The Lifecycle of Stars – Star birth, Planetary Nebulas, and Supernovas

This complex field in Cassiopeia contains six Sharpless objects and a supernova remnant. We call them Sharpless objects because they are listed in Stewart Sharpless’s 1959 catalogue listing 313 emission nebulas. Emission nebulas emit their own light due to the presence of a star or stars near or in the nebula emitting intense radiation. This ionizes the gas in the nebula, and when that ionization occurs, the affected atoms emit photons, which our cameras can detect decades, centuries, millennia, or even millions of years later. Emission nebulas are often stellar nurseries where pockets of gas coalesce due to gravity and start to make new stars. Our own sun was the product of such a process.

Starting from the left and working to the right, the first Sharpless object we encounter is Sh2-163—sometimes called the False Tulip Nebula for its resemblance to the Tulip Nebula (Sh2-101). Distance estimates for this object are all over the map ranging from 7,500 to 12,500 light years. This object, and the other Sharpless objects in the frame, are star-forming regions where new stars are born.

The much smaller (from our point of view) Sh2-164 is located to the upper right of Sh2-163. Distance estimates for it are also uncertain, ranging from 9,200 to more than 16,000 light years.

Moving down and to the right, Sh2-165 makes its home. Distance estimates don’t improve with this one with one at 5,200 and another at 7,800 light years.

Traveling up and slightly to the right, we arrive at Sh2-166. Underscoring the difficulty of ascertaining distances to such objects, here the range is from 7,800 to 11,500 light years.

Above and right of that, we find the pair Sh2-168 and Sh2-169. The distances here are a little more certain with Sh2-168 landing at a distance of around 11,000 light years and Sh2-169 coming in much closer at a mere 7,500 light years.

There’s at least one planetary nebula in the frame—unfortunately, it’s tiny from our view. And it has the creative name PNG 114.4+00.0. “PNG” denotes that it’s from the Galactic Planetary Nebula catalogue, and the numbers are its galactic coordinates.

The name “planetary nebula” is a misnomer. Planetary nebulas have nothing to do with planets. Astronomers in the 18th century thought these objects were planets, but now we know differently. When a star with 80% to 800% of the sun’s mass begins to run out of hydrogen fuel, it starts fusing helium into heavier elements such as carbon and oxygen. As it does this, the additional energy from those stepped-up levels of fusion cause it to expel its outer layers.

Ultimately, the star expels all its outer layers from its core, leaving the core exposed. Once all the helium is burned up, fusion ceases in the core and what is left is a giant, dense ball of carbon (about the size of the earth, but with the mass of the sun)—a diamond in the sky called a “white dwarf.” That core no longer produces new energy, but it’s still extremely hot and gives off copious amounts of radiation, ionizing the gas it expelled earlier. That cloud of gas then eventually dissipates. But before it does, the ionized gas gives off light we can record here on earth with our cameras (if the star is close enough to us).

In the lower-right corner, you can see CTB 1 (CTB stands for “Caltech Observatory list B." It is a compendium of radio sources in the galactic plane published in 1960 by D.R.W. Wilson and J.G. Bolton. In the image overlay, it is labeled as LBN 576, an example of several times scientists have misclassified this object. We now know this is a supernova remnant—thanks to the work of Wilson and Bolton. Evidence suggests that the pulsar PSR J0002+6216 is the remnant of the star that exploded. Scientists surmise that it exploded about 10,000 years ago and is between 6,000 and 10,000 light years away.

CTB 1 has several nicknames—the Medulla Nebula, the Garlic Nebula, and the Popped Balloon Nebula.

While there are many classifications of supernovas, the most common are Type Ia and Type II supernovas. Type Ia supernovas occur when a white dwarf orbiting a larger star starts to bleed off matter from the larger companion. That mass, added to the white dwarf can eventually cause it to exceed what is called the “Chandrasekar Limit,” which is about 1.4 solar masses. When that occurs, the star becomes a carbon bomb and blows itself to smithereens. Because these supernovas occur in the same way, involving the same amount of material, throughout the universe, they are considered “standard candles” for determining the distance to faraway galaxies.

Type II supernovas occur when a star eight times, or more, the mass of the sun runs out of hydrogen fuel and starts burning heavier elements. But rather than stopping at carbon, the gravitation created by the larger mass causes the star to continue fusing heavier and heavier elements until it gets to iron. Unlike lower levels of fusion, fusing iron actually requires more energy than it produces. At that point equilibrium between gravity trying to crush the star and fusion trying to hold the star up is lost. The outer layers rush in toward the core at a significant percentage of the speed of light. That material smashes into the core and rebounds, producing a cataclysmic supernova explosion. The good news is that this seeds the area with heavier elements—those required to create and support life, build concert halls, and fill footballs with air.

What’s left behind is generally either a neutron star or a black hole.

So this single image contains many aspects of the lifecycle of stars—from their formation in an emission nebula to their ultimate fate, whether that’s a planetary nebula or supernova.