We’ve thrown a lot of disasters at Earth over the years: from asteroids and aliens, to the Sun’s death. But hey, the Universe is a dangerous place, and we’re not done yet. Now, we’re going to throw a black hole into the mix, and not just any black hole, but a blazar.

How old is the oldest blazar? Why can we see some blazars, but not others? What is the Doppler effect and how would it let us know that a blazar is on its way?

Black holes are found at the center of most galaxies. The Milky Way has a black hole that is four million times the mass of the Sun. But that’s small compared to other black holes out there. For instance, there’s PSO J0309+27.

This galaxy is the oldest ever discovered at around 13 billion years old. And since the Universe is only 13.8 billion years old, PSO J0309+27 has basically been around almost since the beginning.

At the center of this galaxy is a supermassive black hole that could fit 250 black holes from the Milk Way. Because of this crazy big black hole, PSO J0309+27 is considered to be a blazar. And blazars are the most extreme type of active galactic nuclei, or AGN for short. But what are active galactic nuclei?

Well, there are different types, a blazar being just one of them. But basically, they’re galaxies with supermassive black holes that spew tons of energy, which could range from radio waves to gamma rays.

Because they giveo off so much energy, they are the brightest objects in the Universe. So what makes a blazar special? For one thing, they’re rare.There are anywhere from 200 billion to 2 trillion galaxies in the Universe, and only one out of every 10,000 galaxies are blazars.

But what makes them really spectacular are the jets of material shooting out from the top and bottom of their rotating disks. These jets are supercharged with magnetic fields and radiation. The material inside them can travel just under the speed of light.

And their lethal radiation beams are pointed straight at Earth. Let me be clear. It’s not just some blazars that are pointed toward us, but all of them. Any blazars that aren’t pointing their deadly jets in our direction are invisible to us.

So what blazar would be the most likely to enter our Solar System? Let’s go with the closest one to Earth, Markarian 421 at 400 million light-years away.

How would we know it’s coming? And when would it start affecting Earth? Scientists would know that Markarian 421 is getting closer to Earth almost right away. The light from the blazar would change from red to blue.

Changes to the light’s wavelength would cause this. And it has to do with the Doppler effect. This is a hard one to explain, so I’m going to use a metaphor.

You’ve just pulled over to clear a path for an ambulance. As the ambulance approaches you, its siren has a higher frequency. But once it passes and speeds away from you, its siren has a lower frequency.

Both sound and light behave like waves. So you can use the same idea for the Doppler effect. When a galaxy moves away from you, the frequency of its wavelengths become lower and shift toward red.

When a galaxy moves toward you, its wavelengths have a higher frequency, and shift toward blue. Most galaxies in the Universe are redshifted because the Universe is expanding.
So, if Markarian 421 suddenly changed to blue, astronomers would know right away that it was heading toward us. How long would we have?

Well, even at the speed of light, it would take Markarian 421 400 million years to reach Earth. But it doesn’t have to reach Earth to cause damage. Its jet could be hundreds of thousands of light-years long.

Our Galaxy is only 100,000 light-years across. So Markarian 421 would begin blasting our Solar System with radiation before it even entered the Milky Way. Blazars are also known to blast off from time to time. This giant flare can last from minutes to months.

If Markarian 421 blasted off within range of our Galaxy, its jet would punch a hole straight through our Solar System. It would be a doomsday scenario, and there wouldn’t be anything we could do to save ourselves. Luckily, this Solar System-ending event is low on our list of things to freak out about.

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