Extragalactic Distance Scale

An optical and X-ray image of Kepler's supernova, a type Ia supernova described by Johannes Kepler in 1604. Type Ia supernovas are the explosions of white dwarfs, which astronomers use to measure extragalactic distances and the expansion rate of the universe.

X-ray: NASA/CXC/SAO/D.Patnaude, Optical: DSS

How I Wonder Where You Are

It’s easier to measure distances to nearby stars than far-off galaxies. That’s an obvious statement, but it contains something helpful: if we can reliably find how far nearby objects are, we can use those distances to calibrate measurements to things a little farther away, and so on. This method of successive measurement is called “bootstrapping”.

For relatively close stars, astronomers measure distances using parallax, using the orbit of Earth around the Sun. Over the course of six months, Earth moves about 300 million kilometers, or 186 million miles. That difference in vantage point makes nearby stars appear to be in different places in the sky, which can be used to calculate those stars’ distances geometrically.

Using Predictable Phenomena to Standardize Measurements

The parallax effect is small for far-off objects, so it’s no good for measuring extragalactic distances. However, astronomers have identified certain objects which emit light in predictable ways. If we know how bright something is intrinsically, we can calculate its distance.

The expansion of the universe also provides some help, by stretching the wavelength of light to a larger degree depending on how far the object is away. That changes the color of that light in a predictable way, known as redshift, since red light has longer wavelength than blue light. Astronomers look for the spectrum of certain atoms shifted to different colors, which lets them measure the distance from the redshift.

Using these known quantities, astronomers have a set of reliable objects to use for measuring extragalactic distances. These include:

• A particular type of pulsating star, known as Cepheid variables. In 1908, Harvard astronomer Henrietta Swan Leavitt discovered that the rate of pulsation of a Cepheid variable was connected to its intrinsic brightness. Measuring that rate along with its apparent brightness reveals how far it is from Earth. Later astronomers used Leavitt’s discovery to measure the distances to many Cepheid variables, proving that certain nebulas were actually other galaxies. Cepheids are useful for measuring distances to relatively close-by galaxies, but become less reliable at larger distances simply because it’s hard to pick out individual stars the farther we look from the Milky Way.

• Certain stars and black holes turn molecules into masers, which are the microwave version of lasers. These are very bright objects that emit light in a predictable way, which astronomers can use to measure distances to nearby galaxies for stellar masers. For some supermassive black holes at the centers of galaxies, astronomers can harness the power of multiple radio telescopes together to measure the orbit of the water molecules driving the maser. That allows a geometrical measurement of the distance to the galaxy, much like the parallax method.

• Measuring the distance of galaxies by seeing how much their spectrum of light has redshifted due to the expansion of the universe. This measurement is most useful for galaxies within a certain distance of the Milky Way, but becomes less reliable for farther-off galaxies due to the acceleration of the expansion of the universe.

• Type Ia supernovas, which are believed to be the explosions of white dwarfs. Most stars in the universe, including our Sun, will become white dwarfs at the ends of their lives. These stellar remnants have a maximum mass, and explode if something causes them to exceed that limit: either material transferred from a companion star, or a merger with another white dwarf. The resulting explosion is so bright that they can be seen from billions of light-years away. The similarities between all white dwarfs mean they explode in very similar ways, which is why they are known as “standard candles.”

• Baryon acoustic oscillations, which are the remnants of sound waves in the very early universe. These waves produced patterns in the galaxies that formed later, which astronomers can map by surveying thousands of galaxies. The size of these waves was fixed in the early universe, and has stretched as the cosmos expanded, providing a “standard ruler” for extragalactic measurements on the largest scale.