Lake Sonoma Friday night 3/19/04

by Steve Gottlieb

As the weather was so pleasant on Friday, I decided to brave the afternoon rush hour traffic up 101 and head to Lake Sonoma. Actually, it turned out to be a quite pleasant drive after crawling through San Rafael. As the forecast was more promising than Saturday night, we had a nice turnout with 5 other observers from the North Bay, San Francisco and the East Bay. We had several SCT's in attendance and a couple of 18-inch dobs (Robert Leyland and myself). Conditions were nearly optimal, particularly early on, with excellent seeing, low humidity, perfectly calm and fairly dark skies.

Mark Wagner had posted a question last week about visually observing HII regions in other galaxies, so I corralled Robert into spending some time trying to detect and identify HII regions in NGC 2403. Here's the result from roughly 30 minutes of observing --

NGC 2403
18" (3/19/04): at 160x, this chaotic spiral displayed a tremendous wealth of detail with two broad, diffuse spiral arms, dark lanes, mottling and a few obvious giant HII regions. A number of stars are superimposed including two mag 11 stars. I focused on observing the HII regions were best viewed at 323x: The brightest is the HII complex N2404 on the east side of the core 1.5' from center and 1.5' N of a mag 11 star to the SW of the core. This knot is fairly bright at 323x, perhaps 15" diameter and irregularly round. On the NW side of the halo is a collinear string, consisting of two stars along with a fuzzy knot, oriented SW-NE. This HII knot (identified later as IRAS 07315+6543 using NED) forms the SW end of the string and is clearly nonstellar at 323x, ~15' diameter. It can also be pinpointed 2.4' NW of the mag 11 star west of the core (middle of 3 in a E-W string). At the NNW edge of the core is a mag 13.5 "star" which does not focus and appears to be another HII knot. Close following is a fainter, but definite nonstellar knot ~10" diameter. Finally, returning to the E-W line of three stars on the west side of the galaxy, the eastern star in this trio is just on the SW edge of the core and close south is a fainter mag 14 "star" which has a weak nebulous glow attached.

NGC 2403

18" (3/19/04): at 160x, this chaotic spiral displayed a tremendous wealth of detail with two broad, diffuse spiral arms, dark lanes, mottling and a few obvious giant HII regions. A number of stars are superimposed including two mag 11 stars. I focused on observing the HII regions were best viewed at 323x:

The brightest is the HII complex N2404 on the east side of the core 1.5' from center and 1.5' N of a mag 11 star to the SW of the core. This knot is fairly bright at 323x, perhaps 15" diameter and irregularly round.

On the NW side of the halo is a collinear string, consisting of two stars along with a fuzzy knot, oriented SW-NE. This HII knot (identified later as IRAS 07315+6543 using NED) forms the SW end of the string and is clearly nonstellar at 323x, ~15' diameter. It can also be pinpointed 2.4' NW of the mag 11 star west of the core (middle of 3 in a E-W string).

At the NNW edge of the core is a mag 13.5 "star" which does not focus and appears to be another HII knot. Close following is a fainter, but definite nonstellar knot ~10" diameter.

Finally, returning to the E-W line of three stars on the west side of the galaxy, the eastern star in this trio is just on the SW edge of the core and close south is a fainter mag 14 "star" which has a weak nebulous glow attached.

Also nearby NGC 2403 is the amazing galaxy NGC 2366 -- which has an HII region more prominent than the galaxy itself!

NGC 2366
18" (3/19/04): at the SW end of the galaxy is a very bright knot (HII region), roughly mag 12.5 and perhaps 15" in size which responds to a UHC filter at 160x! At 323x this knot is irregular in shape (~20"x15", SW-NE) and brightness and at moments resolves into two or three components. The galaxy itself is fairly faint, large, and quite elongated SSW-NNE, 3.5'x1.0', with a low surface brightness. The galaxy was first discovered by William Herschel and described as "vF, vS, has a vF branch nf". Actually, the first part of the description refers to the high surface brightness HII region at the SW end and the galaxy itself it the "vf branch nf"!

NGC 2366

18" (3/19/04): at the SW end of the galaxy is a very bright knot (HII region), roughly mag 12.5 and perhaps 15" in size which responds to a UHC filter at 160x! At 323x this knot is irregular in shape (~20"x15", SW-NE) and brightness and at moments resolves into two or three components. The galaxy itself is fairly faint, large, and quite elongated SSW-NNE, 3.5'x1.0', with a low surface brightness.

The galaxy was first discovered by William Herschel and described as "vF, vS, has a vF branch nf". Actually, the first part of the description refers to the high surface brightness HII region at the SW end and the galaxy itself it the "vf branch nf"!

Posted on sf-bay-tac Sun Mar 21 16:53:03 2004 PT
Converted by report.pm 1.0 Thu Jul 8 17:44:44 2004 PT