Alpha Cam - Runaway Star (APOD 24 Nov 2006)

[Galactic Groove]

From today's APOD pic, how do i interpret 3x2 degrees?
http://antwrp.gsfc.nasa.gov/apod/ap061124.html

[kjardine]

To give you a sense of scale, the full moon is about 0.5 x 0.5 degrees in size.

[BMAONE23]

that would put it at the approx size of 24 full moons

[Galactic Groove]

oh i didn't read it right, i was thinking the star was 3x2 degrees from the center... oops Thanks for clearing it up!!!

[ta152h0]

is it possible to draw a line thru the centerline of the curve and project it to a galaxy or something big, black and spinning ?

[ta152h0]

is it possible for the APOD stars to point an arrow to the runaway star ? I assume it is the bright one but we all know what happens when you assume and are wrong

[Andy Wade]

is it possible for the APOD stars to point an arrow to the runaway star ? I assume it is the bright one but we all know what happens when you assume and are wrong

It is that bright one, it's 'arrowed' in an image on this webpage linked from the APOD page:

http://www.galaxyimages.com/AlphaCam.html

[johnnyyooper]

first time (and confused) poster here. hi.
regarding this runaway star. why am i not impressed that it is traveling at 60km/sec? that doesnt seem that fast considering the earth is moving at 30km/sec around the sun and the sun is moving at something like 200km/sec around the center of the galaxy (fairly wild guess). i assume most all stars are moving thru space at some fairly high speed similar to this runaway star. is not empty space in the confines of a galaxy the same empty space between galaxies? shouldnt most stars be producing such a shockwave? jy

[rigelan]

Evidently if the star is compressing the dust in its path, it could be going 60km/s faster than the material around it is. Our Earth on the other hand, rotates together with a bunch of dust that is in our orbit. Even though we are going pretty fast, the dust is too. And so with the sun. Most of the material around the sun stays with the sun, the sun is not moving 200 km/s faster than the material around it.

So maybe this is about relative speed instead of absolute speed.

[Galactic Groove]

Even though we are going pretty fast, the dust is too. And so with the sun. Most of the material around the sun stays with the sun, the sun is not moving 200 km/s faster than the material around it.

So maybe this is about relative speed instead of absolute speed.

Is that what defines it as a "runaway" star, the fact that it isn't moving in a direction relative to its surrounding environment?

[johnnyyooper]

ahhhhhh......i see. ty. after reading more about runaway stars and interstellar medium, it seems you are right. its all about the runaway star going against the grain/flow of the surrounding material. interesting. jy

[djm]

How does physics explain a 10 light year bow wave generated by a star rapidly moving through interstellar material. Presumably there is a repulsive force : radiation and subatomic particles, magnetism? and counteracted by gravitation. I just do not see from a relativistic viewpoint how it can build to 10 light years ahead.

I suppose the radiation emitted 10 years ago is at sufficient intensity after 10 years radiative dilution to activate the interstellar material to radiate light, whereas the radiation emitted say 11 years ago is too diluted to retain sufficient energy / area to excite material to radiate light. However if this is the explanation, I would think that the light radiation of the interstellar material would increase exponentially between the bow wave and the star. It does not seem to.

[BMAONE23]

Or could it be that this star is large enough for its winds to do the work
Or possibly have a 10ly backup of interstellar medium that it is pushing against.

[iamlucky13]

I hope I'm understanding the question right. If not, I apologize.

The area behind the bow wave is actually relatively free of matter, due to the solar wind and radiation pressure pushing free gas and dust outward. At the edge of the heliopause, the outward pressure from the sun is approximately equal to the, shall we say, ambient pressure of the surrounding gas. Charged particles entering this region are slowed by encountering extrasolar particles. As they lose kinetic energy, the emit photons. So not only is there less matter to warm and emit or reflect radiation behind the bow shock, but the solar wind is just streaming through without emitting either, until it encounters the instellar gas. A little more info:
http://en.wikipedia.org/wiki/Solar_syst ... st_regions

As far as the extreme size of the bubble: Alpha Cam is a blue-white supergiant. One of the links from the APOD states that it has an intensity 530,000 times as great as our sun. For comparison, our sun's bow shock occurs about 230 AU's out, which is ~0.004 light years. The intensity of radiation drops off with the distance squared (an area relation). By my math, to achieve the equivalent pressure at 10 light years, our sun would have to be a frightening 7 million times brighter, assuming all other factors are equal.

This number sounds a little high to me (I'm fairly confident in my back of the napkin math), but it gets down to about a factor of 10, and I'm not sure I accounted right for the kinetic energy of stellar wind particles. It's also possible that the interstellar medium is more dense in our region than around Alpha Cam, and likely that Alpha Cam has a disproportionately intense stellar wind (note that it is 530,000 times more intense, but only 30 times more massive).

I admit I'm pushing some of my knowledge a little. I welcome any nitpicks others might have.