It's ! Do you know what an X-ray binary is?

An X-ray binary is one of the most common ways we see black holes and neutron stars, which are dead stars with extreme physics.

[Side note: Black holes and neutron stars together are called "compact objects" because at a macroscopic distance, they're more alike than different!]

The "binary" part of "X-ray binary" comes about because the compact object has a star friend who's a regular star like our Sun, and the compact object and star friend orbit around each other. But! the compact object is a bad friend. The outer bits of the star friend are more gravitationally attracted to the compact object than to its own star, so the compact object slowly drains and eats its friend.

If the star friend is a smol star just like our Sun, the star stuff swirls towards and around the compact object. It's as if the compact object is sitting in the middle of its dinner plate, greedily gathering up star stuff to eat.

If the star friend is big & fluffy, it suffers from a lot of wind (don't be rude) and regularly loses little bits of gas in clumpy winds. These clumpy winds often intersect the orbit of the compact object, which sweeps through and eats them up like Pacman.

In both cases, we get a bunch of hot star stuff falling towards and swirling around very close to the compact object. It gets thick and has a lot of friction, and just like the friction of rubbing your hands together, that generates a LOT of heat ❤️‍🔥

Everything that is warm shines heat light (in physics jargon, blackbody radiation), and the warmer the thing, the more energetic the heat light.

Let's talk about heat light for a sec. The "empty space" in the universe is still a little warm from the Big Bang, and it shines its heat light in microwave light (we call the cosmic microwave background), and yes these are the same microwave energies that your microwave oven in your kitchen uses to reheat leftovers!

The human body is warm (warmer than the empty space in the universe), and we shine our heat light in infrared light. That's how night vision goggles work. Conventional kitchen ovens also use infrared energy light to heat food, so you can call your appliances "the infrared" and "the microwave" if you're a smartass.

The Sun is hot (hotter than a human body), and it shines its heat light in visible light. That's how and why eyeballs work! 👀

Around the compact object, this very hot star stuff is about 100x hotter than the surface of the Sun (like 20 million °F, or AlsoMillionsThoughLessThan20 °C), so it shines its heat light in X-ray light! That's where the "X-ray" part of "X-ray binaries" comes from.

When we look at X-ray binaries, they appear as a tiny dot or single pixel (also called a "point source").

If we wanted to take a picture of the event horizon of a black hole in an X-ray binary in our galaxy (like how the Event Horizon Telescope did for the supermassive black hole M87*), the equivalent imaging resolution would be like trying to see the width of a human hair on the surface of Mars (at its closest approach to Earth).

Math about that here:

Also! the X-rays from X-ray binaries (and any other X-ray and gamma-ray radiation from outer space) cannot get through Earth's atmosphere (yay for the evolution of multicellular life, sad for X-ray astronomers like me), so we have to put X-ray telescopes on satellites that get launched into space on rockets. We'll talk more about X-ray telescopes another time 🚀🛰

@sapoverde Glad you enjoy 😄 check back next Thursday! Likely non-science until then 😂

@alstev incredibly cool thread and impressive use of straightforward language for such an advanced subject, thank you!

@LazyTechsupport Yay, thank you! I really love communicating science in an easy-to-understand way 😊

@alstev > yay for the evolution of multicellular life, sad for X-ray astronomers

🎶 It's like raaa-aa-aain…on your wedding day… 🎶

@alstev this is very cool! if you don't mind: heat light?

i know heat as the energy with the making-atoms-vibrate, and light is somehow a particle and a wave (? which means almost nothing to me lol) and happens at different frequencies = preceived at different colours. Any chance you'd be up for explaining the bit I'm missing here? (no worries if not! it's not important, im just curious)

@certifiedperson The proper term is "blackbody radiation", and it comes about when there's a difference in temperature (temperature being internal energy from the atoms vibrating) between the thing and its surrounding environment. Since the laws of physics want things to go to equilibrium, the hotter thing will radiate energy ("pure energy"=light) into the surrounding environment!

@alstev oh my god that is simultaneously cool yet wack haha

thank you for explaining & for the link!

Is my memory correct when I say the first suspected (or proven?) Black hole was Cygnus X-1 back in the early 90's? And it was detected though the interaction you describe? Your post really jostled some dusty memories!

@paulgatling Yes, the first black hole we ever saw is called Cygnus X-1 and it's in a high-mass X-ray binary! It was first seen in the 1970s with X-ray detectors flown on high-altitude balloons and sounding rockets.

Off by a few years! I remember researching this stuff in high school and the very idea of a black hole at the time was almost a supernatural idea, but then I learned about neutron stars and my mind was blown!

I definitely did not know about the balloons used for x-ray imaging, how cool!

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