According to records from a Chinese historical text, “Song Hui Yao,” on the morning of July 4, 1054, a very bright “guest star” appeared near the Tian Guan (Heavenly Gate) of the Taurus constellation. For the first 23 days, its brightness surpassed that of Venus, making it visible even during the day. Later, this star gradually faded, completely disappearing by April 6, 1056. This is the earliest detailed record in the world of a supernova explosion. The Crab Nebula is the remnant of this significant supernova event.
In the West, the Crab Nebula was discovered by English astronomy enthusiasts in 1731. In 1884, Lord Ross (UK) noted that the shape of the nebula resembled a large crab crawling across the sky, thus naming it the Crab Nebula. This quirky name has persisted to this day.
Crab Nebula (Photo: astroa.physics)
Since its discovery, the Crab Nebula has continually attracted the attention of astronomers. In 1921, American astronomer Dunken compared two photographs of the nebula taken 12 years apart and found that it was expanding. In 1928, American astronomers measured the expansion rate of the Crab Nebula to be 1,100 km/s. Subsequently, many astronomers confirmed that the Crab Nebula is a cloud of gas formed by the explosion of the supernova in July 1054.
The temperature of the Crab Nebula is not as hot as the Sun, but its radiation is incredibly strong, emitting X-rays, radio waves, visible light, and gamma rays at high intensities. It is truly a “cosmic broadcast station” of significant power. Where does this radiated energy come from? In 1968, astronomers discovered a peculiar “pearl” from the “large belly” of the Crab Nebula, identified as a neutron star. This star has a diameter of less than 20 km but weighs as much as the Sun. It possesses an extremely strong magnetic field, with a surface temperature reaching 10 million degrees Celsius and an internal temperature soaring to several hundred million degrees Celsius. Its rotation speed is 30 revolutions per second, and it emits periodic pulses, which can be detected on Earth as “pulsar signals.” Consequently, neutron stars are also referred to as pulsars.
X-rays in the Crab Nebula (Photo: chandra.harvard)
