Nova, Nova, Supernova…

As has been widely reported, Earth’s inhabitants are looking forward to a rare event in the sky. It’s a “nova”, predicted to be visible sometime in the coming weeks.

The word “nova” is simply Latin for “new.” Coined during the Renaissance, it initially meant “a new bright thingy in the sky that isn’t just another Starlink satellite.” Nowadays it means a very particular type of new bright thingy.

But let’s not confuse it with a “supernova.” That’s something different. How different?

• Nova: Your house has an electrical blackout for an hour
• Supernova: Your entire city goes dark for a week

• Nova: A meteor kills the dinosaurs and lots of other creatures
• Supernova: A meteor melts the Earth and blasts part of it skyward to create the Moon
  
  

What will we be able to see?

There’s a star out there, observable through big telescopes but far too dim for the human eye, that is expected to brighten suddenly. It may become as bright as the North Star, making it easy for us to see it.

However, its moment of visibility will be brief. It will glow only a week or so before it fades away again.

Where should we be looking?

See the figure below. Vega is a bright star that is overhead (in northern mid-latitudes) a bit before midnight. If you start there, and look to the west past the nearly-square parallelogram formed by four stars in the Hercules constellation, you’ll find an isolated semicircle of stars, the Corona Borealis constellation. Just outside that semicircle is where the nova will appear.

What will cause the dramatic brightening of this star?

An explosion, of course. Nature is a bit of a pyromaniac.

• Nova: A summer thunderstorm.
• Supernova: A Category 5 hurricane.

• Nova: You post a new video.
• Supernova: Your video goes viral.
  
  

Supernovas are the most famous type of star explosion, to the point that they have entered popular culture. (“Let’s get some distance before that thing goes supernova!”) A supernova is a far more ferocious beast than a nova, much brighter and more violent. Because of this, supernovas can be visible to the naked eye from much greater distances than is the case for novas.

But supernovas are relatively rare; few stars end their lives this way. For this reason, naked-eye novas are more common than naked-eye supernovas. The twentieth century gave us a handful of visible novas, but only one naked-eye supernova (which occurred in 1987 and could be seen only in the southern hemisphere).

• Nova: A small tree branch lands on your roof.
• Supernova: A giant sequoia crashes through your roof.

• Nova: You take an airplane round the Earth.
• Supernova: You take a rocket to Mars.
  
  

Supernovas actually come in several types. It’s natural to compare novas to Type Ia supernovas, because both explosions involve the same type of object: a “white dwarf”.

A star like the Sun, or one a bit less hefty, will end its life as a white dwarf star, a hot object made of an extremely dense exotic material (a million times more dense than water). That material is composed mainly out of the nuclei of carbon and oxygen atoms, drenched in a sea of electrons. A strange object indeed.

A white dwarf can be the source of either a nova or a Type Ia supernova, each of which is a thermonuclear explosion (a runaway process of nuclear fusion — similar to that which drives a hydrogen bomb.) What’s the difference?

• Nova: a certain amount of material on the surface of the white dwarf star goes thermonuclear.
• Supernova (Type Ia): the entire bloody white dwarf star goes thermonuclear!

For obvious reasons, any particular white dwarf can only go supernova once.

But novas can sometimes recur on the same white dwarf, over and over, sometimes at roughly repeated time intervals, or with a precursor clue, or both. It’s these factors that have provided us with a prediction for a summer 2024 nova.

By contrast, no one has yet correctly and convincingly predicted a supernova of any type, though scientists are trying. These titanic explosions don’t give easily interpreted warnings, and they don’t do repeat performances. (Though sometimes the universe does rebroadcasts — sadly, not for the naked eye.)

• Nova: Your credit card bill this month.
• Supernova: Your nation’s budget deficit this year.

• Nova: You say something embarrassing at the dinner table.
• Supernova: You say something embarrassing on social media.
  
  

Why do these explosions and super-explosions occur? Rain. But… not water rain.

[Caution! This is an area of active, ongoing research, so wear your hard hat. What I’ll say below represents and oversimplifies still-incomplete knowledge… so please understand that it leaves out important details, and some parts might turn out to be significantly wrong. It’s not like we can check these ideas by just building white dwarfs and running these explosions in somebody’s basement lab!]

There’s another star nearby the white dwarf, a big blobby one called a “red giant”, and the two orbit each other like one of those married couples that you just can’t figure out. If they’re close enough, material at the edge of the red giant, mostly hydrogen, can stream off and rain down on the white dwarf.

When enough material has built up, creating high temperature and pressure on the white dwarf’s surface, boom: some of it ignites and explodes, via the same mechanism as a hydrogen bomb. But it makes a hydrogen bomb look almost cute. The power of a nova is billions of trillions of times greater.

However, if the white dwarf is large enough to start with, and if the rain is sufficiently heavy — if enough material collects on its surface over time — then something more dramatic can happen. As the mass of the white dwarf gradually increases, its temperature does too, and at some point it becomes hot enough to start nuclear fusion of its own carbon and oxygen. Then the entire star detonates, using its body as a source of nuclear fuel. KAAABOOOOOM!!! A supernova, worth roughly a million novas.

[Again, this is how type Ia (and Iax) supernovas work. Types II, Ib and Ic work differently: the core of a big star suddenly collapses, rebounds, and blasts the star’s outer layers into space. The only reason these diverse processes were all given the same name “supernova” is that they happen, by chance, to release similar amounts of energy and to look similar to telescopes. Their different origins only became clear in the later 20th century.]

• Nova: I write a new blog post.
• Supernova: Taylor Swift writes a new song.
  
  

Our culture likes to put “super-” in front of everything. “Supervolcanoes” are a very new addition to the language; in the past, we called them “big.” Even “superstars” and “supermarkets” are only a century old. The same is true of “supernova.” But not everything needs to be super. Sometimes it’s okay just to be awesome.

And so, for now, we wait. . . for a moment of awe.