Remnants of star death in the eastern sky
We’ve watched the center of our galaxy set with the autumn in the western sky. Now the plane of the galaxy arcs up from west to east through the zenith in the early evening, and the galactic anticenter rises in the east. Though not as rich and dense as the galactic “urban center” in Sagittarius, the plane of the Milky Way is visible even as we look away from the center toward the more rural regions of our home galaxy.
Some of the stars in the eastern sky are in our own “Orion Spur” portion of a galactic spiral arm, but many are in the next outer arm named for the hero, Perseus triumphantly holding the head of the slain Medusa. Beware of looking too long at Algol, the “demon star” as it is Medusa’s eye!
Below Algol in Figure 1, the Crystal Ball Nebula is a planetary nebula that looks like a jellyfish in infrared images (cf. www.nasa.gov/mission_pages/WISE/news/wise20101117.html) due to rings of dust that have been ejected by the dying star. In the top of Orion, NGC 2022 is a planetary nebula that looks like a smaller, younger Ring Nebula (in Lyra). In Gemini, the Eskimo Nebula acquired its name due to looking like a person wearing a fur-fringed parka hood. It’s one of the youngest known planetary nebulae at about 1,000 years old and is a challenge for astronomers to explain. It appears much more explosive than the others with multiple, complex shells of ejected gas and long radial streamers showing gas heated and blown away by strong stellar winds from the remnant white dwarf star.
About the time the progenitor star of the Eskimo nebula was starting to lose its outer layers, a violent Type II supernova explosion took place in Taurus, creating a “guest star” noted by Chinese astronomers and the Anasazi of northwestern New Mexico, who marked its appearance on a petroglyph in Chaco Canon. The explosion that started in 1054 is on-going. Unlike on Earth, where explosions end when the material falls to the ground, explosions in space never end. The gas and dust from the giant star that exploded continues racing away from the core at over 900 miles per second showing us changes in the nebula over the many years we have been observing it. As shown in Figure 2, we observe the Crab in every band of the electromagnetic spectrum and each band reveals different material and processes. In visible light we see gas that has been enriched with very heavy elements that can only be created in giant stars and in these tremendous explosions. Infrared observations reveal heated dust and extremely energetic electrons caught in magnetic fields. Radiation from such electrons is called synchrotron radiation and is also observed in radio wavelengths. In X-ray images, extremely hot gas (millions of degrees) is concentrated near the remnant core of the once-giant star.
It’s this core that really sets the Crab Nebula apart, however. The nebula had been observed by 1939, and astronomers struggled to identify the core of the progenitor star. In the 1960s, radio observations revealed a source that flashed thirty times a second. Later, optical pulses were also observed. When the first of these rapidly pulsing stars was observed by Jocelyn Bell of Cambridge University, it was thought to be spurious noise in the antennas and jokingly named Little Green Men-1 (LGM-1). Further observations and theoretical work, however, revealed these to be pulsars, rapidly rotating neutron stars.
As the iron core in a giant star collapses, the pressure of the outer layers of the star can compress it past the degenerate electron gas (white dwarf stage) where the atoms are crammed together as densely as possible, to a degenerate neutron gas. To do this, the electrons in atoms must be “melted” into the protons in their nuclei to create neutrons. The neutrons (that don’t repel each other) can then be crammed together in a material so dense that a cubic inch of it would weigh over 3 trillion pounds. In the process of becoming so compressed, the star’s magnetic field becomes incredibly strong and its rotation “spins up” like a figure skater pulling in her arms and legs. Most of them, like the Crab pulsar, rotate many times a second.
The cause of the pulses of these stars is that their magnetic fields are so strong that material most easily escapes the incredibly hot, dense surface of the star along the magnetic axes. As with the Earth, the magnetic field axis is not, necessarily, aligned with the rotation axis so the bright beam of escaping particles rotates like the beam of a lighthouse. We see the bright flash when it points toward us. As bizarre as this sounds, many of these sources are known and regularly observed, including the one in the Crab Nebula.
Deep in the center of the image in Figure 2, the jet from the pulsar can be seen emerging from the center and pointing down and to the left. The bright torus the jet seems to emerge from is where particles from the neutron star blown off the stars equatorial region slam into the surrounding gas, heating it to X-ray emitting temperatures. A Google search on the Crab Nebula Pulsar will return links to videos of this very active cosmic wonder.
The volunteer astronomers at the Adirondack Public Observatory are eager to show you the Crab, Eskimo, Crystal Ball other amazing objects. The Roll Off Roof Observatory (RORO) is open to the public on the first and third Fridays of each month approximately one half-hour after sunset. Whether you’re an avid amateur astronomer or have never visited an observatory, come and view through our telescopes and learn about the Wilderness Above. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO at 518-359-6317 to talk with one of our astronomers.