# how do we measure the brightness of stars

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). local because the background can vary ! –1.4. Step 2: Figure out the surface temperature. Astronomers "pretend" to line up stars exactly 10 parsecs (about 32.6 light years) away from Earth. 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. Step 1: We can figure out the total power output of the star by knowing its brightness at Earth and its distance. He called the brightest star in each constellation "first magnitude." Indeed, the star Vega has the magical magnitude of zero. For this technique to work correctly, though, astronomers must first use the parallax method to … What if we could line up all of the stars the same distance away to do a fair test of their brightnesses? … It took astronomers until the 21st century to apply gravitational lensing to measuring stellar masses. Typical Star Mass Measurements . Before that, they had to rely on measurements of stars orbiting a common center of mass, so-called binary stars. Apparent magnitude is as the star appears in the sky. 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. Stars do tend to slow down as they age, but researchers still aren’t sure exactly how much. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.. That is, we might say for a certain star L star = 5.2 x L sun, meaning that the star has 5.2 times the energy output per second of the Sun. A dim star that is nearby looks bright, while a very bright star that is far away looks dim. The process of measuring the apparent brightness of stars is called photometry (from the Greek photo meaning “light” and –metry meaning “to measure”). Take Sirius, the brightest star in the night sky, which is mag. Measuring Stellar Brightness" Aperture Photometry! Apparent brightness In this class, we will describe how bright a star seems as seen from Earth by its apparent brightness. Hipparchus, a Greek astronomer who lived in the second century BC, is usually credited with formulating a system to classify the brightness of stars. The method we use today to compare the apparent brightness of stars is rooted in antiquity. • An offset annulus can be used to subtract the background! 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. We will often measure luminosities of stars in units of the luminosity. That is what absolute magnitude is all about. 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. And the confusion does not end there, for stars can pass zero into brighter magnitudes where the numbers become negative. For instance, a few stars have jumped into a higher brightness magnitude than 1. • If you just look at the brightness of the star in a given aperture you will also be including the background! 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. 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