The USA's national day is Independence Day, a federal holiday celebrated on July 4th. Here's a bit of astronomical history and some colossal cosmic fireworks to mark the day.
In 1776 the Second Continental Congress was the de facto government of the British colonies in what's now the USA. They had been at war with the mother country for over a year. The Congress debated the document drafted by Thomas Jefferson that set out their grievances against King George III. On July 4th the Declaration of Independence was approved.
The day is celebrated in very much the way the summer solstice is in many countries. This includes picnics, music, fireworks and other family and communal events.
A planet called George
George III – the wicked King George of my childhood history lessons – was of the German House of Hanover. He didn't handle the business with the colonies very well and he certainly got a bad press in America. Yet, as kings go, he wasn't a bad sort. He was faithful to his wife and adored his large family. He was more interested in science than in politics. For his pursuit of scientific agriculture, satirists dubbed him “Farmer George”.
The king also loved astronomy. This is where William Herschel enters the story. He was a musician from Hanover who had settled in the English city of Bath. Herschel made his own telescopes and was a very keen astronomer. While observing one night, he discovered what seemed to be a comet. But it turned out to be the first new planet ever discovered.
The King rewarded Herschel with a salary, and Herschel gratefully named the new planet Georgium Sidus (George's Star). Not surprisingly, the name didn't catch on outside England, particularly in America. Eventually, of course, the classical name Uranus was adopted.
However King George and William Herschel had a big fan in Maximilan Hell, the director of the Vienna Observatory. He devised a new constellation out of a few stars from Eridanus which he called Psalterium Georgianum (George's Harp). A psalter was an ancient type of harp. Johann Bode included it in his 1801 star atlas Uranographia under the name Harpa Georgii. The constellation hasn't been in use for well over a century.
We always associate the Fourth of July with fireworks, so here are some impressive bangs.
On July 4, 2005 NASA's Deep Impact spacecraft released a probe that crashed into Comet Tempel-1 with an energy equivalent to about five tons of TNT. The result was carefully studied to learn about the structure of comets. This is important for understanding the origins of the Solar System, as well as for learning how to protect Earth from impacts.
The Crab Nebula (Messier 1) is a supernova remnant 6500 light years away. It's one of the few that we can connect to observations of the original supernova. A supernova is produced when a massive star reaches the end of its life, and they are some of the most powerful explosions in the Universe.
Throughout its life two forces act on a star. Gravity pulls it inward, but it doesn't collapse. It's balanced by the outward pressure from the energy released by nuclear fusion in the core. But when the star finally runs out of fuel and nuclear reactions stop, gravity wins and the collapse is rapid. Even the structure of the atoms in the star collapses, and the protons and electrons are squeezed together to form neutrons. When the matter can't be contracted anymore, the tremendous energy created by the collapse rebounds in a shock wave. It blasts away the star's outer layers and speeds through space. What's left is a neutron star (or sometimes a black hole). The matter in a neutron star is so dense that if you could get a teaspoon of it to Earth, it would weigh millions of tons.
A supernova can, for a time, be more luminous than an entire galaxy. The one that made the Crab Nebula was so bright that it was visible in daylight for 23 days. Chinese astronomers first observed it on July 4, 1054. That was a bit early for the USA's independence, but the nebula is still there and the remnant of the star is a pulsar, a spinning neutron star.
In 1987 there was a supernova in a nearby galaxy that was visible without a telescope. This was the first since 1604. The Milky Way has an average of three supernovae in a century, but few can be seen with the unaided eye. So if we want fireworks, we need the Fireworks Galaxy (NGC 6946).
NGC 6946 was discovered by William Herschel. It's over 22 million light years away, so it was only a fuzzy blob to him. However a big telescope shows the Fireworks Galaxy to be a face-on spiral galaxy with a high incidence of supernovae - nine have been found in less than a century.
Happy Fourth of July to the United States, and greetings to readers from other countries on your own national days.