JWST improves, surpasses Hubble’s view of Pismis 24

Within the plane of the Milky Way, dark dust lanes are omnipresent, representing dense neutral gas clouds usually found within the galaxy’s spiral arms. Here, nebula NGC 6357, also known as the Lobster Nebula, shows the pink signatures of excited hydrogen, a telltale feature of new star formation, along with the blue glow of the reflected light from hot, newborn stars off of neutral matter.

This wide-field image of the Lobster Nebula, NGC 6357, was taken with the Dark Energy Camera aboard the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory. A series of brilliant, blue stars are visible, along with ionized hydrogen gas glowing in red. Just to the right of center, a brilliant glowing area of the haze silhouettes the star cluster Pismis 24, which contains some of the youngest, most massive stars known in the Milky Way.

The open star cluster Pismis 24 contains many brilliant, young massive stars, clustered together at the center of this image. The surrounding clouds both reflect and absorb light, as well as emit their own light due to the excitations of the atoms within them. The radiation emitted by the hot stars works to evaporate the neutral gas, albeit on timescales of millions of years.

The brilliant collection of stars in the top half of this Hubble image represent the stars of the young cluster Pismis 24, with many of these stars weighing in at dozens of times the mass of our Sun, and at least four of them exceeding 50 solar masses. The brightest point of light in the image is actually two stars: Pismis 24-1NW and Pismis 24-1SW, of 74 and 66 solar masses, respectively.

The Advanced Camera for Surveys (ACS) instrument on board Hubble acquired this image of the star Pismis 24-1, at left, demonstrating that it is in fact a double star. Instead of 200-300 solar masses, Hubble was able to resolve that this was a binary whose components had masses of 74 and 66 solar masses, respectively.

What appears to be a craggy mountaintop with wispy clouds and a blue, star-rich field above it is actually a newborn star cluster, Pismis 24, shining brilliantly and photoevaporating the surrounding gas and dust in its vicinity. This JWST view, constructed from five separate color filters in near-infrared light, showcases a variety of features inside this nebula, many of which have never been seen before.

This side-by-side image shows the same regions of Pismis 24 from JWST (left, cropped and rotated) and Hubble (right), with many features easy to match up from one image to the other. However, the JWST view, because of its unique wavelength and resolution capabilities, reveals many features that are unseen by Hubble’s vision.

Although these dual views showcase the same region of the same star cluster in space, Pismis 24, the JWST view, at left, reveals more than an order of magnitude more stars than the Hubble view, at right. This is because of the presence of light-blocking dust, which is very efficient at the short wavelengths that Hubble is sensitive to, but JWST can largely peer through it, seeing the huge populations of stars inside and behind the dust.

The white, wispy clouds seen here aren’t accelerated electrons, as the smoke-like features would indicate surrounding a neutron star, but rather are wisps of neutral gas that is reflecting the intense, brilliant starlight generated by the hottest, brightest stars in this young cluster. Although hints of these reflection features are apparent in the Hubble imagery, JWST’s NIRcam filters are able to expose them in their full glory.

The same region of space, just to the right of the central bright cluster of stars in Pismis 24, shows several stars against a black background in the Hubble image (at right), but many more stars and an eerie blue haze in the JWST image (at left). The blue color is caused by the emission of neutral hydrogen atoms, recombining after being ionized by the intense ultraviolet radiation emanating from the young surrounding stars.

The orange colored features seen here are tiny dust particles: similar to the particles that make up smoke in combustion reactions here on Earth. They stream out of the densest gas-and-dust-rich regions here, loading the interstellar medium with tiny particles that are no longer dense enough to collapse to participate in ongoing star-formation.

The cooler and denser the gas and dust in this nebula is, the more the colors tend from orange to red to even black. And yet, in all but the absolute densest regions, stars still poke through the neutral material, showcasing the incredible sensitivity of JWST to reveal the light sources behind and inside these collections of neutral matter.

Whereas only the brightest stars, and the star in the foreground of neutral material, are visible to primarily optical observatories like Hubble, JWST reveals bright stars, faint stars, and even background stars of the Milky Way, despite the enormous quantities and densities of gas and dust. For studying the properties and evolution of these young, massive stars, there is no better tool than JWST for the endeavor at present.