Few night sky sights are as majestic as the arching band of light that is the Milky Way galactic plane. Viewed from a dark location, the galactic plane’s diffuse light can be observed, interspersed with tendrils and filaments of dust. That light originates from some 100 billion stars – although some estimates push that figure up to 400 billion – residents of the Milky Way galaxy, just like our own star, the Sun.
An image of the Milky Way galactic plane, shot from Two Rocks, Perth, Australia by Josef Borg.The Milky Way is a barred spiral galaxy, with the Earth and the rest of our solar system located on one of the outer spiral arms, named the Orion arm, some 26,500 light years from the galaxy’s core. For context, one light year is the distance that light covers in one year, travelling at almost 300,000 kilometres every second, putting one light year at a staggering 9.46 trillion kilometres. While traditional estimates suggested a diameter of 100,000 light years for the entire galaxy, more recent estimates suggest that the actual diameter of our galaxy lies between 1.5 to two times that amount. The innermost 10,000 light years of the galaxy form the galactic bulge, with a high concentration in stars, gas and dust. Our view of the Milky Way as an arch of light in the night sky is therefore due to our point of view from the outer reaches of our galaxy, looking towards the galactic core. Although the galaxy is indeed spiral, our position within the galaxy provides us a cross-sectional view of the spiral structure, with the brightest parts of the Milky Way being those corresponding to the regions of highest concentration of stars, towards the core.
What lies at the core of our galaxy? This was a question that evaded our understanding for quite some time, due to the difficulty in observing past all the stars, gas and dust present in higher concentrations towards the galactic core. Today, thanks in large part to the use of telescopes which can observe outside the visible spectrum of light, we know that our core plays host to an object of massive dimensions – a supermassive black hole, some four million times the mass of the Sun, named Sagittarius A*. The Sun, and indeed the rest of the stars in our galaxy, orbit the galactic core. The Sun, together with the rest of the solar system, takes an astounding 240 million years to complete one orbit around the galactic core – meaning that the Sun and the Earth have orbited the core 20 times since their formation around 4.6 billion years ago.
Even with its sheer size and number of stars, each possibly with their own systems of planets, the Milky Way is but one of an estimated two trillion galaxies in the observable universe. Some of these galaxies are also substantially larger than our own, albeit several are also smaller. Each of these galaxies contains millions or billions of stars of its own, with each of these stars possibly playing host to their own systems of planets as well!
Josef Borg is currently a PhD student within the Institute of Space Sciences and Astronomy, University of Malta, and also the president of the Astronomical Society of Malta.
Did you know?
• Mass present in our galaxy is largely unaccounted for. From the calculation of the orbital velocities of different stars in our Milky Way galaxy, it is possible to calculate the mass that needs to be present in our galaxy for those stars to reach such velocities at their distance from the galactic core. Results obtained indicate that the Milky Way needs to be approximately six to seven times as massive as observed with all normal matter accounted for using different telescopes. This strange, unaccounted for mass has been attributed to dark matter. The same is also true for other galaxies!
• The Milky Way has satellite galaxies. Two dwarf galaxies in particular are well known – the Small and Large Magellanic clouds. These two irregular dwarf galaxies are not visible from Malta, being always below the horizon from our location since they are Southern hemisphere objects. Even just a short exposure from lower latitudes, however, reveal these two cosmic neighbours, located approximately 150,000 (Large Magellanic cloud) and 200,000 (Small Magellanic Cloud) light years away.
• You don’t need a telescope to observe the Milky Way! The Milky Way arches across the night sky, predominantly visible in the summer months due to our planet’s location with respect to the Sun and the galactic core. Covering such an extensive portion of the sky, no telescope is required to view the Milky Way – indeed, a telescope would hinder observation of the full galactic plane as it would only show a smaller portion at a time! Simply find a dark location, away from light pollution, and lift your gaze up towards the south on a moonless summer night. However, binoculars and telescopes can be used to view portions of the galaxy under higher magnifying power.
Sound bites
• Astronomers spot the fastest spinning magnetar ever seen
Far-off in the Milky Way galaxy, 21,000 light-years from Earth, astronomers have spotted the fastest-spinning magnetar (and possibly youngest, too) ever seen. Magnetars are a unique type of neutron stars, which are the collapsed cores of supergiant stars that died in supernova events. What sets magnetars apart from other neutron stars is that they possess extremely powerful magnetic fields — in fact, the most powerful ones in the known universe.
https://www.space.com/fastest-youngest-magnetar-discovery
• Rocky ‘super-Earth’ planet spotted orbiting one of the Milky Way’s oldest stars
One of the oldest stars in the Milky Way galaxy hosts an unusually hot, rocky ‘super-Earth’ planet, a new study reports. Known as TOI-561b, this exoplanet is about 50 per cent larger and three times more massive than Earth, researchers said. It whips around its host star in a very close orbit, taking less than 12 hours to complete one lap.
https://www.space.com/super-earth-exoplanet-old-star-milky-way
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