The most fundamental truth about stars is that when they run out of fuel to burn, they die.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. The overwhelming majority of stars today are M-class stars, with only 1 known O- or B-class star within 25 parsecs. Our Sun is a G-class star, and more massive than 95% of all stars in the Universe. The most massive stars burn through their fuel the fastest and live the shortest; the older a star cluster is, the redder the stars inside are. (WIKIMEDIA COMMONS USER LUCASVB, ADDITIONS BY E. SIEGEL)

The least massive stars burn their fuel the most slowly, living the longest, while the most massive ones burn out the fastest.

High resolution near-infrared imaging has led to the discovery of three stellar superclusters at the Galactic Center. Since near-infrared wavelengths cut through the dense dust between Earth and the Galactic Center, we are able to see these superclusters. They include the Central Parsec, Quintuplet, and Arches clusters. But all the stars found there, and in the galactic center in general, are quite young.(GEMINI OBSERVATORY)

New stars form in large clusters, creating stars of all different masses simultaneously.

A stellar nursery in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. By surveying star clusters and field stars in and out of our galaxy, as well as measuring the extent of the Milky Way, we can simply determine the number and types of stars that exist. (NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA)-ESA/HUBBLE COLLABORATION)

As they age, the more massive stars die first, leaving only the lower-mass ones behind.

The open star cluster NGC 290, imaged by Hubble. These stars, imaged here, can only have the properties, elements, and planets (and potentially chances-for-life) that they do because of all the stars that died before their creation. This is a relatively young open cluster, as evidenced by the high-mass, bright blue stars that dominate its appearance. (ESA & NASA, ACKNOWLEDGEMENT: DAVIDE DE MARTIN (ESA/HUBBLE) AND EDWARD W. OLSZEWSKI (UNIVERSITY OF ARIZONA, USA))

We can date star clusters by examining which stars remain when we plot out stellar color vs. temperature.

The stars present within a newborn cluster will typically run from the upper-left to the lower-right: the main sequence. As a cluster ages, the stars ‘turn off’ the main sequence, as the upper-left stars die first. Based on where this turn-off appears, we can date the age of the cluster. However, in all open and globular star clusters, a few blue stragglers, stars higher-up on the main sequence than normal, can be found. (CHRISTOPHER TOUT, NATURE 478, 331–332 (2011))

The older a cluster is, the redder, lower-mass, and less bright its surviving stars are.

The ancient globular cluster Messier 15, a typical example of one of these incredibly old globular clusters. The stars inside are quite red, on average. (ESA/HUBBLE & NASA)

Globular star clusters are the oldest; some haven’t formed stars in ~13 billion years.

The globular cluster Messier 69 is highly unusual for being both incredibly old, at just 5% the Universe’s present age, but also having a very high metal content, at 22% the metallicity of our Sun. The brighter stars are in the red giant phase, just now running out of their core fuel, while a few blue stars are these unusual blue stragglers.(HUBBLE LEGACY ARCHIVE (NASA / ESA / STSCI), VIA HST / WIKIMEDIA COMMONS USER FABIAN RRRR)

Yet if we look closely inside these ancient relics from the young Universe, we’ll find a few blue stars.

The stars inside a globular cluster are tightly bound at the center and frequently merge, which could explain why there are larger populations of blue straggler stars in the innermost regions of globular clusters. (M. SHARA, R.A. SAFER, M. LIVIO, WFPC2, HST, NASA)

These “blue stragglers” have lifetimes of 2 billion years or less: incompatible with the cluster’s age.

The clusters, stars, and nebulae in our Milky Way are useful for coming up with an age estimate for the stars located in various parts, but bluer stars than we expect will be present in a large number of clusters. (ESO / VST SURVEY)

But there’s an explanation: many stars have companions.

When massive objects in binary systems get close to one another, they can either merge, creating a new object with their combined mass, or one can siphon mass off of the other, growing into a significantly larger object. (MELVYN B. DAVIES, NATURE 462, 991–992 (2009))

By siphoning mass or merging, the net product will be a new, more massive star.

The old open star cluster, NGC 188, has a few blue stragglers (circled). About a third of the blue stragglers we know of in this cluster have white dwarf companions, suggesting the mass-siphoning scenario as a major contributor to the formation of these stellar oddities. (K. GARMANY, F. HAASE NOAO/AURA)

Every old cluster we know of has at least a few blue stars.

Blue straggler stars, circled in the inset image, are formed when older stars or even stellar remnants merge together or siphon mass from one to another. After the last stars have burned out, the same process could bring light to the Universe through the merger of brown dwarfs to create red stragglers. (NASA, ESA, W. CLARKSON (INDIANA UNIVERSITY AND UCLA), AND K. SAHU (STSCL))

Blue stragglers exist because, in dense environments, stars can’t help but interact.