# (Belated) Happy Birthday, SN1987A!

I don’t know many people who have a favorite supernova, but I do:
SN1987A.
Its light first reached Earth 19 years and a day ago (sorry, I meant to write this entry yesterday, but was too busy), on Feb. 23, 1987.

Every so often, you may run across someone who says that scientists know the laws of physics now, or in our part of the universe, but that the laws may be different in other galaxies, or that they were different in the past (in particular, some creationists have claimed that light used to go a lot faster in the past, which would explain why we can see light from stars more than 6000 light years away).

One of the cool things about astronomy in general, and SN1987A in particular, is that it does allow us to observe the distant and ancient universe. The orange ring in the photo above is a ring of dust and debris. It wasn’t ejected by the supernova; rather, the star ejected it while it was still a red giant. The ring is about 1 light year in radius.

Astronomers can directly measure the angular size of the ring. And since they know that it’s about 2 light years in diameter, simple trigonometry allows them to calculate the distance to the star at 168,000 light years.

But how do they know that the ring is 1 ly in radius? The light from the supernova first reached Earth on Feb. 23, 1987. A year later, the ring lit up with light reflected from the explosion. That tells us that the reflected light took a one-year detour relative to the light that came directly to Earth, so therefore the distance from the star to the ring is 1ly.

Okay, but what if the speed of light was different at the time? Imagine two cars that start in New York and drive to Los Angeles. Let’s say the first one starts out at 10:00 going 50 mph, and the second leaves an hour later, i.e., 50 miles behind. Let’s further say that they are always both at the same speed. If one speeds up, the other speeds up at the same time by the same amount. If one slows down or stops, the other one has to do so as well. Under these conditions, since the second car started 50 miles behind the first one, it’ll always be 50 miles behind. The same logic applies to the light beams from the star and ring.

The speed of light in 1987-1988 was 300,000 km/s, and didn’t change at all in that time, as far as we can tell with very sensitive instruments. So we know that when the light from the star arrived on Earth, the light from the ring was 1 light year behind it (and would arrive in 1988). Since they were 1ly apart when they arrived, they must have been 1ly apart when they started, so the radius of the ring is 1ly.

So we know that SN1987A really is 168,000 ly from here, and that it blew up 168,000 years ago. And by analyzing the spectrum of light from the explosion and how it changed over time, astronomers can tell something about what was going on inside it.

A supernova is an unimaginably violent event, and all sorts of interesting things happen in it. The spectrum of light from SN1987A showed that it
formed 0.1 solar masses of radioactive nickel-56.
It also showed nickel-56 decaying, over time, to cobalt-56 and then iron-56. According to the spectra, this decay occurred the same way and at the same rate as it does on Earth today.

The fact that radioactive decay worked the same way 168,000 years ago, 168,000 light years away as it does here and now is strong confirmation of the fact that scientists do, in fact, have a clue, and know something about how the universe works.

The Astronomy Picture of the Day archive has an
animation
of light echoes coming from the supernova, and bouncing off of two sheets of gas between it and us.

The Hubble Heritage Project has
a much fuller page