Image Credit: ESA/Hubble, NASA and H. Olofsson (Onsala Space Observatory)
U Camelopardalis, or U Cam for short, is a bright red giant – a once Sun-like star nearing the end of its life – surrounded by a shell of gas, located about 1,400 light-years away from Earth in the constellation of Camelopardalis (The Giraffe), near the North Celestial Pole. It is expanding with a velocity of approximately 43.3 meters per second while it is approaching us at around 3 kilometers per second.
For a Sun-like star death is a long process. After billions of years, the hydrogen fuel that powers the star begins to run out. The star balloons to great size and becomes a red giant. Eventually, however, the star collapses back on itself. This increases the temperature at its core and most of the stars material is catapulted into space, enveloping itself in clouds of gas, called a protoplanetary nebula, but the core is not yet hot enough to make the gas itself glow on its own. Instead, the gas is merely reflecting the light from the star.
But as the star continues to evolve, it becomes hot enough to emit strong ultraviolet light. At that point it will have the power needed to make the gas glow, and will become a real full-fledged planetary nebula. But before the nebula begins to shine, fierce winds of material ejected from the star will continue to shape the surrounding gas into intricate patterns. Then the star cools down and all that is left after this process is the exposed, hot and dead core, known as a white dwarf.
A protoplanetary nebula is a relatively short-lived phenomenon, what means there are relatively few of them in existence at any one time. Moreover, they are very dim, requiring powerful telescopes to be seen. This combination of rarity and faintness means they were only discovered comparatively recently. The nebula around U Cam is one of only a handful of such nebulae known.
We are seeing U Camelopardalis as it begins to run low on fuel, the star is becoming unstable. Every few thousand years, it coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse. The gas ejected in the star’s latest eruption is clearly visible in this picture as a faint bubble of gas surrounding the star. Measuring the expansion rate, it looks like this shell was ejected about 700 years ago, and the event only took 50 years!
U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen. This is due to its low surface gravity, which is typical when as much as half of the total mass of a carbon star may be lost by way of powerful stellar winds.
The star itself is actually a cool red giant with a temperature around 2000 degrees Kelvin and much smaller than it appears in this picture. In fact, the star would easily fit within a single pixel at the center of the image. Its brightness, however, is enough to overwhelm Hubble’s detector making the star look much bigger than it really is. The red giant has only a few hundred years left before it collapses into a white dwarf.
The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail in this image. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable, the shell of gas expelled from U Cam is almost perfectly spherical.
The image was produced with the High Resolution Channel of the Advanced Camera for Surveys onboard the Hubble Space Telescope, using three different color filters.