Starship Asterisk*

by **neufer** » Sun Jan 17, 2021 7:05 pm

: NGC 2174 lies near the front of the Perseus Arm just past the Rosette Nebula.

WWW wrote: ↑Sat Jan 16, 2021 11:44 pm
The caption claims the "scene spans about 6 light-years". If that is correct there seems to be an extraordinary number of stars for that small a span. Obviously a few of the stars may be in the foreground, but many are partially darkened meaning they must be within the same local gas and dust cloud, (maybe some behind it too, but how thick could it be?).

Thick enough

... background stars would be twice as obscured as any embedded stars.

At 6,350 ly away NGC 2174 lies near the front of the Perseus Arm
just past the Rosette Nebula.

by **johnnydeep** » Sun Jan 17, 2021 3:50 pm

WWW wrote: ↑Sat Jan 16, 2021 11:44 pm The caption claims the "scene spans about 6 light-years". If that is correct there seems to be an extraordinary number of stars for that small a span. Obviously a few of the stars may be in the foreground, but many are partially darkened meaning they must be within the same local gas and dust cloud, (maybe some behind it too, but how thick could it be?).

Stellar density varies enormously depending on where you are in the Milky Way. From https://en.wikipedia.org/wiki/Stellar_density

Stellar density is the average number of stars within a unit volume. It is similar to the stellar mass density, which is the total solar masses (MSun) found within a unit volume. Typically, the volume used by astronomers to describe the stellar density is a cubic parsec (pc3).

In the solar neighborhood, this value can be determined from surveys of nearby stars, combined with estimates of the number of faint stars that may have been missed. The true stellar density near the Sun is estimated as 0.004 stars per cubic light year, or 0.14 stars pc−3. When combined with estimates of the stellar masses, this yields a mass density estimate of 4 × 10−24 g/cm3 or 0.059 solar masses per cubic parsec. The density estimate varies across space, with the density decreasing rapidly in the direction out of the galactic plane.[1]

The locations within the Milky Way that have the highest stellar density are the central core and the interior of globular clusters. A typical mass density for a globular cluster is 70 MSun pc−3, which is 500 times the mass density near the Sun.[2] In the solar neighborhood, the stellar density of a star cluster must be greater than 0.08 MSun pc−3 in order to avoid tidal disruption.[3]

by **Ann** » Sun Jan 17, 2021 4:32 am

WWW wrote: ↑Sat Jan 16, 2021 11:44 pm The caption claims the "scene spans about 6 light-years". If that is correct there seems to be an extraordinary number of stars for that small a span. Obviously a few of the stars may be in the foreground, but many are partially darkened meaning they must be within the same local gas and dust cloud, (maybe some behind it too, but how thick could it be?).

NGC 2174 (bottom right), star cluster M35 (top right) and the Jellyfish Nebula,
IC 443, at center. Photo: Joachim Korb.

Here you can see a widefield image of NGC 2174 (bottom right). Star cluster M35 is at top right, and IC 443, the Jellyfish Nebula, is at top.

As you can see, if you click on the image, the region is extremely star-rich. That is because the Milky Way runs through this part of the sky, which is the lower part of Gemini and the top part of Orion (from a northerner's perspective).

If you look closely, you can see that the "stellar density" is at least as high outside of NGC 2174 as inside of it.

Ann

by **WWW** » Sat Jan 16, 2021 11:44 pm

The caption claims the "scene spans about 6 light-years". If that is correct there seems to be an extraordinary number of stars for that small a span. Obviously a few of the stars may be in the foreground, but many are partially darkened meaning they must be within the same local gas and dust cloud, (maybe some behind it too, but how thick could it be?).

by **orin stepanek** » Sat Jan 16, 2021 2:20 pm

So much beauty shown by APOD; I never tire of logging in each day!

by **Ann** » Sat Jan 16, 2021 6:33 am

Today's APOD is fine and fascinating. For myself, however, I also want more of "the whole picture" - and, as usual, I want the "RGB picture".

: 'The whole picture' - well, more or less. The small square in the upper left image is the area that is today's APOD. The above set of images is Hubble's 24th Anniversary Image of 2014. NASA Goddard Space Flight Center.

NGC 2174 in RGB + Hα + OIII. Photo: Mark Hanson. Click for a larger image.

The bright blue star in Mark Hanson's picture is HD 42088. It is a star of spectral class O6V, and it is, or so I believe, the one bright massive star inside NGC 2174, and it does almost all of the ionizing of the nebula.

NGC 2174 is clearly a site of ongoing star formation. Note in Mark Hanson's image a star to the upper left of HD 42088 a star sitting inside a small reddish nebula bisected by a dust lane. That star is clearly about to break out of its natal cloud. (Actually, it's more like the star is pushing its dusty birth nebula to one side.) Also note a yellowish patch to the lower left in Mark Hanson's image, where more star formation is most likely taking place.

Ann

by **APOD Robot** » Sat Jan 16, 2021 5:09 am

The Mountains of NGC 2174

Explanation: This fantastic skyscape lies near the edge of NGC 2174 a star forming region about 6,400 light-years away in the nebula-rich constellation of Orion. It follows mountainous clouds of gas and dust carved by winds and radiation from the region's newborn stars, now found scattered in open star clusters embedded around the center of NGC 2174, off the top of the frame. Though star formation continues within these dusty cosmic clouds they will likely be dispersed by the energetic newborn stars within a few million years. Recorded at infrared wavelengths by the Hubble Space Telescope in 2014, the interstellar scene spans about 6 light-years. Scheduled for launch in 2021, the James Webb Space Telescope is optimized for exploring the Universe at infrared wavelengths.

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