Most stars burn with a steady and relatively unvarying flame for billions of years. But many live out the ends of their lives by pulsating, changing their apparent brightness in periods that can vary from minutes to years.
Such stars are called variables, and our autumn skies possess some of the most important and brightest of each class of such stars.
The group of pulsating stars with the most members is the long-period variety, sometimes called Mira variables. Their periods of pulsation often last for months or even years.
They are usually red stars, suggesting that they are old and have burned up much of the hydrogen fuel that stokes their thermonuclear reaction.
The best example of those beating red giants is Mira in the constellation Cetus, which stretches low across the southeastern sky in the early evening. The stars of Pisces, right above Cetus, point downward at Mira.
In Latin, “Mira” means “the Wonderful One.” English words like “miracle” and “miraculous” derive from the Latin term, and with good reason.
Red giants like Mira typically expand to gigantic sizes when their lives are coming to an end. At over 300 million miles wide, Mira is one of the most voluminous stars known. (One light-year is equivalent to about 5.9 trillion miles.)
Compare that to the mere 900,000-mile diameter of the sun. If the sun suddenly expanded to Mira’s size, it would swallow up all the inner planets, including Earth and Mars.
Mira pulsates with a period of about 332 days. At its maximum, it is almost as bright as the North Star and is easily visible to the naked eye.
At its minimum brightness, it takes a telescope or binoculars to see it, and, to the naked eye, it seems to have disappeared entirely from the constellation.
Mira is the first long-period periodic variable discovered in modern times. Its variability was first noticed, sans telescope, in 1596 by German pastor and astronomer David Fabricius.
In 1638, Dutch astronomer Phocyclides Holwarda determined that the star disappeared and reappeared over eleven months. Around the same time, Polish astronomer Johannes Hevelius noticed its unusual variability. He eventually named it Mira in 1662 because he thought its pulsations were unique.
Soon thereafter, French astronomer Ishmael Boulliau observed its period at 333 days, remarkably close to the contemporary value.
Mira is a six-million-year-old red giant star about 350 light-years from the sun at the end of its life. Stars get their explosive power by converting hydrogen to helium deep in their cores.
As stars age, the hydrogen fuel supply in their cores gets used up, causing the core to contract and heat up. The high temperatures cause the star to fuse the hydrogen in the shell outside the core. The new round of hydrogen fusion causes the star to expand dramatically into a red-giant star.
At the same time, the core continues to collapse and heat up. Its intense temperatures cause its helium to fuse into carbon and oxygen.
At Mira’s current state of development, its carbon/oxygen core is inert. No more fusion is occurring in the core. However, the shell around the star is still converting hydrogen to helium.
The resulting helium ash cools the star and causes it to shrink. As it shrinks, the increased pressure and temperature causes the helium to fuse to carbon. That explosive power causes the star to expand until another layer of ash forms around the core, and the cycle repeats.
One result of all that expanding and contracting is that decrepit Mira is shedding a considerable portion of its outer shell into space. As Mira zips through the galaxy, it leaves a comet-like tail of hot gas.
An image from NASA’s Galaxy Evolution Explorer space telescope revealed a tail an astounding 13 light-years long. The long tail suggests that its visible portion must have taken tens of thousands of years to form.
Watching these stars dim and brighten over time is fun — and sometimes quite startling. Occasionally, I’ve looked at the constellation Cetus and become momentarily confused. It looks different. There is a star missing. What happened to it?
But then I remember that only Mira is gone, its light reduced to a minimum — a beautiful, red star beating slowly like the heart of the universe.
Tom Burns is the former director of the Perkins Observatory in Delaware.
