Black Hole Sun - Replacing Our Star With A Black Hole

The Earth orbits the Sun at a distance of about 150 million km (96 million miles). We move at a speed of almost 30 kilometres per second (18.5 miles per second) and it takes one year to complete an orbit (duh, obviously).

Our Sun has a mass of 2 x 10³⁰ kg which is about 330,000 times the mass of the Earth. The radiant energy received on the Earth at the equator is about 1.36 kW/m² (less as you move towards the poles). It is this energy basically drives life on the Earth barring some minor amount of chemical energy from hydrothermal vents.

So what would happen if we replaced our beloved Sun with a black hole of the same mass?

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<<< Soundtrack to this post: Soundgarden - Black Hole Sun >>>

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How Would Our Orbit Change?

Basically there would be no change whatsoever to the Earth's orbit. At distances far from a black hole the solutions to General Relativity and Newton's Law of Gravity give basically the same answer.

It is only when you get close to a black hole that the gravity well deepens and the effects of relativity start to become noticeable.

So the Earth would continue orbiting as if nothing had changed.

A Quiescent Black Hole

As far as I understand most black holes are what you would call quiescent (quiet and well behaved). That means they have no accretion disk and give off no radiation whatsoever (i.e. no light and no heat).

This would spell disaster for life on Earth except possibly for the life huddled around those hydrothermal vents that I mentioned earlier.

The Event Horizon for a black hole with the same mass as our Sun is calculated to be about 3 km. At our distance of 150 million km we would be completely unable to see even the distorted star field effect that the black hole would generate.

The sky would be black, the atmosphere would be condensed onto the cold hard ground (except for the trace amounts of helium) and eventually even the oceans would freeze solid. Bye, bye hydrothermal vent creatures.

A Black Hole With An Accretion Disk

When a black hole gobbles up large amounts of matter we know that this matter can sometimes form what is called an accretion disk. This is a large, circular and flat disk of swirling matter that orbits a black hole.

The particles (either dust or gas) will constantly bump into one another and lose energy via this frictional force and slowly drop closer and closer to the Event Horizon.

The closer the matter gets to the Event Horizon the faster it will orbit. The speeds and the collisions become so enormous that this heats up the inner parts of the disk and it glows brightly. This glow is what astronomers detect when they 'see' black holes in their radio telescopes.

The accretion disk of a black hole is said to radiate primarily in the X-ray part of the spectrum but no doubt it will also radiate in the visible bands as well.

This means that the Earth orbiting a black hole with an accretion disk might get some light and heat. Whether it would be enough to sustain life depends on the vigour of the accretion disk itself. Also accretion disks don't last forever, they will eventually be consumed and disappear leaving you with a quiescent black hole and a frozen Earth.

The accretion disk would be a sight to behold in the 'daytime' sky. A large, swirling and glowing mass of matter swirling down to its doom.

A Black Hole With An Accretion Disk and a Relativistic Jet

The accretion disk will be a hot glowing plasma (i.e. electrically charged ionized gas). This means strong magnetic fields which also means that some of those ions will be ejected out at relativistic speeds from the disk along the lines of magnetic force.

There will be two jets, the positive ions in one direction and the negative ions in the opposite direction. If those jets are pointed away from the Earth then it would just add to the light show and the wondrous sight of the Black Hole Sun in the sky.

If those jets are pointed at the Earth then you can kiss your backside goodbye as nothing good will come out that situation. Very likely the Earth's atmosphere will be ablated away but before that all life on Earth will be fried by the intense radiation.

It would likely be a PhD project to calculate, but I suspect that the Earth would be heated up and eventually turned into a giant lava ball.

Fun times.

Closing Words

It needs to be said that our Sun does not have the mass required to turn it into a black hole. It is simply not heavy enough to overcome the electron degeneracy pressure that would resist that final collapse when the nuclear fuel finally runs out.

As a thought experiment though, if the Sun did turn into a black hole and the Earth somehow survived that process the resulting effect would leave our orbit alone but not much else.

I can't see any good ending to that story for life on Earth. It would be an awesome event to witness though before the inevitable end.

Thank you for reading my post.

Post Sources

Inspiration for this post:
[1] Soundgarden - Black Hole Sun
Information references:
[2] Earth data reference
[3] Sun data reference
[4] Solar Irradiance
[5] Hydrothermal vents
[6] Black holes
[7] Event horizons
[8] Black hole Schwarzschild radius calculator
[9] Accretion Disk
[10] Electron Degeneracy Pressure