Starship Asterisk*

Chris Peterson wrote:

Ann wrote:Isn't it sloppy language to claim that there is an embedded star cluster of massive hot stars in the Eagle Nebula that is hidden from Hubble's view at optical wavelengths?

Why? Am I missing something here? That seems like a perfectly reasonable assertion.

Well, you are missing that the cluster in the Eagle Nebula, NGC 6611, is admittedly a bit reddened, but it is very far from hidden from Hubble's sharp-eyed view. The brightest stars in NGC 6611 are about ninth magnitude in visual light, with typical B-V indexes of around +0.4. It's a piece of cake for Hubble to detect this star cluster through any optical filter.

Yes, I agree that there are hidden stars in the Eagle Nebula. But these are not massive stars, but itty bitty little low-mass stars, the last hurrah of a spent star formation region.

Or maybe not? Am I missing something here? Maybe I am. I would love to hear about a hdden cluster of massive embedded protostars in the Eagle nebula. So if you know about the kind of cluster that APOD Robot appears to talk about today, then please enlighten me, Chris!

Ann

Peeratz wrote:
Dont all readings (Xray, IR, etc) have the same look-back time as visual time (lightspeed)?

Only the readings (Xray, IR, etc) that can actually observe the supernova remnants
have the same look-back time (of 6,500 years).

Visual methods can't observe the supernova remnants at all.

Peeratz wrote:
It sounds like they are trying to say this supernova explosion is 6,000 years old, 500 years less than the current look-back time that we see M16.

I agree that it sounds pretty bad.

Ann wrote:Well, you are missing that the cluster in the Eagle Nebula, NGC 6611, is admittedly a bit reddened, but it is very far from hidden from Hubble's sharp-eyed view.

Well, if you are correct in your interpretation, I wouldn't say there was anything sloppy about the language used, but that the statement is simply incorrect.

I interpret the caption differently. First of all, NGC 6611 is the Eagle Nebula, not some internal cluster. That is, it is the entire object, which consists of an open cluster surrounded by gas and dust. So my reading of the caption is that within the dust, a small cluster of young, hot stars has been identified in x-ray images that cannot be seen in visible light. I don't know if that interpretation is correct, but I think it would be unusual to find any dusty nebulas that don't contain hot stars which can't be seen in visible light.

Peeratz wrote:Dont all readings (Xray, IR, etc) have the same look-back time as visual time (lightspeed)? It sounds like they are trying to say this supernova explosion is 6,000 years old, 500 years less than the current look-back time that we see M16.

It's exactly because somebody is trying to incorporate when an event "really" happened, with when it was observed to happen, and how far away something is estimated to lie, that we are left with a sentence that, by itself, cannot be unambiguously interpreted. Not good.

The actual interpretation (as stated earlier) can only be made by using additional sources. As observed, the supernova happened recently (within a few hundred years at most), and will dramatically alter the structure of the nebula soon (within another few hundred years). The distance to the nebula doesn't matter, and the suggestion that the event happened 6000 years ago is basically wrong in any context that matters.

This discussion has raised a question in my mind: just how dusty is this nebula? If it can hide a supernova
it must be pretty dusty!

In general, how much dust is needed to completely block that much visible light?


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Sam

Sam wrote:This discussion has raised a question in my mind: just how dusty is this nebula? If it can hide a supernova :shock:
it must be pretty dusty!

It probably didn't hide the supernova. What's hiding is a supernova remnant, or a bubble of shocked gas and dust produced by an earlier supernova. That is many, many orders of magnitude dimmer than the supernova itself was during its brief flare.

In general, how much dust is needed to completely block that much visible light?

That depends not just on the density of the dust, but on how much of it you're looking through. Dusty nebulas are billions of times less dense than the Earth's atmosphere. But we are typically looking through them for many light years.

Consider typical window glass. It seems pretty transparent. Indeed, for many people, glass is the very definition of transparency. But a meter thick piece of window glass is about as opaque as a rock. You can't see any light through it at all. One of the early challenges of fiber optics was to develop glasses a million times more transparent than ordinary glass. That's why light travels for kilometers practically unattenuated in a fiber optic cable.

Or consider our atmosphere. When you look straight up at the stars, you are only looking through a few kilometers of dense air, and it hardly attentuates those stars at all (stars from the ground don't look much dimmer than they would from space). But when you look towards the horizon, you are looking through hundreds of kilometers of air- enough that even the Sun is attenuated enough to be viewed directly. Same atmosphere, more of it.

A dusty nebula represents a hard vacuum. If you were on a planet circling a star in such a nebula, you could look around you and see nearby stars easily, and you wouldn't even know you were in a dust cloud based on your view of other planets in the system, or of the star itself. Over distances of a few AUs, such a nebula seems completely transparent.

Chris Peterson wrote: It probably didn't hide the supernova. What's hiding is a supernova remnant, or a bubble of shocked gas and dust produced by an earlier supernova. That is many, many orders of magnitude dimmer than the supernova itself was during its brief flare.

Thanks, Chris. Is there any observation record of this supernova 100 or so years ago?

Chris Peterson wrote:Or consider our atmosphere. When you look straight up at the stars, you are only looking through a few kilometers of dense air, and it hardly attentuates those stars at all (stars from the ground don't look much dimmer than they would from space). But when you look towards the horizon, you are looking through hundreds of kilometers of air- enough that even the Sun is attenuated enough to be viewed directly. Same atmosphere, more of it.

Last night the fog was so dense that I could barely see 2 blocks, yet I could still see the moon and a couple bright stars.

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Sam

Sam wrote:Thanks, Chris. Is there any observation record of this supernova 100 or so years ago?

Not that I'm aware of.

Chris Peterson wrote:

Ann wrote:Well, you are missing that the cluster in the Eagle Nebula, NGC 6611, is admittedly a bit reddened, but it is very far from hidden from Hubble's sharp-eyed view.

Well, if you are correct in your interpretation, I wouldn't say there was anything sloppy about the language used, but that the statement is simply incorrect.

I interpret the caption differently. First of all, NGC 6611 is the Eagle Nebula, not some internal cluster. That is, it is the entire object, which consists of an open cluster surrounded by gas and dust. So my reading of the caption is that within the dust, a small cluster of young, hot stars has been identified in x-ray images that cannot be seen in visible light. I don't know if that interpretation is correct, but I think it would be unusual to find any dusty nebulas that don't contain hot stars which can't be seen in visible light.

Sorry for nitpicking again, Chris. Please understand that I'm not doing it in order to question you, but because I'm so interested in high-mass star formation. I just haven't heard that there is any high-mass star formation going on anywhere in the Eagle Nebula. (Thanks for setting me straight about NGC 6611, by the way.)

I realize that there is low-mass star formation going on inside the Eagle Nebula. I believe that even low-mass star formation may produce X-rays during some time of its formation process and thus be visible in X-rays when it is still hidden at optical wavelengths. Am I wrong about that? Is it only, or mainly, high-mass star formation that produces X-rays?

Is anything more known about whether there is a cluster of massive unborn stars hidden behind the visible optical cluster? Or are the X-ray sources proof enough? We found a cluster of X-rays, ergo, there are massive unborn stars here that we can't see?

The reason why I've been so extra skeptical is that I haven't heard of a discovery of a massive second generation of star formation in the Eagle Nebula, and that I think that many of the X-ray sources appear to be coincident with bright optical optical sources. Admittedly that is not true of all the sources. Does that mean, in case there really is massive new star formation here, that the new stars in the Eagle Nebula are being born more or less directly behind the optical cluster from our point of view, in the same way as the new, hidden star formation in Orion is taking place almost directly behind the Orion Nebula?

Ann

All our information comes from Electromagnetic Waves, just different wavelength/frequencies - but they all travel at the same speed.
But a supernova sends out "information" in two ways: in Electromagnetic Waves (EW), and Physical material. The physical material travels at a much slower speed that the EW, and so we can detect the EW long before the effects that the physical material causes.

A good example of this is SN1972 in Large Magellanic Cloud. It was detected by it's EW in 1972, but over the years we have recorded the different events that have been caused by the physical material passing, at a much lower speed, through its "local" region.

Hope this clears up some confusion.

astrofs wrote:
All our information comes from Electromagnetic Waves, just different wavelength/frequencies - but they all travel at the same speed.
But a supernova sends out "information" in two ways: in Electromagnetic Waves (EW), and Physical material. The physical material travels at a much slower speed that the EW, and so we can detect the EW long before the effects that the physical material causes.

A good example of this is SN1972 in Large Magellanic Cloud. It was detected by it's EW in 1972, but over the years we have recorded the different events that have been caused by the physical material passing, at a much lower speed, through its "local" region.

Hope this clears up some confusion.

Just an addition:
SN1972 was detected in 1972, but since then we have recorded the different events caused by the physical material passing, at a much lower speed, through its "local" region. The physical material trigger events whose EW (Light, IR, X-Ray, UV, etc..) travels out to us at the speed of light - a much greater speed than the physical material is traveling.

Ann wrote:Is anything more known about whether there is a cluster of massive unborn stars hidden behind the visible optical cluster? Or are the X-ray sources proof enough? We found a cluster of X-rays, ergo, there are massive unborn stars here that we can't see?

I have no idea... you'd need to track down the references to get those details. Looking at the original press release that the APOD caption seems to be derived from, it sounds like there is no separate small cluster embedded within the Eagle nebula, but rather that the cluster being referred to is the entire open cluster that defines the stellar component of the object, that the x-ray data was used for identifying a SNR, and the IR data for identifying stars not seen in the visible light data. So the release seems to be saying something a bit different from what the APOD caption says. However, the press release is not well written, and contains some ambiguous and outright incorrect statements itself. Unfortunately, that is all too often the case in these releases that come straight from the institutions sponsoring the research. I'm afraid that the people hired for public relations are simply not very well qualified for their work.

Anyway, my point was only that the language used in the caption for this assertion was just fine- nothing sloppy about it (unlike the language describing time and distance). I wasn't voicing an opinion one way or the other on the accuracy of the statement.

For those willing to wade through the dense science prose, you can click the "evidence" link in the description and get a PDF copy of the paper from which this is derived. (It's under the Download link in the top-right corner.) That paper only suggests that a supernova is one of the two mechanisms they propose that created the "bubble" in the nebula. They clearly indicate much more research is needed to confirm one or the other hypothesis.

Rob