This is a fascinating and delightful image of a low-mass star forming region. The fact that the region is "low-mass" means that the most massive of the stars that form here are no more than, say, two or three times more massive than the Sun.
When you look at todays APOD, start by noticing the dull, dark "mist" that seems to permeate much of the field of view. (But this mist isn't all over the image, not where the background sky seems black and full of stars.) The "mist" is a mixture of gas, which is invisible, and dust, which is not. (The "dust" generally consists of tiny particles of carbon and silicon, coated with ice.)
Where the "mist" looks dark brownish gray but relatively transparent, the concentration of gas and dust is higher than average in the Milky Way, but not high enough to start forming stars.
But now you should be able to find what looks like patches and "tentacles" that are pitch black and opaque. Here the gas and dust that we have already detected in the area has started to collapse under its own weight, due to self-gravitation. The black color is due to the concentration of the dust. When patches of highly condentrated gas and dustg become opaque and pitch black the temperature inside them drops. This allows the gas inside to fall towards the center of the cloud at a faster pace. You get a "core" of concentrated gas (and dust). In fact, you get... a proto-sun.
But not everywhere. The fact that there are large black patches of concentrated gas and dust in this picture doesn't mean that there are stars forming everywhere inside this blackness.
This is an infrared image of the NGC 7129 region, taken by the Spitzer Space Telescope:
You should be able to recognize the general shape of the NGC 7129 nebula. I'm not extremely good at reading infrared images myself, but please not the largish green area which corresponds to an area of utter blackness in the image that we see in visible light. That means that something is going on inside this dark cloud that is hidden from view in visible light.
Let's return to today's APOD, which shows NGC 7129 is visible light.
There are four newborn stars in the visible ligth image that seems to be of particular interest. One is an orange source of light in the upper right part of the large blue reflection nebula. The orange source is surrounded by some visible orange turbulence, which bears some resemblance to a "hurricane". Clearly this must be a young star which is emerging from the deep dark blackness where it formed, but it is still in a turbulent stage and not fully formed. Another thing which is obvious is that this star is relatively faint. It is a low-mass star, probably more light-weight than the Sun. You can find it in the Spitzer image as a relatively faint slightly yellowish star.
Below this star, if you look at the visible light image, is another light source which is much more deeply orange. This is another star which is in the process of emerging from its dust cocoon, but this star may be slightly more massive than the one above it. In the Spitzer image this very orange source looks quite blue, indicating a more energetic source.
But two other stars are also emerging from their dust cocoons, and they are much more energetic and much brighter than the two orange ones. In the visible image, both these bright stars look white-to-peach. In the Spitzer image, the source on the right is bluer than the one to the left. In the visible-light image, a meandering hydrogen emission pink jet is emerging from this star. The jet forms the shape of an arc as it passes another embedded bright star, a star which looks bright blue in the visible image. Clearly the stellar wind from this blue star pushes the jet to one side. It is also likely that magnetic forces in the nebula helps shape the jet. The bright red crescents at the top of the image may be the ends and the "bow shocks" of this jet, as it slams into gas at the end of what may be a magnetized bubble around the nebula.
In any case, the stars with the visible pink jet emerging from it should be the highest-mass star here, or at least it should be of higher mass than the other stars which have not yet peeked out of their dusty cocoons.
Ann
Young Suns of NGC 7129
Gitchy gitchy goo.
