On March 18, Japanese amateur astronomer Yuji Nakamura (who must have a seriously keen eye) spotted something within the constellation Cassiopeia that did not show up on any star maps. In fact, at magnitude +9.6, it was at least 20 times brighter than anything ever spotted at that location.
It was confirmed shortly afterwards by the University of Kyoto, and by the spectrum of it, it is most likely a classical nova. Now it’s called Nova Cas 2021.
At last check, late on March 19, it had brightened to magnitude +7.5. That’s a quick increase in brightness of 6 or 7 times since its discovery less than 48 hours earlier. That means it can be spotted quite easily with binoculars and is approaching the point (magnitude +6.0 or so) where it can be spotted with the naked eye. I haven’t seen any data posted over the weekend on it, but I’d imagine we’ll see some more early this upcoming week.
Just to refresh your memory on the magnitude thing, a decrease of one unit in magnitude means an increase in brightness of about 2.5x:
And here is a quickie guide to some real magnitudes of objects you know:
Let’s see where in the sky this thing is! One of my favorite constellations up there is Cassiopeia, because I can easily spot its ‘W’ shape. (I went to school at Wisconsin, so maybe that has something to do with it, too?) Plus you can always see it from the Northern Hemisphere because it’s circumpolar (goes around and around the North Star).
First, the zoomed-out version. Here, thanks to Awesome Guy Dominic Ford at in-the-sky.org, are two of Cassiopeia’s stars pointing right to the Bubble Nebula, the object in the little green crosshairs that the nova is close to:
Now, let's zoom in to see where it is…
See that little T-shaped arrangement of stars to the left of the Bubble Nebula, where you have four across the top and then one bright one (a star called HIP 115691) at the bottom? The red dot is about where the nova is.
Now let’s check out Yuji Nakamura’s own before-and-after discovery photos, and you’ll see that T-shaped thing. The picture on the left is the “after” picture, with Nova Cas 2021 pointed out by the white line:
A guy named Filipp Romanov caught Nova Cas 2021 a day later, and you can tell it had already brightened (he shows it at magnitude +7.9):
Someone on reddit named Charge_parity got a very detailed before-and-after comparison and was pretty happy about it:
It seems to have only been discovered less than 48 hours ago and continues to brighten as we speak. I'm so bloody happy, this is my first transient event.
Hey, I’m happy, too! Now the question is, how much brighter will it get, and how long will it be around?
Now, a note about classical novae: These aren’t quite like supernovae, where a white dwarf star explodes completely because of a runaway thermonuclear reaction within it. Instead, a white dwarf pulls in lots of material from a super-close companion star, and when that material reaches a high enough temperature and pressure, you get a runaway thermonuclear reaction, but only around the surface of the white dwarf. So both stars live to do the whole thing over, which they generally repeat every 30,000 to 100,000 years.
Below are before-and-after images of Nova Cygni 1992, where the layer of material around the white dwarf’s surface gets blasted away:
While we’re in this area of the sky, we should take a minute to check out the Bubble Nebula (NGC 7635), which is pretty darn cool in its own right:
The Hubble Space Telescope’s site describes it this way:
The seething star forming this nebula is 45 times more massive than our sun. Gas on the star gets so hot that it escapes away into space as a "stellar wind" moving at over 4 million miles per hour. This outflow sweeps up the cold, interstellar gas in front of it, forming the outer edge of the bubble much like a snowplow piles up snow in front of it as it moves forward.
As the surface of the bubble's shell expands outward, it slams into dense regions of cold gas on one side of the bubble. This asymmetry makes the star appear dramatically off-center from the bubble, with its location in the 10 o'clock position in the Hubble view.
I should mention that the Bubble Nebula was discovered by William Herschel, who also discovered Uranus 240 years ago, on March 13, 1781. There’s a really nice article about that by Ben Evans over at Astronomy Magazine.
So we’ll see what this week brings with Nova Cas 2021! I know it won’t affect your life that much, but if you’re like me, the spectacle of this sort of uncommon event is fascinating enough, and it’s also a strangely comforting reminder that whatever your fleeting problems may be right now, they’re no match for the Universe.