What if we could line up all of the stars the same distance away to do a fair test of their brightnesses? –1.4. He called the brightest star in each constellation "first magnitude." The process of measuring the apparent brightness of stars is called photometry (from the Greek photo meaning “light” and –metry meaning “to measure”). In order to find the diameter of a star you really need three pieces of information about the star 1) distance 2) brightness 3) and color. There is no direct method currently available to measure the distance to stars farther than 400 light years from Earth, so astronomers instead use brightness measurements.It turns out that a star's color spectrum is a good indication of its actual brightness. Step 1: We can figure out the total power output of the star by knowing its brightness at Earth and its distance. The dip in brightness that these star spots create are very difficult to spot, as they only account for about a 1-2% reduction in the total light output from the star, but Kepler can handle the task. And the confusion does not end there, for stars can pass zero into brighter magnitudes where the numbers become negative. As we saw Observing the Sky: The Birth of Astronomy, astronomical photometry began with Hipparchus. Take Sirius, the brightest star in the night sky, which is mag. local because the background can vary ! It took astronomers until the 21st century to apply gravitational lensing to measuring stellar masses. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.. Hipparchus, a Greek astronomer who lived in the second century BC, is usually credited with formulating a system to classify the brightness of stars. For instance, a few stars have jumped into a higher brightness magnitude than 1. Before that, they had to rely on measurements of stars orbiting a common center of mass, so-called binary stars. … • Best if the background determination is ! Typical Star Mass Measurements . We will often measure luminosities of stars in units of the luminosity. Measuring Stellar Brightness" Aperture Photometry! The method we use today to compare the apparent brightness of stars is rooted in antiquity. So astronomers can predict the absolute magnitude (i.e., the mean intrinsic luminosity) of any given Cepheid by measuring the time it takes to rhythmically change its brightness. In measuring the brightness of a star, one must distinguish between apparent magnitude and its luminosity which is the power that it emanates in the form of light (like our Sun). Apparent brightness In this class, we will describe how bright a star seems as seen from Earth by its apparent brightness. Measuring a Cepheid's apparent brightness -- how bright it looks from Earth -- allows astronomers to calculate its true brightness, which in turn reveals its distance. • An offset annulus can be used to subtract the background! Step 2: Figure out the surface temperature. A dim star that is nearby looks bright, while a very bright star that is far away looks dim. Apparent magnitude is as the star appears in the sky. In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths. • If you just look at the brightness of the star in a given aperture you will also be including the background! That is what absolute magnitude is all about. Indeed, the star Vega has the magical magnitude of zero. 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