Stellar Astronomy: Part 8 – Stellar Clusters and Populations

Stars often exist as binary pairs – two or more stars that orbit their common center of mass.  However, even larger numbers of stars can exist in close proximity to each other – a star cluster – which can consist of hundreds to hundreds of thousands of stars.  Single stars, binaries, and star clusters exist as part of a much larger structure – a galaxy – which can contain millions to trillions of stars.  Furthermore, the composition of stars varies slightly in a way that depends on where they are found.

Stellar Astronomy: Part 7 – Black Holes

In the previous article, we examined white dwarfs and neutron stars, objects with the mass of a star compressed into a very small volume.  Some of these objects could be what remains of a star that has collapsed in on itself.  But what happens when mass is compressed into an even smaller volume?

Stellar Astronomy: Part 6 – White Dwarfs and Neutron Stars

In a star, the outward flow of energy generated by nuclear fusion in the core balances the inward pull of gravity.  Without such fusion, a star would collapse into a very small volume.  And indeed, the universe contains objects with a mass comparable to that of a star, but with a size comparable to the Earth.  These are called white dwarfs.

Stellar Astronomy: Part 5 – Variable Stars

How do stars change over time?  In parts 3 and 4, we discussed some of the scenarios secular astronomers have proposed for stellar evolution.  These included the now-discredited idea that stars evolve along the main sequence from blue to red, along with modern ideas of giants and supergiants being aged stars.  Unfortunately, such long-term changes cannot be observed and therefore are beyond the scope of operational science.  Nonetheless, some stars change in ways that have been observed in history, and some types of changes are even observable in the present.  In particular, stars can change in luminosity, appearing either brighter or fainter over time.  These are called variable stars.

Stellar Astronomy: Part 4 – Of Dwarfs and Supergiants

It is rather amazing what we can know about stars simply by analyzing their light and with rigorous application of logic and mathematics.  We can measure the distance to any nearby star using parallax.  And by multiplying a star’s apparent brightness by the square of its distance, we can determine its actual luminosity.  Furthermore, we can know both the composition and the surface temperature of a star by analyzing the absorption lines of its starlight using a spectroscope.  This led to the field of stellar classification and the discovery of the main sequence.  But why does this sequence exist?  Why are blue main sequence stars so much brighter than red main sequence stars?