Kepler's Supernova

Kepler’s Supernova was a violent stellar explosion that was observed from Earth in the early seventeenth century. The supernova occurred in the Milky Way galaxy—the home galaxy of our solar system—and was the last such event in the galaxy to be visible from Earth. The supernova grew in brightness over the span of several weeks and was clearly visible even during the daytime. It was named after astronomer Johannes Kepler, who observed the phenomenon and believed it to be a new star. Modern astronomers have determined the explosion to be a type of supernova caused by the violent detonation of a white dwarf, which is the dying remnant of what was once a larger star. They believe the explosion was caused by the white dwarf drawing in matter from a larger companion star until it blew itself apart.

Background

Stars are born when large amounts of interstellar gas and dust clump together and collapse under gravity. The heat and pressure at the core of the collapsing gas cloud eventually cause hydrogen atoms to fuse together. This nuclear reaction forms helium and creates vast amounts of energy in the process. A star continues to shine by converting its hydrogen fuel into helium over the course of its lifetime. However, the length of a star’s lifespan and its ultimate fate depend on the size and mass of the star.

A run-of-the-mill star about the size of our sun will continue to convert hydrogen into helium until it runs out of hydrogen fuel. At that point, it will briefly convert helium into heavier elements, becoming so hot that it expands to become a red giant. Gravity will eventually compress the star’s core into an extremely dense white dwarf, while the star’s outer layers will blow out into space. A star more than eight times the mass of the sun will continue to fuse together heavier elements long after its hydrogen fuel runs out. This process continues until the star begins converting elements into iron. At that point, the star cannot produce enough energy to continue the fusion process. The star’s outer layers suddenly collapse, detonating the star’s core in a violent death explosion called a supernova. This process is the most common form of supernova explosion and classified by astronomers as a Type II supernova.

Overview

The explosion that caused Kepler’s Supernova was a much rarer Type Ia supernova, which can occur only when a white dwarf and another star orbit each other in a binary star system. While astronomers are still unsure of the exact cause of these types of stellar explosions, the most likely scenario involves the white dwarf stealing away matter from a larger companion. Because the extremely dense white dwarf has very strong gravity, it pulls in matter from the larger star until it accumulates enough to reach about 1.4 times the mass of our sun. The white dwarf then suddenly collapses in on itself, creating a massive explosion that obliterates the white dwarf. Another possible theory is that a Type Ia supernova is caused by a collision between two white dwarfs.

On October 9, 1604, stargazers across the world looked up to see a bright, new object in the southwestern night sky. The object increased in brightness for twenty days, outshining all the other objects in the sky but the moon and sun. At its peak, the object was visible during the day before it slowly faded over the course of a year.

German astronomer Johannes Kepler noted the object in his observations, believing it was a new star that may be of “exalted importance” to humankind. In an era where science was just beginning to understand the true nature of Earth’s place in the cosmos, the appearance of the supernova provided evidence that the universe was not an unchanging and fixed place. When later astronomers determined that the object Kepler had recorded was a supernova, they named it in his honor.

The remains of Kepler’s Supernova are about twenty thousand light years away, meaning light from the object took twenty thousand years to reach Earth. The object was located closer to the more crowded regions of the Milky Way galaxy, where more abundant interstellar gas and dust allows stars to form at a faster rate. The original star that became the white dwarf likely had a mass of more than ten times that of the sun. More massive stars burn themselves out faster and have much shorter lifespans. The white dwarf that caused Kepler’s Supernova was believed to be about one billion years old when it blew apart. In comparison, our sun is about 4.6 billion years old and only about halfway through its lifespan.

In the twenty-first century, NASA’s Chandra X-ray Observatory provided scientists with valuable information on the remnant left behind by Kepler’s Supernova. Astronomers determined that the explosion was caused by a white dwarf syphoning off matter from a nearby red giant star. Observations of the region show an abundance of stellar material that could only have come from a larger star. Debris from the supernova explosion at the center of the remnant seems to be plowing through the material left over by the red giant before the explosion. Knots of the debris are still expanding at the same rate as when they were first created more than four hundred years ago. The blast wave is moving at about 15 million miles per hour (24 million kilometers per hour) with the fastest knots reaching speeds of 23 million mph (37 million km/h).

In theory, the red giant star could have survived the blast of its smaller companion; however, the remnant of Kepler’s Supernova shows no sign of a red giant star. Scientists speculate that the larger star could have become a white dwarf itself and was annihilated along with the exploding star. Another idea suggests that the blast so severely damaged the larger star that it can no longer be recognized or detected.

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