Rick Robinson

Aug 9th 2021

A Supermassive Black Hole Turns Red Stars Blue


Something odd seems to be happening to red giant stars that stray too close to the center of our galaxy. According to, it was roughly 30 years ago that astronomers noticed the first hints of a red giant shortage in the center of the Milky Way.

Further studies have confirmed that the galactic core, though packed with stars, has proportionally fewer red giants than most regions of space. The likely culprit, astronomers now suspect, is the supermassive black hole that sits at the very center of the galaxy.

But why is the black hole selectively doing away with red giant stars? And what exactly is it doing to them? A newly proposed theory suggests that the black hole is turning them into blue stars.

Not Finicky Eaters

Black holes are perfectly capable of dining on entire stars. CBS News reports that astronomers were eyewitnesses to one such meal, a star being “sphagettified” — torn into narrow streams of glowing gas — then consumed by a black hole.

But black holes, unlike house cats, are not finicky eaters. Red stars, blue stars or yellow stars like the sun — a supermassive black hole at the center of a galaxy will scarf them all down with equal gusto, or anything else caught in its gravitational maw.

No one could come up with a plausible hypothesis for how a black hole would selectively favor consuming red giants over other types of stars. But according to, our galaxy’s star-packed center has about 1,000 fewer red giant stars than would be expected from the overall prevalence of these stars in the stellar population.

On the other hand, dining on stars is only the final act of a supermassive black hole’s interactions with the stars and other matter surrounding it. And it’s not even the most spectacular part of this interaction.

When Our Galaxy Exploded

Currently, the supermassive black hole at the center of the Milky Way galaxy is fairly quiescent, as far as black holes go. But two vast X-ray sources, located above and below the disk of the Milky Way, indicate that this was not always the case.

Per, these X-ray regions are called Fermi bubbles. They indicate that a few million years ago, the central black hole — Sagittarius A* (pronounced “A-star”) — pulled in so much surrounding matter so quickly that it triggered an explosive eruption, producing energetic jets of particles. According to NASA, these eventually gave rise to the Fermi bubbles, a combined 50,000 light-years across.

A galaxy-sized explosion would certainly affect stars near Ground Zero, producing intense cosmic riptides that could disrupt the outer layers of stars. And a distinct feature of red giants is that they have very large and very distended outer layers.

If such a star were hit by a cosmic fire hose of streaming particles, the dense core of the star would be little affected. But as Science News reports, the thin outer layers would be caught up in the turbulence and swept away.

“The jet preferentially acts on large red giants,” says astronomer Michal Zajaček of the Polish Academy of Sciences, who proposed this hypothesis, per Science News.

From Red to Blue

Those thin outer layers are the key to the mystery of the missing red giants. Normally, these stars are red because, as stars go, they are relatively cool — merely red-hot, not white-hot, and much less the bluish color of the very hottest stars. In spite of their modest temperatures, these stars are bright because they are giants, up to hundreds of times larger than our sun.

But what happens if an exploding black hole disrupts and sweeps away the thin outer layer of a red giant? The star remains bright because it is still generating energy near its core. But with the outer layers gone, the much hotter interior is exposed to view — and to a distant observer, such as an astronomer on Earth, the star will now appear blue.

However, even the explosion of our galaxy may not provide a complete explanation. As UCLA astronomer Tuan Do told Science News, “It may take a combination of several of these kinds of mechanisms to fully explain the lack of the red giants.”

Do goes on to explain that a disk of gas surrounded Sagittarius A* a few million years ago, perhaps a buildup to the coming galactic explosion. Red giant stars orbiting the central supermassive black hole will have passed repeatedly through this disk, gradually losing their swollen outer layers and, thus, their red coloring.

Much more work will be needed to sort out all the complex mechanisms operating in the turbulent region of space near our galaxy’s central black hole. And chances are we’ll find much more going on alongside red giant stars being turned blue.