A stellar nursery is like a quiet place where baby stars go about their business undisturbed. But, of course, a stellar nursery is not like that. (Babies are noisy and cry a lot.) These are dynamic places where powerful elemental forces rage wildly and bend around to their will. And this one, even though its name is the sleepy-sounding Smiling Cat Nebula, is no exception.
The Smiling Cat Nebula, aka Sh2-284, is about 15,000 light-years away in the constellation Monoceros (Unicorn.) Its bright central region is about 150 light-years across, and a group of young star sits in the middle. It’s a massive HII region, meaning it’s made of ionized hydrogen. And the powerful young stars in the middle do the ionizing.
The group of young stars is called Dolidze 25, and it is an open cluster. It contains O-type stars, hot, blue-white stars with temperatures as high as 30,000 K. The surface temperature of our Sun is much lower, only about 5,700 K.
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The stars emit a lot of powerful UV energy that ionizes all that hydrogen. Without ionization, the nebula would not show these colors. But that’s not all the stars do.
They also blow gas and dust away from the central region with their powerful stellar winds. But that is an uneven process. Thicker clumps and knots of material can block the wind, carving out what appear to be giant columns or fingers of gas and dust. There is a prominent one on the right side of the picture.
There’s a lot going on in Dolidze 25, the open cluster at the heart of the Smiling Cat nebula. It’s a stellar nursery, which means some of the gas and dust is swept up and squeezed, triggering the birth of more stars. It is a chaotic process, as if there is an unquellable storm raging for millions of years.
Active star-forming regions like this are of great interest to astronomers. They have many questions about the star formation process, which often takes place hidden within veils of gas and dust.
In a 2018 paper, researchers found that the stars in Dolidze 25 have a lower metallicity than our own Sun. In fact, they have the lowest known metallicity of any star-forming region in the entire Milky Way. Why is that?
Our galaxy has a metallicity gradient. Stars closer to the galactic center have a higher metallicity than stars closer to the periphery. The higher metallicity is because they formed from material enriched by previous generations of stars. Because more stars are born and die near the galactic center, stars recently born in that region are formed from material enriched by the stellar birth/death cycle. But Sh2-284 and Dolidze 25 are located near the galactic anticenter. They are somewhat isolated at the end of one of the Milky Way’s spiral arms.
The same research showed that the stars are young, not older than about 3 million years. That’s just youth for the stars.
As the stars in Dolidze 25 years old, they probably left each other’s company. As an open cluster, their gravitational bonds are relatively weak. Stars in open clusters often have higher velocities than the escape velocities of the cluster, meaning they disperse over time. Over time, the gas in Sh2-284 will also dissipate, blown away by the relentless winds created by young stars.
What does the future of Sh2-284 look like? Planets are likely to form around stars in Dolidze-25. Thanks to NASA’s Kepler and TESS missions, we now know that almost all stars have planets.
Unfortunately for those of us rooting for other life in the Universe, the planets that form around these stars may not be rocky. The low metallicity of the region means that elements such as oxygen, silicon, and iron—all of which are considered astronomical metals—are scarce. And those three elements make up most of planet Earth, more than 75% in fact.
On rocky planets that are unlikely to form, life is unlikely to exist. But if they do, and they have, perhaps one day, on one of the planets, life will follow the same long, complicated path to sentience that life on Earth has followed. Perhaps there will be astronomers looking our way in a few billion years to see the nebula created by our Sun when it reaches its end. They might see the likeness of some animal faces in our nebula.
But that was all just a dream. The following are more likely.
In a few million years, the region will look different. The stars in Dolidze-25 will likely disintegrate, the gas will continue to disappear, and eventually, the region will stop giving birth to new stars. The Smiling Cat will be gone.
That’s fine. It doesn’t really look like a cat.
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