Chloe Adams
Standard candles are objects with known absolute magnitudes and luminosities. They are used to measure the distance between galaxies, which is typically expressed in megaparsecs. The most commonly used standard candles are Cepheid Variable stars, RR Lyrae stars, and Type Ia supernovae. Cepheid Variables were discovered in 1912 by Henrietta Leavitt, who found that the period of a Cepheid could be used to measure cosmic distance. Type Ia supernovae are also useful standard candles because they are extremely bright and can be seen at vast distances. Kilonovae have been proposed as another type of standard candle due to their spherical explosions, which allow astronomers to compare the apparent and actual sizes of the explosions. The challenge with standard candles is determining their luminosity, as it requires already knowing the distance to the object. To address this, astronomers use a distance ladder approach, starting with nearby standard candles with known distances and working outward to those with unknown distances.
| Characteristics | Values |
|---|---|
| Definition | Standard candles are objects with known intrinsic luminosities |
| Formula | Flux (F) = Luminosity (L) x [1 / (4 x π x Distance (d)^2)] |
| Units | Luminosity (L) is measured in watts; distance (d) is measured in meters, light-years, or parsecs |
| Examples | Cepheid variables, Type Ia supernovae, planetary nebulae, RR Lyrae stars, kilonovae, carbon stars, etc. |
| Discovery | Discovered by Henrietta Leavitt in 1912; named by Henrietta Swan Leavitt |
| Challenges | Determining luminosity; standardness or homogeneity of objects; infrequency of Type Ia supernovae |
| Techniques | Parallax, Doppler shift, Very Long Baseline Interferometry (VLBI), gravitational lensing |
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What You'll Learn
Parallax measurements
Parallax is the observed displacement of an object caused by the change in the observer's point of view. In astronomy, it is an important tool for calculating the distances of far-away stars. The parallax method is only effective for measuring the distances of nearby stars, but space telescopes like Gaia have significantly expanded its effectiveness. Parallax remains the most direct and reliable method for measuring stellar distances, forming the basis for calibrating more indirect methods to measure distances to galaxies and beyond.
The closer the star is to the observer, the larger the angle would be. Parallax measurements rely on the same effect as stereoscopic vision. To understand how parallax works, you can perform the following experiment: hold out your hand, close your right eye, and place your extended thumb over a distant object. Now, switch eyes, so that your left eye is closed and your right eye is open. Your thumb will appear to shift slightly against the background. By measuring this small change and knowing the distance between your eyes, you can calculate the distance to your thumb using trigonometry.
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Cepheid variables
Standard candles are objects with known intrinsic luminosities, meaning that the amount of light or radiation emitted by the object at its source is known. By comparing the known luminosity with the amount of light from the object that reaches Earth, astronomers can calculate how far away the object is.
In the 1950s, Walter Baade discovered that the nearby Cepheid variables used to calibrate the standard candle were of a different type than those used to measure distances to nearby galaxies. The nearby Cepheid variables were population I stars with much higher metal content than the distant population II stars. This discovery led to a correction in the estimated distances to globular clusters, nearby galaxies, and the Milky Way's diameter.
The calibration of the Cepheid period-luminosity relation was historically challenging due to the absence of Cepheid variables in nearby star clusters. Astronomers had to compare more distant clusters containing Cepheids with the Hyades cluster via main sequence matching, which allowed for an absolute calibration of Cepheids and, consequently, the determination of the SMC's absolute distance.
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Kilonovae
Standard candles are a class of astronomical objects with known brightness. By comparing the known luminosity of these objects to their observed brightness, astronomers can compute the distance to the object using the inverse-square law. This technique forms the basis for calibrating more indirect methods to measure distances to galaxies and beyond.
The first definitive observation of a kilonova was made in October 2017 when astronomers reported that observations of AT 2017gfo showed that it was the first conclusive observation of a kilonova following a merger of two neutron stars. The existence of a kilonova had long been hypothesized but had never been definitively witnessed until then. The kilonova was observed with large telescopes and monitored spectroscopically to determine its composition, geometry, and velocity.
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Type Ia supernovae
The use of Type Ia supernovae as standard candles was pioneered by the Calán/Tololo Supernova Survey, a collaboration between Chilean and US astronomers. 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 can measure relative distances to 7% accuracy.
The standardisation of Type Ia supernovae is typically done by comparing their light curves and spectra. The light curve width is believed to be determined primarily by the nickel-56 mass, but recent analyses have shown that there is also a connection with the ejected mass or the amount of nickel-56 created in a particular supernova. By comparing the brightness of Type Ia supernovae with known intrinsic luminosities, astronomers can calculate the distance to the object using the inverse-square law.
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Gravitationally lensed quasars
Standard candles are objects of known brightness used to measure the distance to distant astronomical objects by comparing their known luminosity to their observed brightness. The most commonly used standard candles are Type Ia supernovae, which are extremely bright and can be used to measure the distance to galaxies.
The key advantage of quasars as standard candles is the tight correlation between their X-ray and ultraviolet emissions, which is independent of the processes that occur in supernovae. By analysing the X-ray measurements of 2332 distant quasars from the Chandra Source Catalog and comparing them to ultraviolet results from the Sloan Digital Sky Survey, astronomers found that this correlation was consistent over 85% of the age of the universe, becoming even tighter at earlier times. This discovery dramatically extends the range of standard candle redshifts and provides a new tool to measure the properties of the evolving universe.
The use of quasars as standard candles also benefits from the discovery of hundreds of thousands of quasars in recent years. The proposed method relies on the non-linear relationship between the UV and X-ray luminosity of quasars, which can be used to build a Hubble diagram up to redshift z~7.5. This technique provides distance estimates comparable to those from supernovae up to z~1.5 and shows no redshift evolution up to z~5.
In conclusion, gravitationally lensed quasars offer a promising new approach to measuring cosmic distances, providing independent measures of cosmological parameters and extending the range of standard candle redshifts.
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Frequently asked questions
A standard candle is an astronomical object with a known absolute magnitude or intrinsic brightness.
The known luminosity of a standard candle, combined with its observed brightness, can be used to calculate its distance. This is done using the inverse-square law.
Standard candles include Cepheid Variable stars, RR Lyrae stars, Type Ia supernovae, planetary nebulae, and kilonovae.
One challenge is determining the luminosity of the standard candle. Another issue is the standardness of the objects, i.e., how homogeneous they are in their true absolute magnitude. For example, Type Ia supernovae, despite being extremely useful standard candles, do not all have the same peak brightness.
