Backyard Voyages: Last Sunset: The Final Bloom

Helix Nebula (Eye of God).
Credit: Purnendu Gupta

End of Days

Where do stars like the sun go when they die? At five billion years old, our sun is a middle aged star. It is also a middle class star, in the stellar spectral classification (yellow G type). When stars of this type reach the end of life, they do what humans do at the end of their working lives. They retire.

That’s right. They do not explode into a supernova like their big brothers, or collapse into black holes like their superstar cousins. They gently walk into ... the sunset. And when they do, they leave behind what is known as a planetary nebula like the one you see in the image above. The Helix Nebula is also known as the Eye of God, for its unique shape. The nearest example of its kind, it is at about 650 light years distant in the constellation of Aquarius. The image above was shot from my backyard in November of 2022 and is light gathered over an hour and a quarter. The image below is a false-color composite of the same, in visible and invisible “light” as seen by four special purpose NASA telescopes that see in different wavelengths.

Helix Nebula, Composite X-Ray, UV, Optical, Infra Red Source: Nasa

Stars are nuclear fusion reactors that exist at a self adjusting balance between the inward pull of gravity and the outward pressure of hot fusing matter. A little bit like a balloon that exists at the sweet spot between the squishing pull of its skin, and the swelling push of the air inside.

A star first lights up when fusion is triggered in the core of the proto-star, among the simplest atom of all - primordial Hydrogen that have atomic number one. This is forced by enormous gravity to combine with its neighbor transforming into the next heavier element in the Periodic Table, Helium, with atomic number two. As all hydrogen is exhausted, three of these Helium atoms will combine to form Carbon with atomic number six and so on. The nuclear alchemy goes on to magically cook heavier elements from lighter ones releasing energy in the process.

About 12,000 years ago, the progenitor star of the Helix nebula started to run out of its hydrogen fuel. The star cooled some and lost some of its ability to hold itself up against gravity. The core shrank, and the outer layers slipped away from its grip into space cooling and expanding into a red giant. This was the end of the “middle class sun” phase of life of the star. Any planets in that solar system experienced the last sunset as the shell of expelled gas engulfed the inner planets just as our own sun would engulf Mercury, Venus and possibly the Earth.

The Star Within a Star

Once the outer veil got lifted what lay exposed was the star's stopped nuclear engine. While no longer undergoing fusion, it is held up by another force known as electron degeneracy pressure. A bright core made mostly of a soup of electrons, carbon nuclei and some other elements, on its way to becoming a white hot midget star - a white dwarf.

A white dwarf contains about half the mass of the original star but is compressed by gravity into a ball merely the size of the earth or only 1/100th of the sun’s original diameter. As a result the densities are enormous. A tea spoon could weigh as much as an average car on earth. For comparison, a spoon ful of the sun’s core would be much lighter at only 750 grams.

By the time the white dwarf forms, the expelled shell has not had time to go very far - and is perhaps a light year or two away from the core. Instead of being a perfect sphere, the expanding shell takes on beautiful shapes due to pre-existing asymmetries. The radiation from the white dwarf core lights up the expanding gas in concentric shells of colors. The Hydrogen glows red and the Oxygen glows blue and green. Like a cosmic flower in final bloom. While the “retired” white dwarf will last many tens of billions of years, the shells of the nebula around it will quickly fade away.

Forget Me Knots

In 1996, a closer look into the Helix through the Hubble Space Telescope revealed some knot like structures, that were observed to lie in a ring pattern within the outer shell. These appear to be comet-like with a head and a tail pointing radially away from the core of the system. First discovered in the Helix, they were later found in many planetary nebulae. These knots were found to be the size of our solar system formed as the surrounding lighter gas got eroded and stripped back by the stellar wind and radiation from the central white dwarf forming finger like shapes due to a phenomenon known as the Rayleigh-Taylor instability.

There are some theories that suggest that the origin of these knots may result from interactions of the progenitor star with its binary partner. But since the shell is only a couple of light years in diameter wide, the distances are perhaps too small for typical two star systems. Yet another theory suggests these could be remains of supersized comets the size of planets - the likes of which have not been seen before. A more likely explanation may have to do with remains of large planets that orbited the original star. As the star expanded to a red giant, these bodies may have continued to orbit "inside" the parent star. Over thousands of years, as the stellar wind and radiation from the white dwarf stripped away the lighter gas, all that remained are these comet like streams with the "tails" like little flags of existence.

Was there life on any of these worlds? Evolution that spanned millions of years and civilizations that thrived for millennia? If so, these cometary knots may be the only signs that they were ever here.

Imaging Specs:

————————

25x180s light frames (total 1 h 15 min integration) with calibration frames. Optolong L-enhance narrowband filter. ASI294MC Pro cooled camera. Guided exposure with Celestron 127 SLT Mak-Cass on AVX mount

References:

-------------------------------

The Hubble Helix Observations from stsci.edu