Editing Standard Candles (section)

== What to Say ==

When you look at something, how can you tell how far away it is? If you're really close to the object, your two eyes can use parallax for depth perception (see parallax demo). If you're farther away, we need a different technique. When you see two stars in the sky, do you have any idea which one is farther away?

Astronomers use special objects in the sky called standard candles to measure distance. Just by observing these objects, we can tell how far away they are. How does this work?

This demonstration has lots of opportunities for the audience to make observations and participate as volunteers. Make sure to call on audience members that raise their hands politely.

Part 1 of the demonstration: Here we prove that the two flashlights have the same brightness. The observer should agree that each flashlight looks the same.

Part 2: The farther away the object is, the dimmer it looks! The observer should agree that the near flashlight looks much brighter. This is because the light spreads out from the flashlight; as it gets further and further away, it gets spread out over a larger and larger area, so it appears dimmer and dimmer. So, we can tell how far away an object is if we know how bright it really is and we know how dim it looks to us on Earth.

Part 3: But how can we be sure that an object is really as bright as we think it is? The observer should note that the flashlight in the back can be turned on so bright that it appears even brighter than the close object. If we thought these objects were the same brightness, we would misjudge which one is farther away. Astronomers need to be very careful to make sure that any two objects are really at the brightness we think they are - that they really are standard candles. There are all sorts of objects that have other special properties that we can use to tell how bright they really are.

Part 4: So, if we know how bright something really is and how bright it appears to us on Earth, can we definitely determine its distance? Not always! There's another problem to consider. The observer should note that the flashlight covered in plastic wrap appears dimmer, even though its at the same distance. If there is material in between us and the light source, it can appear dimmer than it would simply due to its distance. Is there anything out in space to obscure our vision of stars? Yes! The Interstellar Medium (ISM) is a bunch of gas and dust that fills the space in our galaxy. In some places, the ISM is very thick and the stars behind it appear very dim. In other places, the ISM is thinner and doesn't obscure the stars as much. Notice also that the ISM, modeled by out plastic wrap, makes the light source appear more red. This is a property that always seems to be true of the ISM - it absorbs lots of blue light, but lets most of the led light go through. Astronomers call the effect of the ISM "extinction."

Part 5: So how can we overcome this problem? When you add or remove layers of plastic wrap, note that the extinction either increases or decreases, respectively. This means that the light source appears more or less bright and more or less red. When we look into space, we don't know if the light source is dim because it is far away or because it is highly extincted. However, we can tell how high the extinction is by how red the object is! If the object is really red, we know we're looking at it through lots of ISM. We can measure this reddening and use it to figure out how much dimming is caused by the ISM. Then we know how bright the object would appear to us on Earth if it were unobstructed, so we can tell how far away it is just like we did earlier in the demonstration.