Age may only be a number, but when it comes to the age of the universe, it's a pretty important one. According to research, the universe is approximately 13.8 billion years old. How did scientists determine how many candles to put on the universe's birthday cake? They can determine the age of the universe using two different methods: by studying the oldest objects within the universe and measuring how fast it is expanding.

The universe cannot be younger than the objects contained inside of it. By determining the ages of the oldest stars, scientists are able to put a limit on the age.

The life cycle of a star is based on its mass. More massive stars burn faster than their lower-mass siblings. A star 10 times as massive as the sun will burn through its fuel supply in 20 million years, while a star with half the sun's mass will last more than 20 billion years. The mass also affects the brightness, or luminosity, of a star; more massive stars are brighter. [Related: The Brightest Stars: Luminosity & Magnitude]

Dense collections of stars known as globular clusters have similar characteristics. The oldest known globular clusters have stars with ages between 11 and 18 billion years old. The wide range comes from problems in pinpointing the distances to the clusters, which affects estimates of brightness and thus mass. If the cluster is farther away than scientists have measured, the stars would be brighter, thus more massive, thus younger than calculated.

The uncertainty still creates a limit to the age of the universe; it must be at least 11 billion years old. It can be older, but not younger.

The universe we live in is not flat and unchanging, but constantly expanding. If the expansion rate is known, scientists can work backwards to determine the universe's age, much like police officers can unravel the initial conditions that resulted in a traffic accident. Thus, finding the expansion rate of the universe — a number known as the Hubble constant — is key.

A number of factors determine the value of this constant. The first is the type of matter that dominates the universe. Scientists must determine the proportion of regular and dark matter to dark energy. Density also plays a role. A universe with a low density of matter is older than a matter-dominated one.

To determine the density and composition of the universe, scientists rely on missions such as NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and The European Space Agency's Planck spacecraft. By measuring the thermal radiation left over from the Big Bang, missions such as these are able to determine the density, composition and expansion rate of the universe. The leftover radiation is known as the cosmic microwave background, and both WMAP and Planck have mapped it. [INFOGRAPHIC: Cosmic Microwave Background: Big Bang Relic Explained]

In 2012, WMAP estimated the age of the universe to be 13.772 billion years, with an uncertainty of 59 million years. In 2013, Planck measured the age of the universe at 13.82 billion years. Both of these fall within the lower limit of 11 billion years independently derived from the globular clusters, and both have smaller uncertainties than that number.

Correction: This article was updated on Dec. 27, 2013, to correct a statement about the universe's age as it relates to the objects contained inside of it.