Bad Astronomy

Mi Cas A es su Cas A

In the year 1680, a star in our Galaxy blew up. It wasn’t noticed on Earth, because even though it’s relatively close as these events go – 10,000 light years away – it was behind a thick layer of gas and dust, so its light was dimmed (see the footnote).

But now, 300+ years later, the expanding debris from the explosion is visible. It was first seen as a radio source, and was named Cas A, the first radio source detected in the constellation Cassiopeia. It’s now been observed across the electromagnetic spectrum, and is a pretty cool object. The image displayed here was from Hubble, and was just released today.

We think the star that blew up was about 40 times the mass of the Sun. It lived a violent life, shedding most of its outer layers before it finally exploded. All that sloughed-off gas expanded outward, and when the core exploded the ejecta screamed outward much faster, slamming into the gas, ramming it, making it glow. It’s now a roughly spherical shell of material expanding at tens of millions of kilometers per hour.

As it happens, Cas A was the subject of an educational activity I developed with the help of a team of other folks, both at my home school of Sonoma State University and at Gettysburg College. While developing the activity I calculated some amazing things about the nebula.

The amount of iron in the nebula – just the iron – is over 1000 times the mass of the Earth. That iron was created in the blast, alchemically converted from lighter elements. In fact it’s thought that all the iron in the Universe was made in such explosions. The supernova that made Cas A also created more than enough iron to account for all the iron in a star like the Sun. In the long run, over the lifetime of the the Galaxy, these explosions (which occur roughly once or twice per century) generated enough iron to make 300 million stars like the Sun!

Calcium (as well as a slew of other elements) was created as well. The mass of the calcium in Cas A is very roughly 1029 grams, enough to make 1028 gallons of milk! If you’re curious, there are about 5 grams of calcium in a gallon of milk (I’m rounding the big numbers here pretty severely).

I think that’s all pretty amazing. The iron in your blood and the calcium in your bones were forged in the fires of the death throes of some unknown massive star billions of years ago. The matter was ejected, shot into the Galaxy, where it slammed into and merged with a cloud of gas and dust. That floating junk – the “ash”, essentially, from a tremendous explosion – formed the Sun, the planets, and eventually… you.

Footnote: Yes, I know– some of you will think that it didn’t blow up in 1680, that’s just when the light reached us, and it really blew up 10,000 years ago. I disagree. We cannot say anything about that event until the light reaches us, and in a real sense that event has not happened until the light reaches us. Time flows like light, I sometimes say, meaning that the event itself happens when the light reaches us. So it is acceptable to say that the explosion actually happened in 1680.

Image credit: Robert A. Fesen (Dartmouth College, USA) and James Long (ESA/Hubble), NASA, ESA, and the Hubble Heritage (STScI/AURA)- ESA/Hubble Collaboration