Is there currently a comet in Triangulum?

Mike,
Thanks for the link. Wonderful photos.
If I am not mistaken, this is the largest or one of the largest, emission nebula in the
known universe, many time larger than M42 for example.
Bill Meyers


Mike Wagenbach wrote:

David Knisely wrote in message ...

shows some hints of structure. If you want a *really* close-up look at it,
the Hubble Space telescope got a very nice image of it which is probably on
their web site.

The search function at Hubble Heritage didn't work out for me. SEDS
came through nicely:
http://www.seds.org/messier/more/m033_n604.html

If I am not mistaken, this is the largest or one of the largest, emission
nebula in the known universe, many time larger than M42 for example.

30 Doradus, the Tarantula nebula, in the Large Magellanic Cloud probably holds
the title of "Largest known HII Region" in the universe. NGC 604 in M33 is up
there and obviously a fine target for amateur telescopes. But consider this, if
30 Doradus were positioned where M42 is, it would cover 30-degrees of sky.

But getting back to NGC 604, there's a nice article in the November
_Astronomy_, "Exploring the Pinwheel," with some great images. One has the
major known clusters and nebulae in M33 labelled.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond

"Stephen Paul" wrote in message
...
"Bill Meyers" wrote in message
...
This is as good a time as any to say that thanks are due to you [David
Knisely]
and Chris L. Peterson and Michael Covington and
some others who are able to give authoritative answers to inquiries on
SAA.

I couldn't agree more. I don't know where else I would have turned. The
time
it would have taken to get reasonably correct information would have been
discouraging through other channels.

Thanks for the kind words! When time permits, I always enjoy hanging around
here.


--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope

"Bill Ferris" wrote in message
...

But getting back to NGC 604, there's a nice article in the November
_Astronomy_, "Exploring the Pinwheel," with some great images. One has the
major known clusters and nebulae in M33 labelled.

So Astronomy magazine is worth the subscription price? I get S&T, but for
some reason I can't think of, I don't get Astronomy.

-Stephen

Bill Ferris wrote:

30 Doradus, the Tarantula nebula, in the Large Magellanic Cloud probably holds
the title of "Largest known HII Region" in the universe.

Well, lets see, the main portion of the Tarantula Nebula (NGC 2070) is about
18' arc in its longest angular dimension, although there are fainter tendrils
to the southeast which may or may not be part of the main mass (I have seen
diameters listed to 30' arc, but looking at a picture of it makes me think 18'
is a more reasonable figure). At the distance of the LMC (163,000 light
years), this works out to be about 850 light years across. If we take the
upper limit in possible size (30' arc), the entire HII region would be 1400
light years. NGC 604 has fainter tendrils as well, but its main mass is about
1.5' arc in angular size. At about 2.2 million light years away, so that
works out to about 960 light years in diameter, so it seems to be very
comparable to the Tarantula Nebula. In any case, it is remarkable that we can
see an emission nebula which is over 2 million light years away with just a 4
inch aperture.
However, there may be even bigger "fish" out there. There are HII
regions in M101 like NGC 5461 (0.8'x0.4') which are quite impressive even at a
distance of nearly 25 million light years (works out to be on the order of of
5,000 light years for a size). Exactly how much of this object's dimension is
emission nebulae and how much is the underlying star cloud is unclear.
However, it is fairly clear that while the Tarantula Nebula is *one* of the
largest known HII regions, it may not be the very largest. Clear skies to you.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

"John Steinberg" wrote in message
...
Oh, no! Not the Astronomy v. S&T debate again?!

This kind of comment from a guy that tries to instigate the Mac v. Windows
debate on a regular basis?!

[ducking & running with a big grin on my face]
Craig in Tampa

So Astronomy magazine is worth the subscription price?

Since Dave Eicher is back as chief editor, the magazine has improved
dramatically! I buy it and S&T and am not disappointed.

Don't be so elitist ;-)


--
25° 45' S
28° 12' E

Victor,

I also subscribe to both without the slighest disappointment and,
yes, with David Eicher as chief editor, Astronomy has improved quite a
bit. Their section on the month's events is one of the greatest
improvements, in my opinion, for they provide lots of pages on comets,
asteroids, the planets etc.

Anthony.

Victor wrote:

So Astronomy magazine is worth the subscription price?


Since Dave Eicher is back as chief editor, the magazine has improved
dramatically! I buy it and S&T and am not disappointed.

Don't be so elitist ;-)

Thanks, David, for taking the time to compose another very knowledgeable and
informative posting. I am interested in comparative sizes of emission nebulae so I
have saved this posting to my hard drive.
Clear skies,
Bill Meyers

David Knisely wrote:

Bill Ferris wrote:

30 Doradus, the Tarantula nebula, in the Large Magellanic Cloud probably holds
the title of "Largest known HII Region" in the universe.

Well, lets see, the main portion of the Tarantula Nebula (NGC 2070) is about
18' arc in its longest angular dimension, although there are fainter tendrils
to the southeast which may or may not be part of the main mass (I have seen
diameters listed to 30' arc, but looking at a picture of it makes me think 18'
is a more reasonable figure). At the distance of the LMC (163,000 light
years), this works out to be about 850 light years across. If we take the
upper limit in possible size (30' arc), the entire HII region would be 1400
light years. NGC 604 has fainter tendrils as well, but its main mass is about
1.5' arc in angular size. At about 2.2 million light years away, so that
works out to about 960 light years in diameter, so it seems to be very
comparable to the Tarantula Nebula. In any case, it is remarkable that we can
see an emission nebula which is over 2 million light years away with just a 4
inch aperture.
However, there may be even bigger "fish" out there. There are HII
regions in M101 like NGC 5461 (0.8'x0.4') which are quite impressive even at a
distance of nearly 25 million light years (works out to be on the order of of
5,000 light years for a size). Exactly how much of this object's dimension is
emission nebulae and how much is the underlying star cloud is unclear.
However, it is fairly clear that while the Tarantula Nebula is *one* of the
largest known HII regions, it may not be the very largest. Clear skies to you.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

David Knisely wrote:
[snip]
However, there may be even bigger "fish" out there. There are HII
regions in M101 like NGC 5461 (0.8'x0.4') which are quite impressive even at
a distance of nearly 25 million light years (works out to be on the order of
of

5,000 light years for a size). Exactly how much of this object's dimension
is emission nebulae and how much is the underlying star cloud is unclear.
However, it is fairly clear that while the Tarantula Nebula is *one* of the
largest known HII regions, it may not be the very largest. Clear skies to
you.

David, your post sparked an interest to dig a little deeper into the question,
what's the largest known HII region. I spent a couple of hours surfing the
bibliographic section of the SIMBAD Astronomical Database
(http://simbad.u-strasbg.fr/Simbad ) and found several papers on HII regions,
which reference Robert C. Kennicutt, Jr.'s article, "Structural Properties of
Giant HII Regions in Nearby Galaxies." Originally published in _The
Astrophysical Journal_ (1984), here's the URL for the scanned version:
http://cdsads.u-strasbg.fr/cgi-bin/n...pJ...287..116K

Table 2 (pg. 122) provides a "Summary of Integrated Properties" for the 30 HII
regions discussed by Kennicutt. Column three lists approximate diameters
(parsecs) as measured by visual inspection of H-alpha images of the regions.
Kennicutt points out that "many of these objects do not even possess physical
edges, but an approximate visual diameter is useful for comparative purposes."

The eleven largest in Table 2 are all estimated at 360 pc or larger in
diameter:

OBJECT...................DIAM (PC).........NOTES
M101.B..(NGC 5461).........1000............Multiple core complex, giant HII
region
M101.A..(NGC 5471)..........800............Multiple core complex, giant HII
region
M101.C..(NGC 5455)..........750............Giant HII region
NGC 2403.A..................600............Multiple core complex, giant HII
region
NGC 2366.A..................560............Giant HII region
M81.A.......................450............Giant HII region
M82.A.......................450............Giant HII region
M33.A..(NGC 604)............400............Giant HII region
M33.B..(NGC 595)............400............Giant HII region
30 Doradus..................370............Giant HII region
M33.C..(NGC 592)............360............Multiple core complex, giant HII
region

Table 3 (pg. 129) lists "Some HII Regions Scaled to a Common Distance of 5
kpc." Ten regions are included and, as Kennicutt states, "the diameters listed
is an isophotal diameter, corresponding to an emission measure of 5000 pc
cm^-6, roughly the limit of most Galactic surveys." Here are the objects, which
would have angular sizes greater than 100 arc minutes:

OBJECT.................DIAM (ARCMIN).........NOTES
NGC 5471...................340...............If viewed from a distance of 5 kpc
30 Doradus.................170...............If viewed from a distance of 5 kpc
M82.A......................140...............If viewed from a distance of 5 kpc
NGC 604....................105...............If viewed from a distance of 5 kpc

So which is it; is 30 Doradus the largest *known* single core HII region, as
suggested by Table 3, or one of the top 10 as suggested by Table 2? The answer
probably lies somewhere in-between.

Kennicutt describes 30 Doradus as the prototypical supergiant-class HII region,
"In luminous late-type spirals with large numbers of HII regions, there seems
to be a continuous distribution of nebular sizes and luminosities, from the
largest 30 Doradus-class complexes down to regions at least 100-times fainter."

Of the multiple HII region complexes (e.g. NGC 5471 in M101), Kennicutt
concludes that, though they have total luminosities greater than "those of
large single objects like 30 Doradus, the characteristic brightness of the
single components is usually of the same order as that of 30 Doradus (or less).
This appears to be the case for the major complexes in M101 (NGC 5447, NGC
5455, NGC 5461, NGC 5462, NGC 5471) as well as for NGC 2366 A, a double-core
object."

That said size estimates do vary by method and according to the distance
estimate for the host galaxy. So while it would be presumptive to crown 30
Doradus as king of the known HII regions, it's clearly among the largest and
brightest among single core regions.

FWIW, the 30 objects in Table 2 might make an interesting observing list for
amateur observers. It could be called the "Large HII Region List" and would
provide plenty of opportunity to see structure and detail in our home Galaxy
and others beyond.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
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