Astronomers Discover Oldest Planetary Nebula Inside 500 Million-Year-Old Galactic Cluster

An international team of astronomers led by researchers from The University of Hong Kong have discovered a rare celestial jewel inside a 500-million-year-old Galactic Open Cluster. The celestial jewel is the oldest planetary nebula ever found. The Galactic Open Cluster is called M37, also known as NGC2099. Galactic open clusters are loosely bound groups of a few tens to a few hundred stars, and are found in spiral and irregular galaxies. A planetary nebula is a region of cosmic gas and dust formed from the cast-off outer layers of a dying star, and has nothing to do with planets. The discovery of the oldest planetary nebula in the open galactic cluster is a very rare finding of high astrophysical value.  The study describing the findings was recently published in the prestigious journal Astrophysical Journal Letters

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What Are Planetary Nebulae?

Planetary nebulae are the ejected, glowing shrouds of dying stars that shine with a rich emission line spectrum, and as a result, display their distinct colours and shapes that make them photogenic magnets for public interest. One of the first James Webb Space Telescope images to be released — a picture of the Southern Ring Planetary Nebula — was a planetary nebula. 

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Oldest Planetary Nebula Ever Found

The newly discovered planetary nebula in the galactic open cluster is the only third example of an association between a planetary nebula and an open cluster out of the 4,000 planetary nebulae known in our Milky Way galaxy. The newly discovered planetary nebula is also the oldest planetary nebula ever found. 

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What Is The Kinematic Age Of The Newly Discovered Planetary Nebula?

The researchers determined some interesting properties for their discovery. They found that the planetary nebula has a “kinematic age” of 70,000 years. In astronomy, stellar kinematics is the measurement of motions of stars through space. 

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How Was The Kinematic Age Determined?

The scientists estimated the kinematic age of the planetary nebula on the basis of how fast the nebula is expanding, as determined from the emission lines. Also, the astronomers assumed that the speed has remained effectively the same since the beginning. The kinematic age is the time elapsed since the nebular shell was first ejected by the host, a dying star. 

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The Newly Discovered Planetary Nebula Is A “Grand Old Dame”

Typical planetary nebulae have kinematic ages of 5,000 to 25,000 years. In terms of planetary nebulae, the newly discovered planetary nebula is a “grand old dame”, according to The University of Hong Kong. However, in terms of the life of the original star itself that runs to hundreds of millions of years, the planetary nebula is a mere “blink of the eye”. 

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The “grand old dame” is a “rare beauty with a hot blue heart”, according to the university. Since it lives in a stellar cluster, the environment enables the team of astronomers to determine powerful additional parameters not possible for the general galactic planetary nebula population.

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What Is Stellar Main Sequence? How Is It Useful?

These parameters include estimating the mass of the planetary nebula’s progenitor star with the help of the stellar main sequence. The stars which fuse hydrogen atoms to form helium atoms in their core are known as main sequence stars. Most stars lie on a line known as the “main sequence”, which runs from the top left to bottom right. At the top left, the hot stars, which are brighter, are present. Meanwhile, at the bottom right, the cool stars, which tend to be dimmer, are located. The main sequence is represented on plots of stellar colour vs brightness. 

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The star that exploded into a supernova is known as a progenitor star. The stellar main sequence for the galactic open cluster was derived from the observed properties of thousands of stars in the cluster when plotted in a colour-magnitude diagram. 

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The Planetary Nebula Has A Hot, Blue Central Star

Through the observed properties of the hot, blue central star, the team can estimate the residual mass of the central star that ejected the planetary nebula.

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Therefore, the astronomers figured how massive the star was that ejected the planetary nebula gaseous shell when it was born, and how much mass is now left in its residual, contracting hot core. The hot core of the planetary nebula is a so-called “White Dwarf” star, according to the study.

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What Is The Size Of The Oldest Planetary Nebula?

Data for the hot blue, planetary nebula central star from the Gaia space observatory provide a good distance estimate allowing the planetary nebula’s actual size at this extreme age to be determined as 3.2 parsec in diameter. A parsec is an astronomical unit of measure for interstellar space. One parsec is equal to 3.26 light-years. 

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The diameter of the newly found planetary nebula is at the extreme end of known planetary nebula physical sizes.

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