Mira

Mira is a remarkable binary star system located in the constellation Cetus.

It is famous for being the prototype of Mira variables, a class of stars that change in brightness. The system is actually made of two different stars working together. The primary star, Mira A, is a massive red giant. Its companion, Mira B, is a small and hot white dwarf. Because these two stars interact closely, scientists call this a symbiotic system. This is the closest symbiotic pair to our own Sun.

The main star, Mira A, is currently in a stage called the asymptotic giant branch. This means it is a highly evolved star that is undergoing a thermally pulsing phase. During these pulses, which can last a decade or more, the star's luminosity increases. These cycles happen roughly every 10,000 years. These pulses create dynamic instability within the star. This instability leads to dramatic changes in the star's size and brightness over shorter, irregular periods.

Mira's variability is driven by its physical pulsations. The star's surface expands and contracts, which changes its temperature and brightness. As the star pulses, its radius can vary by over 20 percent. The temperature also shifts, though by less than 10 percent. Interestingly, the highest luminosity occurs when the star is at its hottest and smallest. The star's brightness can swing wildly. Its visual magnitude can reach as high as 2.0 or drop as low as 4.9. This represents a brightness range of almost 15 times.

Humans have been observing Mira's strange behavior for centuries. In 1596, the astronomer David Fabricius began recording its changes. He originally thought he was observing the planet Mercury. By 1638, Johannes Holwarda determined the star reappeared every eleven months. In 1662, Johannes Hevelius gave the star its name. He chose the Latin word Mira, meaning "wonderful" or "astonishing." He felt the star acted like no other object in the sky.

There are many impressive numbers associated with this star. Mira A is estimated to be about six billion years old. It is one of the coolest bright red giants, with temperatures between 3,000 and 4,000 degrees Fahrenheit. In 1925, Francis G. Pease measured its diameter using interferometry. He found it was between 250 and 260 million miles wide. At that time, it was the second largest star known to science. The two stars in the system are currently separated by about 70 astronomical units.

The interaction between the two stars is a constant process of mass exchange. Mira A is undergoing significant mass loss. Its gaseous material is very thin, about one-thousandth as dense as the air on Earth. This material travels along a bridge of matter toward the companion star. The Chandra X-ray Observatory has shown this direct exchange. Mira B, the white dwarf, is accreting this mass from the primary star. This transfer of material is a key feature of their symbiotic relationship.

As Mira moves through space, it leaves a massive trail behind it. The star travels at a high speed of 22 kilometers per second. This motion creates a hot bow wave of compressed gas. This interaction results in a tail that is 13 light-years long. The tail has been forming from shed material over the last 30,000 years. This tail is a vital resource for the galaxy. It drops off carbon, oxygen, and other elements. These elements are the building blocks needed to form new stars, planets, and perhaps even life.