Cassiopeia A

What is Cassiopeia A?

Neutron stars & pulsars· Distance ~11,000 ly. The remnant of a star that exploded some 340 years ago and the brightest radio source beyond the solar system. The neutron star it left behind appeared at the center of the Chandra X-ray telescope's very first image in 1999. Next to Cassiopeia's W, it is one of the youngest supernova remnants in our galaxy.

Where Cassiopeia A is in your sky right now: https://nowsky.org/solar/sky?sel=cassiopeia a — location and time set automatically, no login.

Its place in the sky

A pulsar, in the direction of Lacerta.

Spin, distance, discovery

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Discovery and name

Remnant of a supernova around 1680 that almost nobody recorded — Flamsteed may have glimpsed it.

Frequently asked

What kind of object is Cassiopeia A?

supernova remnant with a central compact object (neutron star).

How far is Cassiopeia A from Earth?

~11,000 ly.

When was Cassiopeia A discovered?

radio source found 1948; neutron star seen in Chandra's first light image, 1999.

Where in the sky is Cassiopeia A?

In the direction of Lacerta.

Where is Cassiopeia A in the sky right now?

Open the sky view for your location and time and it shows the position in the sky right now.

The kind of explosion, learned from light that arrived 300 years late

The light of Cassiopeia A's explosion reached Earth around 1680, but with almost no records from the time the type of supernova could not be known. In 2008 researchers found a way around this. Some of the explosion's light had bounced off nearby dust clouds and was arriving now, more than 300 years late, and a spectrum of that light echo showed the light of the explosion itself.

The answer was a Type IIb supernova: a massive star that had lost nearly all its outer hydrogen before its core collapsed, meaning the star had shed its envelope from a red supergiant before it died. Echoes from different directions carried light from different moments and revealed that the explosion was lopsided. The dust had stored an observation missed three centuries earlier, and the method is now applied to other old supernovae.

The neutron star in depth

Cassiopeia A is a supernova remnant about 11,000 light-years away toward Cassiopeia, among the youngest in our galaxy. Light from the explosion should have reached Earth around 1680, but thick dust hid it and almost no records survive. The debris still expands at 4,000–6,000 km/s and now spans roughly 10 light-years; in radio waves it is the brightest object in the sky outside the solar system.

When the Chandra X-ray telescope aimed at this remnant for its very first test image in August 1999, an unexpected point of light appeared at the center: the neutron star left by the explosion, a "central compact object" with no radio pulses and a hot, cooling surface. Later observations showed its temperature dropping about 4 percent in a decade — the first direct view of a neutron star cooling rapidly as the neutrons in its core turn superfluid. In 2023 the James Webb Space Telescope mapped the chemistry of the debris in unprecedented detail.

Cassiopeia is the W-shaped constellation opposite Polaris, visible from Korea all year round. Select Cassiopeia A in the Sky view and it marks the spot near the left end of the W. The remnant itself is very faint in visible light and needs a large telescope with special filters, but on an autumn evening, knowing that the scar of a 340-year-old explosion and its neutron-star heart lie beside the W overhead is enough to change how the sky looks.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.