Delta Cephei helps measure cosmic distances
Delta Cephei is a pulsating star
Delta Cephei, in the constellation Cepheus the King, is a variable star that changes in brightness with clocklike precision. It doubles in brightness and fades back to minimum brightness every 5.366 days. With careful observation under a dark sky, you can see this star change in brightness over several days. This star, and others like it, are important players in establishing the distance scale of our galaxy … and our universe.
Cepheid variable stars, also called Cepheids, dependably change their brightnesses over regular intervals ranging from a few days to a few weeks. In 1912, astronomer Henrietta Leavitt discovered that the star’s periodic change in brightness was directly related to its intrinsic brightness (or actual luminosity). She found that the longer the brightness pulsation cycle, the greater the intrinsic brightness of the star. This Cepheid period-luminosity relationship is now sometimes called the Leavitt law.
Why are these stars varying in brightness? It’s thought that these stars vary because they expand (get brighter) and then contract (get fainter) in a regular way.
Cepheids help measure cosmic distances
The regularity of Cepheids’ brightening and dimming is a powerful tool in astronomy. It lets astronomers probe distances across vast space. You might know that the surest way to measure star distances is via stellar parallax. But, for the parallax method to work, the stars have to be relatively nearby (within about 1000 light-years). Luckily, in recent years, astronomers have been able to make direct parallax measurements of more distant stars, thanks to space-based telescopes such as Gaia.
Still, the problem remains. How can we find the distance to stars that are too faraway to give us a reliable distance measurement via parallax? Suppose you measured the distance to a nearby Cepheid star using the parallax method. Then suppose you watched its pulsations, which you know are correlated with the star’s intrinsic – real – brightness. Then you know both its distance and how bright the star looks at that distance. Armed with this information, you can then look farther out in the universe, toward more distant Cepheids, those too far for parallax measurements. You can measure the apparent brightness – which is fainter – and pulsation rate of such a star. With a few simple steps of math, you can then find the distance to it.
The Cepheid variable stars are used to measure distances across space. For this reason, they’re known as standard candles by astronomers.
In 1923, the astronomer Edwin Hubble used Cepheids to determine that the then-called Andromeda nebula is actually not a nebula but a giant galaxy lying beyond our Milky Way. It released us from the confines of a single galaxy and gave us the vast universe we know today. This work in understanding the size of the universe is sometimes called the cosmic distance ladder.
The work continues today, not just with Cepheids but also with other astronomical objects and phenomena.
Cepheids in other galaxies
Distance determinations using Cepheids in other galaxies, as well as other techniques, is an active area of research in astronomy. Astronomers are constantly improving distance accuracies to further constrain the value of the Hubble Constant that indicates the expansion rate of the universe.
Cepheids have been observed as far away as 100 million light-years in the galaxy NGC 4603, by the Hubble Space Telescope. However, measuring them at distances of 30 million light-years and farther is difficult because it’s hard to isolate Cepheids from their neighboring stars. At such distances, astronomers transition to other methods to determine distances, such as observing type 1a supernovae.
How to spot Delta Cephei in the night sky
The original Cepheid, Delta Cephei, is circumpolar – always above the horizon – in the northern half of the United States.
Even so, Delta Cephei is much easier to see when it’s high in the northern sky on autumn and winter evenings. If you’re far enough north, you can find the constellation Cepheus by way of the Big Dipper. First, use the Big Dipper “pointer stars” to locate Polaris, the North Star. Then jump beyond Polaris by a fist-width to land on Cepheus.
You’ll see the constellation Cepheus the King close to his wife, Cassiopeia the Queen, her signature W or M-shaped figure of stars making her the flashier of the two constellations. They’re high in your northern sky on November and December evenings.
How to watch Delta Cephei vary in brightness
The real answer to that question is: time and patience. But two stars lodging near Delta Cephei on the sky’s dome – Epsilon Cephei and Zeta Cephei – match the low and high ends of Delta Cephei’s brightness scale. That fact should help you watch Delta Cephei change.
So look at the charts above, and locate the stars Epsilon and Zeta Cephei. At its faintest, Delta Cephei is as dim as the fainter star, Epsilon Cephei. At its brightest, Delta Cephei matches the brightness of the brighter star, Zeta Cephei.
Bottom line: Cepheid variables are a famous class of stars, used in establishing the distance scale of the universe. They’re helpful in this way because their brightness pulsation rate is correlated to their intrinsic brightnesses. So we can see how bright they look, and determine their distance. The stars are named for Delta Cephei in the constellation Cepheus, the first of its type to be identified, in 1784.