Greetings stargazers.
Occasionally, the calendar aligns so that I get to complain about daylight saving time in this column on the same weekend it goes into effect. I need to take the chance to do that whenever I can. Just to clarify my position, year-round standard time is the only way to go. I also think year-round daylight saving time would be worse than the current system when we get the time warp twice per year.
This month’s thanks goes to fellow columnist Dick Grossman for suggesting I discuss Leavitt’s Law, or the period-luminosity relationship found with Cepheid variable stars. These stars play an important role in determining the distance to galaxies outside the Milky Way and thus help us determine the size of the universe.
All stars spend most of their lifetimes in a stable equilibrium on what is called the main sequence. However, during the relatively short time they are very young or very old, stars (just like humans) sometimes have problems finding their equilibrium. The instabilities often lead to changes in the luminosity of the star. It could get brighter and dimmer over the course of a few hours, days, weeks or months.
The name Cepheid for that class of variable stars comes from the star Delta Cephei in the constellation Cepheus. It was identified as a variable in the late 18th century and other stars with similar changes in brightness were all classified in the same group of very luminous giant stars. We now know the changes in brightness for this type of variable are due to both a change in diameter and a change in temperature. Polaris, the North Star, is one of these variable stars. Its period is about four days, but you would need something more sensitive than your eyes to notice its changing brightness.
What Henrietta Leavitt did early in the 20th century was to study a lot of variable stars in the Magellanic Clouds. These “clouds” are relatively nearby dwarf galaxies, companions to the Milky Way. She noticed and plotted a definite relationship between the period and the brightness of the Cepheid variables there. Because they were all in the same dwarf galaxy, they were all very nearly the same distance. This is important because it means you only need to measure the period of the variation in order to determine how much light is coming from the star. At least compared with all the other Cepheid type variable stars. This includes those in the Milky Way and the ones that can be seen in other galaxies, such as M31 in Andromeda.
The step to tie the distance to any visible Cepheid to some absolute scale was to find one or more relatively nearby Cepheid and find its distance via parallax. Parallax means using triangulation as the Earth moves around the sun. The relatively nearby star appears to shift positions slightly against more distant stars. Knowing a star’s absolute magnitude, or how bright it appears at a known distance, means you can use its apparent magnitude to get its distance. The dimmer it appears, the farther away it is.
Useful links
Some of you may have enjoyed all the AI generated astronomical impossibilities flooding social media last month. I did not. I saw annular eclipse images showing a corona (not visible during annular eclipses) with very picturesque non-Antarctic locations, and planetary alignment images either showing Mars in the string (it was not) or showing Neptune easily visible next to Jupiter (also not the case). I mostly find such things annoying, as they can raise expectations unrealistically for what one might see, and lead to disappointment and even more skepticism of science in the general public.
This month, the planets are still aligned along the ecliptic as they have been since the formation of the solar system Jupiter is the one in the best position to see now.
If you have a medium-sized telescope, use it to look at Polaris. You won’t see its variability, but you can see one of its two faint companion stars in an 8-inch telescope.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.
