Chloe Adams
Standard candles are objects and phenomena with a known luminosity due to a characteristic quality. This allows astronomers to determine the distance to extremely distant objects by comparing their apparent brightness to their intrinsic brightness. Examples of standard candles include Type Ia supernovae, Cepheid variables, carbon stars, and TRGB stars. However, there are uncertainties and challenges in observations and theories that affect the accuracy of standard candle measurements. For instance, while Type Ia supernovae were once considered reliable standard candles due to their presumed cosmic uniformity, it was later discovered that factors like dust extinction could impact their brightness and apparent distance.
| Characteristics | Values |
|---|---|
| Type | Carbon star, Cepheid variables, planetary nebulae, Type Ia supernovae |
| Colour | Red or orange |
| Composition | Abundance of carbon in the atmosphere |
| Spectral characteristics | Swan bands of C2 |
| Evolutionary stage | Various stages of the red giant branch |
| Luminosity | Known |
| Distance | Can be calculated using the formula: distance (in cm) = apparent magnitude x absolute magnitude |
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What You'll Learn
Carbon stars
A standard candle is a class of astrophysical objects with known luminosities due to some characteristic quality. Carbon stars are luminous red giants near the end of their lives. Their atmospheres contain more carbon than oxygen, giving them a distinct red or orange colour. These stars are viable standard candles for galaxies large enough to contain one hundred or more carbon stars.
The abundance of carbon in carbon stars' atmospheres is usually due to dredge-up processes, where carbon-rich material from the star's core is brought to the surface. They are characterised by strong absorption bands of carbon compounds, such as the Swan bands of C2. These stars are found at various stages of the red giant branch in the Hertzsprung-Russell diagram.
The J-region Asymptotic Giant Branch (JAGB) method is a standard candle based on the intrinsic luminosities of carbon stars in the near-infrared. The JAGB method is most accurate when measured in the low-reddening outer disks of galaxies. It is a robust standard candle capable of calibrating the luminosities of Type Ia supernovae and providing a high-accuracy measurement of the Hubble constant.
Using near-infrared photometry, carbon stars in the Magellanic Clouds and the Milky Way can be identified. These stars appear as a distinct horizontal feature in the colour-magnitude diagram. By deriving the luminosity function of these colour-selected carbon stars, the median absolute magnitude and dispersion can be determined. This information can then be used to determine distances to galaxies and improve the measurement of the Hubble constant.
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Supernovae
The critical mass that is reached is the same each time, and this means that the explosion is also the same each time. The light curve of a Type Ia supernova always has the same characteristic shape and reaches the same peak absolute magnitude. This consistency means that Type Ia supernovae can 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.
The use of Type Ia supernovae to measure precise distances was pioneered by a collaboration of Chilean and US astronomers, the Calán/Tololo Supernova Survey. In a series of papers in the 1990s, the survey showed that while Type Ia supernovae do not all reach the same peak luminosity, a single parameter measured from the light curve can be used to correct unreddened Type Ia supernovae to standard candle values. This correction is known as the Phillips relationship and was shown to be able to measure relative distances to 7% accuracy.
The advantage of using Type Ia supernovae as standard candles is that they are extremely bright. This means they can be used to measure the distance to the furthest galaxies. For example, in 2015, NASA reported that the Kepler space observatory observed KSN 2011b, a Type Ia supernova in the process of exploding. In July 2019, the Hubble Space Telescope took three images of a Type Ia supernova through a gravitational lens. This supernova appeared at three different times in the evolution of its brightness due to the differing path lengths of the light in the three images. A fourth image will be taken in 2037, allowing for the observation of the entire luminosity cycle of the supernova.
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Cepheid variables
In the early 20th century, Cepheid variables played a pivotal role in shaping our understanding of the universe. Harlow Shapley used these stars to set initial constraints on the size and shape of the Milky Way and the position of our Sun within it. Edwin Hubble's work with Cepheid variables in the Andromeda Galaxy (then known as the "Andromeda Nebula") provided conclusive evidence that it was a separate galaxy, dispelling the notion that the Milky Way comprised the entirety of the universe.
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Parallax
The first successful attempt at measuring the distance to a star using the parallax method was made by German astronomer Friedrich Bessel in 1838. Bessel calculated that a star in the Cygnus constellation was about 10 light-years away from Earth. This marked the beginning of the long and challenging process of creating a three-dimensional map of the universe. Over time, improvements in telescope technology have aided astronomers in expanding the catalogue of stellar distances using the parallax method.
The parallax method is particularly useful for measuring the distances of relatively nearby stars. Beyond a certain distance, even the most sensitive technologies struggle to measure parallax. However, insights from the parallax measurements of closer stars can be used to estimate the distances of more distant stars. By measuring the distances to nearby stars, astronomers can establish relationships between a star's colour and its intrinsic brightness, which is the brightness it would have if viewed from a standard distance.
These stars with known brightness are referred to as "standard candles". By comparing the colour and spectrum of distant stars to these standard candles, astronomers can determine their intrinsic brightness. Then, by comparing the intrinsic brightness to the star's apparent brightness, the distance to the star can be calculated. Examples of standard candles include Cepheid variables, planetary nebulae, and Type Ia supernovae.
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TRGB stars
Standard candles are 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. TRGB stars are one such example.
TRGB stands for Tip of the Red Giant Branch. It is a primary distance indicator used in astronomy. TRGB stars are typically helium-burning stars, meaning they have exhausted their hydrogen fuel in the core and have transitioned to helium burning. They are usually found at the tip of the red-giant branch in the Hertzsprung-Russell diagram. This diagram is a plot of stellar luminosity against surface temperature for a population of stars.
Red giants were first identified in the early 20th century when the use of the Hertzsprung-Russell diagram made it clear that there were two distinct types of cool stars with very different sizes: dwarfs (now formally known as the main sequence) and giants. The term red-giant branch came into use during the 1940s and 1950s, initially as a general term to refer to the red-giant region of the diagram.
The TRGB indicator uses stars in old stellar populations. TRGB stars have a very strong connection between core mass, core temperature, and luminosity. The brightness of the TRGB is stable and can be used to accurately determine distances of up to 10 Mpc in over 40 cases.
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Frequently asked questions
A standard candle is a class of astrophysical objects, such as supernovae or variable stars, with a known luminosity due to some characteristic quality possessed by the entire class of objects.
By comparing the apparent brightness of a standard candle to its intrinsic brightness, we can determine how far away it is.
Some examples of standard candles include Cepheid Variables (Cepheids), planetary nebulae, Type Ia supernovae, red giant branch tips, and carbon stars.
The concept of standard candles was pioneered by astronomer Edwin Hubble in 1924 through his observations of Cepheid stars in galaxies beyond our own. Harvard astronomer Henrietta Swan Leavitt earlier discovered the relationship between a Cepheid star's period and its intrinsic luminosity.
Yes, there are some challenges and uncertainties associated with using standard candles for distance measurement. For example, Cepheid stars were found to lose mass, affecting their brightness and distance calculations. Additionally, uncertainties in theories, models, and observations can introduce errors in the determination of standard candle luminosity.
