Brightest Stars

Delta Cephei helps measure cosmic distances

Illustration of lights in a tunnel, getting fainter as they recede into the distance.
Like lights in a dark tunnel, stars in the distant universe are fainter as they’re located farther away. But stars like Delta Cephei pulsate at a rate always correlated to their intrinsic brightnesses. So they reveal their own true distances. Image via The Last Word on Nothing.

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.

Delta Cephei itself looms large in the history of astronomy. An entire class of supergiant stars – called Cepheid variables – is named in this star’s honor.

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.

A plot showing how Delta Cephei changes with time. The changes appear like a wave.
A light curve plot, brightness vs. time, of the changes in brightness in Delta Cephei. At the bottom, the two lowest sections are when the star is at its minimum brightness. The time it takes from one minimum to the other is 5.366 days. The y-axis shows the star’s brightness, in units of magnitude. Image via ThomasK Vbg / Wikimedia Commons.

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.

A plot with blue dots, each representing a star, with intrinsic brightness on the y-axis and pulsation period on the x-axis. The dots lie on a diagonal, showing a linear relationship between intrinsic brightness and pulsation period.
An example of the period-luminosity relationship of Cepheids in the Large Magellanic Cloud, a satellite galaxy to our Milky Way. The plot shows the intrinsic brightness of stars vs. their pulsation periods. Each star, represented by a dot in the plot, is roughly the same distance from us. Henrietta Leavitt discovered, as illustrated in this plot, that the longer the brightness pulsation cycle, the greater the intrinsic brightness of the star. Image via Dbenford / Wikimedia Commons.

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.

Delta Cephei on a star chart.
View larger. | A star map of Cepheus, showing Delta Cephei, as well as Epsilon and Zeta Cephei, at the bottom left corner of the rectangle. Image via IAU / Sky & Telescope / Wikimedia Commons.

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.

Night sky chart of Cepheus shown with several surrounding constellations, including Cassiopeia, Ursa Major, and Ursa Minor.
View larger. | If you’re not able to see the Big Dipper, try using the distinctive W-shaped Cassiopeia to locate the house-shaped Cepheus. The open side of the “W” faces the “roof” of Cepheus. Once you locate the “roof,” look for a rectangle pattern of four stars connected to it. Image via Stellarium.
A close-up of the Cepheus constellation on a star map with Delta Cepheid, as well as Zeta and Epsilon Cephei.
View larger. | A larger view of Cepheus, showing the Cepheid variable Delta Cepheid (in crosshairs) near two other stars, Zeta and Epsilon Cephei. Delta Cephei displays about a two-fold change in brightness (0.23 visual magnitudes) every 5.366 days, ranging from a visual magnitude of 3.48 at its brightest to 4.37 at its faintest. Zeta and Epsilon Cephei are useful comparison stars for noting changes in brightness of Delta Cephei from one night to the next. Zeta Cephei has a visual magnitude of 3.35, which is close to the maximum brightness of Delta Cephei. Epsilon Cephei has a visual magnitude of 4.15, which is close to the minimum brightness of Delta Cephei. Image via Stellarium.

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.

Have fun!

A bright star in the center, Delta Cephei, against a backdrop of fainter stars and wispy red interstellar clouds. To the left of the center star is a reddish nebulous feature, the Wizard Nebula. To the right of the star is another fainter red nebulous region, Sharpless 2-135. Right of Sharpless 2-135 is an orange star, Zeta Cephei.
View larger | Astrophotographer Alan Dyer captured this image of Delta Cephei (center), with the Wizard Nebula on its left, and the nebula Sharpless 2-135 on its right. The orangish star on the far right is Zeta Cephei. Image via Alan Dyer / /Flickr. Used with permission.

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.

September 24, 2021
Brightest Stars

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