An apocryphal tale from anthropology states that members of a tribe who lived exclusively in a dense jungle, when brought out onto the savanna, laughed uproariously upon seeing distant elephants, having no experience with the dangerous animal, seemingly tiny on the horizon and non-threatening. After a lifetime of only seeing things at close range, they couldn’t judge that the elephants in the open were distant, not hilariously small.
Like elephants, stars, with few exceptions, are far too remote for even the most powerful telescopes to see as anything but specks, necessitating indirect means for determining their distances.
Using Earth’s 186-million-mile-wide orbit as the baseline of a surveyor’s triangle allows us to measure star distances out to a few thousand light years. Over six months stars exhibit a tiny “parallax” shift compared to more distant objects, due to being viewed from slightly different vantage points.
Yet a few thousand light years is still a small fraction of the way across the Milky Way, and minuscule compared to the distance to even the nearest galaxies. To measure those distances requires still more indirect methods.
Fortuitously, a star’s energy output (“luminosity”) correlates with its temperature: the hotter the star, the brighter it is. A star’s dominant color gives us its temperature, and thus its luminosity. Comparing that against its apparent brightness yields the distance (just as comparing a “tiny” elephant to a nearby one would).
In practice, it’s more complicated. The temperature-luminosity relationship only works for hydrogen-fusing stars, which may appear nearly identical to heavier element-fusing stars from the outside.
Stellar chemical composition differences also smear the temperature-luminosity relation. And, interstellar dust absorbs blue starlight more than red, making stars appear cooler (and more distant) than they really are. These limit the technique’s range to around ten thousand light years.
Next column: Gravitational wave astronomy gets a reboot.