What would happen if the Sun turned into a black hole?

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Imagine for a moment that the Sun—our source of life and light—suddenly transforms into a black hole of the same size. What would change in the solar system? Would Earth continue orbiting or fall directly into it? Could moons and planets survive? This article dives into the effects of the collapsed Sun on us and the entire solar system, explaining what would happen to orbits, gravity, and, of course, life on Earth! Get ready for a journey through physics, gravity, and the laws of the universe—with a touch of humor, because if you’re going to crash into a black hole, you might as well do it with a smile.

From Sun to Black Hole

What Is a Black Hole?

A black hole is a region of space–time in which gravity is so intense that nothing, not even light, can escape it. This immense gravitational force arises from a large amount of mass concentrated in an extremely small volume. Black holes generally form when a very massive star collapses under its own gravity at the end of its life.

There are different types of black holes, the most common being:

Stellar black holes: Formed by the collapse of a massive star that has exhausted its nuclear fuel.
Supermassive black holes: Found at the centers of most galaxies, including the supermassive black hole at the center of the Milky Way.
Intermediate black holes: A middle type believed to have masses between those of stellar and supermassive black holes.

A black hole has three main parts:

Event horizon: The “surface” that marks the boundary beyond which nothing can escape. Once something crosses this boundary, it cannot return.
Singularity: The point at the center of the black hole where density is infinite and the laws of physics as we know them cease to apply.
Photon sphere: Although everything inside the event horizon is dark, light just outside it can orbit or move without being swallowed. The hot plasma nearby emits streams of photons that, under normal circumstances, would travel in straight lines, but the intense gravity is strong enough to bend their paths.

Black holes are fascinating objects that remain an active area of research in astrophysics.

The Black Hole

The Sun itself cannot instantly turn into a black hole, but if it did—while retaining its mass—nothing interesting would happen. To make the scenario more intriguing, let’s assume the Sun becomes a black hole with the Sun’s radius, which implies its mass will change.

When we speak of a black hole’s radius, we mean the Schwarzschild radius, which is the radius of its event horizon and has a direct relation to the black hole’s mass.

A black hole of one solar mass (M☉) has a radius of approximately 3 km. Although one can calculate our black hole’s mass using that relation, there is a more exact formula:

r\_s\ =\ \frac{2GM}{c^2}

where r_s is the Schwarzschild radius, G is the universal gravitational constant, M is the black hole’s mass, and c is the speed of light. Rearranging, we obtain:

M=\frac{r\_sc^2}{2G}

Substituting the variables with their respective values, we find that the mass of our black hole is 4.6914 × 10³⁵ kg, about 236,000 times the Sun’s mass:

\frac{6.9634\cdot{10}^8\times1.989\cdot{10}^{30}}{{299,792,458}^2}=4.6914\cdot{10}^{35}

Initial Assumption

With our black hole’s characteristics established, we can begin the scenario.

The Sun instantaneously transforms into a black hole with the same radius (≈ 696,340 km) and a mass of about 4.6914 × 10³⁵ kg.

This hypothesis does not violate any physical law, but it is quite extreme.

What Happens to Earth?

Gravitational Force

Earth, like the other planets, orbits the Sun because there is a balance between the planet’s centripetal force and the Sun’s gravitational force. However, when the Sun becomes a black hole, its mass increases—so does its gravitational force. This upsets the balance, and the black hole pulls the planets inward instead of allowing them to continue orbiting.

\frac{M_{Earth}\cdot v^2}{r}=G\cdot\frac{M_{Earth}\cdot M_{Sun}}{r^2}

Balance between Earth’s centripetal force and the Sun’s gravitational force.

\frac{M_{Earth}\cdot v^2}{r}<G\cdot\frac{M_{Earth}\cdot M_{BlackHole}}{r^2}

Imbalance between Earth’s centripetal force and the black hole’s gravitational force.

Acceleration Toward the Black Hole

This imbalance and gravitational force produce an acceleration of Earth directed toward the black hole. One way to calculate this acceleration is via the gravitational force exerted by the black hole, then using Newton’s second law. Another way is to apply the gravitational acceleration formula, which combines the two and eliminates the attracted object’s mass; thus, gravitational acceleration is independent of the object’s mass.

F_{Grav}=G\frac{M_{BlackHole}\cdot m_{Earth}}{r^2}

Universal gravitation formula.

F_{Grav}=m_{Earth}\times a \;\to\; a=\frac{F_{Grav}}{m_{Earth}}

Newton’s second law.

a=\frac{G\frac{M_{BlackHole}\cdot m_{Earth}}{r^2}}{m_{Earth}}\;\to\;a=\frac{G\cdot M_{BlackHole}}{r^2}

Gravitational acceleration formula.

Since the gravitational constant and the black hole’s mass are constant, we can substitute them to reuse the formula:

g_{BlackHole}=\frac{6.67\cdot10^{-11}\times4.6914\cdot10^{35}}{r^2}=\frac{3.1292\cdot10^{25}}{r^2}

Gravitational acceleration.

To find Earth’s acceleration, substitute r with the distance between Earth and the Sun (or black hole), one astronomical unit (≈ 149,597,870,700 m), giving:

a_{Earth}=\frac{3.1292\cdot10^{25}}{(149597870700)^2}=1398.2425

Comparing this with Earth’s surface gravity (9.81 m/s²) yields about 142.5G.

Effects on People

A force of about 142.5G is extremely high. An average person can tolerate up to about 5–6G, and a trained pilot up to 9G, but 142.5G would cause:

Blood failing to reach the brain properly.
Loss of consciousness in under a second.
Catastrophic damage to internal organs, tissues, and blood vessels.

One might think people would be thrown off the ground or crushed against it, but gravity affects all material bodies equally—including people, animals, etc.—so we would move along with Earth. However, due to Earth’s large radius, there is a slight acceleration difference between the side closer to the black hole and the side farther away. This tidal variation is about 0.126 m/s² between the nearest and farthest points, being zero at the midpoint. This difference wouldn’t throw you off because Earth’s own gravity counters it.

How Long Until the Planets Fall into the Black Hole?

Free-fall Time Formula

The simplest approach would be to use the equations of uniformly accelerated linear motion:

d=d_0+v_0t+\frac{1}{2}at^2

where d is the final distance traveled, d₀ the initial distance, v₀ the initial velocity, a the acceleration, and t the time.

The problem is that acceleration isn’t constant—it depends on distance to the black hole—so this yields a significant error. To correct for this, we use the radial free-fall time formula:

T_{Fall}=\int_{r_{final}}^{r_{initial}}\frac{dx}{\sqrt{2GM_{BlackHole}\left(\frac{1}{r}-\frac{1}{r_0}\right)}}

With r_Final = 0 and r_Initial as the object’s starting distance from the Sun, solving the integral gives:

T_{Fall}=\frac{\pi}{2}\sqrt{\frac{r_0^3}{2GM_{BlackHole}}}=\frac{\pi}{2}\sqrt{\frac{r_0^3}{6.2584\cdot 10^{25}}}

Earth’s Free-fall Time

With r₀ = 1 AU, the fall time is 3 h 11 m 29 s (11,488.91 s):

T_{Fall,Earth}=\frac{\pi}{2}\sqrt{\frac{(149597870700)^3}{6.2584\cdot10^{25}}}=11488.91

Other Planets Fall Times

Using:

T_{Fall} = \frac{\pi}{2}\sqrt{\frac{(149597870700 \times d_{AU})^3}{6.2584\cdot10^{25}}}

Planet	Distance	Fall Time
Mercury	0.387 AU	0 h 44 m 30 s (2,670.03 s)
Venus	0.722 AU	1 h 57 m 28 s (7,048.30 s)
Mars	1.524 AU	6 h 0 m 15 s (21,615.05 s)
Jupiter	5.203 AU	37 h 52 m 31 s (136,351.39 s)
Saturn	9.539 AU	94 h 1 m 20 s (338,480.08 s)
Uranus	19.192 AU	268 h 19 m 20 s (965,960.13 s)
Neptune	30.058 AU	525 h 54 m 58 s (1,893,297.79 s)

Tidal Deformation

Since acceleration depends on 1/r², there is a difference in acceleration between an object’s near and far sides, causing tidal deformation. This makes planets and other bodies stretch (“spaghettify”) with an inverse-square profile—more stretching on the side facing the black hole than on the far side. A cubic object would appear conical or triangular in its elongation.

This effect intensifies as the object approaches the black hole.

Tidal deformation isn’t unique to black holes; moons around planets also experience slight gravitational deformation. That is why the Moon’s rotation period matches its orbital period.

Other Curious Effects

Extreme Ocean Waves from Weight Deformation

The black hole’s gravitational pull on Earth’s oceans could generate “extreme tides.” These tides would deform the water and alter wave frequency and amplitude.

Normally, ocean waves form from wind action on the water’s surface. Under a black hole’s gravity, waves would also be influenced by an additional acceleration. This could make waves not only larger in amplitude but also faster and more destructive. Depending on proximity and gravitational strength, waves could reach unimaginable heights.

Probable Atmospheric Resonance → Instantaneous Climate Effects

The atmosphere would also be affected. Sudden acceleration might induce gravitational resonance in atmospheric layers, resulting in pressure waves, extreme cyclones, and possibly “apocalyptic” cosmic thunder from violent air deformation.

Moreover, as the atmosphere moves at a different pace than solid ground, unpredictable storms and supersonic winds could appear—perhaps the final weather forecast would read: “Today: 3,000 km/h winds with a chance of one-way levitation.”

Interesting Visual Effects Watching Other Planets Fall

As Earth falls into the black hole, you’d witness bizarre sights in the sky (assuming you still have functional eyes and haven’t lost consciousness from 142.5G):

Inner planets like Mercury and Venus would fall faster than we do, so you might see them streaming toward the black hole in nearly straight lines, visually stretched by relativistic effects.
Outer planets would lag behind. From our vantage, they’d appear more “leisurely,” but as light bends and gravity distorts space, they might seem to loop or wobble as if intoxicated.

All of this is thanks to gravitational lensing, which warps light and makes watching the solar system in free fall feel like being inside an Einstein-designed kaleidoscope.