APOD: Hydrogen in M51 (2010 Jun 11)

[flash]

I assume that the stars in galaxies orbit "inward" toward the center of the galaxy in much the same way that winds flow inward towards the center of a low pressure system. If that's the case, then the stars in this galaxy (and perhaps also the entire galaxy) orbit in a counter-clockwise direction (as seen from here). But the spiral is described as clockwise (spiraling outward). This seems counter-intuitive. Why would the spirals be described in the outward direction when the stars are being influenced by gravity in the inward direction? I have also wondered whether the motions of the individual stars in their orbits are necessarily moving in the same direction as the spiral arms... I have heard that the spiral arms are merely waves of concentrated mass. Couldn't that allow that the stars and the spiral arms are moving in opposite directions?

[Chris Peterson]

I assume that the stars in galaxies orbit "inward" toward the center of the galaxy in much the same way that winds flow inward towards the center of a low pressure system.

They do not. They are in elliptical orbits like all bodies in closed orbits. That is, the distance of stars from the center does not change over time (outside the variations caused by eccentricity and perturbations). The spirals are just density fluctuations moving through the disc of stars. They move slower than the stars- that is, stars move into spirals from behind, pass through, and then exit from the front.

[Guest]

WOW!! Just WOW! Amazing creations that we discover all the time in our vast universe. I asked my father what he thought this galaxy looked like. He said a shell, my sister said a shell also, my mother said a tornado and I thought it looked like a jewel, priceless and worth a thousand words, hanging upon the lace of gravity in it's "small" place in the universe. The universe is so large, it goes to show how small we really are! And funny thing is, the more we discover, the more we really don't know!

-H-

[Guest]

Oh how bright and beautiful
Your majesty is!
A tear drop shining in amongst
Diamonds of the night.
Dark and dense forest is around you,
But you sparkle blue and bright.
Red pure jewels- hanging
Upon the necklace
Of the heavens.
Strewn with diamonds and
Shaped as a shell,
The wonder of creations like you
Haunt me even more.
I chase it and still the less I know,
The farther I become
And the smaller my home is from you.
You're shaped in His hand
And move with time
Like the waves of the sea.
How majestic and beautiful
You have come to be!
My jewel of the night.

[flash]

Chris,
Thanks for the reminder. I realize that the individual stars orbit in ellipses and do not move inward. Winds around a low pressure center move similarly I believe. In both cases there is a central force (gravity or air pressure) and conservation of angular momentum causes tangential velocity to be larger toward the center in both cases. The similarities of the appearences of cloud formations and galaxy spirals is obvious. When I imagine a galaxy with a hypothetical 'initial state' to be with the spiral arms oriented radially (linear) instead of spiraling and then consider the evolution in fast forward, I imagine the radial arms would bend into spirals because the stars in the inner orbits move faster than those in the outer orbits. But perhaps this thinking is flawed because as you say "stars move into spirals from behind". Can you clarify?

[Chris Peterson]

In both cases there is a central force (gravity or air pressure) and conservation of angular momentum causes tangential velocity to be larger toward the center in both cases.

I don't think it is useful, or entirely correct to consider the conservation of angular momentum to be the reason that tangential velocities are higher near the center of galaxies. Conservation applies to the change in velocity you get if something forces a body inwards. But that isn't happening. The orbital speed of stars within the galaxy is determined by simple orbital mechanics, which doesn't depend on the conservation of angular momentum.

The similarities of the appearences of cloud formations and galaxy spirals is obvious. When I imagine a galaxy with a hypothetical 'initial state' to be with the spiral arms oriented radially (linear) instead of spiraling and then consider the evolution in fast forward, I imagine the radial arms would bend into spirals because the stars in the inner orbits move faster than those in the outer orbits. But perhaps this thinking is flawed because as you say "stars move into spirals from behind". Can you clarify?

A spiral arm is somewhat analogous to the density fluctuation you see in traffic jams. A jam moves forward, but it moves forward slower than any individual cars. Cars come up on a jam from behind, work their way through it, and emerge on the front. In the frame of reference of the highway, the jam moves forward, but in the frame of reference of the cars, it moves backwards. It's the same with stars and spiral arms. As stars orbit, they will move into a spiral arm, through it, and out again. In the galaxy's frame of reference, both the stars and the arms are moving in the same direction. But in the frame of reference of a star, the part of a spiral arm at its radius is moving backwards- slower in its "orbit" than the star is moving.

[neufer]

ALL RIGHT!!!! - Flower - Power!!!!

BP Solar is a leading producer of solar panels since
its purchase of Lucas Energy Systems in 1980.
--------------------------------------------------------------------

<<The Lucas numbers are an integer sequence named after the mathematician François Édouard Anatole Lucas (1842–1891), who studied both that sequence and the closely related Fibonacci numbers (both are Lucas sequences). Like the Fibonacci numbers, each Lucas number is defined to be the sum of its two immediate previous terms, i.e. it is a Fibonacci integer sequence. Consequently, the ratio between two consecutive Lucas numbers converges to the golden ratio.

However, the first two Lucas numbers are L₀ = 2 and L₁ = 1 instead of 0 and 1, and the properties of Lucas numbers are therefore somewhat different from those of Fibonacci numbers.

The sequence of Lucas numbers begins:

2, 1, 3, 4, 7, 11, 18, 29, 47, 76, 123, ...
--------------------------------------------------------------------

Here is a succulent (cactus) with an arrangement of

11 spirals in one direction and 18 in the other:

--------------------------------------------------------------------

[Beyond]

A-A-A-A-A-A Chooooo Yes - i did write that -- didn't i Silly me

[Sal]

Interesting that all previous images of M51 APOD has issued (that I have come across) shows the galaxy spinning in the opposite direction as this one.

[Chris Peterson]

Interesting that all previous images of M51 APOD has issued (that I have come across) shows the galaxy spinning in the opposite direction as this one.

How the spiral appears in the raw image depends on the design of the telescope- lenses versus mirrors, and for mirrors, how many reflections. Because north up, east left (or a rotation of that) is most common, people often adjust their images to match. In this case, they didn't.

[b17]

Is it possible that instead of interaction b/t 2 galaxies we have a massive wandering black hole (probab;y swallowed up its own galaxy) and has now found a new buffet. It just doesn't have the same features as 2 galaxy interaction...instead of galactic arms maybe those are jets typical of a black hole swallowing a whole lot of matter?

[Chris Peterson]

Is it possible that instead of interaction b/t 2 galaxies we have a massive wandering black hole (probab;y swallowed up its own galaxy) and has now found a new buffet. It just doesn't have the same features as 2 galaxy interaction...instead of galactic arms maybe those are jets typical of a black hole swallowing a whole lot of matter?

Even the most massive black holes have a tiny gravitational influence compared with entire galaxies. I don't think there is any way that a black hole could have this effect. Also, simulations of interacting galaxies do show the same sort of structure seen in this image.

[Ann]

The red hydrogen features which can be seen beyond the smaller galaxy, NGC 5195, are clearly no emission nebulae. Instead, they appear to be signs of a relatively strong outflow of hydrogen, which may be caused both by the strong star formation in M51 and by the interaction of the two galaxies. Possibly it may have something to do with processes going on in or near the nucleus of NGC 5195.

Compare this APOD of M51 and NGC 5195 with an APOD of M106. In the image you can see if you click on the link, there are two curved jets entirely made up of ionized hydrogen apparently being expelled from near the center of M106.

http://apod.nasa.gov/apod/ap110319.html

Ann