James Webb Space Telescope Images Helix and PMR 1 Nebulae, Providing Insights into Stellar Evolution

The James Webb Space Telescope (JWST) has captured detailed images of two distinct nebulae, the Helix Nebula and the PMR 1 Nebula. These observations contribute to understanding the final stages of stellar life cycles, from the formation of planetary nebulae around Sun-like stars to the complex structures surrounding massive dying stars, and the distribution of material into the cosmos.

The Helix Nebula: A Glimpse into the Sun's Future

The Helix Nebula, also known as NGC 7293 or Caldwell 63, is a planetary nebula located approximately 650-655 light-years from Earth in the constellation Aquarius. Discovered before 1824 by Karl Ludwig Harding, it is recognized as one of the closest and brightest planetary nebulae observable from Earth, sometimes referred to by the nicknames 'Eye of God' or 'Eye of Sauron'.

The Helix Nebula, also known as NGC 7293 or Caldwell 63, is recognized as one of the closest and brightest planetary nebulae observable from Earth, sometimes referred to by the nicknames 'Eye of God' or 'Eye of Sauron'.

Stellar Evolution and Significance

The Helix Nebula offers insights into the eventual fate of stars similar to our Sun, which is projected to undergo a similar process in roughly 5 billion years. This process involves a star exhausting its hydrogen fuel, expanding into a red giant, and then shedding its outer layers to form a planetary nebula, leaving behind a dense, hot white dwarf core.

The nebula is estimated to be between 10,000 to 12,000 years old, with its progenitor star beginning to shed its layers 15,000 to 20,000 years ago. It is expected to continue expanding and dimming over the next 10,000 to 20,000 years, eventually dispersing into the interstellar medium approximately 50,000 years after its formation.

The material dispersed from these dying stars, including elements such as carbon, oxygen, and nitrogen, contributes to the interstellar medium and can potentially be incorporated into future star and planet formation.

JWST Observations

Using its Near-Infrared Camera (NIRCam), the JWST provided a detailed view of the nebula's structure, revealing several key features:

Pillar-like structures, also known as globules, which number an estimated 40,000 within the nebula. Some of these knots are potentially larger than the extent of our Solar System to Pluto's orbit.

These structures are formed as powerful winds of hot gas from the dying star collide with slower-moving, colder shells of dust and gas previously shed, sculpting the nebula's distinct appearance. They feature a well-illuminated head and a trailing tail of less energized gas.

The image shows a clear transition between hot ionized gas closer to the white dwarf and cooler molecular hydrogen further out. Colors indicate temperature and chemical composition:

• Blue hues denote the hottest gas, energized by ultraviolet light from the central white dwarf.
• Yellow regions represent cooler gas where hydrogen atoms form molecules.
• Reddish tones at the outer edges trace the coolest material where gas thins and dust can form.

The observations highlight how more complex molecules can form in shielded zones within the nebula's dust clouds, creating environments suitable for chemical development.

When combined with images from the ground-based Visible and Infrared Telescope for Astronomy (VISTA), it was observed that the comet-like figures move outward, cool, and disperse into space as a red haze, which is expected to contribute to the formation of new stars and planets in the distant future.

The PMR 1 Nebula: An "Exposed Cranium" in Space

The PMR 1 Nebula, informally known as the "Exposed Cranium" nebula due to its resemblance to a brain within a transparent skull, is a cloud of gas and dust located approximately 5,000 light-years away in the constellation Vela. It was discovered in the late 1990s by Parker, Morgan, and Russell using the U.K. Schmidt Telescope, with its distinctive nickname originating from Spitzer Space Telescope observations in 2013. The nebula measures about 3.2 light-years across.

The PMR 1 Nebula, informally known as the "Exposed Cranium" nebula due to its resemblance to a brain within a transparent skull, is a cloud of gas and dust located approximately 5,000 light-years away.

Stellar Evolution and Structure

The PMR 1 Nebula forms from a dying star nearing the end of its fuel-burning phase, shedding layers of material into space. JWST's observations capture a moment in this stellar decline. The nebula exhibits distinct regions reflecting different evolutionary phases:

• Outer Shell: Primarily composed of hydrogen gas, believed to have been expelled earlier from the central star and subsequently cooled, forming the 'cranium' structure.
• Inner Cloud: Contains a more structured, complex mixture of ionized gases, which were emitted later and form the 'brain' structure.

JWST Observations

The JWST utilized both its Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) to capture the nebula:

• NIRCam's view highlights more stars and background galaxies.
• MIRI's mid-infrared view accentuates cosmic dust more prominently.

Both NIRCam and MIRI images show a distinct dark lane running vertically through the nebula, which contributes to its brain-like appearance by resembling the longitudinal fissure of a brain. This feature is hypothesized to be caused by an outburst or outflow from the central star, potentially involving twin jets ejected in opposing directions. The MIRI image shows ionized gas exiting through the hydrogen envelope at the nebula's upper region, supporting this hypothesis.

The Central Star

The precise nature of the dying star at the center of PMR 1 remains an area of investigation.

The star's ultimate fate depends on its mass; if sufficiently massive, it will undergo a supernova explosion. Otherwise, a less massive, Sun-like star would continue shedding layers, leaving a dense white dwarf core that would cool over time.

Earlier observations in the 1990s and a 2001 paper suggested the nebula's emission features were consistent with a Wolf-Rayet star, a type of massive and unstable star known for rapidly shedding mass through powerful stellar winds. However, other evidence points towards a smaller, Sun-like star that would eventually become a white dwarf.

One theory proposes that the star may be a rare type of planetary nebula central star that mimics a Wolf-Rayet star but is actually an exposed core of a Sun-like star transitioning to a white dwarf.

Astronomers theorize that white dwarfs may eventually cool over extended periods to become black dwarfs, though the universe is considered too young for any to have fully formed.

The James Webb Space Telescope, an international collaboration led by NASA with partners ESA and CSA, continues to contribute to understanding celestial structures and the processes of stellar evolution across the universe.