Why don't we see more supernovae in our galaxy?

Question

I found this on the Wikipedia page on supernovae:

The total supernova rate in our galaxy is estimated to be about 4.6 per century, or one every 22 years, although we haven't actually observed one for several centuries.

That sounds like a freak occurrence to me.

It made me curious about the numbers so I refreshed my knowledge on the Poisson distribution. It is not specified how many centuries we have not seen a supernova, but let's assume it's only two.

If on average 9.2 supernovae happen per 2 centuries then the chance of having exactly 9 in any two century period can be calculated with:

$$p(k) = {\lambda^k \times e^{-\lambda} \over k!}$$

Filling in $k=9$,

$$p(9) = {9.2^9 \times e^{-9.2} \over 9!}=0.1315$$

The chance that there is not a single supernova in a two century period is:

$$p(0) = {9.2^0 \times e^{-9.2} \over 0!}=0.00010104$$

That's about a 1 in 10,000 chance. If "several" means 3 centuries, then the chance of not having any supernova is as little as 1 in 100,000.

So from this I can only come to the conclusion that supernovae did in fact happen in the Milky Way in the last two centuries, but that we didn't see any of them.

But now I'm confused. I read about supernovae that have happened in galaxies billions of light-years from here that lit up as the brightest source of light in the sky for days. Surely we would be able to see all of the supernovae that happen in our Milky Way (no more than 100,000 light years away) with the naked eye, let alone with telescopes?

Are some supernovae so much more super than others? Are all of the Milky Way's supernovae visible to the naked eye?

Answer 1

So from this I can only come to the conclusion that supernovae did in fact happen in the Milky Way in the last two centuries, but that we didn't see any of them.

But now I'm confused. I read about supernovae that have happened in galaxies billions of light-years from here that lit up as the brightest source of light in the sky for days. Surely we would be able to see all of the supernovae that happen in our Milky Way

One problem with estimating how many supernovae have happened in the Milky Way in the last two centuries, is that the remnant from a supernova is far dimmer than the nova itself and far harder to find. Several supernovae could have happened in the Milky Way in the last 2 centuries and remain undiscovered — and just for clarity, we define the date of a supernova by the date the light from the explosion reaches the Earth, so saying saying a nova "happened" in the last 200 years refers to the date the light from the event reached Earth, not the actual date of the event, which you probably already know, but just to clarify.

So for argument's sake, let's say that a supernova's light reached the Earth about 50 years ago, but it took place on the far side of the galaxy. To find that, we'd have to look for a nebula on the far side of the galaxy and that's a hard thing to see. Similar to looking for the theoretical Planet Nine, finding old supernova remnants on the other side of the Milky Way takes a lot of looking. Even with modern telescopes, it's still a needle in a haystack, and especially if the view is blocked by dust like much of the Milky Way is.

A Milky Way supernova remnant was discovered in 1985, supernova remnant G1.9+0.3. It was thought to have "happened" around 1868, though it went unobserved and probably wasn't visible to the naked eye at the time. There's probably been several others more recent than that one. G1.9+0.3 would have been visible if not for interstellar dust. From article above:

It was a type Ia supernova believed to have exploded about 25,000 years ago, and the signal began reaching Earth around 1868. The light from the supernova would have been visible to 19th century astronomers, had it not been obscured by the dense gas and dust of the Galactic Center.

I'm not sure the 4.6 supernovae per century from your article is accurate. It might be, but the number I'm used to hearing is about one per century. But regardless of which number is actually correct, it still doesn't imply high mathematical improbability because many Milky Way novae would have gone unnoticed if they were far enough away. In short, I agree with what you said here:

I can only come to the conclusion that supernovae did in fact happen in the Milky Way in the last two centuries, but that we didn't see any of them.

Here's a related article on Milky Way visibility.

Observation of a distant galaxy nova is possible if we have telescopes looking in that direction. A nova is much more detectable at the time it goes nova. Much less so, years later.

Today, however, with neutrino detection in 7 locations around the globe, I think it's virtually impossible that we'd miss a supernova in the Milky Way, so we have a good chance of seeing one in our lifetime and it's likely that none have occurred since somewhere around 1980. A supernova was detected in the Andromeda galaxy in 1987 by that method, and our neutrino detection has improved since then to give us early warning and pinpoint location.

As far as visibility, size matters, but what matters more is how close and how much dust is in the way. Most of the recorded supernovae were quite bright and fairly noticeable to someone who was familiar with the stars in the sky (list of known supernovae).

Eight Milky Way novae have been recorded by history and observed by the naked eye in the last 2,000 years, well below the number that should have happened in that time. Five of those eight had brightness greater than $-3$, which is brighter than Jupiter and would have been immediately noticed by anyone familiar with star charting. Two others had magnitudes around zero, which is still brighter than most stars. SN386 was less bright, but is still easily visible and recorded by Chinese astronomers. And finally, Cassiopeia A was quite dim, but it was still observable. Only about 10% (ballpark estimate) of the Milky Way is close enough and unobstructed enough to provide supernovae that would visibly get noticed. Most would have gone unnoticed until recently.

Hope that's not too wordy, I can try to clean up if needed.

Answer 2

Firstly, the figure you quote is disputed and must have very substantial uncertainties. The same wikipedia article also cites 3 per century.

Second, and more importantly, you have not accounted for dust obscuration. Most type II supernovae will occur in massive stars close to the Galactic plane and will be afflicted by huge amounts of extinction.

If we assume an peak absolute magnitude of about -18 and visual extinction of 2 mag per kpc in the galactic plane, then a supernova would have to be within a few kpc to be a notable naked eye object.

If we consider the Milky Way disc to be of radius 15kpc, then a horizon at say 3kpc only encompasses 4% of the possible volume. Thus if there are say 4 per century, only 0.16 per century would be notable naked eye objects.

Answer 3

Some supernovæ are more super than others, but that is not the reason that we have not observed one in 300 years. There have been 5 or 6 naked eye supernovae in the last 1000 years, There have certainly been others (such as Cassiopeia A and Supernova remnant G1.9+0.3 But they have have been hidden by the dense dust and gas in the plane of the galaxy. Some regions of the centre of the galaxy have 30 magnitudes of extinction in visible light. That means that a SN with a magnitude of 0, would appear to have magnitude 30, and be essentially invisible.

So while we have been unlucky: we should probably expect to have seen a couple of supernovae since Kepler's star, the probability is not so very small.

Answer 4

Also, you might be forgetting that the distances of the stars that exploded will cause us to see them at different times. Say three stars exploded in a given year but one was 1,000 light years away another was 1,500 light years away and the third was 2,000 light years away. We wouldn't see those novae here on Earth in the same year. So, while it's quite possible that there are 2 to 3 novae a year, we may (i.e. likely will) see them many years apart.

On the flip side, a star that exploded 100 years ago that was 100 light years away and another star that exploded 500 years ago and was 500 light years away would appear to us in the same year here on Earth.