Holmes Comet Coma Grows Bigger Than The Sun

coondoggie passed us a NetworkWorld article, as he does, noting that there is now an object in our solar system bigger than Sol. The Holmes comet has a huge coma, with a diameter scientists are now calculating to be larger than our own middle-sized star. "Scientists don't seem to have a guess as to how big it will ultimately become. The Holmes coma's diameter on Nov. 9 was 869,900 miles (1.4 million kilometers), based on measurements by Rachel Stevenson, Jan Kleyna and Pedro Lacerda of the University of Hawaii Institute for Astronomy. The sun's diameter, stated differently by various sources, is about 864,900 miles (1.392 million kilometers)."

Comments

Diameter? A bit hard to define.

[gardyloo]

Yes, this is pedantic, but by practically any definition (even leaving aside unbounded electromagnetic radiation or mass), Sol is FAR larger than the comet. The very fact that the comet HAS a tail of sorts speaks to the influence of the solar wind.

      I'd guess that the diameter that most people talk about when they're discussing the sun is that determined by the mean-free scatting path length of photons produced within the sun. Once the photons' probability for escaping the sun is higher than that for being scattered back into the interior, that's what we usually call the "diameter", and it accounts for the relatively sharp "edge" to the sun.

      I could release a bunch of helium atoms on Earth's surface, and eventually they'd diffuse enough to be effectively larger than the sun's "diameter" as defined in the articles. It still doesn't mean a whole lot.

Re:Diameter? A bit hard to define.

[Anonymous Coward]

But the term "large", when used without specifying which measurement is being referred to, usually refers to length. Thus, for solar objects, diameter. By that measurement, Holmes is technically larger than the sun.

This depends - as the parent post already pointed out - entirely on how you define the diameter of the object in question. And - again, as the parent post already pointed out - the diameter of the sun is usually defined by that part of the sun which emits most of the photons that reach us from the sun. By most other definitions - and there are lots of them that do make sense - the diameter of the sun is larger. Much larger, in fact.

Incidentally, if you define the diameter of the sun using a measure that more-or-less matches the one used for defining the diameter of a comet's coma - namely, the diameter of the gas cloud emitted by the object in question - you'll get the entire heliosphere. Which is way larger than the coma of 17P/Holmes.

Got a photo of it last night

[yeremein]

I took a picture of the comet just yesterday. Posted here.

In fact, I have several photos of the comet taken over the past few weeks. They're not all cropped the same, but it's still quite apparent how much the comet is expanding. One of these days I plan to put together a composite photo, fixing the stars in place, and showing not only the expansion of the comet but also its motion relative to the stars.

Comet P/17 Holmes visibility, naked eye aspect

[fyngyrz]

So if it's bigger that Sol, does it look as big as Luna from Terra?

It looks exactly like this.

That's a shot with a 50mm portrait lens - no telescope, no magnification, nothing. The comet is plainly visible as an orb, yes, just as the sun is.

Re:Comet P/17 Holmes visibility, naked eye aspect

[fyngyrz]

I'm quite serious. Read the text I supplied with the image. The "orbs" you refer to are not resolved images of the edges of stars, they are simply burned out areas on the camera's sensor as the atmosphere's lensing moves the stars around over the many seconds of the exposure. Those stars are points of light, no more.

The burnout on many star occurs because stars, in relation to one another, can be literally millions of times different in brightness. Comet Holmes, while large, is not nearly as bright as many of those stars at any one sensor location, hence is does not burn out the imager and you get a considerably more accurate representation of it. Likewise, the dimmest stars still appear as pinpoints because when they are lensed to a random location, one or two passes over a sensor site aren't enough to result in a bright reading - that only happens at the centroid of the focus, essentially right where the star actually is.

Re:Comet P/17 Holmes visibility, naked eye aspect

[fyngyrz]

Also, re your remark about post processing, I did three and only three things:

1) I used levels to push very faint background stars and sensor noise (which is fairly significant at ISO 3200) down towards or into black;

2) I pushed the color saturation up by about double to bring out the colors of the stars.

3) I clipped the portion of the image containing the comet and a few stars out and posted that fragment only, as opposed to the entire 10 megapixel shot.

None of which affected the capture of the comet in any significant manner. It isn't reflecting any particular color, so the change in saturation had no effect. The comet is quite bright, so pushing down the bottom-most brightness levels only served to trim a few pixels at the darkest edges of the comet. It appears just a trifle larger in the original.