The cosmic distance ladder: how we measure stars and galaxies

Step 1: Measuring distances to nearby stars

Parallax

As the Earth orbits the Sun, nearby stars appear to shift slightly against the background of more distant stars.

From this angular shift, astronomers can calculate the distance to the star.

For example, Proxima Centauri, the closest star to the Sun, is located approximately 4.25 light-years away.

Today, the Gaia spacecraft has measured the parallax for over a billion stars in the Milky Way.

Step 2: Measuring distances to nearby galaxies

Cepheid variables

Certain variable stars called Cepheids have a direct relationship between their pulsation period and their intrinsic luminosity.

By simply measuring their cycle of brightening and dimming, astronomers can infer their true brightness and, from there, calculate their distance.

Cepheids helped confirm that nebulae like Andromeda are actually galaxies located outside the Milky Way.

Type Ia supernovae

When the distance is too great to observe Cepheids, astronomers use Type Ia supernovae.

These explosions reach extreme brightness and have nearly identical peak luminosities, making them "standard candles" for measuring distances of billions of light-years.

It was this method that led to the discovery that the universe is expanding at an accelerating rate.

Step 3: Measuring distances on a cosmic scale

Redshift

At very large distances, instead of measuring brightness directly, scientists use the phenomenon of redshift.

Because space is expanding, light from distant galaxies is stretched toward the red end of the spectrum.

The further away a galaxy is:

• The greater its redshift.
• The higher its recession velocity.

This relationship is described by:

v = H₀ × d

Where:

• v is the recession velocity.
• d is the distance.
• H₀ is the Hubble constant.

Why is measuring distance so difficult?

Each step of the distance ladder depends on the one below it.

For example:

• Parallax calibrates Cepheids.
• Cepheids calibrate Type Ia supernovae.
• Type Ia supernovae calibrate the Hubble constant.

Therefore, if one step has an error, that error propagates to all higher steps.

This is why astronomers are still debating the precise value of the Hubble constant and the expansion rate of the universe.

🌌 From the nearest stars to galaxies at the edge of the observable universe, every distance measurement is built like a series of rungs on a ladder. Understanding distance is the key to determining the size, age, and evolutionary history of the entire universe.

Further reading

• Galaxy: definition and classification
• Why astrology is not a science
• Zodiac constellations