Westerlund 1-26: Hubble finds one of universe’s largest stars

Despite coyly hiding most of its light emissions in the longer wavelength infrared part of the spectrum, Westerlund 1-26 has been caught on camera by Hubble’s powerful zoom.

When the light in this photo left the Westerlund 1 super star cluster 15,000 years ago, the woolly rhinoceros was just going extinct, and the woolly mammoth still had 10,000 years to go. The star of the cluster is young and bright: at only 3 million years old, it’s more than a thousand times younger than our sun, and its luminosity suggests a radius 1,500 times wider.

To put that in perspective, if Westerlund 1-26 took the place of the sun at the center of the solar system, Mercury, Venus, Earth, Mars, and Jupiter would all be swallowed up inside the so-called “hypergiant,” the boundary of which would extend almost a third farther than Jupiter.

When Bengt Westerlund first imaged the star cluster in 1961, he called it “a heavily reddened cluster” in the Ara constellation. Barely visible outside the infrared, he was unable to determine the spectral types present.

Half a century later, astronomers know quite a bit more. By studying how much light the star emits in a variety of wavelengths, scientists have calculated how hot it burns, much as we might guess that a filament glowing white-hot is warmer than a cool blue flame.

The other key piece of information is how much light the star gives off, called its luminosity. Just like campfires, stars can be bright because they’re hot, because they’re big, or both.

Arranging stars on a graph with luminosity on one axis and temperature (wavelength) on the other creates what’s called the Hertzsprung-Russell Diagram, which organizes stars into classes. A relatively smooth continuum stretching from hot, bright stars to cool, dim stars represents the unsurprising “main sequence” stars, but a few outlying clusters have revealed the existence of more exotic varieties, like hot but dim white dwarfs and cool but bright red giants.

By combining luminosity and wavelength, physicists have estimated Westerlund 1-26’s size, and found that it is in the top handful of biggest stars known to science.

Many of these cool-burning colossuses earn the nickname “red hypergiant,” although the boundary with “red supergiant” is a fuzzy one. These stars tend not to get much bigger than 1,000 to 2,000 solar radii because of a natural cap called the Eddington Limit.

Gravity pulls constantly inwards, while the act of shining creates a force pushing outwards, and each star must maintain a balance of sorts. Red hypergiants toe the line of too much radiation pushing outwards, occasionally blowing off a chunk of their outer shell, an event sometimes called a “supernova imposter.”

An ionized nebula surrounding Westerlund 1-26 suggests that it too suffers similar outbursts, making it a target of great interest for future observation.