Not long after JWST started observing the universe, it saw something weird: objects that were unresolved (appearing as dots in images with no surrounding fuzz, like a galaxy has) and extremely red. Like, so red they’re not even red anymore; they’re actually bright only in the infrared, well outside the wavelength range of our eyes.

Astronomers dubbed these objects Little Red Dots, which is descriptive if not terribly enlightening. It wasn’t clear what they were at first. The immediate thought was that they are extremely distant but luminous galaxies. This makes sense for a few reasons. We know all big galaxies have a supermassive black hole in their core, and when material falls in it forms a flattened disk called an accretion disk, and the matter in it heats up and can blast out radiation so fierce it can outshine all the stars in the galaxy.

In the early universe, when galaxies were first forming, this would’ve been more common than it is today, and these galaxies emitted energy even more enthusiastically — we call these active galaxies. The light emitted is extremely blue (even ultraviolet), and the expansion of the cosmos would redden it. Also, after a billion years or so, there could be a lot of dust in these galaxies — tiny grains of carbon or silicaceous molecules — that preferentially absorbs blue light, making the galaxy centers look redder (this is similar to why sunsets can look red).

It’s also possible that what we’re seeing is a super starburst galaxy, one that was extremely fecund and made zillions of stars in a very short time and in a very compact volume of space. This kinda sorta matches what the Little Red Dots looked like, too.

There were other ideas of what they could be, like brown dwarfs for example, but the colors didn’t match (and here, by “colors”, I mean how bright the objects are at different wavelengths; an astronomer will call something blue if its brighter at short wavelengths than longer ones). The problem is, even an active galaxy doesn’t quite match the observations. The starburst scenario is a little unrealistic, too, because there had to be so many stars so close together that they would collide all the time, which would generate a lot of X-rays. Yet no X-rays are detected from these objects.

If you’re looking at physical models of different kinds of objects to explain what Little Red Dots look like you wind up adding so many caveats (like, surrounding the central black hole with incredibly thick, maybe even unreasonably thick dust clouds) that the model becomes unwieldy. Scientists prefer simpler explanations, because the universe tends to bend that way; Occam’s Razor and all. That’s not always the case, of course, but it’s a nice rule of thumb.

So. A team of astronomers has come up with a new idea, and it’s weird, but not so weird it shouldn’t be considered. They wonder if the Little Red Dots might be what they call black holes stars [link to journal paper].

This idea is similar to the active galaxy core I discussed above. However, instead of a disk of material flowing in to the black hole creating the light, the black hole is surrounded by a huge and very thick sphere of hydrogen gas, heated up by the black hole in the center. That’s similar to a star, though not one by definition — a star is something that generates (or at some point generated) energy by nuclear fusion in its core. But it still does look like a star from a distance.

Why does this make sense? Because in important ways, the spectra of Little Red Dots look more like a star than anything else.

[Warning: very science stuff to follow]

In a star, the heat comes from under the surface and moves outward. At the top of the star the hydrogen gets thinner and thinner (like our atmosphere does with height above the ground), and eventually lets the light out into the cosmos at large.

But the hydrogen atoms still absorb a lot of the light from below before re-emitting it. And as I’ve written about many times (and go over in a little more detail in my Crash Course Astronomy episode about light), those atoms absorb light at very specific wavelengths. The electron in a hydrogen atom needs a specific amount of energy to jump from one energy level to the next (this is the so-called “quantum leap”), so it sucks up the wavelengths of light corresponding to those energies. There’s a series of these wavelengths depending on what levels are occupied by the electron, so we get sequences of absorbed wavelengths called the Lyman series, the Balmer series, and others.

We see these in stellar spectra! The light we see is spread across all wavelengths, but there are dips in brightness at some wavelengths where the electrons absorb the light. And here’s a fun thing: at high enough energy the electron actually gets blown off the atom entirely, stripped away like shrapnel — we then call the atom ionized. So any light with this energy or above (or with shorter wavelengths, if you prefer) gets absorbed, and we see a big drop bluer of that wavelength. It doesn’t drop to zero because the hydrogen isn’t 100% efficient at absorbing all the light, but we do see a drop called the Balmer Limit. It’s more pronounced in hotter stars, which have enough energy to ionize hydrogen, and the strength of the jump depends on the kind of star.

OK, phew. So with that explained, the fun bit here is that one of the Little Red Dots seen shows this behavior pretty well. Nicknamed The Cliff, spectra taken with JWST show it’s pretty far away; the redshift of its light indicates we see it as it was nearly 12 billion years ago when the universe was less than two billion years old (and it’s one of the closer Dots to us, and brighter as well). Those same spectra reveal it has a very sharp drop in light blueward of the Balmer Jump, way steeper than we see in stars, even though it superficially looks similar.

The astronomers found that’s hard to replicate in models with tons of stars in the galaxy core, but it does fit well if you instead have a supermassive black hole encased in a huge ball of hydrogen. That’s not proof this is what we’re seeing, but it’s pretty interesting.

Of course we’re still in the early days of all this. These objects are faint, and need a lot of time to observe (the astronomers got 60 hours of JWST time to observe the dots, which is a lot on the overly subscribed observatory), so it’s not easy to get the data. Hopefully over time they’ll get more observations that might be able to support or disprove their black hole star model.

The idea is peculiar, but not so peculiar I’m willing to dismiss it out of hand. And we’re sure to see lots of other weird stuff as we build and use bigger telescopes that see the universe in a different way. That always happens! And as weird ideas go this isn’t too weird. It may not be right, but that’s something astronomers will have to investigate further.

And if it is right, well! That tells us the cosmos is a pretty bizarre place, and what we see in the immediate universe around us is not the whole of what’s out there. That’s a conclusion I actually quite enjoy.

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