A standard candle refers to any entity with a distinctive brightness that relies solely on physical processes and is independent of distance. The two prevalent standard candles used are Cepheids and Type Ia supernovae.
Cepheids are oscillating variable stars whose luminosity typically fluctuates over a span of numerous days. The absolute magnitude of these entities is contingent on the pulsation period. By merely observing a Cepheid and measuring its variability period, which is inferred from the light curve, one can determine its absolute magnitude. From this and its apparent magnitude, a distance estimate can be derived.
Naturally, in reality, things are not as straightforward, since the period-luminosity relationship initially needs calibration to Cepheids of a known distance, thereby introducing a degree of uncertainty to the measurements.
Cepheids can be detected up to several tens of millions of light-years from Earth, beyond which their faintness eludes telescope detection.
For galaxies further away, Type Ia supernovae, some of the brightest entities in the Universe, are employed. These supernovae occur in a binary system when a white dwarf amasses material from its companion star until it surpasses a critical mass and explodes, annihilating itself. As the physics behind the explosion remains constant, all Type Ia supernovae possess the same absolute magnitude. Comparing this value with its apparent magnitude allows for a distance estimate to the galaxy it resides within.
UGC 9391, depicted in the image, offers a prime case for calibrating these distance indicators. Situated 130 million light-years from Earth, multiple Cepheid variables and even a Type Ia supernova have been detected within this minor irregular galaxy. Comparing the values acquired from both methods thus sheds light on their respective shortcomings and strengths.
Image Credit: NASA, ESA, and A. Riess (STScI/JHU) et al.
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