Photometry

FIELDS OF STUDY: Astronomy; Stellar Astronomy; Space Technology

ABSTRACT: Photometry is the science of measuring the brightness of celestial objects, including stars. It is important as a means of knowing more about very distant objects. The data can help scientists understand what bodies are made of, their temperatures, and other information. The information gathered may be very general or, with more measurements, highly detailed.

Photometry

Photometry is the science of measuring the brightness of celestial objects. It is used to learn about distant objects in space that cannot be observed easily otherwise. The measurements taken may be very basic, and provide very general information, such as a star’s temperature. With more focused study and technology, scientists are often able to learn more specific information, such as an object’s elemental composition.

Ancient astronomers first studied stars that were visible to the human eye. They measured the brightness of these stars using magnitude. Greek astronomer Hipparchus (ca. 190–120 BCE) divided the stars he catalogued into six classes. The magnitude reflected how soon the stars became visible during twilight. The first stars to be discerned were the brightest, and were placed in the first class. However, this classification relied on an individual’s eyesight and observation. It did not provide a measurement that could be compared, and visibility was affected by changes in the atmosphere. The Almagest (ca. 170 CE) by Ptolemy of Alexandria was thought to be based on the star catalogue of Hipparchus. The Almagest served as the basis of astronomy until the sixteenth or seventeenth century. Starting in the early seventeenth century astronomers tried to measure the brightness of stars by comparing their sizes as seen through a telescope, but this again was imperfect. For example, atmospheric conditions affected the appearance of stars. The atmosphere blocks out many wavelengths, is not transparent under the clearest conditions, absorbs and scatters some light, and acts as a prism. Even the telescope used by an astronomer might perform differently under varying conditions.

The invention of astronomical photography in the nineteenth century allowed observers to maintain records of what they were seeing for future reference. By comparing multiple images of the same object over time, researchers could learn which were most accurate, and use the information to compare with other objects for more precise measurements. As early photography became more sophisticated, these images provided researchers with a wealth of information.

In 1856 English astronomer Norman Robert Pogson (1829–91) calculated magnitude in terms of the density, or flux, of the radiation it emits. Flux is the number of photons passing through a unit of area within a unit of time. Pogson found that the magnitude scale corresponds to the logarithmic response of human retina receptors. He also found that the difference between five levels of magnitude is about 100 in flux.

In 1910 Edward Pickering suggested a method for heating or lighting photographic plates. He did so to measure the amount of heat or light they let through. Pickering matched these measurements with stellar brightness. He was thus able to calculate the brightness of celestial objects. Harlan Stetson and Jan Schilt each furthered Pickering’s work by creating plate photometers. Their photometers could measure the intensity of light that passed through a photographic plate.

Beginning in the late twentieth century, telescopes were launched into space to beam back even more information. Unhindered by Earth’s atmosphere, these telescopes and their successors can capture data using x-rays and gamma rays. The new technology opened up whole new areas of study for scientists. Through optics, or the study of how electromagnetic radiation waves act and interact, researchers could view things previously unseen. These space-based telescopes include the Hubble Space Telescope, launched in 1990, which measures visible, UV, and near-infrared wavelengths; the Chandra X-Ray Observatory, launched in 1999 to measure x-rays; the Spitzer Space Telescope, launched in 2003 to measure infrared wavelengths; the Herschel Space Observatory, which has been measuring far-infrared radiation since 2009; and the Planck Observatory, launched in 2009 to measure microwave wavelengths.

Studying Light

Electromagnetic radiation includes visible light as well as many other kinds of radiation on what is called the electromagnetic spectrum. Electromagnetic radiation includes gamma rays, x-rays, ultraviolet (UV) radiation, visible light, infrared radiation, and radio waves.

Radiation travels in waves. Wavelength is measured from the crest, or highest point, of one wave to the crest of the next. Usually, smaller wavelengths mean the radiation has greater energy. For example, gamma rays have very small wavelengths, and very high energy, while radio waves may be very large—sometimes even longer than a football field—and have much lower energy. The wavelengths of the visible light spectrum, which includes the radiation detectable by the human eye, fall between these extremes. Red has the longest wavelength in the visible light spectrum, while violet has the shortest wavelength. The energy of visible light is also somewhere in the middle range. X-rays and UV wavelengths are shorter than those of visible light, while infrared and microwave wavelengths fall between visible light and radio waves.

When scientists know the flux of a celestial object, and the approximate distance to the object, they can calculate the total energy output, or luminosity, of the object. Researchers can also glean the size, temperature, and other properties of the object.

Though stars emit a great deal of radiation, scientists can also study objects that are far colder or hotter. Planets, molecular clouds, and even interstellar dust emit radiation in the longer wavelengths. Ionized gas clouds emit radiation in the ultraviolet range of the spectrum.

Because hotter objects radiate energy with shorter wavelengths, this appears to the human eye as color within the visible spectrum. Scientists can use this information to measure the temperatures of stars. For example, among stars we can see, red stars are the coolest, while blue stars are the hottest. Other stars are so cold they produce almost no visible light, so they can only be observed through infrared telescopes. At the opposite end of the spectrum, astronomers can find the hottest stars by searching for ultraviolet light. UV telescopes can filter out other wavelengths and focus on the nurseries where stars are being born.

Classifying the Stars

The temperature of a star is related to its apparent brightness. Astronomers measure the luminosity, or the total amount of energy a star emits from it surface, because energy in this case refers to light. Astronomers refer to stars in terms of both apparent magnitude—how bright it appears from Earth—and absolute magnitude, which refers to how bright it appears at a standard distance of 10 parsecs (32.6 light years). Stars that are far away and very bright might appear fainter than closer, less-bright stars. Classifying stars by absolute magnitude eliminates this difference. The modern five-magnitude scale classifies stars by a brightness ratio of 100, using Vega as a reference point. Vega, which is 25 light years from Earth, has an absolute magnitude of 0.6. A star with an absolute magnitude of 1 is 100 times as luminous as a star with an absolute magnitude of 6. However, classification is not so simple, because the astronomers must also know which wavelength is being measured for the classification. Some stars might be highly luminous when measuring x-rays, but emit much less infrared radiation. Astronomers also rely more on newer measurements. As technology has improved, so has the accuracy of classifying celestial bodies.

The Sun is just 93 million miles from Earth and is the closest star. Its absolute magnitude is 4.2. One of our nearest neighbors at just 4.3 light years from Earth, Alpha Centauri (Rigil Kentaurus) in the Centaurus constellation has an absolute magnitude of 4.4, and is about 1.3 times as luminous as our Sun. Rigel, in Orion, is 1,400 light years away, and has an absolute magnitude of −8.1. Betelgeuse, also 1,400 light years from Earth, has an absolute magnitude of −7.2.

Viewed from Earth, many stars appear to be different colors. This is due to the amount of radiation in the visible light spectrum that the stars emit. The surface temperature of the Sun is about 5,800 kelvins (9,932 degrees Fahrenheit). At this temperature, it produces mostly yellow light. Scientists know this without sending a probe to take the star’s temperature because they can measure the light waves. They also know that the star Rigel, which looks blue-white, is about 11,000 kelvins (19,000 degrees Fahrenheit), while Betelgeuse, at a mere 3,600 kelvins (about 6,000 degrees Fahrenheit), appears blue.

Seeking Answers among the Stars

Scientists even use measurements of wavelengths to see beyond and behind celestial bodies. For example, they can study something cold, such as a gas, which is obscured by interstellar clouds. These long wavelengths cut right through dense matter that would prevent it from being seen by any other means. Microwave telescopes can detect wavelengths left over from the Big Bang, the event that gave birth to the universe and all celestial objects.

The BRITE (Bright Target Explorer) nanosatellite photometry project began in 2005 with the funding of an Austrian BRITE nanosatellite by the University of Vienna. On February 25, 2013, the first two of a planned six nanosatellites were launched. The sixth BRITE nanosatellite was launched on August 19, 2014. The satellite constellation is intended to study the brightest of stars. Austria, Canada, and Poland are participating in the joint effort.

Exoplanets, planets orbiting stars in far-off galaxies, are being located and studied using photometry. For example, scientists can detect the briefest dimming of a star when a planet crosses it in orbit because they know the luminosity of the star and can accurately measure any changes. A passage between Earth and a far-off star is called a transit. When such dimming is detected regularly, scientists know a planet is likely in transit around the star. The degree of the dimming helps them to know the general size of the planet. When they detect distant planets, scientists can focus their efforts on them to learn about their temperatures, atmospheres, and other information. They do this by waiting for the planet to disappear behind the star. Then they compare the light spectrum of the star when the planet is in transit with the spectrum when the planet is hidden behind it to get the planet’s spectrum. This research allows astronomers to find and explore new places from Earth. For example, in 1998 astronomers found a large planet orbiting Gliese 876, a red dwarf not far from Earth. In 2001 a second large planet was found. In 2005 astronomers discovered a much smaller third planet orbiting Gliese 876. They deemed it the most Earth-like exoplanet to be found so far.

Scientists are looking for exoplanets on which humans might survive. They are also looking for planets that support forms of life. The first planet to be discovered orbiting a star like the Sun was found in 1995. As of January 2015 NASA’s Kepler Telescope has found more than one thousand exoplanets since its launch in 2009.

Other research in the field of photometry is seeking to understand the origin of the universe and the celestial bodies. For example, quasars emit tremendous amounts of energy while at the same time sucking matter into the black holes at their centers. Scientists hope that data gathered about quasars and other objects will help explain the physical processes of the Big Bang.

PRINCIPAL TERMS

• electromagnetic radiation: the flow of waves generated by periodic electric and magnetic field variations; examples include gamma rays, radio waves, visible light, and x-rays
• flux: the amount of electromagnetic radiation from an object passing through a detector
• luminosity: a celestial object’s total energy output per second
• optics: science of the properties and behaviors of light
• visible light spectrum: portion of the electromagnetic spectrum visible to the human eye

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