35 Aquilae

Lore & Background

35 Aquilae is an A-type main sequence star with a spectrum classified as A0 V. It has 238% of the Sun's mass and 155% of its radius, emitting 17 times the Sun's luminosity at an effective temperature that gives it a white-hot hue. The star spins rapidly, with a projected rotational velocity of 110 km/s. Its apparent visual magnitude of 5.8 makes it visible to the naked eye from dark suburban or rural skies, though interstellar extinction dims it by 0.26 magnitude. Annual parallax of 16.34 mas places it about 200 light-years away, with a margin of error of 4 light-years.

In 1994, 35 Aquilae was identified as a variable star, specifically a Delta Scuti variable, which lies on the instability strip and undergoes short-period pulsations. Its pulsation period is only 30 minutes. Observations with the Spitzer Space Telescope revealed an excess of infrared radiation beyond what is typical for its class, possibly due to heating of nearby interstellar dust in a diffuse cloud the star is passing through. This interaction may also account for its classification as a Lambda Boötis star.

Reader's Guide

35 Aquilae holds significance as a Delta Scuti variable, a class of pulsating stars that help astronomers study stellar structure and evolution through their short-period oscillations. Its 30-minute pulsation period is among the shorter ones for this type, making it a useful target for asteroseismology. The star's infrared excess, detected by the Spitzer Space Telescope, suggests an ongoing interaction with a diffuse interstellar cloud, which may also explain its Lambda Boötis classification—a rare spectral type characterized by underabundances of certain metals. This interaction provides a natural laboratory for understanding how stars can alter their surrounding environment and how interstellar material can affect a star's observed spectrum. The star's rapid rotation and A-type main sequence status further contribute to its interest, as such stars often exhibit complex pulsation behaviors. Its faint naked-eye visibility makes it accessible to amateur astronomers with modest equipment, while its professional study continues to yield insights into stellar variability and interstellar processes.

Did You Know?

• 35 Aquilae has a pulsation period of just 30 minutes.
• It is classified as both a Delta Scuti variable and a Lambda Boötis star.
• The star's infrared excess may be caused by heating of nearby interstellar dust.
• Its apparent magnitude of 5.8 is dimmed by 0.26 due to interstellar extinction.

Historical Discovery and First of Its Kind

On September 10, 1784, the astronomer Edward Pigott recorded a change in the brightness of Eta Aquilae, making the star the very first known representative of the classical Cepheid family. This class of young, Population I variable stars pulses radially in a remarkably regular rhythm, with periods stretching from mere fractions of a day out to several weeks and visual amplitudes ranging from a few tenths of a magnitude up to roughly two full magnitudes. The broader group would later take its common name from Delta Cephei, whose variability John Goodricke identified, and the stars are also catalogued under the labels Type I Cepheids, Population I Cepheids, and Delta Cepheid variables. Pigott's observation of Eta Aquilae thus opened the door to an entire stellar category that would go on to reshape how we measure the scale of the Universe.

The Signature Bump in Its Light Curve

Eta Aquilae is frequently cited as a textbook example of a peculiar feature astronomers call the "bump." In a typical classical Cepheid light curve, brightness climbs quickly to a peak and then fades more gradually toward minimum, a shape produced by the phase offset between the star's changing radius and its shifting surface temperature. For stars pulsating near a six-day period, however, the descending portion of that curve can develop a brief pause or even a small secondary brightening. This bump is believed to arise from a resonance interaction between the fundamental pulsation mode and the second overtone. Eta Aquilae, sitting in that period range, displays the bump on its declining branch. As periods lengthen toward ten days the bump migrates closer to maximum light and can split it into a double peak, while at still longer periods it shifts onto the ascending side. Beyond roughly twenty days the resonance fades entirely.

Physical Nature and Pulsation Mechanics

As a member of the classical Cepheid family, Eta Aquilae belongs to a population of stars whose masses range from four to twenty times that of the Sun and whose luminosities span a staggering factor of one thousand to fifty thousand solar outputs. Spectroscopically these stars present as bright giants or low-luminosity supergiants with spectral types falling between F6 and K2, and their radii can reach several tens or even hundreds of solar radii. During each pulsation cycle the star's temperature, spectral appearance, and radius all shift in concert; for longer-period members the radius can swing by roughly a quarter, driving brightness variations of up to two magnitudes that are most visible at shorter wavelengths. The majority of classical Cepheids, including the archetype Eta Aquilae, are thought to pulsate in the fundamental mode, which produces the characteristic asymmetric light curve. A smaller subset oscillates in the first overtone or, rarely, in mixed modes, and those overtone pulsators tend to be more luminous and larger than a fundamental-mode star of the same period.

A Cosmic Yardstick: The Broader Legacy of Its Class

The true significance of Eta Aquilae extends far beyond its own sky position. Classical Cepheids possess a tightly defined relationship between their pulsation period and intrinsic luminosity, a property that transforms them into natural standard candles. By measuring a Cepheid's period and apparent brightness, astronomers can calculate its distance with remarkable precision, anchoring both the galactic and extragalactic distance scales. Observations with the Hubble Space Telescope of classical Cepheids have sharpened the constraints on Hubble's law, the relationship describing how the observable Universe expands. The same class of stars has helped map the local spiral-arm structure of the Milky Way and pin down the Sun's distance above the galactic plane. Today roughly 3,600 classical Cepheids are catalogued in our own galaxy, nearly ten thousand in the Magellanic Clouds, and hundreds more in distant systems; the Hubble Space Telescope has even spotted them in NGC 4603, a galaxy lying one hundred million light-years away.