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.

[dotancohen]

Measuring by mass, the sun is the largest object in the solar system. 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. The sun remains, however, more massive.

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.

Re:No fear

[ackthpt]

I'm going back to freshman level science classes here but since most comets never come near the sun, but stay way out in never-never land, we have nothing to be afraid of her, right? Also, when comets DO come in for a pass at the sun (which is in theory possible for this guy) they condense and their centres form masses of ice and other solids. So this thing would shrink significantly if it ever did come in for a visit. Very cool piece of trivia though. I'm going to try using it this weekend

Shrink? Well, loose mass it would. The tail is the matter leaving the body of the comet. It is possible, while at aphelion the comet has sufficient gravity to attract more matter in the Oort cloud. But it is most likely comets have a limited life. So many trips around and they have evaporated.

The cool thing about these comets is they may leave a legacy. A legacy of meteor showers. (c=

OBLIGATORY

[i_want_you_to_throw_]

EVERYBODY PANIC!!!!!!

Re:Get out your Nikes and brew up a batch of Kool

[Anonymous Coward]

No, you've got it all wrong. All you religious fruitcakes^wfanatics^wadherrants, this a sign from God. You should kill yourselves immediate to return to His grace.

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.

Re:Got a photo of it last night

[yeremein]

It is certainly visible to the naked eye, and looks distinctly not-star-like. It's spectacular in binoculars.

Right now it's in Perseus, quite close to the brightest star. It's in the northeast as dusk falls, below Cassiopeia.

Re:Name

[spun]

And we'll just call you 'person' rather than calling you by your name. A sun is a type of thing, Sol is the name of the sun that happens to be closes to us. In any language used by creatures living on planets, there will be a word for 'sun.' But 'Sol' will still be unique. When we have colonies on Mars, what will the people living there be referring to when they say, "The moon has just risen?"

And 'we' do not call our sun, 'the sun,' except in English. Stop being so parochial, I mean, don't you think we should be referring to it in Chinese or something, to be fair? There is nothing pompous about calling it 'Sol.' Except to anti-intellectual types, and I wouldn't think they'd frequent a place like Slashdot.

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.

Why would it ever stop expanding?

[Tango42]

It's nonsense to talk about how big it's going to get. It's expanding in space - it's going to keep expanding forever (the solar wind might stop it expanding symmetrically, though). It will just get thinner and thinner. The interesting thing is that it's still easily visible, but it will start to dim eventually as it gets too thin to reflect much light. The question is how big it will get while still reflecting enough light for it to be visible to the naked eye (in fact, I think only the middle can be seen without a telescope already), and that should be fairly easy to calculate.

Re:Name

[LordSnooty]

Exactly, people use the non-rude pronunciation precisely to avoid sniggering and off-topic giggles about butts. It's not about sounding scientific as such.