Cassiopeia A

Cassiopeia A, often called Cas A, is a massive supernova remnant located in the constellation Cassiopeia. A supernova remnant is the expanding cloud of debris left behind after a star explodes. This specific object is highly significant in astronomy. It is the brightest extrasolar radio source in the sky at frequencies below 1 GHz. This means it emits very strong radio waves from outside our solar system. The remnant sits in the Perseus Arm of the Milky Way galaxy. This arm is the next-nearest arm outward from our own Orion Arm.

The explosion that created this remnant was a Type IIb supernova. This type of event occurs when a massive star undergoes internal collapse. The star was likely a red supergiant with a helium core. Before the explosion, the star had lost almost all of its outer hydrogen envelope. When the core collapsed, it triggered a violent outward blast. This process is known as supernova nucleosynthesis, which creates new chemical elements. During this explosion, the star's material was thrown into space at incredible speeds.

Scientists have studied the movement of this expanding shell for many years. The shell has a temperature of approximately 30 million K. It expands at speeds between 4,000 and 6,000 km/s. However, the expansion is not perfectly uniform. Observations from the Hubble Space Telescope revealed high-velocity eject knots. These knots move at transverse velocities between 5,500 and 14,500 km/s. The fastest movement occurs in two nearly opposing jets. When astronomers use colors to show different chemical compositions, they see that similar materials often stay gathered together in the debris.

We do not have a definitive record of the original explosion reaching Earth. Calculations suggest the light arrived around the 1660s. Some astronomers believe John Flamsteed might have seen it in 1680. He recorded a sixth-magnitude star named 3 Cassiopeiae. However, no star currently exists at that recorded position. It is possible that interstellar dust absorbed the visible light before it reached us. The massive star may have also been cloaked by its own ejected outer layers. This would have absorbed much of the visible-light emission during the collapse.

The discovery of Cas A happened in stages through different types of light. In 1948, astronomers Martin Ryle and Francis Graham-Smith reported its discovery as a radio source. They used the Long Michelson Interferometer to make these observations. The optical component was identified in 1950. Later, researchers used X-ray technology to study the remnant. In 1999, the Chandra X-Ray Observatory found a central compact object. This object is a neutron star, which is the dense remnant left at the center of the explosion.

Cas A provides a unique way to study the past through light echoes. In 2005, the Spitzer Space Telescope observed an infrared echo. This echo was caused by thermal emission from dust. The dust was heated by the radiative output of the supernova during its shock breakout. This light echo allows scientists to reconstruct astronomical events that happened long ago. By studying the spectrum of this light, researchers confirmed the supernova was Type IIb. A 2011 study used these echoes to prove the explosion was asymmetric.

This remnant also helps us understand how the universe is built. In 2013, astronomers detected phosphorus within the cloud. This discovery confirmed that supernovae are responsible for producing phosphorus. The phosphorus-to-iron ratio in Cas A can be up to 100 times higher than in the general Milky Way. Furthermore, the remnant is constantly changing. Its flux density at 1 GHz is decreasing as the remnant cools. Because of this cooling, Cas A is now less intense than Cygnus A at frequencies below 1 GHz. However, it remains the brightest extrasolar radio source above 1 GHz.