# B The unusual motion of stars in galaxies

#### Wallsy

Ive been lying in bed this morning wondering about the unusual motion of stars in galaxies.The whole disc rotating like a record at the same speed.Does it mean that the effects of gravity are simply altered by speed?Much like a wonder-wall.So the faster the speed of the object,or stars,the less gravity pulls downwards and the more it pulls sideways.Holding the stars in place.Just like a motor cyclist on a wonder wall?

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#### Wallsy

Ive been lying in bed this morning wondering about the unusual motion of stars in galaxies.The whole disc rotating like a record at the same speed.Does it mean that the effects of gravity are simply altered by speed?Much like a wonder-wall.So the faster the speed of the object,or stars,the less gravity pulls downwards and the more it pulls sideways.Holding the stars in place.Just like a motor cyclist on a wonder wall?

#### phinds

Gold Member
Ive been lying in bed this morning wondering about the unusual motion of stars in galaxies.The whole disc rotating like a record at the same speed.
Read up on the effects of Dark Matter.

I take it you didn't like the answers you received over a year ago in

The answers and the mechanism haven't changed and just asking the question again isn't going to make them change.

#### Janus

Staff Emeritus
Gold Member
Ive been lying in bed this morning wondering about the unusual motion of stars in galaxies.The whole disc rotating like a record at the same speed.Does it mean that the effects of gravity are simply altered by speed?Much like a wonder-wall.So the faster the speed of the object,or stars,the less gravity pulls downwards and the more it pulls sideways.Holding the stars in place.Just like a motor cyclist on a wonder wall?
1. Galaxies do not rotate like a record. With a record, the parts further out have to travel faster in order to complete a revolution because they have to travel a larger circle in the same time. In galaxies with a flat rotation curve, the velocities of the stars don't change as you move outward in the disk. Thus the stars further out take longer to orbit the galaxy than the ones closer in, having to travel a longer distance at the same speed.

2. There are a couple of effects in play with the car on the wall. The wall is tilted at an angle so some of the outward centrifugal effect is given a upward component which works against gravity( which remains in full force and does not pull down any less). Then there is the friction between the car tires and wall. For the car to slip down the wall, the car would have to skid on its tires sideways against this friction. The outward centrifugal effect (or centripetal acceleration) increases the force between tire and wall, which increases the friction between them, making it so that the car has less of a tendency to succumb to the relentless pull of gravity and slide down the wall.
Neither of these effects have any bearing on stars orbiting in a galaxy.

Mentor
Gold Member

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#### berkeman

Mentor
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Mentor

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