Brightness Brouhaha

Announcements at the American Astronomical Society meeting in Seattle reflected some interesting tweaks to the process by which astronomers make sense of the size of the Universe around us.

When we look at the sky, we see objects at various distances:  planets relatively nearby (minutes or hours of light travel time), stars much farther away (light years, in fact), and galaxies and quasars way far out (billions, yep, billions of light years away). Astronomers must face the challenge of disentangling the distances to all these objects, and they use a variety of techniques to do so.

Astronomers refer to this as a “distance ladder” with various objects positioned as rungs that allow us to judge distances farther and farther from home.  It turns out that an important rung on the ladder includes variable stars, including Cepheid variables.

These stars brighten and dim over time, and the longer they take to do so, the greater their intrinsic brightness. Why is this important? If you know something’s intrinsic brightness, then you can use the inverse-square law to calculate its distance.

Astronomers call Cepheids and other, similar objects “standard candles,” which you can think of as light bulbs (instead of candles) of a consistant brightness.  A 100-watt bulb will appear dimmer farther away, but if you know it’s putting out 100 watts of light, then you can calculate its distance if you know how bright it appears to be…  That’s the concept behind a standard candle.

So… What happens if we fid out that the intrinsic brightness ain’t what we expected? If the standard candles aren’t so standardized? Well, recent observations of Delta Cepheid (the star that gives Cepheid variables their name) suggest that it has a cloud of material surrounding it. In fact, that the surrounding material could change its brightness slightly in visible light! As it turns out, this effect won’t radically transform the scale on which the stars help determine distances, but it’s a sobering thought to realize that these objects, which seemed utterly predictable, have sordid lives of their own…

In not unrelated news, the Crab Nebula appears to vary over time… Not in a regular pattern like Cepheids, but in a seemingly random series of bursts, brightenings, and dimmings. Yikes!  X-ray astronomers considered the Crab so constant that they referred to “millicrabs” as units of brightess. This isn’t quite the same as the concept of a standard candle, but it still has implications for the day-to-day work of astronomers.

That’s all for now!  The conference ends tomorrow, and I’ll send one last update before heading home to San Francisco…