From anywhere on the earth’s surface, in the absence of light pollution, looking up at the night sky can reveal upwards of 6,000 stars to the dark-adjusted naked eye. It is also easy to note that different stars come in different apparent brightness levels as well as different colours – all of which can tell us a lot about the stars themselves.
While the colours of different stars can be used to infer their surface temperatures, the observed brightness of stars from earth is itself a function of two contributing factors – the actual luminosity of the star and its distance from our planet. An extremely luminous star located some 1,000 light years away might look significantly dimmer than a less luminous star located a mere 10 light years away, simply because the latter is much closer to us. The measurements of distances to nearby stars is crucial in understanding the true nature of these stars themselves and other similar stars at further distances.
Stars can be classified in several manners; classification by stellar mass and radius is typically preferred. Red dwarf stars are typically the smallest kind of star, with stellar masses up to 0.25M (where 1M is the mass of the sun). Low to intermediate mass stars can range anywhere between 0.25M and 10M, while giant stars are typically those with a mass greater than 10M. Such stars evolve in significantly different ways, and last for varying periods of time.
Red dwarfs, though the smallest, are the most resilient – lasting upwards of one trillion years, with their small mass contributing to a lower rate of hydrogen fusion and their fully convective nature meaning that all hydrogen in the star will be available for fusion. On the other hand, low to intermediate mass stars and giant stars last for significantly less – a few billion or a few million years each, respectively.
So what is the fate of a sun-like star? After around 10 billion years of fusing core hydrogen to helium, the sun will run out of hydrogen at the core and start fusing hydrogen located just outside the helium core instead.
After this outer hydrogen envelope is also used up, the core contracts, reaching high enough temperatures to fuse helium instead.
This causes a significant rise in core temperatures, consequently heating up and expanding the star to a red giant. After a few million years, the red giant sun starts to lose its outer layers, eventually shedding them all to reveal its remnant core – a white dwarf.
And what about giant stars? Their fate is even more catastrophic – after several successive core contractions and stellar expansions, giant stars become supergiants, fusing silicon to iron in their core, which process takes up more energy then it releases. This eventually results in the collapse of the massive supergiant star on its own core, crushing the atomic nuclei at its core to form a ball of neutrons. It releases so much energy at this point that the collapsing material reverses its inward collapse and is expelled outwards instead in a massive explosion – a supernova. Depending on the remnant core’s mass, this will leave behind either a neutron star or a black hole!
Josef Borg completed a PhD in astronomy at the Institute of Space Sciences and Astronomy, University of Malta, and is currently a researcher at the Faculty of Health Sciences at the University of Malta.
Sound Bites
• Book released recounting anecdotes from Maltese who experienced the lunar landing of Apollo 11: A book describing the series of events leading up to the Apollo lunar landings has been released by Gordon Caruana Dingli. The book, named We Went to the Moon, includes a series of anecdotes by Maltese individuals who remember the events unfolding on the night of the first lunar landing in 1969.
• Possible observation of the first extragalactic exoplanet: The first observation of an exoplanet located outside of the Milky Way was announced in Nature Astronomy, but confirming the presence of such an exoplanet will prove significantly difficult. If confirmed, the exoplanet itself is located in a rather inhospitable place – an X-ray-emitting binary system, with a neutron star or black hole accompanying a nearby star.
https://skyandtelescope.org/astronomy-news/did-we-find-a-planet-in-another-galaxy/
For more soundbites, listen to Radio Mocha every Saturday at 7.30pm on Radju Malta and the following Monday at 9pm on Radju Malta 2 https://www.fb.com/RadioMochaMalta/.
DID YOU KNOW?
• Supernovae have been witnessed with the naked eye from earth! While several supernova explosions have been witnessed even in very faraway galaxies, using telescopes, supernovae occurring in our own galaxy have been observed with the naked eye even from significantly far away! An example is the supernova explosion witnessed in 1054AD, which is thought to have resulted in the formation of a neutron star in the Crab Nebula – the supernova remnant.
• The sun is only halfway through its estimated supply of hydrogen at the core! At around 4.6 billion years old, the sun still has another five billion years until it expands to a subgiant, fusing hydrogen in the stellar envelope just outside the core!
For more trivia, see www.um.edu.mt/think.