Editing Cosmic distance ladder (section)

== Galactic distance indicators ==
{{see also|Distance measure}}

With few exceptions, distances based on direct measurements are available only out to about a thousand parsecs, which is a modest portion of our own Galaxy. For distances beyond that, measures depend upon physical assumptions, that is, the assertion that one recognizes the object in question, and the class of objects is homogeneous enough that its members can be used for meaningful estimation of distance.

Physical distance indicators, used on progressively larger distance scales, include:
* [[Dynamical parallax]], uses orbital parameters of [[visual binary|visual binaries]] to measure the mass of the system, and hence use the [[Mass–luminosity relation#In astronomy|mass–luminosity relation]] to determine the luminosity
**[[Binary star|Eclipsing binaries]] — In the last decade, measurement of eclipsing binaries' fundamental parameters has become possible with 8-meter class telescopes. This makes it feasible to use them as indicators of distance. Recently, they have been used to give direct distance estimates to the [[Large Magellanic Cloud]] (LMC), [[Small Magellanic Cloud]] (SMC), [[Andromeda Galaxy]] and [[Triangulum Galaxy]]. Eclipsing binaries offer a direct method to gauge the distance to galaxies to a new improved 5% level of accuracy which is feasible with current technology to a distance of around 3 Mpc (3 million parsecs).<ref name="Bonanos2006">
{{cite journal |last=Bonanos |first=A. Z. |year=2006 |title=Eclipsing Binaries: Tools for Calibrating the Extragalactic Distance Scale |journal=[[Proceedings of IAU Symposium]] |volume=240 |pages=79–87 |arxiv=astro-ph/0610923 |doi=10.1017/S1743921307003845 |bibcode=2007IAUS..240...79B |citeseerx=10.1.1.254.2692 |s2cid=18827791}}</ref>
* [[RR Lyrae variable]]s — used for measuring distances within the [[Milky Way|galaxy]] and in nearby [[globular cluster]]s.
* The following four indicators all use stars in the old stellar populations (Population II):<ref name="Ferrareseetal2000">
{{cite journal |last1=Ferrarese |first1=L |display-authors=etal |year=2000 |title=A Database of Cepheid Distance Moduli and Tip of the Red Giant Branch, Globular Cluster Luminosity Function, Planetary Nebula Luminosity Function, and Surface Brightness Fluctuation Data Useful for Distance Determinations |journal=[[The Astrophysical Journal Supplement Series]] |volume=128 |issue=2 |pages=431–459 |arxiv=astro-ph/9910501 |bibcode=2000ApJS..128..431F |doi=10.1086/313391 |s2cid=121612286}}</ref>
** [[Tip of the red-giant branch]] (TRGB) distance indicator.
** [[Planetary nebula luminosity function]] (PNLF)
** [[Globular cluster luminosity function]] (GCLF)
** [[Surface brightness fluctuation]] (SBF)
* In galactic astronomy, [[X-ray burster|X-ray burst]]s (thermonuclear flashes on the surface of a [[neutron star]]) are used as standard candles. Observations of X-ray burst sometimes show X-ray spectra indicating radius expansion. Therefore, the X-ray flux at the peak of the burst should correspond to [[Eddington luminosity]], which can be calculated once the mass of the neutron star is known (1.5 solar masses is a commonly used assumption). This method allows distance determination of some low-mass [[X-ray binary|X-ray binaries]]. Low-mass X-ray binaries are very faint in the optical, making their distances extremely difficult to determine.
* [[Astrophysical maser#Distance determinations|Interstellar masers]] can be used to derive distances to galactic and some extragalactic objects that have maser emission.
* [[Cepheid variable#Use as a "standard candle"|Cepheids]] and [[Nova#Novae as distance indicators|nova]]e
* The [[Tully–Fisher relation]]
* The [[Faber–Jackson relation]]
* [[Type Ia supernova]]e that have a very well-determined maximum absolute magnitude as a function of the shape of their [[light curve]] and are useful in determining extragalactic distances up to a few hundred Mpc.<ref>
{{cite journal |last=Colgate |first=S. A. |year=1979 |title=Supernovae as a standard candle for cosmology |journal=[[Astrophysical Journal]] |volume=232 |issue=1 |pages=404–408 |bibcode=1979ApJ...232..404C |doi=10.1086/157300}}</ref> A notable exception is [[SN 2003fg]], the "Champagne Supernova", a Type Ia supernova of unusual nature.
* [[Redshift]]s and [[Hubble's law]]

=== Main sequence fitting ===
{{main|Spectroscopic parallax}}

When the absolute magnitude for a group of stars is plotted against the [[Stellar classification|spectral classification]] of the star, in a [[Hertzsprung–Russell diagram]], evolutionary patterns are found that relate to the mass, age and composition of the star. In particular, during their hydrogen burning period, stars lie along a curve in the diagram called the [[main sequence]]. By measuring these properties from a star's spectrum, the position of a main sequence star on the H–R diagram can be determined, and thereby the star's absolute magnitude estimated. A comparison of this value with the apparent magnitude allows the approximate distance to be determined, after correcting for interstellar extinction of the luminosity because of gas and dust.

In a gravitationally-bound [[star cluster]] such as the [[Hyades (star cluster)|Hyades]], the stars formed at approximately the same age and lie at the same distance. This allows relatively accurate main sequence fitting, providing both age and distance determination.