Date: May 12, 2011

Title: How We Measure Distances to Stars and Galaxies

Podcaster: Damian Lima

Organization: Ciencia Limada – http://www.ciencialimada.com.ar/

Description: How do we measure distances to stars and galaxies?
We’ve all heard or read about the incredible distances between Earth and distant stars and galaxies. But, how astronomers manage to measure with such precision those distances? In this podcast we’re going to explain three of the most important methods for measuring stellar and cosmological distances: the parallax, the Cepheid stars and the type Ia supernovae.

Bio: Damian Lima is studying a degree in physics at the University of Buenos Aires. He works at a major technology company. Is a passionate amateur astronomer and writes a blog on science, critical thinking and skepticism, called “Ciencia Limada”.

Sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2010, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.

Transcript:

Hello. Welcome to this “365 Days of Astronomy” podcast. My name is Damian Lima; I’m a physics student and amateur astronomer from Argentina.

You have probably heard or read, at some point of your life, that “certain star is thousands of light years away”, or “this galaxy is about a million light years away”. Then you probably you asked yourself: how astronomers manage to measure with such precision the distance to an object that is out there, in the incredibly vast distances of space?

To measure distances to relatively close objects, this means up to one hundred light years away, we use a method called “parallax”. To explain it very briefly, parallax is the apparent change in the position of an object when you look at it from two different points of view. It’s on that difference in the position of the object, which is known as apparent position, where an angle is drawn, and then applying trigonometric calculations we determine the distance to that object.

When using the parallax method for interstellar distances, the two different points of view correspond with two different places on Earth, or two different moments of our planet’s orbit. Measuring the difference in the apparent position of a star in relation with the background, we can determine the distance to that star.

But when measuring distances to stars and galaxies over one hundred light years, the parallax method fails. That’s the reason why the genius Edwin Hubble came up with a technique that we could call “the standard candles method”.

Let’s do an imagination exercise. Suppose we are in a field and we want to measure the distance to another field that is far away. But we can’t use any commons methods of measurement, since we cannot leave our own field (as we cannot measure astronomical distances going out and just measuring). Instead, we’re going to use a rather exotic method: we buy a lot of 60 watts light bulbs and distributed them throughout our field.

We know that all light bulbs shine with equal intensity, because there all have 60 watts. But when we watch the light bulbs from a random position in the field, we will see those closer to us shining more and those further away shining less. This is extremely important: all the light bulbs shine equally, but we see them shine differently.

This allows us to define a methodology for determining the distance to any light bulb. First we measure the distance to a nearby light bulb using a measuring tape (which in our analogy correspond to the parallax method) and then, using a photometer, we measure how much we see it shine. Now we repeat the same procedure for a second light bulb at a different distance, but also close. Then comparing both distances and both apparent brightness, we now know how much the brightness of a light bulb change when the distance is different.

Now we have all the tools that we need to know the distance to any light bulb in our own field. Simply we measure the brightness of any of them using a photometer and then apply the mathematical Rule of Three.

Suppose now that we convince the owner of the other field (which we want to measure the distance) to buy bulbs of all kinds, including some 60 watts light bulbs. Then we could use our original method to measure the distance to the 60 watts light bulbs in the other field. Then we can conclude that the distance between the two fields is equal to the distance between us and any of the 60 watts light bulbs in the other field.

You’re probably wondering: how can this method be useful? The universe is not populated by objects with equal brightness, but by stars, galaxies and other objects, each one shining with its own intensity. Where can we find the 60 watts light bulbs in the sky? Well, those exist and there are called Cepheid stars.

Cepheids are a particular class of variable stars, whose intrinsic brightness (the watts in our analogy) varies rhythmically with a very regular period. This variation in the brightness of the Cepheid is produced by a series of contractions and expansions that occur in the same star.

The important feature of Cepheid stars is that if two of them have the same period (takes the same amount of time to change their brightness) then those two have the same intrinsic brightness (have the same watts). Here we arrive then to the final solution: if we find Cepheid stars in our galaxy (our field) and in the other galaxy far away (the far field) with the same period, we can measure how we see them shine and use them as 60 watts light bulbs, thereby determining the distance to other galaxies.

While the galaxy we want to measure the distance is close enough to identify individual stars, the previous method works perfectly. But what happens if the galaxy is so distant in space that we can only see it as a small spot, without the possibility of identify individual stars?

In the case of such distant galaxies, in the order of billions of light years away, the method of Cepheid stars is not useful. If we cannot identify individual stars, we cannot identify Cepheids and use them as 60 watts light bulbs. In this case, the solution was given to us by one of the most energetic, chaotic and huge events out there: supernovae.

Explained in a very simplistic way, a supernova is the chaotic death of certain types of stars, which experienced a huge explosion and scattered all their stuff through space, releasing a billion times more energy than it is released by the sun in all its life.

Supernovae are so energetic and powerful events that they shine even more than the galaxy they belong. It is the same as saying that a supernova emits in a short period of time more energy and luminosity than hundreds billions of stars together.

There are different types of supernovae, depending on how the process that leads to the explosion occurs. We use a particular kind of supernovae in measuring cosmological distances: the so-called Type Ia Supernovae.

This type of supernovae commonly occurs in binary systems: when two stars orbit each other. But not just any type of stars: the most common scenario of type Ia supernovae occur between the combination of a white dwarf and a massive star, orbiting each other.

A white dwarf is the remnant left after the death of a low-mass star, less than ten solar masses. When the regular lifetime of such stars is completed, instead of exploding, they softly expel its outer layers into space and all that remains is its small core, which we call white dwarf.

When a white dwarf and a massive star orbit each other, a particular phenomenon is produced: the gravitational force created by the white dwarf on the outer layers of the massive star initiates a process through which the white dwarf takes material from the massive star; that is to say, the white dwarf begins to absorb mass from its companion.

And here is what really matters: The white dwarf can take material from its companion until it reaches a certain mass. This amount of mass is the limit value which we know as the Chandrasekhar limit, equivalent to 1.44 solar masses. When the white dwarf reaches this limit, it cannot continue absorbing material, then an uncontrollable nuclear chain reaction is produced, and that leads to the final explosion that we call supernovae.

Chandrasekhar limit sets the maximum amount of mass that exists in the white dwarf before the explosion. Therefore, it establish how big the explosion is and how much mass is involved in it. As a result, these types of supernovae have the same intrinsic brightness and they emit the same amount of light. And the brightness is so intense that we can observe it even in the most distant galaxies: here we have our incredibly powerful 60 watts light bulbs.

If we find this kind of supernovae in distant galaxies and measure their apparent brightness, we can then compare the brightness with the one of a nearby type Ia supernovae. This will determine the distances to these supernovae, and therefore, the distance to the galaxy in which they find themselves.

I hope you enjoyed this podcast about how we measure distances to stars and galaxies. You can find more information about astronomy and science in my blog at www.ciencialimada.com.ar. I wish you all clear skies.

Goodbye.

End of podcast:

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