The evening of Nov. 11, 1572, the Danish astronomer Tycho Brahe was looking up at the sky when he saw something he’d never seen before. Northwest of the lopsided W formed by the constellation Cassiopeia’s brightest stars, and about halfway toward the constellation Cepheus, was a bright star that was not there a week earlier.

This, as we say nowadays, did not compute. The heavens were thought to be perfect and changeless, and astronomers for eons had been calculating the mathematics of that perfection. The sudden appearance of a new star was literally cause for alarm.

Tycho’s supernova remnant seen in a composite of X-ray images taken by the Chandra X-ray Observatory, together with infrared and optical light images. Photo courtesy of NASA

For about two weeks, it was the brightest star in the sky. It then began to fade, and by March 1574 was no longer visible to the eye (the first telescopes were about 35 years in the future). B Cassiopeiae, or Tycho’s Star as it came to be known, was followed spectacularly in 1604 by a second new star in the constellation Ophiuchus that came to be known as Kepler’s Star, after Tycho’s more famous scientific descendant Johannes Kepler.

These stars were called “novas,” Latin for “new.” Following improvements to telescopes and accumulations of mathematical data, the astronomers figured out these were exploding stars, and there are two kinds.

A nova (see Backyard Naturalist column published Jan. 26) is a star that has accumulated so much material from a companion star that it explodes. Nova explosions are so huge that stars thousands of light-years away, which were previously invisible to us, suddenly become visible.

But what Tycho and Kepler (among others) saw were supernovas — explosions 10,000 times more powerful than novas.

There are two kinds of supernova. A Type I supernova occurs when an elderly white-dwarf star accumulates from a companion star so much material — reaching 1.4 times the mass of our sun — that its structure cannot hold up and it collapses and then explodes.

A Type II supernova occurs when an old, large star burns out its nuclear fuel, creating instabilities in the star’s different layers. At a certain point, pressures inside the star become so unbalanced that the star’s core suddenly collapses.

The mathematics of supernova physics suggests this collapse happens in about a second. This is an unimaginable force and speed — the star is millions of miles in diameter. In an instant, the core material crushes to a density where it cannot collapse any further, and then a rebounding shock blasts the outer parts of the star away at thousands of miles per second.

The explosion is so enormous that the energy released by this one star can be equivalent to the energy output of an entire galaxy — billions and billions of stable stars. Such blasts can be seen in galaxies billions of light-years away.

While a nova explosion normally leaves the star intact, in a supernova the star is essentially destroyed. An expanding shell of material called the supernova remnant is left, from which new stars are thought to form. A Type II explosion may leave a neutron star — a dark body about 10 to 20 miles wide with the mass of the sun — or a black hole, so dense that even light can’t escape from it.

Astronomers estimate that a star goes supernova somewhere in the universe about every 10 seconds. Thousands of them have been detected through high-tech telescopes, and historical records indicate nine appear to have been seen. Of those nine, one was seen by Chinese astronomers in 185 AD, and another in 1054.

The brightest supernova yet observed was recorded by Arab and Chinese astronomers in 1006. Another was seen by Chinese and Japanese astronomers in 1181; the remnant, known as Pa 30 in Cassiopeia, is producing an extremely powerful outflow of gas. Two others were in 1572 and 1604. A supernova explosion inside our galaxy, whose remnant is called Cassiopeia A, probably occurred around 1690, but was not seen. The nearest supernova detected recently is SN 1987A, which exploded about 170,000 light-years away in the Large Magellanic Cloud, a small galaxy orbiting the Milky Way, in 1987.

A supernova explosion occurring within about 50 light-years could damage or wipe out life on Earth, according to the European Space Agency, but no stars in our area are likely to explode. The nearest old star that has a chance of exploding in the next 100,000 years is Betelgeuse, which is somewhere between 500 and 750 light-years distant.

What is going on in a universe with events so stupendous they cannot be experienced or even imagined except from a vast mathematical distance? The magnitude of our awe at the facts of supernovas is probably comparable to the magnitude of the awe Tycho Brahe and Kepler felt on seeing new stars where none had been before.

It is a kind of waking up to vastness.

Dana Wilde lives in Troy. His writings on the stars and planets are collected in the e-book “Nebulae: A Backyard Cosmography,” available by download from online book sellers. Backyard Naturalist appears the second and fourth Thursdays each month. You can contact him at dwilde.naturalist@gmail.com.