Red Giant Star Blows Really Big Bubbles

A laser beam passing through a small, circular opening creates a diffraction pattern similar to what a star looks like through a telescope. The bright center dot is the Airy Disk. Wisky / CC BY-SA 3.0

No star looks larger than a point in all but the very largest telescopes. Even then only the largest of stars — Betelgeuse, Antares and the like — show disks. It’s doubtful anyone’s ever seen a star’s true shape through a telescope because special photographic techniques and multiple telescopes are needed to capture an actual outline. Professional astronomers generally don’t look through telescopes. They prefer using their precious hours to record data with cameras and spectrographs that can be carefully analyzed later on the computer.

Instead, what we see when we look at a star under high magnification is a bright point of light called the Airy Disk surrounded by a series of delicate concentric rings of decreasing intensity. It’s a diffraction pattern created when light from the star passes through a small opening — the telescope lens. As the light emerges from the lens, it spreads into waves that overlap and interfere with one another to form a ripple-like pattern of bright and dark fringes. Where the waves reinforce one another, a bright ring or fringe forms; where they cancel one another, a dark fringe or lack of light results.

Astronomers using ESO’s Very Large Telescope have directly observed granulation patterns on the surface of a star outside the Solar System — the ageing red giant π1 Gruis. This remarkable new image from the PIONIER instrument reveals the convective cells that make up the surface of this huge star. ESO

So no one sees a star per se through a telescope but instead the pattern its light creates. Totally nuts, right? That’s what makes this recent photo the Pi-1 (π1) Gruis all the more remarkable. It was taken by the European Southern Observatory’s Very Large Telescope (VLT) and not only shows the star’s shape but granulation patterns on its surface. Pi-1 (π1) Gruis is an aging red giant star 350 times the diameter of the sun 530 light years from Earth in the southern fall constellation Grus the Crane.

This colorful photo shows the red giant π1 Gruis (center)and π2 Gruis (to its left) along with the bright spiral galaxy IC 5201. ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin

The photo reveals large convective cells, similar to those that transfer heat from deep in the sun’s interior to the surface but much larger. Each cell covers more than a quarter of the star’s diameter or 75 million miles (120 million km) across. Just one of them would extend from the sun to beyond Venus! Lest you think Pi-1 Gruis is unique, both it and our sun have similar masses despite their difference in size and follow similar evolutionary tracks. Five billion years from now, when Pi-1 Gruis will have lost its big, gassy envelope and become an Earth-sized white dwarf star, the sun will have ballooned into a red giant.

The sun’s too-bright-to-look-at surface, called the photosphere, contains about two million convection cells or granules with diameters of just 930 miles (1,500 km). You can think of them as bubbles of hot water rising in a pot. Heat from the stove burner heats the water at the bottom of the pot first, causing it to rise in bubbly convection cells. When the bubbles reach the top, they cool and sink back down to be heated all over again. This is the essence of convection.

Even a small telescope fitted with a proper solar filter will show the sun’s convective granules, which give the photosphere a grainy appearance that resembles the look of over-enlarged black and white film photos. If you have a filter, take a look. You’ll be amazed at the sight of our bubbly star.

You can see the grainy texture of this sun in this photo caused by individual cells of rising and sinking gases called granules. Each is about the size of the state of Texas. NASA

The big difference in size between the sun’s cells and those on Pi-1 Gruis has to do with that star’s surface gravity. Its surface is so far from the center compared to the sun that gravity there is much weaker, allowing for more expansive granules.

Stars are simple creatures really. Mostly made of hydrogen, their fate is determined by their initial mass. Little ones are cooler and burn their fuel frugally, sticking around for trillions of years. Medium-sized ones like the sun live for billions of years and expand into red giants when they run out of one fuel and switch to another (from hydrogen to helium). Big ones eat through their reserves in millions of years, run out of fuel and end their lives in titanic explosions as supernovae.

Want to learn more about Pi-1 Gruis? Check out the study just published in the journal Nature. Some of you may know (and some not) that I write a weekly observing blog for Sky & Telescope. This week’s article is about observing the Local Group of galaxies. If you’re interested in learning more about our galactic neighbors have a look here.