Stars can grow huge, but there’s a limit |

Stars can grow huge, but there’s a limit

Orion, the hunter – best-known constellation of winter lies just a bit farther west, and sets just a little bit earlier, every night: welcome to spring.

From Orion’s three-star belt hangs a three-star sword. But the middle “star” of the sword is fuzzy – it’s nebulous. A small telescope reveals, within the Orion nebula (Latin for “cloud”) a clutch of young stars.

Stars are born when clouds of gas and dust collapse in on themselves. Bound by gravity, the nascent star’s core is squeezed and heated, to the point where atomic nuclei fly so fast they fuse – they collide so intimately they overcome their electrical repulsion (all nuclei are electrically positive; like charges repel), enabling the “strong nuclear force” to bind them together into a larger nucleus.

Weigh that larger nucleus, and weigh the smaller nuclei that fused to make it. The parts weigh more than the whole. The “mass deficit” got turned into energy – into starlight. E=mc2.

Squeezing and heating the core requires that the star have at least a certain minimum amount of matter. Any less and the core won’t be squeezed hard and hot enough to sustain fusion. It’ll fizzle, becoming, not a star, but a “brown dwarf.”

At 900,000 miles across, our sun is only a moderate-size star. What makes stars so large?

Stars are hot, and hot gas expands. But temperature is not enough to explain the huge size of stars.

The tail of a comet always points away from the sun, no matter what direction the comet is moving – the tail is not a “swoosh mark” that trails behind the comet.

Johannes Kepler, centuries ago, deduced that a comet’s tail is pushed away from the sun by sunlight.

Radiation pressure is miniscule, yet navigators must take it into account when plotting the trajectories of their spacecraft.

Stars are inflated in part by their heat, and, in equal part, by the pressure of starlight pushing out from their cores.

The larger a star, the stronger its gravity – the hotter its core – the faster it fuses its small nuclei into larger ones – the stronger the radiation that is pouring out of its core.

The stronger the starlight, the stronger the radiation pressure – pushing outward.

Engineers speak of “self-disassembly” when they want to avoid saying their machine blew up.

Could radiation pressure grow strong enough to cause a new-born star to self-disassemble?

The question has bugged astronomers for a century. Last month, astronomers using data from the Hubble Space Telescope – exploiting Hubble’s extraordinary resolution (sharpness) – published, in the journal Nature, evidence for an upper limit in how large a star be.

Looking at Orion, you’re looking out of our galaxy; turn your back on Orion and you’re looking in toward the center. In that direction, a team led by Don Figer of the Space Telescope Science Institute has been studying the densest cluster of stars ever found in the Milky Way. Were our sun within that cluster, we’d be surrounded by so many stars that our sky would never grow dark.

Within such dense star clusters, are found the most massive – the heaviest – stars in the galaxy.

Within the cluster, Figer and his team found stars weighing as much as 130 suns.

But none heavier. And yet, theory says that a cluster as dense as the one they studied should harbor dozens of much heavier stars.

Adding a bit of mass to be conservative, the astronomers propose that no star heavier than 150 suns can survive; as soon as it formed, it would be blown to bits by the outward pressure of starlight.


Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).

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