Day/Night (super) Stars

A supernova is the explosion of a large star that has fused all its hydrogen into helium (and other, heavier, elements). Well, actually, there are two types of supernovae. The first sentence describes a core-collapse supernova, which is all supernova types other than Type Ia. Ia supernovae occur when a white dwarf sucks so much matter from a red giant companion that it collapses under the weight and explodes. This blog is not about the differences between types of supernovae.

This blog post is about how supernovae affect civilisations. I've mentioned them before in the context of sterilising planets and hence halting the development of life. Today, I talk about supernovae that are distant enough to not kill everything while still being clearly visible to the unaided eye.

Historically

In the past millennium, there have been several (obviously non-sterilising) supernovae visible from Earth. We know about them thanks to various historical records, which tend to get more scientific as they become more recent. Don't think that being distant enough not to kill us means that they aren't bright. Most of them have been brighter than all the other objects in the night sky (other than the moon) and some were even still visible during the day.

Some comments on the Milky Way's historical supernovae:

• Lupus is now the remnant of a supernova which exploded in 1006. It is 2.2 kpc away (kpc = kiloparsecs; that distance is 7200 light years). It was visible during the day and apparently illuminated the landscape at night. (Interesting fact: if you take out the moon, it was brighter than the rest of the night sky put together.) It was recorded by Chinese, Arabic and European astronomers of the day.
• Crab, as in the Crab Nebula and the Crab pulsar, is the remnant of the supernova that exploded in 1054. It is about 2 kpc (= 6500 light years) away and was well documented throughout Asia and the Middle East. It was visible in the sky for two years, though it was less bright than Lupus (due to there being more dust obscuring its light in that direction), it was very much visible during the day.
  

  

  Crab Nebula Mosaic from HST
  Image Credit: NASA, ESA, J. Hester, A. Loll (ASU)
  Acknowledgement: Davide De Martin (Skyfactory)

• 3C 58 is one of the less inspiring names for a supernova remnant (pre-dominantly pulsar in this case). It was seen in 1181 by Chinese and Japanese astronomers and was only visible a night albeit as the brightest star in the sky. It's possible that the pulsar in that direction is older than the supernova event, but its hard to know for certain. It is 3.1 kpc (= 10 000 light years) away.
• Tycho is the next supernova on the list. It exploded in 1572 and is named after Tycho Brahe not because he discovered it (how can you "discover" something that everyone can see, even during the day) but because he studied it extensively (some have said obsessively). It inspired him (and others) to revolutionise the astronomy of the day.
• Kepler came next, with his supernova which was first observed in 1604. (4.8 kpc = 15 600 light years away.) It was bright enough to be visible during the day, but not when the sun was high. As with Tycho, Kepler didn't discover it but he wrote a book about it, which led to it being named after him. Wiki says this was the most recent observed supernova in the Milky Way, but there are two more about which less fuss was made because they were less glaringly obvious.
• Cassiopeia A probably exploded in 1680 but that date uncertain. It was noted down in a routine sky catalogue by the first Astronomer Royal, John Flamsteed, then later erased as an erroneous entry because there was no long a star at that location. The modern remnant wasn't discovered until 1947, after which it was linked to the erroneous catalogue entry. Although the remnant is 3.4 kpc (= 11 000 light years) away, it wasn't easily visible because of the large amount of dust in that direction.
• Speaking of dust, this last supernova is an interesting case. It doesn't have a nice name, merely one based on its galactic co-ordinates: G1.9+0.3, or G1.9 for short. No one saw it explode. there is a lot of dust in that direction. Most of the dust in the Milky Way lies in the plane of the disc and, looking towards G1.9, we are looking right through that disc of dust. From the speed of the remnant expansion, we predict that it exploded around 1868. Anyway, this one is less relevant to the thrust of this post, I just thought it was cool.

You may have also heard of supernova 1987a (which exploded in 1987, hence the moniker), but that was actually in the Large Magellanic Cloud, not the Milky Way.

Because I can, here is a little graphic showing the various directions of these supernovae:

Image credit: NASA/CXC/M.Weiss

Auspicious portent?

So supernovae are pretty cool (or incredibly hot, if you want to be literal about it) and now it's time to tie it back into stories.

Before we, as civilisations, knew what supernovae really were (and to be fair, that occurred relatively recently, compared with all those historical supernovae), they were seen as new stars, visiting stars. In fact that's what the "nova" part indicates: newness.

Lacking a physical explanation, imagine what those people must have thought when a new light appeared in the sky and then, incredibly, was visible during the day. It's the sort of thing that, these days, might make someone less abreast of astronomy think of aliens. What would it have been back then? Portents?

We know that comets were often hailed as portentous, so why not an auspicious supernova? Supernovae are even rarer which, one would think, would make them even more significant, mythologically speaking. In a world of myth and legend, what might the appearance of a bright new star lead people to do? I hesitate to suggest that people would panic (unless goats with two heads were born at the same time, maybe) but it would surely affect their lives. Especially if it lit up the night enough to see by (like Lupus probably did).

In a world of myth and legend, what might someone do if a new star lit up the day sky? Would they, perhaps, set out on a quest to follow it? What would their reaction be to whatever they found beneath it on their journey?

The Evolution of a Science

As you may have gathered from the intro of my previous post, I've been at a conference, preceded by a winter school, this week. I had planned to have a post prepared before I left, but it was not to be. Instead you get a post inspired by said winter school. (And incidentally, this was written entirely on an iPad soft keyboard. It wasn't a bad experience, if you're wondering.)

One of the talks I attended spoke about where we are today in understanding galaxies. As part of the talk the presenter also went over the history of the scientific study of galaxies, which got me thinking about how a scientific field evolves in time and what we need to consider when we're inventing a fictional scientific field. Then, obviously, I decided this would make a good blog post and here we are.

This isn't really going to take many societal effects into account, which could be very important in some fields, especially when religion disagrees with scientific discoveries. What I am instead going to talk about is how the scientific progress gets made using examples from galaxy astronomy/astrophysics and a few other fields.

Breaking it down

The way I see it, the development of a scientific field can be broken down into four stages:

1. Discovery
The field first has to be discovered. This is a pretty basic requirement. In the case of galaxies, it was thought a hundred or so years ago that the Milky Way was the entire universe and contained everything we could see in the night sky. Then other galaxies outside of our own were discovered or, more accurately, it was realised that that "spiral nebula" in Andromeda was not actually within the Milk Way) and a field was born.

2. Classification
Once a bunch of galaxies had been discovered, Hubble and others started classifying them based on obvious characteristics of appearance. We actually still use a classification system based on Hubble's. However useful it is to be able to say, "Well, that galaxy there is elliptical, that one is a late-type* spiral," it wasn't quite giving us more information just yet.

3. Analysis
This is the part where instead of just collecting things, we start analysing them in different ways. While Hubble was looking at galaxies with optical telescopes, he also took their spectra. It was at approximately this point when he noticed that all the far away galaxies were moving away from us (looping back to the discovery point) and the field of modern cosmology was born.

Since Hubble, of course, many other people have studied galaxies. As new data became available, thanks to the progression of technology, we discovered dark matter (from studying the dynamic properties of galaxies), we learnt that galaxies can interact and merge and we have been able to observe them at all sorts of different wavelengths leading to the discoveries of a variety of properties of galaxies and other things. We have started mapping the universe (which, if you hadn't guessed, is significantly larger than just the Milky Way), inventing models like hierarchical assembly (sorry, I couldn't find a sufficiently lay link for this one) to fit our data and we are now much better equipped to study the evolution of galaxies.

4. Understanding
The last few points I made in the previous section are tied in with starting to really understand galaxies. This is the stage when we start to understand what's going on and become able to make predictions. As technology develops further, we can test our predictions more and more precisely and, sometimes this leads to discoveries of discrepancies and, again, we loop back to analysing and trying to explain these.

As hinted above, these aren't distinct stages. There is almost always going to be some overlap and a considerable amount of looping as the field progresses. And during the development of the field of galaxies, a whole lot of other (sub-) fields were born such black hole physics (well, more specifically, AGN), dark matter and dark energy (which are actually completely unrelated to each other).


*Don't get me started on why Hubble's ideas of "late-type" and "early-type" galaxies irritate me greatly.


Technology-driven advances

I mentioned above that some of the new discoveries were made when new technology made new data available. In the absence of new data what sometimes happens is that more and more elaborate theories are invented to explain bits of observations that we just don't have enough information to address.

An obvious example that springs to mind is the celestial spheres rotating in the sky which were once used to explain orbital mechanics. The idea was that the stars were embedded in a sphere made of ether or quintessence (or insert fifth element of choice here), surrounding the Earth which rotated around the Earth, accounting for the motions of the stars across the night sky. Then more spheres, with each of the planets, sun and moon embedded in one each, were added to explain the motions of the nearer celestial objects. As observations and measurements improved, more spheres were added to account for things like the precession of the equinoxes/solstices. Even Copernicus, when he came along, kept the celestial spheres and just changed them so that, other than the moon, they rotated around the sun rather than the Earth.

It wasn't until Kepler came along and developed his laws of planetary motion that we moved from celestial spheres to orbits and then, shortly after, Newton came up with gravity and proved Kepler's laws. Kepler was able to do this thanks to the more precise measurements of planetary motions made by Tycho Brahe. New observations made it possible to move forward and, indirectly, contributed to a new field (Newtonian gravity) to be born.

Moving forward

Now we have absolute proof of a lot of things in astronomy and astrophysics (and many other areas of science). Basically, we know stuff now. But we don't know everything, not by a long shot. Remember, just over a hundred years ago, scientists thought that we knew almost everything and only a few small details were left to be filled in. Then quantum mechanics was discovered.

I like to think of the pool of human knowledge as fractal; the more we know, the greater the area of the fractal and the more branches of knowledge we develop, the larger and more visible the infinite perimeter between knowledge and known unknowns becomes.