Jessie Taylor
Standard candles are astronomical objects that have a known absolute magnitude, making them useful for measuring the distance to faraway galaxies. Cepheid variable stars, discovered by Henrietta Leavitt in 1912, are a well-known example of standard candles. Other examples include carbon stars, X-ray bursts on neutron stars, and certain types of supernova explosions. These objects have unique characteristics that allow astronomers to determine their intrinsic brightness and, by comparing this to their apparent brightness, calculate their distance from Earth. However, there are challenges in using standard candles, such as calibration issues and uncertainties in the formation and evolution of stars and galaxies.
| Characteristics | Values |
|---|---|
| Definition | An astronomical object that has a known absolute magnitude |
| Usage | To determine the distance to farther and farther galaxies |
| Formula | Apparent magnitude of the object = absolute magnitude of the object x distance to the object in parsecs |
| Commonly used standard candles | Cepheid Variable stars, RR Lyrae stars, Type Ia supernovae, planetary nebulae, TRGB stars, Carbon stars, Gravitationally Lensed Quasars, X-ray bursts on neutron stars |
| Concerns | The "standardness" of the brightest standard candles, calibration, homogeneity, and accuracy |
| Recent findings | Cepheid variables shrink in mass, affecting their use as standard candles |
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What You'll Learn
Cepheid Variable stars
Cepheid variables are excellent standard candles because their pulsation period is directly related to their absolute magnitude. This means that by observing and measuring the period of a Cepheid variable, astronomers can determine its intrinsic brightness. This relationship is known as the period-luminosity relation. The longer the pulsation period, the more luminous the star.
The period-luminosity relationship was first discovered by Henrietta Leavitt in the early 20th century. This discovery provided astronomers with a reliable method to measure distances to faraway galaxies. By comparing the absolute magnitude of a Cepheid variable (obtained through its pulsation period) with its apparent magnitude (how bright it appears from Earth), the distance to the star or galaxy it resides in can be calculated using the inverse-square law of light.
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RR Lyrae stars
Standard candles are a class of astrophysical objects with known luminosities due to some characteristic quality possessed by the entire class. RR Lyrae stars are one such example of standard candles. They are old, low-mass, Population II stars that are commonly found in globular clusters. RR Lyrae stars are pulsating horizontal branch stars of spectral class A or F, with a mass of around half the Sun's. They are thought to have shed mass during the red-giant branch phase and were once stars at around 0.8 solar masses.
RR Lyrae variables are distinct from classical Cepheids due to their shorter periods, differing locations within the galaxy, and chemical differences. They are also much more common than Cepheids but less luminous. RR Lyrae stars can be used as standard candles for distance measurements, although there are challenges due to their intrinsic faintness, metallicity, and blending.
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Type 1a supernovae
However, Type 1a supernovae are not perfect standard candles because they do not all have the same peak brightness. Their peak luminosities differ, and these differences are correlated with how quickly the light curve declines after maximum light via the luminosity-decline rate relation. This effect can be corrected for, and Type 1a supernovae can be standardised.
The use of Type 1a supernovae as standard candles is an example of how standard candles can be used to build a cosmic distance ladder. By using objects with known luminosities, astronomers can estimate the distance to extremely distant objects by comparing their apparent brightness to their intrinsic brightness. This allows astronomers to determine the distances to celestial objects that are too far away for direct distance measurement.
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Carbon stars
Standard candles are a class of astrophysical objects with known luminosities due to some characteristic quality possessed by the entire class of objects. This means that if an extremely distant object can be identified as a standard candle, then its absolute magnitude (luminosity) is known, and its distance can be calculated from its apparent magnitude.
The use of carbon stars as standard candles is limited by the availability of telescopes. With currently available ground-based telescopes, carbon stars can only be used as standard candles out to a distance of ~2 Mpc. However, with the next generation of telescopes, carbon stars could be detected in MC-type galaxies at distances out to 50-60 Mpc.
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Gravitationally lensed quasars
Quasars are extremely distant cosmic objects with massive black holes at their cores, emitting vast amounts of energy. When a galaxy is very close to the line-of-sight of a quasar, its gravity acts as a lens, deflecting the light and creating multiple images. These gravitationally lensed quasars are rare because they require an almost exact alignment between the foreground galaxy and the background quasar.
The study of gravitationally lensed quasars has provided valuable insights into quasars, lensing, and cosmology. By monitoring the luminosity variations of the quasar images, astronomers can measure time delays caused by the light taking slightly different paths around the lensing galaxy. This helps in detecting dark matter concentrations, as the presence of dark matter alters the apparent brightness and position of each distorted quasar image.
Additionally, gravitationally lensed quasars have been proposed as new "standard candles" to determine cosmic distances. Quasars have been found to exhibit a tight correlation between their X-ray and ultraviolet luminosities, even at large cosmological distances. This relationship can be used to measure the distance to each quasar, which in turn helps test cosmological models and probe fundamental cosmological parameters.
The use of quasars as standard candles offers several advantages. Firstly, they extend the range of standard candle redshifts back to earlier times in the universe's history. Secondly, the physical processes in quasars differ from those in supernovae, providing independent measures of cosmological parameters. Lastly, the discovery of hundreds of thousands of quasars in recent years provides a larger dataset for analysis.
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Frequently asked questions
Standard candles are a class of astrophysical objects with known luminosities due to a characteristic quality possessed by the entire class of objects.
Cepheid Variable stars, RR Lyrae stars, Type 1a supernovae, and Carbon Stars are some examples of standard candles.
If the luminosity of a source is known, the distance can be estimated based on how bright it appears from Earth. Astronomers measure the radiant flux intensity of the electromagnetic radiation arriving on Earth.
