In Our Galaxy’s Center, a Tiny Monster

It’s far from a perfect image. [Note added: if you need an introduction to what images like this actually represent (they aren’t photographs of black holes, which are, after all, black…), start with this.]

EHT’s blurry time-averaged image of the ring of material surrounding the black hole at the center of our galaxy

It’s blurred out in space by imperfections in the telescopic array that is the “Event Horizon Telescope” (EHT) and by dust between us and our galaxy’s center. It’s blurred out in time by the fact that the glowing material around the black hole changes appreciably by the hour, while the EHT’s effective exposure time is a day. There are bright spots in the image that may just be artifacts of exactly where the telescopes are located that are combined together to make up the EHT. The details of the reconstructed image depend on exactly what assumptions are made.

At best, it shows us just a thick ring of radio waves emitted over a day by an ever-changing thick disk of matter around a black hole.

But it’s our galaxy’s black hole. And it’s just the first image. There will be many more to come, sharper and more detailed. Movies will follow. A decade or two from now, what we have been shown today will look quaint.

We already knew the mass of this black hole from other measurements, so there was a prediction for the size of the ring to within twenty percent or so. The prediction was verified today, a basic test of Einstein’s gravity equations. Moreover, EHT’s results now provide some indications that the black hole spins (as expected). And (by pure luck) its spin axis points, very roughly, toward Earth (much like M87’s black hole, whose image was provided by EHT in 2019.)

We can explore these and other details in coming days, and there’s much more to learn in the coming years. But for now, let’s appreciate the picture for what it is. It is an achievement that history will always remember.

13 thoughts on “In Our Galaxy’s Center, a Tiny Monster”

  1. “its spin axis points, very roughly, toward Earth”

    A little surprising to me. I would have thought it was aligned with the galactic axis of rotation. Perhaps there’s some kind of precession involved?

    • Nobody yet understands how supermassive black holes form and grow, so there isn’t a strong prejudice one way or another, as far as I can tell from talking to experts. Even if it started out aligned with the axis of rotation, it may get kicked around as it merges with black holes that were in the centers of galaxies the Milky Way merged with. Definitely an area of ongoing research.

  2. Pretty soon Hubble pics will be quaint. It’s a first step and a valuable ones. If we get enough of different core black holes, it may lead to a theory of their formation and may provide clues to the history of the universe. Not trivial by any means

  3. I have two questions. (1) Why the Milky way BH changes every hour or minute and 2019 M87 BH is much more stable. Only thing I can think of is that M87 BH being old is practically done with its dinner and the recent one is young and is on feeding frenzy! Do you agree?
    Also is BH in our galaxy younger because galaxy itself is younger compared to others?

    • It’s simply a matter of size. A huge object cannot change quickly because the speed of light limits how quickly a change in one location can propagate to another location. For example, if a new chunk of material falls in, it can’t cause a large fraction of the accretion disk to brighten in ten minutes if the accretion disk is ten days across.

      The M87 Black Hole and its accretion disk are light-days to light-weeks across, so any changes to the radio emissions takes place over days and weeks. That of the Milky Way are only light-minutes to light-hours across, so major changes to the radio emissions can occur within an hour.

      Age has nothing to do with it at all. Density and diversity of the local material around the black hole might have some effect. But it’s dominantly just the speed of light combined with the huge size of M87’s black hole compared to our own.

      • Thanks Matt! Do you think our BH will become supermassive by continuing to eat?! Or one has to be born supermassive!
        Also Can you explain how they combine the signals? I understand they cannot use internet. So they have to record data on CDs and send them for analysis. But adding intensities would be of no use. So they must be recording amplitudes and phases and square after adding. Is that how it works? Fantastic computer job!

        • Yes, it involves amplitudes and phases — look up “interferometry”. No one knows how supermassive BH’s initially form but they grow partly by accreting gas and dust, and partly by eating the smaller but still supermassive BH’s of galaxies they collide with. Ours will continue to grow, though right now it is growing very slowly.

  4. Amazing how all these tiny bits of information can come together and create such tremendous sights, stars, black holes, galaxies and above all else us.

  5. However it is a reworked photo of 26,000 year old data. Currently the Black Hole may have changed.

    • You know, after special relativity, the concept of “Currently” is not what it used to be 🙂

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