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This concept is so weird and wonderful that I think a pre-quiz is definitely appropriate!
What is a standard candle?
A standard candle may not sound fun based off of its formal definition, but its applications are both extremely cool and very necessary to our understanding of cosmic distances.
What Astronomical Objects can be Called "Standard Candles"?
There are many different types of standard candles in our universe but the two most common, and therefore the most helpful, are Type 1A supernovae and Cepheid variables.
I will do an a more in-depth explanation of each of these standard candles in another post but first let us explore how and why a standard candle is important to determining astronomical distances.
How Standard Candles Help us Understand Distance in Space
If you stood on a sidewalk at night and looked off into the distance, you would most likely see a street lamp casting a patch of light to help you get safely from one spot of the street to the next. While these lamps are generally spaced equally, let us assume that there is some street where lamps have been placed at distinctly different distances from one another. Some unusual scientist (me) has tasked you with telling me how far away each of these street lamps are from you. The catch is that you aren't allowed to move from where you are to physically measure these lamp's distances and you are only allowed to know one lamp's luminosity.
Many might be stumped- but if you're familiar with the concept of the standard candle, this should be a breeze. Luminosity tells us the power output (energy) of a given object, it is an object's intrinsic brightness.
Unfortunately, this is where the street lamp metaphor breaks down. If you haven't guessed, the lamps represent standard candles and the task of determining their distances from us is a real problem astronomers have solved. There is actually a formula that helps us determine a star's (or a lamp's) distance from us.
Equation's for Determining Distance
The equation is relatively simple:
m = M - 97.5 + 5xlog(D)
Where m is apparent brightness, that can be defined as:
m = -2.5xlog(I) - 11.5
In the equation above we define I as (L/4(pi)d^2), L= Luminosity and D= distance
You might ask- what is the point of requiring two unknown variables (the apparent magnitude and the distance) in a formula when we are ultimately solving for distance?
Fortunately, in actual astronomy we can use tools like parallax to help us determine the distances of relatively close stars and from there determine the apparent magnitude of stars farther from us.
So let us put these ideas together. For stars very far away from us we have no way of accurately measuring their distances, and we certainly can't get up and physically measure them, no, much like you and the street lamp, we are bound to our place.
However, standard candles give give off the same amount of energy anywhere in the universe- they are a consistent and calcluable light sources that we can use to figure out most bodies distances from us.
Armed with luminosities and our trusty equations celestial bodies can no longer shroud their distances from us!