Excerpting the article at this link:
The Nebula Of Uncertain Parentage
Planetary nebulae were discovered in the 1780s. The astronomer William Herschel believed that they were newly forming planetary systems. He was wrong, but the name stuck. They’re actually glowing clouds of gas around a dying star, and they’re often quite beautiful....
Many planetary nebulae are bipolar, meaning that they have symmetrical clouds coming from opposite sides of their star—they’re often compared to an hourglass or a butterfly. Sharpless 2-71 is made of multiple bipolar lobes in different orientations....
The Neutron Stars That Look Too Old
....The neutron star [left from a supernova explosion] in the center is spinning with a period of 6.7 hours per rotation. Since a star’s rotation speed slows down over time, this would normally put this particular neutron star’s age at several million years. However, the parent star went supernova around 2,000 years ago. The variation in the neutron star’s X-rays are also unusually large, so something is going on....
Multiple Messier Mysteries
...The stars of the Messier 15 globular cluster are unusually bunched in its center....
Crab [Nebula] Flares
"...Until 2011, it was thought to be one of the most steady sources of light, radio, and gamma radiationin the sky....
But between 2007 and 2010, astronomers from different observatories detected three powerful gamma ray flares with no change in other wavelengths. This was described by one astronomer as a “big puzzle,” while another called it a “real mystery.” The unexpected flares were the first seen from a nebula and were five times more intense than any other yet observed....."
Aligned Bipolar Nebulae
"...It’s not just the jumbled bipolar nebulae in Sharpless 2-71 that pose a mystery to astronomers. Scientists used the Hubble to examine 130 such objects in the central bulge of the Milky Way and found something weird. The nebulae were in different places, formed at different times, and have never interacted. Yet despite that, most of them appear to be aligned along the same axis...."
The Great Eruption
"....In 1838, the glow of Eta Carinae increased until it became the second brightest star in Earth’s sky. It stayed that way for 10 years...."
Mysterious Magnetars
"...Magnetars are a type of neutron star with a magnetic field quadrillions of times stronger than Earth’s. They’re the most powerful magnets in the universe. They were only theorized in the 1990s and they’re filled with properties that we still haven’t figured out...."
The Sun’s Mysterious Cousins
'...Around a third of Sun-like stars have year-long periods of varied brightness as they approach the ends of their lives. ... The conclusion [of a recent study] was clear, and told us exactly what we already knew: “All the possible explanations for their unusual behavior just fail....”'
Epsilon Aurigae’s Disc
Polaris Is Awkward
as described in an essay entitled "The North Star Mysteries: The Remarkable Brightness Increase of Polaris from Historical and Modern Observations."
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