JWST peers inside a dying star's "exposed cranium"

Whenever stars are born, their masses determines their fates.

morgan keenan spectral classification stars

The (modern) Morgan–Keenan spectral classification system, with the surface temperature range of each star class shown above it, in kelvin. The overwhelming majority of stars today are M-class stars, with only 1 known O- or B-class star within 25 parsecs. Our Sun is a G-class star, along with about 5-10% of total stars. However, in the early Universe, almost all of the stars were O- or B-class stars, with an average mass 25 times greater than average stars today. In general, more massive stars live shorter lives, and die in more explosive fates.

Credit: LucasVB/Wikimedia Commons; Annotations: E. Siegel

Sun-like stars evolve into giants, blow off their outer layers, and contract: forming white dwarfs.

planetary nebulae

From their earliest beginnings to their final extent before fading away, Sun-like stars will grow from their present size to the size of a red giant (~the Earth’s orbit) to up to ~5 light-years in diameter, typically. The largest known planetary nebulae can reach approximately double that size, up to ~10 light-years across. Typically, stars born with 8 solar masses or under can experience this fate, but observing an evolved Sun-like star is often ambiguous as far as determining its ultimate demise.

Credit: Ivan Bojičić, Quentin Parker, and David Frew, Laboratory for Space Research, HKU

Significantly more massive stars become supergiants, destined to violently die in supernovae.

Hertzsprung-Russell diagram showing stellar classification by luminosity, temperature, spectral class, absolute magnitude, and how stars’ mass and energy influence different types.

When stars run out of hydrogen in their cores, they evolve off of the main sequence, becoming subgiants, then red giants, then igniting helium in their cores (which is the tip-of-the-red-giant branch phase), and then evolving onto the horizontal branch and eventually into supergiants (for high-mass stars) or into the asymptotic giant branch (for non-high-mass stars) before dying. The mass of a star determines its ultimate fate, but the rate of fusion is set by other internal properties.

Credit: Starhuckster.com

The line separating the two is blurry, as both star types experience winds and ejecta before dying.

Infographic showcasing various supernova remnants with names, dates, locations, and types in circular images set against a dark background, adding an aura of mystery to these bizarre cosmic explosions.

This infographic shows nine of the ten historical supernovae, along with remnants, that have been identified over the past 2000 years within the Milky Way. Not shown is G1.9+0.3, which occurred near the galactic center approximately 155 years ago, but was only discovered posthumously in 1985. Our best supernova remnant observations all come from NASA’s Chandra X-ray telescope.

Credit: NASA/Chandra X-ray telescope

Around 5000 light-years away, the faint nebula PMR 1 hides a star whose fate is not yet known.

A black and white image shows a star field with numerous stars of varying brightness scattered across the night sky.

This view from the Digitized Sky Survey shows the area of the sky where nebula PMR 1, also known as the Exposed Cranium Nebula, is located. Note that almost no visual signature of the Nebula’s existence can be spotted in visible light wavelengths.

Credit: Digitized Sky Survey/Association for Universities for Research in Astronomy; Animation: E. Siegel

NASA’s Spitzer revealed its structure in 2013, bestowing a nickname: the Exposed Cranium Nebula.

A nebula appears as a brownish, oval-shaped cloud surrounded by numerous blue stars against a dark space background.

This was the first high-quality image of the structure of the nebula surrounding the star at the center of PMR 1. Taken by NASA’s Spitzer in 2013, it was Spitzer scientists who coined the name “Exposed Cranium Nebula” on account of the object’s shape. The ionized gas at the center is surrounded by a glowing reservoir of hydrogen gas: reflecting the maximum amount of science extractable from the Spitzer data.

Credit: NASA/JPL-Caltech/J. Hora (Harvard-Smithsonian CfA)

Gaia data, although powerful, showcased nothing of visual interest.

A dense star field in deep space with numerous stars and faint galaxies scattered against a dark background.

This view of a specific region of space shows data from the Gaia mission, with many objects of interest highlighted in blue circles. The red circle, added for clarity, shows the location of PMR 1 and the Exposed Cranium Nebula, which does not exhibit a visually interesting signature in Gaia data.

Credit: ESA/Gaia, Legacy Surveys/D. Lang (Perimeter Institute); Animation: E. Siegel

All it revealed was a reddened core: evidence of its dust-rich nature.

A color-magnitude diagram with Gaia BP-RP color on the x-axis and Gaia G magnitude on the y-axis, showing labeled stellar evolution tracks, clusters, and notable objects.

Using data from Gaia, the central stars of many nebulae were catalogued and had their properties extracted, including the core of the Exposed Cranium Nebula, marked as PMR 1 here. It is one of the most severely reddened objects, as indicated by its present far to the right on this graph, of all the stars found at the cores of nebulae.

Credit: N. Chornay and N.A. Walton, Astronomy & Astrophysics, 2020

However, JWST just imaged it across many wavelengths, with both NIRCam and MIRI.

Side-by-side images of the Exposed Cranium Nebula taken by the James Webb Space Telescope using NIRCam (left) and MIRI (right) filters, showing colorful nebular structures.

All told, the Exposed Cranium Nebula was imaged in a total of 8 different filters by JWST: at 1.5, 1.87, 4.44, and 4.70 microns in near-infrared wavelengths, as well as 10.0, 11.3, 12.8, and 18.0 microns by the mid-infrared instrument. The nebula itself is about two light-years in diameter, although NIRCam and MIRI views showcase vastly different details from one another: about the Nebula as well as what lies behind/around it.