There’s a New Exploding Star in the Sky

An erupting white dwarf in the constellation Cassiopeia should be visible with binoculars or small telescopes on a dark night.
March 24, 2021, 1:00pm
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There’s a brilliant new light shining in the constellation Cassiopeia, which pinpoints the spot where a star exploded into a radiant nova. This stellar eruption is so bright that it can be spotted with a small backyard telescope or even a pair of binoculars on a dark night.

The transient event was discovered on March 18 by Yuji Nakamura, an amateur astronomer who lives in Kameyama City, Japan, who reported it to the National Astronomical Observatory of Japan. 

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Since then, the nova has been confirmed by several hobbyist astronomers and professional observatories. It has been given the informal name Nova Cassiopeia 2021, as well as a formal designation, V1405 Cas. The nova should be visible for at least the next several days in the Northern Hemisphere, though the glare of the waxing Moon may make it slightly harder to see.

Nova Cas 2021 is a classical nova, which means it was produced by the combustible interactions within a binary system that contains a regular living star and a type of dead star known as a white dwarf. Most stars, including our Sun, will end their lives by collapsing into white dwarfs; these stellar remnants are about the size of Earth, but still contain a star’s worth of mass, which means they are very dense and compact.

For this reason, when white dwarfs end up in systems with regular stars, their intense gravitational force can tug gassy material off of their companion. When this gas build-up around the white dwarf reaches a certain temperature, a fusion reaction is triggered that sparks the nova. What we see on Earth is the white dwarf forcefully ejecting the atmosphere it stole from its companion, at an estimated speed of 1,600 kilometers per second (3.6 million miles per hour).

It’s not yet clear what system Nova Cas 2021 belongs to, which makes it hard to tell how far it is from Earth. However, astronomers have noted that the brilliant eruption is coincident with a known binary system called CzeV3217, which is located 5,500 light years away, so this is the likely source of the nova.

If you search “Nova Cas 2021” on Twitter, you will find many helpful maps and guides to tracking it down in the night sky; it is located at right ascension 23h 24m 48s, declination +61° 11′ 15, according to Sky & Telescope

Perhaps the easiest way to spot the nova is to find Cassiopeia on the sky, and search for a bright star called Caph at the bottom of this W-shaped constellation. This star is basically pointing at the nova, so you just need to follow its direction away from Cassiopeia to see the erupting white dwarf. If you can’t find it for any reason, don’t sweat it: novae are common and you’ll likely get the chance to see one again within a few years.