136a1 is clearly visible, along with dozens of stars surrounding it. This allowed astronomers to get better measurements of the color of R136a1 than ever before.
This part is critical. The colors of a star can be compared to the expected colors given the characteristics of the star: Its mass, its temperature, its size, its elemental abundance, its age, etc.
Ultimately, the new measurements give a mass for the monster star of 196 times the mass of the Sun, with an uncertainty of +34 and -27, so it could reasonably be between 169 and 230 times the mass of the Sun. That’s an incredibly large mass, but still significantly lower than a previous estimate of nearly 300 solar masses at the high end.
The astronomers who did the work warn that they’ve pushed speckle imaging to the limit of what it can do, so they don’t want to draw a line in the sand around the mass. It is still uncertain. But it does suggest that since R136a1 is the most massive star known, the upper bound for star masses may be lower than previously thought.
By the way, that’s how massive the star is now. It’s over a million years old and has no doubt lost a lot of mass over that time in a strong wind; stars like this, called Wolf-Rayet stars, are known to blow away large amounts of material. It was therefore formerly much more massive. How much is not well known.
As an aside, the luminosity they get for the star is 4.6 million times that of the Sun. If you replaced the Sun with R136a1, it would appear the size of your outstretched fist, but be so bright that your fist would also catch fire and the Earth would burn out. So here is. Luckily it’s a few galaxies away from us.
It also changes the way we think certain stars explode. Extremely massive stars can experience a theoretical type of explosion called pair instability supernova, which can result in an ultra-bright explosion. However, if the upper mass limit for stars is 200 rather than 300 solar masses, that reduces the number of such supernovae we can see. Additionally, the amount of heavy elements expelled by extremely massive stars is highly mass-dependent, so this new result could change how we think all those critical elements appeared.
This is not an esoteric result; it literally changes the way we see the Universe and our origins in it.
So how can this new result be confirmed or invalidated? More resolution, and that means bigger telescopes, at least something in the 30 meter range. These do not yet exist, but plans exist to build them. It will take a while, though, so for now astronomers will have to find other ways to zero in on the biggest stellar beasts the cosmos can whip up.