Starship Asterisk*

NGC 346 in the Small Magellanic Cloud

Explanation: How and why are all these stars forming? Found among the Small Magellanic Cloud's (SMC's) clusters and nebulae NGC 346 is a star forming region about 200 light-years across, pictured above by the Hubble Space Telescope. A satellite galaxy of the Milky Way, the Small Magellanic Cloud (SMC) is a wonder of the southern sky, a mere 210,000 light-years distant in the constellation of the Toucan (Tucana). Exploring NGC 346, astronomers have identified a population of embryonic stars strung along the dark, intersecting dust lanes visible here on the right. Still collapsing within their natal clouds, the stellar infants' light is reddened by the intervening dust. A small, irregular galaxy, the SMC itself represents a type of galaxy more common in the early Universe. But these small galaxies are thought to be a building blocks for the larger galaxies present today. Within the SMC, stellar nurseries like NGC 346 are also thought to be similar to those found in the early Universe.

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The colors here are... weird. But take it easy! Don't shoot me! Yes, the colors are strange, but the filters chosen for this image (F555W (V), yellow-green, and F814W (I), infrared) are not without merit!

First, the large blue nebula you can see in the picture is an emission nebula, so it radiates most strongly in Ha light at, if I remember correctly, 656 nm (or was it 658?). That is far into the red region of the spectrum. But the red color of every hydrogen emission nebula is always "diluted" a bit by hydrogen beta emission, which radiates at, oh, 498 nm or something. That is the blue-green part of the spectrum. Together these two emission lines create the pink color that we are familiar with from photographic images of so many emission nebulae.

Yes, but really energetic nebula, energized by some hefty O stars, will also radiate quite a bit of OIII emission at about 501 nm. Together, hydrogen alpha and beta and OIII will create a yellowish light. Look at this picture of the Trapzium region of the Orion Nebula:
See the color of the brightest part of the nebula? It is yellowish, and that is no mistake.

Assuming the F555nm filter used for this image has a relatively broadband response, it should be able to catch both the hydrogen beta and the OIII radiation, plus perhaps a bit of Ha light. That's not so bad. (On the other hand, the nebulosity is shown as blue in this picture, and that is wrong no matter how you look at it.)

As for the F814 nm filter, it is quite possible that faint stars will show themselves well through that filter (although I would have thought that they would show themselves even better through a filter of, say, 750nm). In any case, one of the really good things about this image is how it brings out all the faint stars that are born along with the few bright ones. However, it is not today's APOD which does the best possible job of resolving the small low-mass newborn stars, but rather this one:

http://www.mpia.de/Public/Aktuelles/Bil ... _Aug06.jpg

With a resolution as good as this one, you can start estimating the IMF (initial mass function) of the newborn clusters. That means, in plainer language, that you estimate how many faint low-mass stars are born for each bright high-mass one.

Well, today's APOD portrays an interesting starforming region, that much is certain!

Ann

http://coolcosmos.ipac.caltech.edu/cosm ... m/smc.html

Vapor clouds in Small Magellanic Cloud, a reflection of the seas ran Magellan, the sea water and cloud water is cloudy in the cloud, a sea barrier cliffs, which starfish is born, white to right silver to left

Ann wrote:
The colors here are... weird. But take it easy! Don't shoot me!

Aim for just above the HEART:

http://antwrp.gsfc.nasa.gov/apod/ap030214.html wrote:
Explanation: Looking toward star cluster NGC 346 in our neighboring galaxy the Small Magellanic Cloud, astronomers have noted this heart-shaped cloud of hot, x-ray emitting gas in the cluster's central region. The false-color Chandra Observatory x-ray image also shows a strong x-ray source just above the heart-shaped cloud which corresponds to HD 5980, a remarkable, massive binary star system that lies within the cluster. HD 5980 has been known to undergo dramatic brightness variations, in 1994 briefly outshining all other stars in the Small Magellanic Cloud, and has been likened to the luminous, eruptive variable star Eta Carinae in our own Milky Way galaxy. At about 100 light-years across, NGC 346's heart-shaped cloud is probably the result of an ancient supernova explosion. Alternatively it may have been produced during past eruptions from the HD 5980 system, analogous to the nebula associated with Eta Carinae.

Ann wrote:
Yes, the colors are strange, but the filters chosen for this image
(F555W (V), yellow-green, and F814W (I), infrared) are not without merit!

Ann wrote:
First, the large blue nebula you can see in the picture is an emission nebula, so it radiates most strongly in Ha light at, if I remember correctly, 656 nm (or was it 658?). That is far into the red region of the spectrum. But the red color of every hydrogen emission nebula is always "diluted" a bit by hydrogen beta emission, which radiates at, oh, 498 nm or something. That is the blue-green part of the spectrum. Together these two emission lines create the pink color that we are familiar with from photographic images of so many emission nebulae.

Ann wrote:
Yes, but really energetic nebula, energized by some hefty O stars, will also radiate quite a bit of OIII emission at about 501 nm. Together, hydrogen alpha and beta and OIII will create a yellowish light.

Assuming the F555nm filter used for this image has a relatively broadband response, it should be able to catch both the hydrogen beta and the OIII radiation, plus perhaps a bit of Ha light. That's not so bad. (On the other hand, the nebulosity is shown as blue in this picture, and that is wrong no matter how you look at it.)

The nebulosity is presumably shown in reflected starlight with BOTH the 486 nm Hß light & 656 nm Hα light blocked.

Today's APOD is essentially this false color APOD sans Hα:

http://antwrp.gsfc.nasa.gov/apod/ap080928.html wrote:
Explanation: The massive stars of NGC 346 are short lived, but very energetic. The star cluster is embedded in the largest star forming region in the Small Magellanic Cloud, some 210,000 light-years distant. Their winds and radiation sweep out an interstellar cavern in the gas and dust cloud about 200 light-years across, triggering star formation and sculpting the region's dense inner edge. Cataloged as N66, the star forming region also appears to contain a large population of infant stars. A mere 3 to 5 million years old and not yet burning hydrogen in their cores,the infant stars are strewn about the embedded star cluster.

In the above false-color Hubble Space Telescope image, visible and near-infrared light are seen as blue and green,
while light from atomic hydrogen emission is red.

Ann wrote:
As for the F814 nm filter, it is quite possible that faint stars will show themselves well through that filter (although I would have thought that they would show themselves even better through a filter of, say, 750nm). In any case, one of the really good things about this image is how it brings out all the faint stars that are born along with the few bright ones. However, it is not today's APOD which does the best possible job of resolving the small low-mass newborn stars, but rather this one:

http://www.mpia.de/Public/Aktuelles/Bil ... _Aug06.jpg

With a resolution as good as this one, you can start estimating the IMF (initial mass function) of the newborn clusters. That means, in plainer language, that you estimate how many faint low-mass stars are born for each bright high-mass one.

The F814 nm filter is best to cut through the dust so as to see all types of stars (including red dwarfs).

(The IMF [International Monetary Fund] is the intergovernmental organization that oversees the global financial system by following the macroeconomic policies of its member countries, in particular those with an impact on exchange rate and the balance of payments.)

http://asterisk.apod.com/vie ... =9&t=21604

mexhunter wrote:The picture is great.
A beauty that I hope soon to see with my eyes.

Ratified.
Greetings
César

Could APOD staff consider giving us fabricated photo or artist's rendition of how the Milky Way galaxy would appear in the night sky to residents of a planet on the near side of the Large Magellanic cloud? Maybe also the from Sagittarius and Canis Major dwarf galaxies given their closer proximity? Just curious as to how beautiful their naked eye stargazing would be on a dark (assume moonless) alien night there when the Milky Way is high above their horizons? Thanks!

El Gordo wrote:Could APOD staff consider giving us fabricated photo or artist's rendition of how the Milky Way galaxy would appear in the night sky to residents of a planet on the near side of the Large Magellanic cloud? Maybe also the from Sagittarius and Canis Major dwarf galaxies given their closer proximity? Just curious as to how beautiful their naked eye stargazing would be on a dark (assume moonless) alien night there when the Milky Way is high above their horizons? Thanks!

Think about how the Milky Way looks. Now, shorten it so it doesn't go from horizon to horizon, and make it broader. Instead of dust lanes cutting into the Milky Way, you'd see something similar defining the spiral arms. You'd need to be somewhat dark adapted to see this structure; otherwise, you'll just see a big, faintly glowing gray region in the sky.