standard candlestandard candlesdistanceExtragalactic Distance Scalestellar distanceStandard sirendistance indicatorastronomical distance scaleAstronomical scaleglobular cluster luminosity function
The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects.wikipedia
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### Parallax

trigonometric parallaxsolar parallaxmotion parallax
The most important fundamental distance measurements come from trigonometric parallax.
These distances form the lowest rung of what is called "the cosmic distance ladder", the first in a succession of methods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder.

### RR Lyrae variable

RR Lyr variableRR LyraeRR Lyrae variables
This statistical parallax method is useful for measuring the distances of bright stars beyond 50 parsecs and giant variable stars, including Cepheids and the RR Lyrae variables.
They are used as standard candles to measure (extra)galactic distances, assisting with the cosmic distance ladder.

### Astronomical unit

AUastronomical unitsAUs
Direct distance measurements are based upon the astronomical unit (AU), which is the distance between the Earth and the Sun.
The astronomical unit is typically used for stellar system scale distances, such as the size of a protostellar disk or the heliocentric distance of an asteroid, whereas other units are used for other distances in astronomy.

### Cepheid variable

CepheidCepheidsCepheid variables
This statistical parallax method is useful for measuring the distances of bright stars beyond 50 parsecs and giant variable stars, including Cepheids and the RR Lyrae variables. Beyond the reach of the Wilson–Bappu effect, the next method relies on the period-luminosity relation of classical Cepheid variable stars.
A strong direct relationship between a Cepheid variable's luminosity and pulsation period established Cepheids as important indicators of cosmic benchmarks for scaling galactic and extragalactic distances.

### Parsec

Mpcpckpc
A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs) to Earth.
The parallax method is the fundamental calibration step for distance determination in astrophysics; however, the accuracy of ground-based telescope measurements of parallax angle is limited to about 0.01 arcseconds, and thus to stars no more than 100 pc distant.

### Astronomy

astronomicalastronomerastronomers
The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects.
The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe.

### Gaia (spacecraft)

GaiaGaia Data Release 2Gaia spacecraft
In 2018, Data Release 2 from the Gaia space mission provides similarly accurate distances to most stars brighter than 15th magnitude.

### Henrietta Swan Leavitt

Henrietta LeavittH. LeavittLeavitt, Henrietta Swan
These objects of known brightness are termed standard candles, coined by Henrietta Swan Leavitt.
Leavitt's discovery provided astronomers with the first "standard candle" with which to measure the distance to faraway galaxies.

### Stellar parallax

parallax shiftparallaxparallax measurements
This statistical parallax method is useful for measuring the distances of bright stars beyond 50 parsecs and giant variable stars, including Cepheids and the RR Lyrae variables.
Other techniques, such as standard candles and spectral red-shift, are required to measure the distance of more remote objects.

### Binary star

spectroscopic binaryeclipsing binarybinary
Binary stars which are both visual and spectroscopic binaries also can have their distance estimated by similar means, and don't suffer from the above geometric uncertainty.
This makes it feasible to use them to directly measure the distances to external galaxies, a process that is more accurate than using standard candles.

In particular the distance obtained for the Hyades has historically been an important step in the distance ladder.
The fact that three independent measurements agree makes the Hyades an important rung on the cosmic distance ladder method for estimating the distances of extragalactic objects.

### Astrometry

astrometricastrometristastrometrical
The precise measurement of stellar positions is part of the discipline of astrometry.
Astrometry is an important step in the cosmic distance ladder because it establishes parallax distance estimates for stars in the Milky Way.

### Baryon acoustic oscillations

baryon acoustic oscillationBAOacoustic peaks
More recently the physical scale imprinted by baryon acoustic oscillations (BAO) in the early universe has been used.
In the same way that supernovae provide a "standard candle" for astronomical observations, BAO matter clustering provides a "standard ruler" for length scale in cosmology.

### Milky Way

As a result, the population II stars were actually much brighter than believed, and when corrected, this had the effect of doubling the distances to the globular clusters, the nearby galaxies, and the diameter of the Milky Way.
This value is estimated using geometric-based methods or by measuring selected astronomical objects that serve as standard candles, with different techniques yielding various values within this approximate range.

### Tip of the red-giant branch

tip of the red giant branchTRGB
Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy.

### Planetary nebula luminosity function

PNLF
The planetary nebula luminosity function (PNLF) was first proposed in the late 1970s by Holland Cole and David Jenner.
Planetary nebula luminosity function (PNLF) is a secondary distance indicator used in astronomy.

### Surface brightness fluctuation

The surface brightness fluctuation (SBF) method takes advantage of the use of CCD cameras on telescopes.
Surface brightness fluctuation (SBF) is a secondary distance indicator used to estimate distances to galaxies.

### Hubble's law

Hubble constantHubble parameterHubble flow
In such cases, the redshift of the host galaxy allows a determination of the Hubble constant H_0.
(See cosmic distance ladder for details.)

### Open cluster

open star clusterstar clusteropen clusters
Only open clusters are near enough for this technique to be useful.
Calibration of the astronomical distance scale relies on a sequence of indirect and sometimes uncertain measurements relating the closest objects, for which distances can be directly measured, to increasingly distant objects.

### Planetary nebula

planetary nebulaeplanetarycentral stars of planetary nebulae
If the expansion of a gas cloud, like a supernova remnant or planetary nebula, can be observed over time, then an expansion parallax distance to that cloud can be estimated.

### Period-luminosity relation

period-luminosity relationshiprelationshipLeavitt's law
Beyond the reach of the Wilson–Bappu effect, the next method relies on the period-luminosity relation of classical Cepheid variable stars.
Discovered in 1908 by Henrietta Swan Leavitt, the relation established Cepheids as foundational indicators of cosmic benchmarks for scaling galactic and extragalactic distances.

### Large Magellanic Cloud

LargeLMCGreater Magellanic Cloud
The distance to the LMC has been calculated using a variety of standard candles, with Cepheid variables being one of the most popular.

### Type Ia supernova

type IaType Ia supernovaetype 1a supernova
For example, all observations seem to indicate that Type Ia supernovae that are of known distance have the same brightness (corrected by the shape of the light curve).
The stability of this value allows these explosions to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.

### Standard ruler

Another class of physical distance indicator is the standard ruler.
Distances can also be measured using standard candles; many different types of standard candles and rulers are needed to construct the cosmic distance ladder.

### Redshift

red shiftzred-shift
This will be important for signals originating at cosmological redshifts greater than 1.
Vesto Slipher was the first to discover galactic redshifts, in about the year 1912, while Hubble correlated Slipher's measurements with distances he measured by other means to formulate his Law.