Links in the Cosmic Distance Ladder

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  • 0:02 The Distance Ladder
  • 0:55 Radar & Parallax
  • 1:43 The Variable Stars
  • 2:48 Secondary Distance Indicators
  • 3:53 Tertiary Distance Indicators
  • 4:39 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson goes over the important basics regarding the cosmic distance ladder and primary, secondary, and tertiary indicators. It will also define standard candles and parallax.

The Distance Ladder

You obviously know that the universe is a vast place. Yet, astronomers constantly come up with measurements as to how far away something is from Earth or from another celestial object. How do they do this if they don't have huge measuring sticks or the ability to travel to somewhere far away? Well, astronomers have to rely on workarounds for measuring vast distances in the universe just like surveyors have to on Earth.

The methods that are used by astronomers to determine distances to faraway celestial objects are known collectively as the distance ladder, pyramid, or scale. The true links in the distance ladder are quite complex when it comes to explaining their details and are more nuanced that what this lesson explores. Thus, in this lesson, we'll just focus on the major points of how astronomers measure great distances in the universe.

Radar and Parallax

Using radar ranging, accurate measurements can be made about the distances from Earth to the planets in our solar system. However, if we want to measure the distances to objects farther away, like nearby stars, we need to rely on stellar parallax. Parallax is the apparent change in position of an object when viewed from two different locations.

The word is kind of intimidating, but you actually see it every day. For example, if you drive past a house where there's a tree in the front lawn, the tree will appear to one side of the house as you approach the house, and on the opposite side of the house as you move past the home - that's parallax. The tree didn't move, but it sure seemed to!

The Variable Stars

The problem with measuring stellar parallax is that it doesn't work well for distance beyond a few hundred parsecs. Hence, astronomers have to use a different method for finding the distances to nearby galaxies. For this end, astronomers can rely on a distance indicator (sometimes called a standard candle), which is a celestial object whose luminosity is well-known and used to find the distance to a faraway object, like a galaxy.

The best primary distance indicators are called Cepheid variables, which are variable stars (stars whose brightness varies as seen from Earth). By comparing the apparent brightness of a Cepheid variable with its luminosity (meaning its intrinsic brightness), astronomers can find the distance to the galaxy that contains them. Basically, a standard candle is like a light, be it a lighter or a cell phone, out in the dark reaches of a stadium during a rock concert; the bigger and brighter the light, the farther out we can see it.

Secondary Distance Indicators

But just like parallax is limited to only certain distances, so are the primary distance indicators. That's because these primary distance indicators aren't bright enough to be identified in distant galaxies. Therefore, secondary distance indicators are used. In short, these guys are brighter than the primary distance indicators and can, thus, be seen at very great distances.

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