|Star nursery in constellation Carina|
Image Credit & License: ESO/G. Beccari
The dating of starlight and establishing a chronology of cosmic events is based on very exact high technology and deep theoretical understanding of light and its sources.
Astronomic time scales only began to be known from the work of Edwin Hubble and his colleagues at Mt. Wilson Observatory. Henreietta Swan Leavitt had realized in 1908 that variable stars, cepheids, could be used as stellar milestones because their regularly changing temperature and diameter provides accurate evidence for distances. Analysis of cepheids at Palomar led Hubble to the shocking realization that there are many galaxies out there in the deep space way beyond Milky Way.
Light year is a unit that combines distance and time. Accordingly, measuring the distance to the light source also gives date when photons and other radiation started their cosmic journey before reaching Earth. The current estimation of Big Bang to 13.8 billion years ago relies on this foundation. From evidence gathered from the light and radiation coming from different stars and galaxies it has been concluded that Sun is about 4.5 billion years old and will keep burning hydrogen into helium for another five billion years.
Since the discovery of pulsars in 1967 the research of radiation from these rapidly rotating neutron stars has added highly accurate stellar timers to the toolbox of cosmologists that rival atomic clocks in accuracy.
So the beautiful picture of open cluster NGC 3293 has both distances and dates that make it alive and multi-dimensional. The cluster is at the distance of about 8000 light years. The young blue "baby stars" in it are about 6 million years old and the reddish stars about 20 million years old. The formation of this cluster has taken 15 million years. (APOD).
The enhanced image thus opens in front of our eyes a canvas of extreme richness including dots of light that are indeed very old and dots that are fresh and young in the cosmic chronology.