Cepheid variable

That means you've got to know two things in order to find out the third, uh, and that brings me to the comment I want to make about problem two-A on the current problem set.

Uh, you are asked in this problem to determine the difference between the absolute magnitude of one kind of star called type one Cepheid, so I label it C1, uh, and another kind of star, type two Cepheids, which I label C2.

Uh, and if you're asked to...

And, and this difference is called, I don't know, delta MC or something like that.

But we were only really able to do this in the–starting in the early 1990s, because there were two key advances that were made at that time.

One is that the Hubble Space Telescope measured things like Cepheids and other kinds of stars, other kinds of distance measurements, in galaxies that had historically had Type Ia supernovae.

These things don't occur very often, once every couple of hundred years per galaxy.

There hasn't been one in our own galaxy since before they invented the telescope.

They take a couple weeks to rise to their maximum brightness, and then they decay over a few months.

So, you can see them for a little while.

measured Cepheids.

This meant that we could calibrate them.

At first, it was simply a curiosity, an oddity.

I happened to be studying a Cepheid variable star, one of the huge, very bright stars that pulsate so regularly that you can set your watch by them.

It had attracted my attention because it seemed to be unusually close for a Cepheid, only seven hundred light-years away.

The distance could be easily gauged by timing the star's pulsations.

Footnote one.

Astronomers have been able, since about 1910, to estimate the distances of Cepheid variable stars by timing their pulsations.

The length of this type of star's pulsation is a true measure of its intrinsic brightness.

Comparing the star's actual brightness to its apparent brightness as seen from Earth gives a good value for the star's distance.

When it moves aside, the full brightness is restored.

Variable stars have been important in the history of astronomy because certain types, particularly a kind called Cepheid variables, have a very useful property.

The period of their pulsation is directly related to their intrinsic luminosity.

If you measure how quickly a Cepheid pulses, you can calculate how much light it is actually producing.

And if you know how much light it is producing, you can compare that to how bright it appears from Earth and calculate its distance.

This relationship has been one of the most powerful tools in astronomy for measuring distances to stars and galaxies, and it was instrumental in establishing the true scale of the universe in the early 20th century.

Once only he glanced at the nineteen-point outline of his address.

As there is no point in keeping it a secret, it was as follows: One, Cepheid and Cepheids.

How long is a yardstick? Two, double trouble.

Is ours a binary universe? Three, cerebrum and cortex.

Measuring Cepheid periods in other galaxies gave Hubble their true brightnesses as though undimmed by distance.

Cepheids became what we call standard candles, objects of known luminosity whose observed brightness therefore tells us their distance.

But this calculation involves assumptions and uncertainties.

For one thing, the Cepheid period-luminosity relationship first had to be calibrated based on nearby Cepheids whose distances can be figured using stellar parallax, tracking their tiny motions on the sky as-

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This stepwise determination of astronomical distances is called the cosmic distance ladder.

With each step on the ladder, uncertainties compound.

2.5 million light years.

That number has been measured, verified and refined over a century of observation, from Edwin Hubble's original Cepheid Variable measurements in the 1920s to modern data from the Gaia spacecraft and the Hubble Space Telescope.

It is one of the most precisely known extragalactic distances in astronomy.

And it is a number that, once you truly internalize what it means, changes how you think about every other distance you have ever encountered.

It was resolved four years later, not by argument, but by observation.

Edwin Hubble, working at the Mount Wilson Observatory in California with the 100-inch Hooker telescope, then the most powerful telescope on Earth, turned it toward the Andromeda Nebula and began searching for a specific type of star.

He was looking for Cepheid variables, pulsating stars whose brightness is directly related to their pulsation period.

A Cepheid that pulses slowly is intrinsically brighter than one that pulses quickly.

He found them.

Embedded within the spiral arms of Andromeda, he identified individual Cepheid variable stars.

This was the breakthrough that changed everything.

Cepheid variables are stars that pulse.

They brighten and dim in regular, predictable cycles that can last anywhere from a few days to several weeks.

What makes them invaluable to astronomers is a relationship first established by Henrietta Swan Leavitt in 1912.

The mathematics is straightforward.

The implications were revolutionary.

When scientists use the cosmic microwave background to infer the expansion rate, effectively running the physics of the early universe forward through time and predicting what the Hubble constant should be today, they get a value of approximately 67 km per second per megaparsec.

When scientists use local observations, Cepheid variable stars, Type Ia supernovae, and other distance indicators, In the nearby universe, they measure a value of approximately 73 km per second per megaparsec.

Both measurements are extraordinarily precise.

Both have been checked, rechecked and refined with improved data.

Its sharper resolution and greater sensitivity allowed researchers to remeasure the distance indicators with unprecedented accuracy, eliminating potential sources of error such as stellar crowding and dust contamination.

Some teams, notably the group led by Wendy Friedman, reported measurements using different stellar indicators that landed closer to 69 or 70 km per second per megaparsec, potentially easing the tension.

But other teams, particularly those using Cepheid-calibrated supernovae, continue to find values near 73, ruling out observational errors at high confidence.

As of 2026, the Hubble tension has not been resolved.

These stars brighten and dim at rates directly related to their intrinsic luminosity, making them reliable cosmic distance markers.

By identifying Cepheids within the nebulae and comparing their apparent brightness to their known true brightness, Hubble could calculate the distance to each nebula with unprecedented accuracy.

The result was the discovery that changed cosmology forever.

When Hubble plotted distance against recession velocity, the relationship was unmistakable.