Understanding the Lifecycles of Stars
Stars are a fundamental building block in the universe and dominate the night sky seen with the unaided eye. The Sun is considered a benchmark for middle-aged stars, formed from a cloud of gas and dust that collapsed, with a byproduct of a surrounding planetary system. Such a star-formation scenario seems to be typical and intuitive, but individual quite dramatic variations exist depending upon the circumstances of the star-formation environment.
Detailed observations and computer simulations have been used hand-in-hand to understand the lifecycles of stars, their chemistry, the nuclear processes within them, and the nature of the gas and dust (the interstellar medium or ISM), out of which these objects form. HST has been used to probe the intricate complexity of these environments, and has unveiled stars and planetary systems in the making.
The chemistry of stars depends upon the material in which they originate. In the early universe, stars were formed from material that lacked most elements except for hydrogen and helium. The other chemical elements have been and still are being created in the interior of stars through nuclear fusion processes, and that new material can be dredged to the surface of the star and/or eventually in turn recycled into subsequent generations of stars and planets. The later generations of stars and planets thus inherit chemistry from earlier populations of stars and planets.
Groupings and Clusters of Stars
The other interesting observational evidence is that most stars form in multiple star systems and that many have planets even when multiple stars are present. The detailed nature of this formation process is physically tricky and not completely understood. The groupings of stars that form together can vary from a few stars to many hundreds or thousands of stars. Typically, star clusters that have formed in the last 13 billion years or so form in collections with a few thousand, hundreds, or tens of stars. The stars in each cluster have a variety of masses. Theoretical models agree with the observations that the most massive stars are rare, whereas the least massive stars are the most numerous as a result of the physical process of star formation.
HST observations have probed star clusters of all sizes, and spectroscopy is used to determine the detailed chemistry in star cluster members. Precise HST observations of star cluster members to determine their luminosities and temperatures has helped to refine our understanding of star formation, stellar evolution, and the physics of the theoretical models used to explain these phenomena. How planetary systems form in such close quarters is a surprise, and it appears that large numbers of massive planets end up in very close orbits around their stars.
The most massive star clusters, containing tens and hundreds of thousands of stars, were mostly formed early on in the universe, about 13 billion years ago or so. These massive clusters, called globular clusters, still persist today although the stars in them have evolved over time, and the cluster characteristics are tracers of the earliest times of cosmic star formation. HST observations of globular clusters have revealed subtle differences in globular clusters, their chemistry, and in some cases, evidence that these clusters actually have multiple generations of stars within them.
Demise of Stars
Once formed, stars use up their nuclear fuel, and due to the diminishing amount of energy being produced in their cores countered by the gravitational pressure of the overlying layers, a readjustment of the stellar interior structures occurs. Cepheid variable stars pulsate and change their brightness due to this readjustment. Many Cepheids have been systematically and precisely measured with HST in nearby galaxies, establishing them as signposts for gauging distances in the universe, and assist in mapping the cosmic expansion rate and acceleration.
Average and smaller mass stars can eject their outer shells as beautiful planetary nebulae and then are left as residual hot cinders. HST observations with exquisite image quality have demonstrated the diversity and complexity of the ejection physics. More massive stars can eject a substantial amount of their outer shells more spectacularly, some becoming supernovae.
Many stars are in binary systems, and the ejection of material during the structural readjustments can result in the binaries exchanging matter and often blowing off that material explosively. HST observations have monitored such explosive events and their aftermath to aid in understanding the many mechanisms that can cause stars to explode so dramatically.
Also, the particular type supernova, SN 1a, are very bright when they explode, and are the critical “standard candle” that can be observed throughout the cosmos for measuring the rate of expansion and acceleration of the universe. By measuring Cepheids and Supernova in the same galaxy, the distance scale is extended outwards in the universe, and the precision of those measurements legislate the precision of the derived expansion and acceleration. Many of the more massive stars, having suffered catastrophic explosive events, end up as neutron stars, pulsars, and black holes.