Twinkling stars may look like little pinpricks of light, but in reality they are huge.
The nearest star is, of course, the Sun. It has a mass of about 2 million trillion trillion kilograms (a 2 followed by 30 zeroes). If Earth weighed the same as a paper clip, the Sun would weigh as much as a Harley-Davidson motorcycle.
But while not a lightweight, the Sun is only slightly above average. About 1% of stars weigh more than eight times the Sun, and a handful of stars in the galaxy weigh one or twohundredSuns.
The most massive star known, called R136a1, tips the scales at about 265 solar masses. It is so heavy, its discovery in 2010 forced astronomers to rethink their theories for how massive stars can get.
That in turn forces us to re-examine our ideas about the first stars that ever formed. It turns out that some of these first stars, born just 200 million years after the Big Bang, could have weighed as much as 100,000 solar masses, making them by far the most massive stars ever. The question is, how did R136a1 and the primordial stars ever get so big?
A star's mass is not just a curiosity. It is the star's single most important property, determining how it lives and dies.
Massive stars of a few tens of solar masses burn bright and fast
A star is an enormous ball of hot gas, so massive that its gravity pulls it in on itself. This makes the star's core extremely dense and hot. This triggers nuclear fusion, in which pairs of atoms smush together to form larger ones, generating lots of heat and pressure that pushes back outward.
A star's life hangs in this balance between gravity and pressure. Once it runs out of fuel, nuclear fusion stops and it can't stop itself from collapsing.
The star's fate and how quickly it exhausts its fuel depends on its mass.
Massive stars of a few tens of solar masses burn bright and fast. They live for only a few hundred million years before exploding as a supernova and leaving behind dense, exotic objects such as a black hole or neutron star.
That really is one of the big unsolved questions in astrophysics
In contrast, smaller stars like the Sun smoulder steadily for billions of years before becoming stellar corpses called white dwarfs.
The smallest a star can be is about 0.08 solar masses, according to relatively straightforward calculations. A star like that is just massive enough to ignite nuclear fusion. Anything smaller, and the "star" is just a ball of gas.
But while astronomers have a pretty good handle on the minimum mass of a star, the other end of the range is fuzzier. "That really is one of the big unsolved questions in astrophysics," says Volker Bromm, an astrophysicist at the University of Texas, Austin, in the US.
As recently as ten years ago, astronomers thought the upper end of stars in the current universe was about 150 solar masses. "There was good evidence that suggested that was the limit, both from theory and observations," says Paul Crowther of the University of Sheffield in the UK.
Astronomers could not find any star greater than around 150 solar masses
You would have to be lucky to see a very high-mass star, because their lifetimes are so short. Stars of a hundred or more solar masses would die in a few million years: a cosmological eye-blink.
One promising place to find such a star seemed to be the Arches Cluster, one of the densest known collections of stars in the Milky Way.
The cluster appeared to have formed recently enough that the most massive stars had not yet died. There was also plenty of star-forming material lying around, providing an environment conducive for stellar behemoths.
But astronomers could not find any star greater than around 150 solar masses. Perhaps, they thought, it was impossible for stars to get that heavy.
At some point, a star should get so massive and bright that its radiation blows away its outer layers, preventing it from growing further. This natural mass limit is called the Eddington Limit, and calculations suggested it was close to 150 solar masses.
Exceeding the limit turns out not to be a problem
But in 2010, Crowther and a team of astronomers studied an even heavier group of stars called the R136 cluster. There, they discovered not one but several stars surpassing 150 solar masses. The most prodigious, dubbed R136a1, is an astounding 265 solar masses.
What's more, it might have been even heavier when it was born.
R136a1 is a Wolf-Rayet star: that means it is massive, bright and hot, with powerful radiation that expels its outer layers. It has a temperature of about 53,000 °C and shines nearly 10 million times brighter than the Sun. Even though it is so young, barely over a million years old, it has already lost roughly 50 Suns worth of gas.
That means that R136a1 once weighed more than 300 Suns. So much for a 150-solar-mass weight limit.
Exceeding the limit turns out not to be a problem. Previous estimates of the Eddington Limit were relatively crude, Crowther says, and more detailed calculations reveal that stars can get much more massive – in theory at least.
They must form in as little as a hundred thousand years
As for the Arches cluster, astronomers have discovered that it is older than they thought, which means any really massive stars are long gone. R136, however, is young enough that its original stars are still going.
Still, heavyweights like R136a1 are rare. It may be that fewer than a handful exist in the Milky Way, Crowther says.
"The biggest question is how they got to be so massive," he says.
It takes time for a growing star to accumulate mass. Stars like the Sun take about 10 million years to form. But stars like R136a1 live for only a few million years, so they must form in as little as a hundred thousand years: a cosmic flash.
No one is quite sure how. One idea is that these whopping stars form when long filaments of cold, dense gas collide. In the last couple of years, Europe's Herschel Space Observatory has spotted these filaments throughout the galaxy. Each one can span several light-years.
How stars like R136a1 got so big is a puzzle
When these filaments crash into one another, dense pockets of gas can form and collapse into stars, giving birth to a whole star cluster all at once. Most of the new stars would be small, some would be massive, and sometimes a few could be enormous like R136a1.
But it's difficult to know how exactly this happens. "The details I'd say are pretty sketchy," Crowther says. These regions of massive star formation are obscured in thick clouds of interstellar dust, so even the most powerful telescopes have trouble seeing what is going on.
Enormous stars could also form when stars in orbit around each other merge. Most of the heavier stars are found in pairs anyway, so if a pair of stars each had a mass a few dozen times that of the Sun, they could combine into a single jumbo-sized star.
How stars like R136a1 got so big is a puzzle, but the very first stars are even more mysterious. These were truly humongous.
As early as 200 million years after the Big Bang, there was light. That is when clouds of gaseous hydrogen and helium collapsed into the universe's first stars.
Unlike modern stars, they were all rather more massive. Most weighed tens of solar masses, and some reached a hundred or two. Those first stars were able to bulk up because the cosmic environment was different. In particular, there were no heavy chemical elements.
The black hole feeds on a swirling disk of dust and gas, and blasts out tremendous beams of energy
Heavy elements are important because they help cool gas clouds. In a hot gas, the atoms zip around and crash into one another. Heavy elements can convert this collision energy into light, which is then radiated away. That means the heat escapes.
But heavy elements did not always exist. They were forged from nuclear fusion in the cores of stars, and in the explosive deaths of massive stars. It has taken generation after generation of stars to produce all the elements found in the cosmos today. When stars first appeared, there was only hydrogen and helium, and tiny bits of lithium.
Without heavy elements, gas clouds could not cool off as easily, which made it harder for them to collapse into stars. To compensate, each cloud had to grow even bigger before it gained enough gravity to induce collapse. The resulting stars tended to be more massive than today's stars.
For decades, though, no one was sure how much bigger. Then in recent years, astronomers made a perplexing discovery that suggested the stars could have been significantly bigger.
They found quasars existing a billion years after the Big Bang.
Quasars are enormously bright objects, each one powered by a black hole millions to billions of times more massive than the Sun. The black hole feeds on a swirling disk of dust and gas, and blasts out tremendous beams of energy.
It involves truly gigantic stars weighing a few 100,000 solar masses
The mystery is how those supermassive black holes got there.
Black holes form when stars exhaust their fuel and collapse. For a black hole to become supermassive, it would have to gobble up lots of mass in the form of nearby gas and dust, or merge with other black holes.
The problem is that these quasars existed so early in cosmic history, the supermassive black holes had to have gained all that weight in an incredibly short amount of time. According to theory and computer simulations, even stars of a couple hundred solar masses could not have grown fast enough to become supermassive.
There is a solution to this paradox, but it involves truly gigantic stars weighing a few 100,000 solar masses. Such stars would dwarf even R136a1.
Computer simulations show that a cloud of a million solar masses could collapse into a 100,000-solar-mass star. Conditions would have to be right: no heavy elements, and lots of ultraviolet radiation, which further prevents gas clouds from cooling.
A star this big would be unstable, and would immediately collapse into a black hole. This black hole could then continue increasing its mass by consuming dust and gas or merging with other black holes, until it was massive enough to power a quasar.
By understanding how massive the first stars got, astronomers can figure out what the first galaxies were like
That is the story, at least. "Our computers are very patient at making these objects," says Alexander Heger of Monash University in Australia. "But whether they exist in nature, we don't have any direct evidence of that. They're all theoretical at this point."
We could get some direct evidence, if we could observe black holes merging.
When two black holes collide, they create undulations of space-time called gravitational waves. Europe's Laser Interferometer Space Antenna (eLISA) should be able to detect these, when it launches sometime after 2028. By measuring the waves, astronomers can determine the masses of the merging black holes and whether they came from supermassive stars.
Astronomers are also waiting for the next generation of telescopes, which include the James Webb Space Telescope, the Thirty Meter Telescope, the European Extremely Large Telescope and the Giant Magellan Telescope. These observatories could find the first black holes born from supermassive stars. They might even catch a star in the act of collapsing into the black hole.
Such discoveries could be groundbreaking. By understanding how massive the first stars got, astronomers can figure out what the first galaxies were like.
"This question of the nature of the first stars and how massive they can get tells you about a very special moment in cosmic history," says Bromm. "Before that, the universe was a simple, boring place." For one thing, "there were no sources of light."
Twinkling stars may not be little, but the nursery rhyme did get one thing right: we still wonder what they are.