The beauty of a star-strewn sky greets the eyes of avid stargazers every night, anywhere on the globe where light pollution has not yet deprived us of this natural heritage. A look through a humble telescope reveals the even more numerous collection of stars pertaining to our home galaxy, as well as complex gaseous structures such as nebulae and whole other galaxies, outside our own Milky Way. Notwithstanding the massive distances between different stars, however, several stars do not spend their churning lifetime as solitary fusing bodies. On the contrary, several stars tend to group up – binary, trinary, quadruple and quintuple systems are well known. The famed Northern star, Polaris, is itself a quintuple system, actually composed of five gravitationally-bound stars.
Stars can however group up in significantly larger numbers. Such clusters of stars are formally known as star clusters within the astronomical community. These can be of two types – open star clusters and globular star clusters. The former type of star clusters tends to be the more well known, with two examples of such open star clusters visible to the naked eye. The Pleiades star cluster, also known as the Seven Sisters, and the Hyades star cluster, are both open star clusters easily visible from Malta in our winter skies. Open star clusters are typically short-lived structures, containing a smaller number of stars, with stars in such regions forming together from the same original nebula, eventually ousting the longer-lived smaller stars due to strong gravitational interactions until eventually only the larger, shorter-lived stars remain. For this reason, any open star clusters we see are necessarily ‘young’ – which in cosmic terms, means a few tens of millions of years – otherwise, all the massive stars in such clusters would have already died out, and the cluster would be no more.
Globular clusters, on the other hand, are significantly different. While open star clusters tend to have random star arrangements, globulars tend to have well- rounded, spherical structures with higher concentrations of stars towards their cores. Globular clusters are notoriously large – with some examples having upwards of a hundred thousand stars very tightly bound together and – once again – having originally most likely formed from the same nebula, even though multiple episodes of star formation have been proposed. The strong gravitational force keeping these structures together means that several smaller stars are not lost over time, as opposed to open star clusters. This means that these longer-lived stars outlive their larger counterparts in the cluster itself, happily fusing hydrogen for billions of years. As a result, globular clusters represent some of the oldest structures in our galaxy – at around 10 to 12 billion years old, these have been around since the early days of the Milky Way’s formation.
Famous examples of globular clusters include the Hercules globular cluster, some 25,000 light years away from earth and containing upwards of 300,000 stars, and the famous Omega Centauri globular cluster, some 17,000 light years from earth and a whopping 150 light years across, hosting some 10 million stars alone. The nature of globular clusters provides a unique opportunity for constraining estimates of the age of the universe as well as provide estimates for the presence of the galactic core.
Dr 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
China unveils first Mars photos from Zhurong rover
• China has released its first two photographs taken by its Zhurong rover, which touched down on Mars on Friday, May 14, as part of the country’s Tianwen-1 mission. This mission makes China only the second country to land successfully soft-land on Mars. The photographs released by the CNSA showcase parts of the rover with Utopia Planitia in the background.
https://www.space.com/china-mars-rover-zhurong-first-photos
Osiris-Rex leaves asteroid Bennu as it makes its way home
• NASA’s Osiis-Rex has begun its long journey back to earth, carrying within it precious samples of asteroid Bennu. The spacecraft will orbit the sun twice, in an orbit closer to the sun than Venus, before making a flyby of earth in September 2023, when it will release its payload, which payload will then land on our planet.
https://skyandtelescope.org/astronomy-news/nasas-osiris-rex-leaves-asteroid-bennu-heads-home/
For more sound bites 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?
• On any one night, around 6,000 stars can be seen with the naked eye from anywhere on earth! This figure will, unfortunately, be significantly lower if one lives in light polluted locations, where fainter stars are not visible at all. Indeed, from the most heavily light polluted locations, even brighter stars can be barely visible, with some of the more familiar constellations only barely visible.
• Globular clusters and the blue straggler conundrum. Observations by astronomers showed that globular clusters, thought to contain only very old, smaller stars, did indeed contain the occasional larger, bluish stars. This was at first mysterious, since such large stars should not have survived for billions of years and star formation in these regions should have seized. However, it turned out that these large blue stars formed more recently as a result of collisions between stars in the cluster itself!
• The Pleiades star cluster is currently passing through a dusty nebula. Observations and images of the Pleiades star cluster show that it is surrounded by tendrils of dusty filaments, resulting in a blue reflection nebula. This was initially thought to be the remnant of the nebula through which the Pleiades’s stars formed, but this is not the case – the stars formed elsewhere, but are currently passing through a dusty nebular region, which is unrelated to their formation.