The Skull Nebula is another example of the remains left by a dying Sun-like star that has cast away its outer layers in a cosmic farewell, leaving behind hot, glowing complex shells of ionized gas. I’ve shared examples before of these so-called planetary nebulae, but the Skull is a particularly appropriate nickname for such an object!
The gas in the nebula’s leading edge moves in the same direction as the central star. As it collides with the interstellar medium, the gas slows down as is compressed. It becomes hotter and unstable, giving the nebula’s ring a brighter and sharper appearance and producing an asymmetric appearance. The material on the nebula’s trailing edge expands freely.
NGC 246 (92 min total exposure Jan 11, 2024)
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The Skull Nebula is located about 1600 light-years away from us. It is a fairly small object – it would take about eight of them lined up side by side to equal the apparent width of the full moon. It is relatively old for a planetary nebula, about 6,600 years.
Once the dying star has lost all of its atmosphere, the intense ultraviolet radiation from its exposed luminous core ionizes the expelled material, causing the gas to glow as a bright planetary nebula. The central star of NGC 246 illuminates the expanding clouds of material and will continue to do so for a few more millennia. Eventually, it will cool down and fade, and the clouds of gas and dust will disappear into the interstellar medium.
Note the two radically-different common ways for stars to die and leave behind interesting remnants for us to see:
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One process is via a supernova explosion where the star’s remnants are cast violently away into space. This happens to large, high mass stars. My previous image, that of the Jellyfish Nebula was an example of one such.
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The other process happens to low to intermediate-mass stars. They evolve into red giants near the end of their life cycles. The Skull is an example of this process, a gentle and slow puffing up of stellar material, where layers are cast away to leave behind a hot, glowing stellar remnant. The star loses mass through a strong stellar wind, and the expelled material forms the nebula’s outer shells. As the central red giant continues to lose material, it becomes surrounded by increasingly complex shells of ionized gas. (Our sun in an example of such a star; it will die via this process in about five billion years.)