Jessie Taylor
Standard candles are objects with a known absolute magnitude or intrinsic luminosity. They are used in astronomy to determine the distance to an object by comparing its apparent brightness to its intrinsic brightness. The most commonly used standard candles are Cepheid Variable stars and RR Lyrae stars, whose absolute magnitudes can be determined from their variability period. Type Ia supernovae are also considered standard candles due to their nearly identical peak brightness, although they are more accurately described as standardisable candles since their peak luminosities vary. Other types of standard candles include carbon stars, gravitationally lensed quasars, and X-ray bursts on neutron stars. The choice of the best standard candle depends on various factors, such as the availability of objects with well-known distances for calibration and the homogeneity of their true absolute magnitude.
| Characteristics | Values |
|---|---|
| Name | White dwarf supernova, Type Ia supernova |
| Consistency | Consistent peak luminosity |
| Brightness | Very bright and energetic |
| Mechanism | Explosion of a white dwarf after it accumulates enough mass from a companion star |
| Visibility | Can be detected across billions of light-years |
| Use case | Measuring distances to extremely distant galaxies |
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What You'll Learn
- White dwarf supernovae are the best standard candles for distant galaxies
- Cepheid variable stars are excellent standard candles for nearby galaxies
- Type Ia supernovae have consistent peak brightness
- The Tully-Fisher relation assumes all galaxies have the same mass-luminosity ratio
- Gravitationally lensed quasars allow us to measure time delays between images
White dwarf supernovae are the best standard candles for distant galaxies
A standard candle is a class of astrophysical objects with known luminosities due to some characteristic quality possessed by the entire class of objects. The standard candle technique employs the inverse square law to calculate the distance to an object of known luminosity. The key to using the standard candle method is finding similar objects that all have the same luminosity.
White dwarf supernovae, specifically Type Ia supernovae, are excellent standard candles for distant galaxies. Type Ia supernovae are a subcategory in the Minkowski–Zwicky supernova classification scheme. They are the result of a merger of two white dwarfs, each with 1 solar mass, reaching the mass limit of 1.44 solar masses. This mass limit causes a white dwarf supernova, and the fixed critical mass at which they explode results in a consistent peak luminosity. This consistent peak luminosity allows these explosions to be used as standard candles to measure the distance to their host galaxies. The visual magnitude of a Type Ia supernova, as observed from Earth, indicates its distance from Earth.
Type Ia supernovae are reliable standard candles because we understand the physics behind them. They generally occur in all types of galaxies and show no preference for regions of current stellar formation. They have a characteristic light curve, a graph of luminosity as a function of time after the explosion. The peak brightness reached is nearly the same for all of them, in contrast to other types of supernovae. This property was discovered by astronomers during the 1980s and 1990s.
The intrinsic luminosity of Type Ia supernovae probably varies by a factor of four, and the underlying physical reasons for this are not yet fully understood. However, the standard procedure is to apply empirical corrections to the "raw" absolute magnitude, depending on the width and shape of the light curve, the colour of the light curve, and the stellar mass of the host galaxy. These factors have been found to correlate with the peak luminosity of the explosion.
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Cepheid variable stars are excellent standard candles for nearby galaxies
A standard candle is a class of astrophysical objects, such as supernovae or variable stars, that have a known luminosity due to some characteristic quality possessed by the entire class of objects. The standard candle technique employs the inverse square law to calculate the distance to an object of known luminosity. The key to using the standard candle method is finding similar objects with the same luminosity, so that if we know what type of object we are looking at, we can look up its luminosity, measure its flux, and use the inverse square law to deduce its distance.
Cepheid variable stars are a type of variable star that pulsates radially, varying in both diameter and temperature. They are bright, young stars, abundant in nearby spiral and irregular galaxies. They pulsate with very regular periods on the order of days to months, and their radii can change by millions of kilometers during a pulsation cycle. Cepheid variables are important cosmic benchmarks for scaling galactic and extragalactic distances. A strong direct relationship exists between a Cepheid variable's luminosity and its pulsation period. This allows astronomers to establish the true luminosity of a Cepheid by observing its pulsation period, and then calculate its distance by comparing its known luminosity to its observed brightness.
Cepheid variables were discovered to have radial velocity variation in the early 20th century, and in 1918 they were used by Harlow Shapley to place initial constraints on the size and shape of the Milky Way and the placement of the Sun within it. In 1924, Edwin Hubble used Cepheid variables to establish the distance to classical Cepheid variables in the Andromeda Galaxy, showing that they were not members of the Milky Way. In 1929, Hubble and Milton L. Humason combined Cepheid distances to several galaxies with measurements of the speed at which those galaxies receded from us, formulating what is now known as Hubble's law and confirming that the universe is expanding.
In the mid-20th century, significant problems with the astronomical distance scale were resolved by dividing the Cepheids into different classes with very different properties. Classical Cepheids are younger and more massive population I stars, while type II Cepheids are older, fainter population II stars. Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established. Type II Cepheids are used to establish the distance to the Galactic Center, globular clusters, and galaxies.
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Type Ia supernovae have consistent peak brightness
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 a distant object can be identified as a standard candle, its absolute magnitude (luminosity) is known. Knowing the absolute magnitude, its distance can be calculated from its apparent magnitude.
Type Ia supernovae are a type of supernova that occurs in binary systems where one of the stars is a white dwarf. The other star can be anything from a giant star to another white dwarf. When the white dwarf gradually gains mass from its companion, it eventually reaches a critical mass, known as the Chandrasekhar mass, and explodes. This critical mass is the reason why Type Ia supernovae have a consistent peak brightness.
The peak brightness of Type Ia supernovae is nearly the same for all of them, making them excellent standard candles. This consistency in peak brightness is related to the amount of nickel-56 produced in white dwarfs exploding near the Chandrasekhar limit. The similarity in the absolute luminosity profiles of nearly all known Type Ia supernovae has led to their use as standard candles in astronomy.
The use of Type Ia supernovae as standard candles allows astronomers to measure their distances accurately. By comparing the supernovae's redshifts to their apparent brightness, astronomers discovered dark energy. These studies revealed that explosions at greater redshifts were dimmer than expected, indicating that the expansion of the universe is accelerating.
The Roman Space Telescope will use Type Ia supernovae to measure cosmic distances and gain a better understanding of the universe's expansion and the nature of dark energy. By studying the light from these supernovae, astronomers will be able to determine how quickly they are moving away from us and trace the cosmic expansion over time.
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The Tully-Fisher relation assumes all galaxies have the same mass-luminosity ratio
Standard candles are a class of astrophysical objects such as supernovae or variable stars that have known luminosities due to some characteristic quality possessed by the entire class of objects. This means that if a very distant object can be identified as a standard candle, then its absolute magnitude (luminosity) is known. By measuring the object's flux (apparent brightness), we can use the inverse square law to calculate its distance.
The Tully-Fisher relation is an established fundamental scaling relation for spiral galaxies. It is a correlation between the width of the integrated H I spectrum and the absolute magnitude, and primarily an empirical relation between circular velocity and luminosity. The Tully-Fisher relation assumes that all galaxies have the same mass-luminosity ratio (M/L constant). This assumption is based on well-founded physics: more massive galaxies would be more luminous and rotate faster.
The relation became known as the Tully-Fisher relation after the work of Tully and Fisher in 1977. They suggested the use of two alternatives to the virial theorem, which was commonly used to determine distances at the time. The Tully-Fisher relation has been used to estimate the distance to spiral galaxies by allowing the luminosity of a galaxy to be derived from its directly measurable line width. The distance can then be found by comparing the luminosity to the apparent brightness.
There are several different forms of the Tully-Fisher relation, depending on which precise measures of mass, luminosity, or rotation velocity are used. For example, the relation in terms of stellar mass is called the "stellar mass Tully-Fisher relation" (STFR). The tightest correlation is found when considering the total baryonic mass (the sum of a galaxy's mass in stars and gas), and this form of the relation is known as the baryonic Tully-Fisher relation (BTFR). The baryonic Tully-Fisher relation has been suggested to be more fundamental than the classical Tully-Fisher relation due to its reduced scatter.
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Gravitationally lensed quasars allow us to measure time delays between images
A standard candle is a class of astrophysical objects with known luminosities due to some characteristic quality possessed by the entire class of objects. When a star explodes as a supernova, its brightness rises very rapidly, increasing in about three weeks until it rivals the brightness of the galaxy in which it is located. The brightness then begins to turn over, and the star slowly fades over many weeks or months. In the case of Type Ia supernovae, the peak brightness reached is nearly the same for all of them. This is not true for other types of supernovae.
Gravitationally lensed quasars allow us to measure time delays between multiple images. When a galaxy is very close to the line of sight of a quasar, the light is deflected, and multiple images of the quasar appear. As quasars are intrinsically variable objects, monitoring the luminosity variations of all images allows us to measure time delays between images. These time delays are caused by splitting the light into slightly different paths around the lensing galaxy.
The determination of the time delay between different images of gravitationally lensed quasars is an important step in various studies. It helps in deriving the expansion rate of the universe, conducting microlensing studies, and performing detailed investigations of the structure of a lensed quasar. The Large Synoptic Survey Telescope (LSST) is expected to find more than 8000 lensed quasars, with approximately 3000 of them having well-measured time delays.
The time delays in lensed quasars are believed to be unique numbers, but their measurement is impacted by the quality of light curves and the contaminating contribution of gravitational microlensing. Microlensing by the stars in the lens galaxy can introduce changes in the actual time delays on the scale of the light-crossing time of the accretion disc. These microlensing-induced time delays will vary as the accretion disc moves relative to the stars causing microlensing.
Techniques such as the NMF method and numerical modelling are employed to determine time delays in gravitationally lensed quasars. The NMF method allows for the simultaneous testing of time delays in multiple light curves, providing coherence to the results. However, it is important to note that the determination of time delays in lensed quasars is subject to variability due to microlensing effects, which can impact the accuracy of measurements.
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Frequently asked questions
A standard candle is an astronomical object with a known absolute magnitude or intrinsic luminosity.
Cepheid Variable stars, RR Lyrae stars, Type Ia supernovae, carbon stars, and planetary nebulae are some examples of objects used as standard candles.
By measuring the apparent magnitude or flux of a standard candle, we can use the inverse square law to determine its distance. This is based on the principle that the difference between an object's absolute and apparent magnitudes is its distance modulus, which helps in calculating its distance.
