01 January 2006

About stars for a change

More “whaaaat?” material this morning... in particular, the star FG Saggitae (from the planetary Nebula He 1-5) went from a boring, ordinary hot star around 1920 to a much cooler but brighter (as its spectrum dropped from UV to visible) star in 1955, then by 1970 had picked up a whole stack of new spectral lines. In 1992, it suddenly dropped five magnitudes of brightness, and over the next four years, a further two magnitudes.

You’d have to be au fait with stellar astronomy for the implications of this to really hit you between the eyeballs, but stars aren’t supposed to be capable of changing character in anything less than thousands of years, and even then it’d be considered indecent haste. Worse, the changes FG Saggitae went through have absolutely nothing to do with canonical stellar development.

A star pulling gauche stunts like that just to get attention would be bad enough, but wait! There’s more!

V 605 Aquilae and V 4334 Sagittarii (“Sakurai’s Object” — astronomers demote you to “object” if you misbehave) have also undergone radical changes, Aquilae in under 80 years and Sagittarii in less than 6 years.

V838 Monocerotis

Then we come to V838 Monocerotis. This bad boy started acting up on New Year’s Day in 2002, but “The visible progenitor resembled a somewhat under-luminous F0 main sequence star, that did not show detectable variability over the last half century.” At first it was thought to be a classical nova, but all of the spectral indicators were completely wrong for that. V838 bounced all over the stellar barnyard over the course of a few months, pulling stunts like picking up nine orders of magnitude literally overnight, and then relapsed to being a boring old star again, but for an expanding debris shell and set of light-echoes. And having stepped halfway across the Hertzsprung-Russell diagram in under a hundred days.

This shook astronomers up a bit (to say the least; APOD commentary phrased it thusly: “The transformation defies the conventional understanding of stellar life cycles”) and many of them now consider simpler observations of other “novas” to be suspect. That should produce some more interesting science as they take a bit more care with observations of future stellar delinquency.

The big take-home news, however, is that conventional astrophysics has no explanation at all for stellar behaviour like this.

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