# Why the spiral arms in the galaxy?

We say that the center of the galaxy is a black hole with an event horizon , nothing can escape its gravity, its gravity is responsible for holding the entire galaxy together. Then why do arms always come out the center itself? Why cant the center suck back the stars near to the spiral once it come out, If the spiral centrifuge created the stars the centrifuge is powerless once star comes out of the event horizon. Shoudnt it suck the stars back in?

• Why should it suck in or grow bigger? Why does me swing a brick on a string not cause the brick to suck in toward the center when the string is pulling inward? Or the brick fly out when the centrifugal force pulls it outward? There's a balance of forces and an equilibrium to be found. And not all galaxies have arms, or are even spirals. Nov 25, 2022 at 16:59
• scientificamerican.com/article/…. Centers do suck in matter back. Nov 25, 2022 at 17:01
• so there does seem to be a massive force at the center Nov 25, 2022 at 17:03
• I'm sure you know the strength of gravity decreases with distance. But yes, there is that whole dark matter conundrum. Nov 25, 2022 at 17:03
• The dark matter is spread out, so the force is spread out. There is a (relatively) small spike at the galactic center. Nov 25, 2022 at 17:05

We say that the center of the galaxy is a black hole with an event horizon

You cannot have a black hole without an event horizon.

nothing can escape its gravity

Not true. It's just a large mass object as far as anything else in the universe is concerned.

, its gravity is responsible for holding the entire galaxy together.

No.

The mass of the black hole is "just" $$4\times 10^6$$ solar masses. The mass of the entire galaxy is something like $$10^{12}$$ solar masses. The black hole is just one millionth the mass of the entire galaxy. What holds the galaxy together it all the mass inside and the dark matter halo around it.

Then why do arms always come out the center itself? Why cant the center suck back the stars near to the spiral once it come out,

Stars are in complex motions within the galaxy. There's no reason they should be "sucked" into the core.

If the spiral centrifuge created the stars the centrifuge is powerless once star comes out of the event horizon. Shoudnt it suck the stars back in?

The stars did not come out of the black hole at all. The black hole formed and grew, but remaining stars in the galaxy formed outside the event horizon from material outside the event horizon.

its gravity is responsible for holding the entire galaxy together.

This is a misconception.

Many people may think that some black holes hold galaxies together like the Sun holds the solar system together. In the solar system's case, the Sun has over 99% of the total system's mass. For our galaxy, Sagittarius A* has a mass of about 4.1 million solar masses (give or take), while the Milky Way has a mass of 890 billion solar masses to 1.54 trillion solar masses (or 1.2 to 1.9 trillion solar masses, depending on your sources). So you see, Sagittarius A* has only about 0.00046% of the mass of the total system at most.

A mainstream idea in astrophysics right now is that galaxies (not just our own) are held together by dark matter. This also explains why stars orbit them at different speeds than what we would expect if the galaxies were held together just by the mass we can observe.

And last but not least: things can orbit a black hole beyond its horizon event. It will not suck anything in just in the same way that the Earth does not get sucked into the Sun.

If the spiral centrifuge created the stars the centrifuge is powerless once star comes out of the event horizon

This makes no sense. Nothing comes out of an event horizon of a black hole, and black holes are not the source of the stars in a galaxy.

• Dark energy does not hold galaxies together. Dec 4, 2022 at 19:45
• @PeterErwin true, fixed. Dec 4, 2022 at 21:43
• Is the gravitational barocentre of the galaxy centred at the black hole? Would that add to its mass? Dec 4, 2022 at 21:47
• @AbdulMoizQureshi no and no. Even if the barycenter was centered at the black hole its mass would not be affected. Dec 4, 2022 at 23:43