# Black holes and gravity basic questions

are black holes just dead stars with lots of mass which shrunk to small size and got lots of gravity, or an actual hole in space-time? if they're actually holes, where do they lead stuff that gets in them? if they're just dead stars with lots of gravity, their mass must grow over time because of all the dust and stars they attract, right?

now, about gravity, why do stars with lower gravity just circle around other stars with bigger gravity instead of colliding into each other? if they're attracted to each other because of gravity, what's keeping them apart so they don't collide?

mfb
Mentor
They are not like holes in a material. Their Schwarzschild radius surface is a one-way boundary - you can go inwards, but never come back.

where do they lead stuff that gets in them?
To the center, probably to a singularity there, but a theory of quantum gravity might be necessary to explore this in detail.

if they're just dead stars with lots of gravity, their mass must grow over time because of all the dust and stars they attract, right?
Black holes will grow if mass is falling into them, right.

now, about gravity, why do stars with lower gravity just circle around other stars with bigger gravity instead of colliding into each other?
This is similar to the moon and earth, or satellites and earth, or all other orbits.
Attach a massive object to a rope, whirl it around: The object will feel a constant force towards you, but never reach you. In a similar way, all orbiting objects are constantly "falling" (accelerating) towards the central object, but as they have a tangential velocity, they "always miss it". There are some better descriptions somewhere in the internet.

Drakkith
Staff Emeritus
Remember that in space you continue to move even if you don't have a force applied. So even though gravity is pulling two things together, they already have motion relative to each other. For example, the moon orbits the Earth with a velocity of about 1 km/s. So while the Earth accelerates the Moon towards it, the Moon moves fast enough to avoid actually falling into the Earth. Think of a thrown object. If you barely throw something it will go a short ways before impacting the ground. The harder you throw it the further it will go before gravity can pull it to the ground. However, if you throw it fast enough the object will not impact the ground because gravity cannot pull it hard enough to make up for the ground "falling away" from the object as it travels.

thanks. so stars don't collide into each other because they move fast enough / have enough kinetic energy to partially defy gravity? they have enough energy to stay away from each other but they don't have enough energy to completely get out of each other's gravitational field, so they just orbit around the gravitational field? so if the moon was moving at a velocity of, like, 1cm/year, it would collide into earth?

mfb
Mentor
thanks. so stars don't collide into each other because they move fast enough / have enough kinetic energy to partially defy gravity?
I would not use "defy" here - gravity still works, and accelerates the objects as always.

so if the moon was moving at a velocity of, like, 1cm/year, it would collide into earth?
It would, and it would crash into earth within a few days.

thanks. so stars don't collide into each other because they move fast enough / have enough kinetic energy to partially defy gravity? they have enough energy to stay away from each other but they don't have enough energy to completely get out of each other's gravitational field, so they just orbit around the gravitational field? so if the moon was moving at a velocity of, like, 1cm/year, it would collide into earth?

Some stars will eventually collide with other stars. Especially rogue stars that are flung away from their orbits after being gravitationally disturbed by a black hole swallowing its binary partner.

Such a star is sling-shotted away as its source of orbital stability is consumed and its direction might very well eventually result in either assuming another orbit or else a direct collision with another celestial body-such as a planet, moon, asteroid, or star.

cepheid
Staff Emeritus
Gold Member
thanks. so stars don't collide into each other because they move fast enough / have enough kinetic energy to partially defy gravity?

Just to emphasize what mfb said, it's not defying gravity at all. In fact, gravity is what's needed to keep the planet moving in a bound circular or elliptical orbit around the star. Without gravity, the planet would just fly off in a straight line, never to be seen again. It wouldn't be gravitationally bound to the star.

This is just a result of the way gravity works. If the object is just initially at rest, (no initial momentum) it will just fall straight inward towards the other gravitating body. But if the object has some "sideways" motion i.e. not directly towards or away from the other object, but in the tangential direction (more formally, if the object has some angular momentum), then the effect of gravity is to cause the object to move in a curved path around the star. The shape of the curved path depends on the amount of angular momentum that is present. It could be a circle, an ellipse, a parabola, or a hyperbola. The idea is that the object's path will curve towards the star, but it will not actually hit it. If the path is a closed orbit (like a circle or an ellipse), then you can think of the object as continuously falling "towards" the star but missing it.

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davenn