By Joshua Winn, Princeton University
Cepheids are examples of what is known in astronomy as standard candles; sources for which we somehow know the luminosity. Astronomers love standard candles. An interesting historical factoid is that Cepheids were what first allowed us to zoom out beyond the Milky Way. How? Read on to find out.
In 1923, Edwin Hubble spotted some Cepheids in the Andromeda Nebula, which at that time, was a subject of debate, over whether it was a distant galaxy of billions of stars, or some kind of swirly gas cloud or star cluster that was inside the Milky Way. Hubble used the Cepheids to find the distance to Andromeda, and he showed it was a good fraction of a megaparsec away. Therefore, its angular size of a few degrees corresponds to a true size comparable to that of the Milky Way.
Interestingly though, even to this day, we don’t know exactly why Cepheids are such good standard candles. There’s been a lot of progress, but it’s fair to say that we can’t calculate the period-luminosity relationship from first principles. Of course, we don’t always need to understand a tool for it to be useful. We just need to confirm it works reliably. But it is a strange situation.
Type Ia Supernovas
Cepheids are these powerful beacons that have been used for more than a century to map out our galactic neighborhood. And yet, what’s going on inside the beacons, remains an area of ongoing research.
With our best telescopes, Cepheids can be seen out to a distance of around 50 megaparsecs. But if we want to go further, if we want to reach out to gigaparsecs, we need to find something else. Ideally, we’d find another type of standard candle, one that’s orders of magnitude more luminous than a Cepheid, so we could see it all the way across the universe. Unfortunately, nobody has found such a standard candle. But it turns out there is something equally good, maybe even better: a standard explosion.
In the 1980s, astronomers realized that a certain category of exploding stars—supernovas—produce fireballs that all have nearly the same peak luminosity. They’re called Type Ia supernovas, and they all explode with about the same energy. Or, at least, nearly the same energy; they’re predictable enough that if we measure the color and duration of the afterglow, we can determine its luminosity to within a few percent. And how do we know that? It’s because we’ve spotted Type Ia supernovas in nearby galaxies that also have Cepheids in them.
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Using Type Ia Supernovas as Standard Explosions
A chart showing data for some nearby Type Ia collection illustrates how we can use these supernovas as standard explosions. The horizontal axis, in the chart shows time, in days, and the vertical axis shows the measured luminosity of the explosion. In the chart, they all rise to about the same level, with differences that correlate with the duration. Keeping in mind that the faster the explosion fades, the lower the peak luminosity, when we measure the rise and fall of flux from a really distant type Ia supernova, we can match the observed duration of the event to one on this chart, and then read off the peak luminosity.
That’s how we can use Type Ia supernovas as standard explosions. Their great advantage is that they’re as bright as 5 billion Suns, bright enough to see even when they happen in galaxies, far, far away.
These are stupendous explosions which we don’t know the cause of. What we do know is that they’re almost certainly exploding white dwarfs but the trigger for the explosion remains a topic of active research. What is clear, though, is that they work. We can use them to measure cosmological distances. They’re the last step in our quest.
Different Measurement Techniques
The chart which illustrates different methods for figuring out distances to astronomical objects, is referred to as the distance ladder. As each rung is an order of magnitude in distance, it’s a logarithmic chart. The bars joining the rungs are the different measurement techniques that work over that range of distances. Out to a few AU, we can use radar ranging. To go beyond the solar system, we use parallax, which works out to kiloparsecs; that takes us most of the way around the Milky Way.
From there, we rely on standard candles: Cepheid variables. We measure their pulsation periods, deduce their luminosities, and use the flux/luminosity relation to get the distance. That works out to around 50 megaparsecs, a big part of the universe, containing hundreds of thousands of galaxies.
Even farther away, we rely on standard explosions: Type Ia supernovas. We watch the light from the fireball rise and fall, deduce the peak luminosity, and again use the flux/luminosity relation to get the distance which gets us out to gigaparsecs.
Common Questions about Cepheid variables
Edwin Hubble used the Cepheids to find the distance to Andromeda, and he showed it was a good fraction of a megaparsec away. Therefore, its angular size of a few degrees corresponds to a true size comparable to that of the Milky Way.
In the 1980s, astronomers realized that a certain category of exploding stars—supernovas—produce fireballs that all have nearly the same peak luminosity. They’re called Type Ia supernovas, and they all explode with about the same energy.
Out to a few AU, we can use radar ranging. To go beyond the solar system, we use parallax, which works out to kiloparsecs; that takes us most of the way around the Milky Way.