How Bright Are the Stars?

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It may seem like a trivial question, but to astronomers, it’s anything but. A method to more accurately measure star brightness, or luminosity, could yield big benefits for astronomers — and help crack open some of the universe’s biggest mysteries.

Telescopes gather light from stars and other objects in the cosmos. To turn those light measurements into information such as the distance of a star from Earth, astronomers compare their measurements to sources of known brightness known as “standard candles.”

But even standard candles must be calibrated. Currently, all astronomical luminosity measurements in the visible and near infrared are based on a 1970s-era calibration of a single star.

Recent advances have enabled NIST to measure luminosity much more precisely, and in a way that is intimately tied to the International System of Units, or SI — the bedrock of all physical measurements. Scientists have also gotten much better at measuring, modeling and correcting for the interfering effects of Earth's atmosphere. These developments together could herald a revolution in starlight measurement.

Better Measurement Offers Big Potential Payoffs

More precise luminosity measurement could provide a major boost to astronomers trying to answer two of the field’s biggest questions. One of those questions is: How fast is the universe expanding?

In 1998, two research groups discovered that the universe is flying apart at an accelerating rate. The discovery earned the leaders of those groups the 2011 Nobel Prize in Physics and introduced the concept of “dark energy” — a form of energy that pervades the cosmos and pushes outward.

Today’s leading theory of dark energy treats it as a cosmological constant that ratchets up the universe’s expansion at a steady rate. More recent measurements, however, suggest that the acceleration rate may not be constant and that our cosmic picture could be ripe for a major revision.

Astronomers estimate the universe’s expansion rate by measuring extremely bright, extremely distant exploding stars known as supernovae. Certain types of supernovae have a highly predictable luminosity, allowing them to be used as standard candles. To do this, astronomers must measure how bright these sources are in different wavelength regions or bands.

The precision with which scientists can measure these supernovae is limited by how well they can calibrate their telescopes. Better calibration could therefore lead to better measurements of supernovae — and ultimately to new insights into the fate of the universe.

Starlight measurements are also essential for scientists seeking to answer one of humanity’s oldest questions: Are we alone in the universe?

Currently astronomers search for “exoplanets” — planets outside the solar system — by looking for small variations in the light of stars around which planets orbit. This has yielded a menagerie of thousands of known exoplanets, but so far, no planet likely to support life.

Planned future telescopes will directly measure exoplanets themselves. To search for signs of possible life, they will need to pinpoint planets in what scientists consider the “habitable zone” of a star — the range in which a planet’s surface could be warm enough for water to be liquid, but not so hot that all water would evaporate. (Earth, for example, resides comfortably within the Sun’s habitable zone. Venus is at the inner edge, and Mars is at the outer edge.) 

To estimate a given star’s habitable zone, and to predict the features of planets that might reside in this zone, astronomers must know how much light the star emits. Better calibration of star luminosity measurements could therefore be a critical steppingstone toward answering a question that may be nearly as old as humanity itself.

NIST’s Role

NIST is involved in two projects to better calibrate telescopes’ measurements of starlight. In one of these efforts, NIST researchers are building a special-purpose observatory that will use state-of-the-art luminosity measurement techniques to calibrate the brightness of roughly 10 stars, including Vega and Sirius (the brightest star in the night sky) with exquisite accuracy. These stars will then be able to serve as standard candles for astronomers around the world.

One key to making these calibrations is determining how much starlight is lost to Earth’s atmosphere. To do this, the researchers record the amount of light reaching the telescope as it peers at a star through different thicknesses of the atmosphere during the night. When a star is near the horizon, for example, its light passes through more atmosphere than when it is directly overhead. Changes in the amount of light gathered as the night progresses allow astronomers to calculate and correct for the atmospheric absorption.

In the second project, named for Arlo Landolt, an astronomer who cataloged thousands of secondary standard stars, NIST is working with partners to place an “artificial star” in the sky. This “star” will actually be a satellite placed in a geostationary orbit that will aim calibrated light from a set of lasers toward telescopes on Earth’s surface. Astronomers will be able to calibrate their telescopes by pointing them at the artificial star before observing objects of interest.

Click on the links below to learn more about these projects.

• Landolt Space Mission
• NIST Stars