Nearly a thousand years ago, a “guest star” appeared in the sky, shining almost as brightly as the moon and six times more vibrant than Venus. Chinese, Japanese, Arabic and possibly Native American astronomers recorded the event in 1054, according to NASA. Although it was visible during the day for almost a month, the star faded soon after. Several years later, it was invisible.
What these astronomers witnessed was a supernova in the Taurus constellation, the final, violent act of a dying star and the largest explosion that takes place in space. It formed the Crab Nebula, one of the most famous supernova remnants studied.
We’ve seen images of the beautifully wispy filaments of the ever-expanding gas cloud that constitutes the star’s remains. And now, NASA’s Hubble Space Telescope has peered into the center of this cosmic wonder 6,500 light-years away and witnessed its beating heart. The radiation signature was first detected in 1968.
The heart is actually a rapidly spinning neutron star or pulsar, otherwise known as the crushed core of an exploded star. What makes it strange and unique to scientists is the fact that a neutron star contains about the same mass as the sun but is tightly compressed into a solid ball only a few miles across. It spins 30 times a second, sending off radiation pulses with the precision of a striking clock. These rapid pulses make this core appear just like a heart within the nebula.
“The density of a neutron star can be approximated by stuffing a herd of 50 million elephants into a thimble,” said Frank Summers, outreach astrophysicist at the Space Telescope Science Institute.
In the image captured by Hubble, the red streaks are glowing gas cavities and filaments of star debris, which continue to expand. The blue wisps are really electrons moving at the speed of light, forming an expanding ring. Scientists believe that high-speed wind tsunamis whip off the neutron star and turn into the charged particles forming this wispy magnetic field.
“The Crab Nebula is about 14 light-years across on its long axis,” Summers said. “In comparison, the star Sirius, the brightest star in the night sky, is about 8 light-years away from the sun. Hence, the gas in the nebula has expanded from being part of a star to stretching across interstellar space over the last thousand years. The speed of that expansion is millions of miles per hour.”
The overlapping rainbow colors are mainly due to the fact that this is a time-lapse image capturing the movement of materials within the nebula.
Pulsars are comparable to a lighthouse beam. In the magnetic field of the exploded star, the remaining gas is pushed out at high speeds that form jets of materials. When we can see these pulsing jets, that means the poles are pointed toward Earth.
The fact that the Crab Nebula is one of the closest to us has afforded scientists the opportunity to study it and its features.
“By studying and following the emission of the Crab, we get a ringside seat for understanding how young neutron stars and supernova remnants develop,” Summers said.
“This supernova is only a thousand years old, so we are seeing an early stage on astrophysical time scales.”
But what does this have to do with us? More than you might expect.
“Very massive stars, greater than about 8 times the mass of our sun, end their lives as these titanic explosions,” Summers said. “The heavy elements forged by nuclear fusion in these stars are then spewed forth and become part of the interstellar medium. When new stars form from this enriched material, the collapsing clouds have the heavy materials from which to make rocky planets like Earth. Our planet, and indeed our species, could not have formed were it not for such supernova explosions.”