1596

Mira Variables

David Fabricius (1564–1617), Johannes Fabricius (1587–1615), Johannes Holwarda (1618–1651)

Astronomers of the sixteenth century were aware that occasionally a seemingly ordinary star would flare up and dramatically increase in brightness—what Tycho Brahe had termed a nova. But no one had ever observed a star brighten, then dim, then brighten and dim again. That’s what the Dutch-German pastor and part-time astronomer David Fabricius discovered from his observations of the star Omicron Ceti, also known as Mira, in 1596 and again in 1609. In 1638 the Dutch astronomer Johannes Holwarda discovered that Mira is a periodic or pulsating variable star, with a period of about 330 days.

When Fabricius discovered Mira, it was a magnitude 3 star that dimmed quickly (within a month or so) to beyond the human visual limit of magnitude 6. Later astronomers monitoring Mira telescopically over the years have seen it brighten to magnitude 2 and dim to magnitude 10, a factor of over 1,700 in brightness. We now know that Mira is a puffed-up red giant star about 350 times bigger than the Sun—if Mira were in our solar system it would extend out to the orbit of Mars! Its pulsations are part of normal stellar evolution for relatively low-mass stars near the end of their lives. Nearly 7,000 stars are now known that exhibit pulsations like this, with periods from around 100 to 1,000 days; they are collectively referred to as Mira variables.

Mira was discovered to be in a double star system in 1923, with a smaller white dwarf companion called Mira B. Recent Hubble Space Telescope and Chandra X-Ray Observatory images show that Mira B is gravitationally pulling gas off Mira A and into a solar nebula–like disk that may be accreting planets. Mira is a dying star, yet in its death throes it may be giving life to new planets.

Fabricius and his son Johannes are also credited with being the first astronomers to systematically observe sunspots and to use them to discover that the Sun rotates, as predicted by physicist Johannes Kepler, and has a rotation period of about 27 days. Coincidentally, there’s a Mira connection there as well, as astronomers have discovered that Mira has starspots that, like sunspots, may be related to strong magnetic fields in the star’s outer layers.

Artist’s conception of the red giant star Mira A (right) and its companion star Mira B, a small white dwarf that is surrounded by a disk of gas and dust being drawn off the pulsating red giant.