Hacker News Reader

Comment by trhway | Hacker News Reader

>One of them is the speed with which stars orbit the center of a galaxy

do we really orbit it? I mean is it a stable orbit (that is the option which requires DM ) or a just a spiral trajectory (no DM required)? The spiral galaxies look like a fireworks wheel https://depositphotos.com/69366109/stock-video-fireworks-pin... , and without DM our "orbital" speed being higher than otherwise expected Keplerian orbital speed means that we're just moving away on a spiral trajectory from the center the same way like the sparks fly away on a spiral trajectory from the center of the fireworks wheel.

View on HN

If you assume that almost all the mass of a galaxy is in its nucleus, then the only "orbits" allowed are conical sections (ellipses, parabolas, hyperbolas): there is no such thing as a spiral trajectory. You would need a very unusual mass distribution to have spiral trajectories ... and it would imply that galaxies are not stable which would contradict what we see.

We assume stable orbits because it is unlikely that every galaxy we look at, near and far, exploded at the same time, and we just happen to be seeing them at the same moment in their evolution as they fly apart.

People have really though this stuff through! Some of them for years!

There are plenty of non-spiral galaxies where we observe the same.

Our sun moving away from the galactic center is also unlikely as if it had been much further in during it's creation phase, it would have been unable to create planets that support life. A galactic year is only a few million earth years, so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it. That points to a stable orbit around the galaxy.

If it was a firework wheel our current position would mean that sun would have been traveling to the outside of the galaxy far below escape velocity or would have been unable to produce life-bearing planets.

>A galactic year is only a few million earth years

no. It is 250 million years.

>so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it.

not really. While moving on that spiral trajectory, we've made only 16 rotations since Sun's formation. When the life on our planet started our neighborhood looked very different, not the boonies we're at right now :)

>no. It is 250 million years.

I don't see how that contradicts is being a few million years?

>not really. While moving on that spiral trajectory, we've made only 16 rotations since Sun's formation. When the life on our planet started our neighborhood looked very different, not the boonies we're at right now :)

Orbits don't really do spiral patterns in practice. Atleast not at this scale. The star would either be in a fairly stable orbit or on trajectory to leave the galaxy (minus elliptical orbits but those would carry the sun in and out of the dead zone quite often).

Moving on a spiral trajectory does happen in orbital mechanics but only in one direction; inwards. Outwards spiraling would require quite a few quirks in the galactic gravity field.

The spiral pattern itself isn't the result of a firework wheel, it's quite a quirk in the distribution of stars, though there are various models that explain them, they all involve very stable orbits for most stars.

If they are flying away, something would need to add kinetic energy to the stars over time. Also, galaxies should then start small and disperse as they age.

>Also, galaxies should then start small and disperse as they age.

not exactly. The gravitation gathers the stuff closer, and at the same time the rotational momentum must be preserved. So initially that causes the galaxy matter into elliptical shape and from elliptical galaxy the disk forms and is thinning and spreading further to preserve the total rotational momentum of the galaxy compensating for the loss of the momentum by the matter gathering at the bulge. The radiation pressure seems to be a major component mediating that process (and in particular causes and supports the push away of the stars from the bulge&bar - part of the kinetic energy addition you mentioned).

This is why we don't "observe" that much DM in elliptical galaxies as that momentum spread by means of forming the disk is only starting there.

Would the spinning motion of the galaxy affect us any differently than the way the rotation of a planet affects a spacecraft during a gravity assistance boost maneuver?

You mean, not?

It's not the rotation that assists, it's the planet's linear -- well, bent, but that doesn't help -- that does it.

There is this notion of "frame dragging" in general relativity, but its effect is practically impossible to measure. They lofted a satellite to measure its effect over several years because it took that long to accumulate enough to measure.

Reply on news.ycombinator.com