Stargazing this Week

The rapidly darkening skies will bring stargazing back to the forefront, once these autumn clouds disappear. Assuming they do this week, here are a few things to look up for. If they don’t, then please enjoy the accompanying stories.

Fluctuating Delta Cephei
Remember looking for the Draconid meteors a few weeks back? Their host constellation Draco the Dragon wraps between the dippers Big and Little, just below the boxy constellation Cepheus.

The constellations Cepheus, Cassiopeia, Andromeda, Perseus all tie together both in the Celestial Sphere and Greek mythology. We’ll visit that block of the sky soon, but for this week we’ll focus in on Cepheus the king of Ethiopia.

Actually, we’ll focus in on just one of Cepheus’ stars. The one in the far corner, closest to the Milky Way: Delta Cephei.

Delta Cephei varies significantly in brightness, day to day. This is more than a twinkle, the star’s energy output - the very light it gives out - is oscillating in time. Being one of the first such “variable stars” to be studied, an entire class of star is named after it. These are the so-called Cepheid Variables, and there are a lot of them. Between the Magellenic Clouds and the Milky Way, there are over 2400 known examples.

To find Delta Cephei first find the Big Dipper to the North. Use the far two “dipper stars”, Merak and Dubhe, to guide you to Polaris of the Little Dipper. Just above the Little Dipper, but below the Milky Way, lies a rectangle of stars that from the upper body of Cepheus. The shoulder closest to the Milky Way - and furthest from the Big Dipper - contains a collection of stars including Delta Cephei.

Delta Cephei itself oscillates in brightness a little more than once a week. The difference in magnitude is definitely noticeable to the human eye. If you have access to dark sky this would be an excellent opportunity to test out your astrophotography skills! Even the iPhone is capable of taking reasonable pictures of stars.

Much of the details of Delta Cephei and Cepheid Variable stars has been collected in a nice essay by Bruce McClure and Shireen Gonzaga over at EarthSky.

Color Flashes and Twinkling Stars
While you’re at EarthSky, check out Deborah Byrd’s recent article about the relationship between twinkling stars, color variation and atmospheric refraction.

Unlike the Cepheid variable stars, the twinkling we observe with our eyes has little to do with the inner workings of the stars. Instead, it’s caused by noise in our own atmosphere.

Refraction breaks light into different colors - as happens with a prism. The different colors of light, corresponding to different wavelengths and therefore different energies, travel through the atmosphere at slightly different speeds. Variations in air pressure, wind a temperature all contribute to refraction. The noisy nature of the atmospheric gas gives rise to the “twinkle” observed in starlight.

When stars are lower on the horizon, their light refracts more, as you’ve problem seen as our most familiar star, the sun, crosses the horizon to bookend the day.

The reason for this is simply distance. The shortest path through the atmosphere is straight up, which means the starlight seen directly overhead passes through the least amount of atmospheric gas.

Starlight seen on the horizon pass through much more atmospheric gas, affording much more distortion. It just so happens that they’re a bunch of bright stars near the horizon these days:

Arcturus - of the ancient constellation Bootes - sets around 8pm to the Northwest.

Sirius - the Dog Star - rises to the East after Orion, a bit after 1 am. Even the familiar Pleiades - who rise well before Orion - look fairly distorted as they pass above the Chewuch in the early evening. If you find that familiar star cluster, look to the left for Auriga the crown.

Its brightest star, Capella, will also twinkle with color changing diffraction as it rises shortly before 8pm.

Byrd’s main point in her essay: there’s nothing nefarious or artificial in the sky, it’s just a matter of declination.