Antares is a supergiant star that would fill the Solar System beyond Mars, but its atmosphere is 12 times larger than that.

Antares, the furious red eye of the constellation Taurus the bull, is a red supergiant star near the end of its life. And astronomers with VLA and ALMA have realized that it is much, much bigger than we ever imagined.

So when I say Antares is a big star, I don’t think you really appreciate how big it is. En masse, it’s not the most impressive thing in the universe, just a dozen times more massive than our sun. But Antares is in the last stages of its life cycle, running out of hydrogen in its nucleus. The increasing presence of helium, the byproduct of hydrogen fusion, in the core pushes the hydrogen that burns inside a shell around it. This in turn inflates the rest of the star to grotesque proportions.

He is a great star.

Antares atmosphere
Antares atmosphere determined by multiple instruments. Credit: NRAO / AUI / NSF, S. Dagnello

The photosphere, the layer of a star that emits visible light (and is therefore the best functional definition of “surface” that we can get when it comes to giant balls of plasma) is approximately 700 times wider than the sun. If placed within our own solar system, the Antares photosphere would gobble up the planets Mercury, Venus, Earth, and even Mars, extending roughly to the orbit of the asteroid belt.

And then it just continues.

Using the combined high resolution of ALMA (Atacama Large Millimeter Array) and Karl G. Jansky Very Large Array (VLA), astronomers determined that the atmosphere of that bloated star extends much more than it should.

The first layer of a star’s atmosphere is called the chromosphere. In the sun, the chromosphere is a bare splinter that hugs close to the photosphere, a mere half a percent of the sun’s radius. But in Antares, the chromosphere reaches dizzying heights, extending to 2.5 the radius of the star itself. Furthermore, the Antares chromosphere is divided into two discrete regions and is relatively cold with a temperature of 3,500 degrees Celsius. Compare that to the scorching 20,000-degree Celsius chromosphere of the sun, and you can see that while Antares is massive, it’s not necessarily fierce.

Did I mention that if you put Antares in our own solar system, its chromosphere would almost reach Saturn’s orbit?

Yes big one.

Beyond the chromosphere, Antares becomes even stranger. Spanning twelve times the diameter of the star (enough to reach Uranus in our own system), Antares sports a vast region that serves to accelerate particles faster than the star’s escape velocity, launching winds that leave the system. completely.
Those winds usher in what will eventually become a vast and magnificent nebula once Antares is a star.

We don’t know much about these wind launch regions. And as you can imagine, the same processes in the sun are slightly weaker. By better studying large, nearby stars like Antares, astronomers can better understand exactly how massive stars die and prepare to recycle in the next generation.