Naked and Alone

Try as we might, we just can’t see a black hole. Are they invisible, like Harry Potter in the cloak of invisibility? Well, not quite. They just don’t emit their own light and any incident light hitting them is absorbed. However, a black hole can distort an object behind it.

The black holes found so far are associated with stars. They are found via two methods:

By seeing star stuff getting caught by, whizzing in orbit around the black hole, and shining brightly.

By recording groups of stars whizzing around apparently empty space.

So, how do we find and “see” a naked, lone black hole?

Let me back up a bit and describe what a black hole is for those who need context.

A black hole can be the end product of very massive stars, depending on variable situations. A very massive star’s core will eventually contract, like most stars. Unlike most stars however very massive stars fuse most of their hydrogen, then helium, then oxygen, etc. until the core has a lot of iron. Iron fusion requires more energy than it produces so fusion sputters and the massive core collapses with a bang! Gravity wins! The result of a massive star’s core collapse can take three forms:

A neutron star – not really a star, because fusion is not occurring. Neutron stars exist because atomic neutrons have enough strength, called degeneracy pressure to hold gravity at bay. They are small, around 20 miles across, but with more mass than the Sun.

A black hole – this happens when even neutrons cannot hold back gravity and the core collapses into an infinity – and is called a singularity. Weirdness.

Stardust – the core blows up entirely, spreading newly formed heavy elements throughout the universe.

A black hole then is a strange place, with a singularity of unknown size (infinitely small), and a surrounding intense gravity field, its diameter varying with its mass. A certain distance from the singularity is what’s termed the event horizon, you do not want to go there. Within the event horizon every form of energy is trapped by gravity and cannot escape. That’s why a black hole does not shine by itself, nor reflect light.

Finding a naked black hole is tricky but doable, and teams from the Space Telescope Science Institute and the University of California, Berkeley think they might have bagged one. The evidence comes from images of a known star that briefly dimmed, then brightened. The dimming however was like the microlensing seen when galaxy clusters distort images of objects behind them. The intense gravitational field of a black hole can do that, but on a much smaller scale so they are difficult to spot.

What’s in the Sky?

Comet C2022 E3 (ZTF) is up there but you need binoculars at least to see it. It should brighten in coming weeks. Stay tuned.

Astronomy Night at Tye Preston Memorial Library, Canyon Lake: 01/21/23, 7pm