The first-known Cepheid variable, Delta Cephei, is visible all night. Follow it for several days and you can watch it fade and brighten again.
The house-shaped constellation Cepheus harbors one of the sky’s most famous stars: Delta (δ) Cephei. You can find it just off the house’s southeastern edge. Credit: Astronomy: Roen Kelly.
- Delta Cephei, the archetype Cepheid variable star, is located in the constellation Cepheus, a circumpolar constellation visible in the northern sky.
- Delta Cephei’s position can be determined relative to other stars in Cepheus, notably Zeta Cephei, lying approximately 2.5° to its northeast.
- Delta Cephei exhibits a periodic variation in brightness, ranging between magnitudes 3.6 and 4.3 over a 5.366-day cycle.
- Observational methods suggested include visual comparison with Zeta Cephei and astrophotography to track its brightness fluctuations over several nights.
Cepheid variables are some of the most well-known variable stars in the sky, responsible for helping astronomers accurately measure cosmic distances and famously clueing Edwin Hubble in to the fact that the Andromeda Galaxy was far beyond the Milky Way. So, tonight let’s begin September by finding the Cepheid variable that started it all: Delta Cephei, the archetype Cepheid variable.
If you know how stellar names work, you’ll already know where to find this star — it’s in the constellation Cepheus. The King lies high in the north after dark at this time of year, his house-shaped outline appearing upside-down early in the evening, then slowly rotating onto its side as the hours progress. Cepheus is a circumpolar constellation, meaning it appears to circle the North Celestial Pole (and the North Star, Polaris) rather than rise in the east and set in the west.
Two hours after sunset, Cepheus is 50° high in the north, standing on the top of its peaked roof, marked by magnitude 3.2 Gamma Cep. The two stars marking the eaves of the house (at the top of its square shape when rightside-up) are above Gamma at this time — they are magnitude 3.2 Beta (β) and magnitude 3.5 Iota (ι) Cep. Above Beta is magnitude 2.5 Alpha (α) Cep, and above Iota is magnitude 3.4 Zeta (ζ) Cep. It is near Zeta that we’ll finally find our target, Delta Cep, which lies about 2.5° to Zeta’s northeast, or that star’s lower right early this evening.
Delta Cep varies in brightness between magnitudes 3.6 to 4.3 over a period of 5.366 days. So, you can use Zeta as a comparison once you find the star — is Delta close in brightness to Zeta, or is it notably fainter? Once you’ve noted the relative magnitude, make sure to come back every day for the next several nights and watch whether it brightens or fades. If you’re experienced in taking astrophotos, try taking a photo each night to chart the star’s changes; its cycle may be easier to follow on a series of photos than with your memory and your eyes.
Sunrise: 6:28 A.M.
Sunset: 7:31 P.M.
Moonrise: 3:55 P.M.
Moonset: —
Moon Phase: Waxing gibbous (64%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 12 P.M. local time from the same location.
For a look ahead at more upcoming sky events, check out our full Sky This Week column.